Advances in Agronomy, Volume 68

Agronomy D VA N C E S VOLUME I N 68 Advisory Board Martin Alexander Ronald Phillips Cornell University Universi...

Author: Donald L. Sparks

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Agronomy D VA N C E S

VOLUME

I N

68

Advisory Board Martin Alexander

Ronald Phillips

Cornell University

University of Minnesota

Kenneth J. Frey

Larry P. Wilding

Iowa State University

Texas A&M University

Prepared in cooperation with the American Society of Agronomy Monographs Committee Jon Bartels Jerry M. Bigham Jerry L Hatﬁeld David M. Kral Linda S. Lee

Diane E. Stott, Chairman David M. Miller Matthew J. Morra John E. Rechcigl Dennis E. Rolston

Donald C. Reicosky Wayne P. Robarge Richard Shibles Jeffrey J. Volenec

Agronomy

DVANCES IN

VO L U M E

68

Edited by

Donald L. Sparks Department of Plant and Soil Sciences University of Delaware Newark, Delaware

ACADEMIC PRESS San Diego

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Boston

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∞ This book is printed on acid-free paper. Copyright © 2000 by ACADEMIC PRESS All Rights Reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the Publisher. The appearance of the code at the bottom of the ﬁrst page of a chapter in this book indicates the Publisher’s consent that copies of the chapter may be made for personal or internal use of speciﬁc clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc. (222 Rosewood Drive, Danvers, Massachusetts 01923), for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to other kinds of copying, such as copying for general distribution, for advertising or promotional purposes, for creating new collective works, or for resale. Copy fees for pre-1999 chapters are as shown on the title pages. If no fee code appears on the title page, the copy fee is the same as for current chapters. 0065-2113/00 $30.00 Explicit permission from Academic Press is not required to reproduce a maximum of two ﬁgures or tables from an Academic Press chapter in another scientiﬁc or research publication provided that the material has not been credited to another source and that full credit to the Academic Press chapter is given.

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Contents Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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A LIFETIME PERSPECTIVE ON THE CHEMISTRY OF SOIL ORGANIC MATTER M. Schnitzer I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II. Soil Organic Matter (SOM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III. Humic Substances: Analytical Characteristics . . . . . . . . . . . . . . . . . . . IV. Chemical Structure of Humic Substances . . . . . . . . . . . . . . . . . . . . . . V. Nitrogen-, Phosphorus-, and Sulfur-Containing Components of SOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI. Colloid Chemical Characteristics of Humic Acids and Fulvic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VII. Water Retention by Humic Substances . . . . . . . . . . . . . . . . . . . . . . . . VIII. Reactions of Humic Substances with Metals and Minerals . . . . . . . . . IX. Interactions of Pesticides and Herbicides with Humic Substances . . . X. Functions and Uses of Humic Substances . . . . . . . . . . . . . . . . . . . . . . XI. Conclusions and Outlook for the Future . . . . . . . . . . . . . . . . . . . . . . . XII. Personal Encounters with Outstanding Scientists . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 4 11 21 30 36 38 41 44 45 46 47 54

REPRODUCTIVE DEVELOPMENT IN GRAIN CROPS DURING DROUGHT Hargurdeep S. Saini and Mark E. Westgate I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II. Sensitivity to Drought at Various Reproductive Stages . . . . . . . . . . . . III. Nature of Injury. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV. Water Relations of Reproductive Tissues and Their Inﬂuence on Yield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Physiological and Metabolic Bases for Reproductive Failure under Drought . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI. Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

60 61 62 68 71 85 86

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CONTENTS

ADVANCES IN CHLORIDE NUTRITION OF PLANTS Guohua Xu, Hillel Magen, Jorge Tarchitzky, and Uzi Kafkaﬁ I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II. Behavior of Chloride in Soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III. Chloride in Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV. Chloride in Crops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Chloride Management in Fertilization and Irrigation . . . . . . . . . . . . . VI. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

98 99 103 127 134 139 140

OXISOLS S. W. Buol and H. Eswaran I. II. III. IV. V. VI. VII. VIII. IX.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Historical Background. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Geography of Oxisols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Deﬁnition and Kinds of Oxisols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Processes and Formation of Oxisols . . . . . . . . . . . . . . . . . . . . . . . . . . . Soil–Landscape Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Features and Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ecosystem Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

152 153 158 163 164 167 170 180 187 187

CROP RESIDUES AND MANAGEMENT PRACTICES: EFFECTS ON SOIL QUALITY, SOIL NITROGEN DYNAMICS, CROP YIELD, AND NITROGEN RECOVERY K. Kumar and K. M. Goh I. II. III. IV. V. VI. VII.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Crop Residues and Their Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Decomposition of Crop Residues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Crop Residues and Management Practices. . . . . . . . . . . . . . . . . . . . . . Soil Nitrogen Dynamics and Crop Nitrogen Recovery. . . . . . . . . . . . Nitrogen Beneﬁts to Subsequent Crops . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

198 199 200 230 257 271 278 279

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

321

Contributors Numbers in parentheses indicate the pages on which the authors’ contributions begin.

S. W. BUOL (151), Department of Soil Science, North Carolina State University, Raleigh, North Carolina 27695 H. ESWARAN (151), Natural Resources Conservation Service, U.S. Department of Agriculture, Washington, DC 20013 K. M. GOH (197), Soil, Plant, and Ecological Sciences Division, Lincoln University, Canterbury, New Zealand UZI KAFKAFI (97), Department of Field Crops, Vegetables, and Genetics, The Hebrew University of Jerusalem, Rehovot 76100, Israel K. KUMAR (197),1 Soil, Plant, and Ecological Sciences Division, Lincoln University, Canterbury, New Zealand HILLEL MAGEN (97), Extension Service, Ministry of Agriculture, Tel Aviv 61070, Israel HARGURDEEP S. SAINI (59), Institut de recherche en biologie vegetale, Montreal, Quebec, Canada H1X 2B2 M. SCHNITZER (1), Eastern Cereal and Oilseed Research Center, Agriculture and Agri-Food Canada, Ottawa, Ontario, Canada K1A 0C6 JORGE TARCHITZKY (97), Dead Sea Works Ltd., Potash House, Beer-Sheva 84100, Israel MARK E. WESTGATE (59), Department of Agronomy, Iowa State University, Ames, Iowa 50010 GUOHUA XU (97),2 College of Resources and Environmental Sciences, Nanjing Agicultural University, Nanjing 210095, People’s Republic of China

1Present address: Department of Soils, Punjab Agricultural University, Ludhiana 141 004, Punjab, India. 2Present address: Department of Field Crops, Vegetables, and Genetics, The Hebrew University of Jerusalem, Rehovot 76100, Israel.

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Preface Volume 68 contains ﬁve outstanding and contemporary reviews on topics dealing with soil chemistry, plant physiology, plant nutrition, pedology, and soil and crop management. Chapter 1 is a classic review by one of the great pioneers in soil organic matter chemistry (SOM), Dr. Morris Schnitzer. Dr. Schnitzer clearly and brilliantly summarizes past and present knowledge on SOM, discussing humic substances (HS), analyses, chemical structure, N-, P-, and S-containing components of SOM, colloid chemical characteristics of HS, water retention by HS, interactions of HS with metals, minerals, and organic chemicals, and future prospects, with a lively personal discussion of interactions with other pioneers in the ﬁeld over an almost 50-year distinguished career. Chapter 2 is a comprehensive treatise on reproductive development in grain crops during drought by two leading experts: Hargurdeep S. Saini and Mark E. Westgate. The authors discuss the sensitivity of plants to drought at various reproductive stages, types of injury, water relations of reproductive tissues and their inﬂuence on yield, and physiological and metabolic bases for reproductive failure under drought. Chapter 3, by Guohua Xu, Hillel Magen, Jorge Tarchitzky, and Uzi Kafkaﬁ, presents advances in the chloride nutrition of plants. Aspects of chloride in soil, plants, and crops and chloride management in fertilization and irrigation are extensively discussed. Chapter 4 is another review in our continuing series on the 12 soil orders. S. W. Buol and H. Eswaran, distinguished pedologists, provide a very useful review on Oxisols. They provide a historical background on Oxisols and cogent discussions of the geography, kinds, and processes/formation of Oxisols, soil–landscape interactions, features and processes, and ecosystem management. Chapter 5 is a timely review on crop residues and management practices as they relate to soil quality and nitrogen dynamics. The authors, K. Kumar and K. M. Goh, discuss aspects of crop residues, including uses, decomposition, management practices, and soilnitrogen dynamics and nitrogen beneﬁts to subsequent crops. I thank the authors for their excellent reviews. Donald L. Sparks

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A LIFETIME PERSPECTIVE ON THE CHEMISTRY OF SOIL ORGANIC MATTER M. Schnitzer Eastern Cereal and Oilseed Research Center Agriculture and Agri-Food Canada Ottawa, Ontario, Canada K1A OC6

I. Introduction II. Soil Organic Matter (SOM) A. Deﬁnitions B. Relationship among SOM, Humus, and Humic Substances C. Problems Associated with Extraction of SOM from Soils D. Direct Analysis of SOM in Whole Soils by 13C Nuclear Magnetic Resonance (NMR) and Pyrolysis–Field Ionization Mass Spectrometry (Py-FIMS) E. How Is SOM Affected by Long-Term Cultivation? III. Humic Substances: Analytical Characteristics A. Chemical Methods B. Infrared (IR) and Fourier Transform IR Spectroscopy C. 13C NMR Spectroscopy D. Electron Spin Resonance Spectroscopy E. Electron Microscopy IV. Chemical Structure of Humic Substances A. Oxidative Degradation B. Reductive Degradation C. Py-FIMS of Humic Acid (HA), Fulvic Acid (FA), and Humin D. Curie-Point Pyrolysis Gas Chromatography/Mass Spectrometry (GC/MS) E. A Two-Dimensional Structure of HA F. A Three-Dimensional Structure of HA, SOM, and Whole Soil V. Nitrogen-, Phosphorus-, and Sulfur-Containing Components of SOM A. Origins and Functions of Soil Nitrogen B. Nitrogen Distribution in Soils and Humic Substances C. Amino Acids in Soils and Humic Substances D. Amino Sugars in Soils and Humic Substances E. Nucleic Acid Bases in Soils and Humic Substances F. 15N NMR Analyses of Soils and Humic Substances G. Detection of Nitrogen Compounds in Soils and Humic Substances by Pyrolysis GC/MS

1 Advances in Agronomy, Volume 68 Copyright © 2000 by Academic Press. All rights of reproduction in any form reserved. 0065-2113/00 $30.00

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VI.

VII. VIII.

IX. X.

XI. XII.

H. Phosphorus in Soils and SOM I. Sulfur Compounds in Soils and Humic Substances Colloid Chemical Characteristics of Humic Acids and Fulvic Acids A. Surface Tension, Surface Pressure, and Viscosity Measurements on HAs and FAs Water Retention by Humic Substances Reactions of Humic Substances with Metals and Minerals A. Formation of Water-Soluble Complexes B. Mixed Ligand Complexes C. Adsorption and Desorption D. Dissolution of Minerals E. Adsorption on External Mineral Surfaces F. Adsorption in Clay Interlayers Interactions of Pesticides and Herbicides with Humic Substances Functions and Uses of Humic Substances A. Functions in Soils B. Uses and Potential Uses Conclusions and Outlook for the Future Personal Encounters with Outstanding Scientists References

The author has researched the chemistry of soil organic matter for almost 50 years. In this chapter, he presents a personal account of how soil organic matter chemistry has evolved during the second half of this century from wet to computational chemistry. The chapter begins with a deﬁnition of soil organic matter and how it relates to humus and humic substances. Problems associated with the extraction of organic matter from soils, separation of the extract into humic substances, and puriﬁcation of the resulting fractions are then discussed. New experimental approaches to the in situ analysis of organic matter in whole soils to overcome these problems are described. Investigations on the chemistry of soil organic matter are outlined in terms of (a) an analytical and (b) a structural approach. The analytical approach involves determinations of the characteristics of humic substances by chemical methods, infrared, 13C nuclear magnetic resonance, electron spin resonance spectroscopy, and electron microscopy, whereas the structural approach consists of oxidative and reductive degradations, pyrolysis–ﬁeld ionization mass spectrometry, and Curie-point pyrolysis–gas chromatography/mass spectrometry. The author recounts how the results of the analytical and structural studies led to the formulation of a two-dimensional humic acid model structure and how the latter was converted with the aid of computational chemistry to a threedimensional humic acid model structure and later to three-dimensional model structures of soil organic matter and whole soils. The next topics discussed by the author are advances in the chemistry of N-, P-, and S-containing components of soil organic matter. Especially noteworthy is progress in the chemistry of N in soil organic matter, which points to a prominent role of heterocyclic N. As far as colloid-chemical characteristics of humic substances are concerned, the three parameters that control the molecular characteristics (molecular weight, size, and shape) of humic and fulvic acids are (a) the concentration of the humic substance, (b) the pH of the system, and (c) the electrolyte concentration of the medium. In the last

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part of the chapter, the author discusses how humic substances interact with water, metals, minerals, pesticides, and herbicides; lists functions and uses of humic substances; and describes personal encounters with outstanding scientists who inﬂuenced his research. © 2000 Academic Press.

I. INTRODUCTION I thank Dr. D. L. Sparks for not only inviting me to write this chapter but also for suggesting the title. After almost 50 years of continuous research on the chemistry of soil organic matter (SOM), I have learned a lot about this complex material, and I am pleased to have the opportunity to communicate some of this knowledge to readers. This chapter is not a literature review but a personal account of how SOM chemistry has evolved during the second half of this century and what the prospects for the future are. Over the years, the success of SOM chemists in dealing with these complex materials depended to a large extent on how well they could adapt newly developed methods and instruments to SOM. In the late 1940s, wet chemistry done in beakers, ﬂasks, and test tubes was predominant. The major instruments that were available to me at that time were pH meters, powered by batteries, and colorimeters requiring ﬁlters for changing wavelengths. In the early 1950s, recording ultraviolet (UV) spectrophotometers became available, and in the mid-1950s, I remember convincing my director to purchase an infrared (IR) spectrophotometer. The early 1960s saw the arrival of gas chromatographs. This was an important development because it allowed us to separate complex mixtures of humic acid (HA) and fulvic acid (FA) oxidation products, along with organic soil extracts containing alkanes, alkenes, fatty acids, and esters. In the mid-1960s we purchased a mass spectrometer, which we attached to a gas chromatograph. This allowed us to not only separate complex mixtures of organics but also to identify the separated compounds. About the same time, we saw the arrival of an electron spin resonance (ESR) spectrometer, which enabled us to measure concentrations of free radicals in humic materials and to obtain information on the nature of free radicals. ESR also helped us throw light on the symmetry and coordination of paramagnetic metals such as Fe3+, Cu2+, and Mn2+ bound to HA and FA. In the early 1980s, we purchased a liquid-state 13C nuclear magnetic resonance (NMR) spectrometer. After we had learned how to use this instrument properly, we realized that 13C NMR was of great importance to SOM chemists. It showed, for the ﬁrst time, that aliphatic C in HAs and FAs was as important as aromatic C and that the older theories that HAs were almost completely aromatic were no longer valid for SOM in most agricultural soils. Finally, the mid-1980s saw the arrival of pyrolysis–ﬁeld ionization mass spectrometry (Py-FIMS), which we applied to the in situ

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analysis of SOM, i.e., the direct analysis of SOM in whole soils, without extractions, puriﬁcations, etc. Another application of mass spectrometry was Curie-point pyrolysis–gas chromatography/mass spectrometry (GS/MS), which we used in structural studies on HAs and which resulted in proposing a two-dimensional structural HA model. We then converted the latter by computational chemistry into a three-dimensional HA model structure. We similarly generated three-dimensional model structures for SOM and a whole soil with both inorganic and organic constituents. Thus, during the past 50 years, I was fortunate to have participated, along with other scientists, in the evolution of SOM chemistry from wet chemistry to computational chemistry. In addition to studies on the chemical structure of humic substances, I also worked on determining colloid chemical properties of these materials, mechanisms of water retention, reactions with metals and minerals, and with pesticides and herbicides. Thus, the overall objective of my research was to investigate the chemical structure and reactions of humic substances. It was and still is my hope that the results of this research will assist soil scientists, agronomists, and farmers in the development of more efﬁcient management and production systems so that they can grow sufﬁcient food for an increasing population. At the end of the chapter, I describe personal encounters with some outstanding scientists of the past 50 years.

II. SOIL ORGANIC MATTER (SOM) A. DEFINITIONS The term “soil organic matter,” as used in this chapter, refers to the sum total of all organic carbon-containing substances in the soil. SOM consists of a mixture of plant and animal residues in various stages of decomposition, substances synthesized microbiologically and/or chemically from the breakdown products, and the bodies of live and dead microorganisms and their decomposing remains (Schnitzer and Khan, 1978). Solid-state 13C NMR spectra of whole soils show the presence of parafﬁnic C, OCH3-C, amino acid-C, C in carbohydrates and aliphatic structures bearing OH groups, aromatic C, phenolic C, and C in CO2H groups (Arshad et al., 1988). From the 13C NMR spectrum, aromaticity and aliphaticity of SOM can be calculated. Resulting data show that the C aromaticity of SOM seldom exceeds 55% and that the aliphaticity of SOM is often greater than its aromaticity (Schnitzer and Preston, 1986). Similarly, Py-FIMS of SOM in whole soils indicates the presence of carbohydrates, phenols, lignin monomers, lignin dimers, alkanes, fatty acids, n-alkyl mono,di, and tri esters, n-alkylbenzenes, methylnaphthalenes, methylphenanthrenes, and N compounds (Schnitzer and Schulten, 1992). Carbohydrates, proteinaceous materials (amino acids, peptides, proteins), and

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lipids (alkanes, alkenes, saturated and unsaturated fatty acids, alkyl mono, di, and tri esters) in SOM appear to be strongly retained by the aromatic SOM components and can only be separated from them with great difﬁculty. For example, even after exhaustive extractions with n-hexane, followed by chloroform, Schnitzer and Schuppli (1989) could remove only 10% of the total lipids from three agricultural soils sampled in western Canada. The separation of carbohydrates and proteinaceous materials from SOM requires prolonged hydrolyses with relatively strong acids under reﬂux. Thus, the different chemical components of SOM are closely associated to form a complex structure.

B. RELATIONSHIP AMONG SOM, HUMUS, AND HUMIC SUBSTANCES There is some confusion among soil chemists about the meanings of SOM, humus, and humic substances. Do these terms depict different materials? According to Stevenson (1994), SOM is synonymous with humus. In my opinion, the term total humic substance is also synonymous with SOM and humus as long as losses occurring during the extraction and separation procedures are held to a minimum. My deﬁnition of humic substances is that it is the sum of humic acid fulvic acid humin. While essentially each of the three terms can be used, I personally, as a SOM chemist, prefer use of the term SOM.

C. PROBLEMS ASSOCIATED WITH EXTRACTION OF SOM FROM SOILS The SOM content of agricultural soils usually ranges between 1 and 4% (w/w), with most soils containing between 2 and 3% SOM. In the soil, because SOM and inorganic soil constituents are closely associated, it is necessary to separate the two before either can be examined in greater detail. This separation is usually achieved by extracting the SOM with either dilute base (0.1–0.5 M NaOH solution) or by a neutral salt solution such as aqueous 0.1 M Na4P2O7. Extraction of SOM with a dilute base works reasonably well and was originated by Archard in 1786. Separation of the alkaline extract into HA, FA, and humin was ﬁrst carried out by Sprengel (1826). The three fractions into which the alkaline SOM extract is partitioned are (1) HA, which is that fraction of SOM that coagulates when the alkaline extract is acidiﬁed; (2) FA, which is the SOM fraction that remains in solution when the extract is acidiﬁed, i.e., it is soluble in both acid and alkali; and (3) humin, which is that SOM fraction that remains with the soil, i.e., it is insoluble in both alkali and acid. Over the years, many objections have been raised against the use of alkaline solutions, which are still the most efﬁcient SOM extractants today. Stevenson (1994) lists the following objections: (1) silica is dissolved from the

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mineral matrix, which contaminates the SOM extract; (2) protoplasmic and structural components of fresh organic tissues are dissolved, and these mix with the SOM extract; (3) autooxidation of some organic components occurs in contact with air when the extracts are allowed to stand for extended periods of time; and (4) other chemical changes can occur in the alkaline solutions, including condensation between amino acids and CuO groups of reducing sugars or quinones to form Maillard reaction products. Some of these changes can be minimized by doing the extractions in the presence of an inert gas such as N2, but not all possible changes can be excluded. Another serious difﬁculty with the extraction of SOM from soils and the partitioning of the extract into HA, FA, and humin is that these are laborious and timeconsuming procedures that are not suitable for the analysis of large numbers of soil samples. A new approach, not involving the use of wet chemical methods, is required to overcome these problems.

D. DIRECT ANALYSIS OF SOM IN WHOLE SOILS BY 13C NUCLEAR MAGNETIC RESONANCE (NMR) AND PYROLYSIS – FIELD IONIZATION MASS SPECTROMETRY (PY-FIMS) In recent years we have witnessed the development of two analytical methods, based on “high technology,” that appear to be suitable for the direct analysis of SOM in situ, i.e., in whole soils. These methods are solid-state 13C NMR and PyFIMS. The solid-state 13C NMR analysis of whole soils has been described by Wilson (1987) and Arshad et al. (1988). This type of analysis requires that the soil contain at least 3% C and that the concentration of paramagnetic ions, e.g., Fe3+, in the soil not be too high because paramagnetic ions interfere with the recording of acceptable 13C spectra. According to Arshad et al. (1988), the C:Fe (w/w) ratio is an important indicator for obtaining satisfactory solid-state 13C NMR spectra of whole soils and particle-size fractions separated from them. If the C:Fe ratio is 1, the quality of the spectrum will be good; if the ratio is 1, a reasonable spectrum will be obtained, but if the ratio is 1, the spectrum will be poor. The quality of the spectrum can be improved by reducing the Fe3+ to Fe2+ by dithionite and then removing it. Another option is to separate the soil into particle-size fractions and running 13C NMR spectra on the fractions (Wilson, 1987). Arshad et al. (1988) report that SOM-rich soil and particle size fractions can be prepared by ﬂotation and that these yield well-deﬁned 13C NMR spectra. Another approach that can be used is to separate the soil on a magnetic separator into magnetic and nonmagnetic fractions, but this method requires specialized equipment and is too timeconsuming. Figure 1 shows solid-state 13C NMR spectra of particle-size fractions separated from Culp and Rycroft soils from northwestern Alberta, enriched in SOM by ﬂotation (Arshad et al., 1988). Flotation increased the C content of the

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Figure 1 Solid-state 13C NMR spectra of (a) OM-enriched ﬁne sand fraction isolated from Culp soil, (b) OM-enriched sand fraction isolated from Rycroft soil, and (c) OM-enriched silt and clay fraction separated from Rycroft soil. From Arshad et al. (1988), with permission of the publisher.

sand fraction separated from the Culp soil from 2.05 to 20.15%, the C content of a similar fraction separated from the Rycroft soil increased from 3.16 to 15.62%, whereas the C content of the silt and clay fraction separated from the same soil increased from 3.08 to 8.59%. The three resulting spectra are well deﬁned and are characteristic of SOM or humic materials (Schnitzer and Preston, 1986). The major signals are at 30 ppm (parafﬁnic C), 73 and 102 ppm (carbohydrate C), 130 ppm (aromatic C) 150 ppm (phenolic C), and 173 ppm (C in CO2H groups). The aromatic C content is lower than that of many soil HAs. It is hoped that with improvements in 13C NMR equipment and technology, it will be possible to analyze soils that contain 3%C, which would include many agricultural soils.

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Although 13C NMR spectroscopy provides information on the different types of C in SOM, a method that yields data on SOM at the molecular level is Py-FIMS. This method is more sensitive than 13C NMR and can also be used for the direct analysis of SOM in soils. The Py-FIMS spectrum of the whole Armadale soil (Schnitzer and Schulten, 1992), shown in Fig. 2, is dominated by carbohydrates (m/z 60, 72, 82, 84, 96, 98, 110, 112, 114, 126, 132, 144, and 162), phenols (m/z 94, 108, 110, 122, 124, 126, 138, and 154), monolignins (m/z 164, 166, 178, 180, 182, 194, 196, 208, 210, and 212), dilignins (m/z 246, 260, 270, 272, 274, 284, 286, 296, 298, 300, 310, 312, 314, 316, 326, 328, 330, 340, 342, and 356), and suberin derived esters (m/z 446, 474, 502, and 530). The signals at m/z 170 and 184 arise from tri- and tetramethylnaphthalene, respectively, whereas m/z 178, 192, 206, 220, and 234 are due to phenanthrene, and methyl-, dimethyl-, trimethyl-, and tetramethylphenanthrene, respectively. Also, n-C10 to n-C18 alkyl diesters are present. Normal alkylbenzenes range from m/z 442 (C6H5C26H53) to m/z 470 (C6H5C28H57). The occurrence of N-containing compounds is indicated by m/z 59 (acetamide), 67 (pyrrole), 79 (pyridine), 81 (methylpyrrole), 93 (methylpyridine), 103 (benzonitrile), 117 (indole), 131 (methylindole), and 167 (not identiﬁed). The two methods just described allow SOM chemists to obtain signiﬁcant information on the chemical composition of SOM in whole soils, i.e., in situ. They also make it possible to study the chemistry of SOM without extracting it from the soil, without partitioning it into HA, FA, and humin, and without having to lower the ash content of each of these fractions. It is noteworthy that while both 13C NMR and Py-FIMS provide similar chemical information on SOM, the problem that faces SOM chemists at this time is to decide whether the analysis of SOM in whole soils by advanced methods is the path to follow or whether they want to continue using the “classical” approach that involves the extraction and separation of HA, FA, and humin. As shown in Section III,C, 13C NMR spectra of HA and FA demon-

Figure 2 Py-FI mass spectra of the whole Armadale soil. From Schnitzer and Schulten (1992), with permission of the publisher.

A LIFETIME PERSPECTIVE

9

strate that the main structural features, as well as the aromaticity and aliphaticity of the two humic fractions, are quite similar. Also, the 13C NMR spectrum of humin, after deashing, is similar to that of HA (Preston et al., 1989). These ﬁndings do not support conclusions of earlier workers (e.g. Sprengel, 1826) that HA, FA, and humin are different substances that are separated by the “classical” extraction procedure. The terms HA, FA, and humin do not stand for distinct chemical substances. Both 13C NMR and Py-FIMS show that the three are closely related materials and that the separation scheme proposed by earlier workers has no chemical validity. The obvious solution to the problem is the direct SOM analysis of the whole soil or soil fractions by 13C NMR or Py-FIMS. Data so obtained tell us about the chemical composition of SOM in terms of aliphatics, proteins, carbohydrates, aromatics, phenols, heterocyclic N compounds, etc. These are chemical classes of compounds, and analytical data are readily understood by all chemists. It is essential that we start to express ourselves in the language of chemistry and no longer use terms that have no chemical meaning. I propose that SOM chemists use the term SOM for all C-containing compounds in the soil, high molecular weight SOM for HA, low molecular weight SOM for FA, and insoluble SOM for humin. Similarly, water chemists could use the term natural organic matter (NOM) for SOM, high molecular weight NOM for HA, low molecular weight NOM for FA, and insoluble NOM for humin. While it is true that enormous literature on the chemical and physical properties of humic substances (HA, FA, humin), consisting of thousands of scientiﬁc papers, has accumulated over the past 200 years, it is not necessary to abandon or disregard this huge literature. Older data can be reinterpreted and will help us to better understand the information generated by the new analytical approaches.

E. HOW IS SOM AFFECTED BY LONG-TERM CULTIVATION? Little is known about how SOM is affected by long-term cultivation. Schulten et al. (1995) employed Py-FIMS of whole soils, a method described in the previous section, to throw light on this problem. Soil samples originated from a Typic Haploboroll under a long-term crop rotation established in 1910 at Lethbridge, Alberta. One soil sample, collected in 1910 from the Ahorizon after breaking the native grassland, had been stored. Another soil sample was collected in June 1990 from the A horizon under a wheat-fallow, nonfertilized, rotation. This sample had been under cultivation for 80 years. The native sample (collected in 1910) contained 3.03% SOM but the cultivated sample (collected in 1990) contained only 2.23% SOM. Percentages of sand, silt, and clay and the exchange capacities of the two samples were, however, almost identical. The aggregate stability of the native sample was 65%, whereas that of the cultivated sample was only 42%. There were signiﬁcant reduc-

10

M. SCHNITZER

tions in enzyme activities after 80 years of cultivation. The activity of dehydrogenase dropped by 60%, that of acid phosphatase by 77%, and that of urease by 82%. While qualitatively the Py-FIMS spectra were similar, (total ion intensities) (TII), which are related to SOM concentrations, were dramatically different for the two samples. The TII value per milligram of soil for the native sample was 31.25 104 counts compared to only 3.96 104 counts for the cultivated sample. As illustrated in Fig. 3, TII values of the summed signals characteristic of the major SOM components that are carbohydrates (carboh), phenols and lignin monomers (phenols), alkylaromatics (alkylar), N-containing compounds (N-comp), peptides, lipids, and lignin dimers (lignin) show quantitatively different compositions. Despite similar SOM contents, the TII for the SOM in cultivated soil is only one-sixth of that in the native soil. Signals in Py-FIMS spectra indicating the presence of carbohydrates, phenols and lignin monomers, alkylaromatics, and N-compounds in SOM of the cultivated sample are only between one-ﬁfth and one-seventh of the intensities generated by the same compound classes in the SOM of the native sample. TII values of the signals for peptides, lipids, and dimeric lignins in the cultivated sample constitute even smaller proportions of similar components of the SOM in the native sample.

Figure 3 Total ion intensity (TII) values of summed signals for major SOM components that are carbohydrates (carboh), phenols and lignin monomers (phenols), alkylaromatics (alkylar), N-containing compounds (N-comp), peptides, lipids, (alkanes, alkenes, fatty acids, esters), and lignin dimers (lignin). (a) Native sample, (b) cultivated sample. From Schulten et al. (1995), with permission of the publisher.

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11

A possible explanation for data obtained is that the components identiﬁed by Py-FIMS in the cultivated sample originate from more thermally stable and higher molecular weight SOM than those present in the native sample. Thus, cultivation causes increased polymerization and cross-linking of the major SOM components, leading to the formation of larger molecules with higher molecular weights, stability, and complexity (Schulten et al., 1995). Increases in molecular size and chemical complexity of the SOM in the cultivated sample may explain the observed decreases in enzyme activities involving the C, N, and P cycles. Thus, anthropogenic disturbances through cultivation may induce signiﬁcant changes in the quality, chemical composition, and molecular size of SOM. While these changes may help preserve and maintain SOM in agricultural soils, little is known on how they affect soil biology.

III. HUMIC SUBSTANCES: ANALYTICAL CHARACTERISTICS A. CHEMICAL METHODS Humic substances contain per unit weight relatively high concentrations of oxygen-containing functional groups (CO2H, OH, CuO). It is through these groups that these materials are capable of attacking and degrading soil minerals by complexing and dissolving metals, transporting these throughout the soil, and making them available to plant roots and microbes. Metal–humic complexes with widely differing chemical and biological stabilities and characteristics are formed. Interactions between humic substances and metal ions have been described as ion exchange, surface adsorption, chelation, peptization, and coagulation reactions (Schnitzer, 1978). FA at any pH and HA at pH 6.5 can form stable water-soluble metal complexes in competition with hydrolysis reactions. HA is water insoluble at pH 6.5 but exhibits sorption properties that lead to the concentration of metals, especially trace metals, and organics on its large surface. The elemental composition and functional group content of a typical HA (extracted from the A horizon of a Haploboroll) and a FA (extracted from the Bh horizon of a Spodosol) are shown in Table I. A more detailed analysis of data shows that (1) the HA contains approximately 10% more C, but 36% less O than the FA; (2) there are quantitatively smaller differences between the two materials in H, N, and S contents; (3) the total acidity and CO2H content of the FA are signiﬁcantly higher than those of the HA; (4) both materials contain per unit weight signiﬁcant concentrations of phenolic OH, total CuO, and OCH3 groups, but the FA is richer in alcoholic OH groups than the HA; (5) about 74% of the total O in the HA is accounted for in functional groups, but

12

M. SCHNITZER Table I Analytical Characteristics of a Haploboroll HA and a Spodosol FA HA Element (g kg1) 564 55 41 11 329 Functional groups (cmol kg1) Total acidity 660 COOH 450 Phenolic OH 210 Alcoholic OH 280 Quinonoid CuO 250 Ketonic CuO 190 OCH3 30 E4 /E6 4.3

C H N S O

FA

509 33 7 3 448 1240 910 330 360 60 250 10 7.1

all of the O in the FA is similarly distributed. The E4 /E6 ratio of the FA is almost twice as high as that of the HA, which means that the FA has a lower particle or molecular weight than the HA (Chen et al., 1976).

B. INFRARED (IR) AND FOURIER TRANSFORM IR SPECTROSCOPY IR and FTIR spectra of humic substances show bands at 3400 cm1 (H-bonded OH), 2900 cm1 (aliphatic C–H stretch), 1725 cm1 (CuO of CO2H, CuO stretch of ketonic CuO), 1630 cm1 (COO, CuO of carbonyl and quinone), 1450 cm1 (aliphatic C–H), 1400 cm1 (COO), 1200 cm1 (C–O stretch or OH deformation of CO2H), and 1050 cm1 (Si–O of silicates). The bands are usually broad because of the extensive overlapping of individual adsorbances. IR and FTIR spectra reﬂect the preponderance of oxygen-containing functional groups, i.e., CO2H, OH, and CuO in humic materials. While IR and FTIR spectra provide worthwhile information on the functional groups and their participation in metal–humic interactions, they tell us little about the chemical structure of humic materials. Celi et al. (1997b) applied FTIR to the analysis of CO2H groups in a number of HAs. Concentrations of CO2H groups in HAs were determined directly from FTIR spectra by totaling adsorbances at 1720–1710 cm1 (CO2H) and 1620–1600 cm1 (COO). Good correlations were found between total carboxyl groups determined by FTIR, a wet chemical method, and by 13C NMR. Thus, depending on

A LIFETIME PERSPECTIVE

13

the equipment and facilities available, soil chemists have a choice of methods that can be used for measuring CO2H groups in HAs.

C.

13C

NMR SPECTROSCOPY

Until about the mid-1980s, when the use of liquid- and solid-state 13C NMR became more widespread, most soil chemists thought that the chemical structure of humic substances was predominantly aromatic. 13C NMR demonstrated that aliphatic structures in humic substances were often as important, and sometimes even more important than aromatic structures (Schnitzer and Preston, 1986; Wilson, 1987; Norwood, 1988; Schnitzer, 1991). This was a very signiﬁcant development in our understanding of the chemistry of humic substances. Aromaticities of HAs extracted from soils of widely differing pedological origins range from 30 to 60% (Schnitzer, 1991). A substantial portion of aliphatic carbons in HAs consists of parafﬁnic carbons. Of considerable interest are the prominent resonances in both liquid- and solid-state 13C NMR spectra of humic substances near 130 and 132 ppm, which can be assigned to C in aromatic rings that are not substituted by strong electron donors such as O and N but by C. Alkaylaromatics are typical structures that produce such resonances (Breitmaier and Voelter, 1978). Of special interest in the context of this discussion is a comparison of solid-state 13C NMR spectra of a HA (extracted from the Ah horizon of a Haploboroll) and a FA (extracted from the Bh horizon of a Spodosol). The HA spectrum in Fig. 4 shows several distinct peaks in the aliphatic (0–105 ppm), aromatic (106–150 ppm), phe-

Figure 4 Solid-state 13C NMR spectra of HA (extracted from the Ah horizon of a Haploboroll) and FA (extracted from the Bh horizon of a Spodosol).

14

M. SCHNITZER

nolic (155–160 ppm), and carboxyl (170–180 ppm) regions. The signals at 17, 21, 25, 27, and 31 ppm are likely due to alkyl C. The resonance at 17 ppm is characteristic of terminal CH3 groups and that at 31 ppm of (CH2)n in straight parafﬁnic chains. The resonance at 40 ppm could also include contributions from both alkyl and amino acid C. The broad signal at 53 ppm and the sharper one at 59 ppm may be due to C in OCH3. Amino acid C may also contribute in this region (Breitmaier and Voelter, 1978). Carbohydrates in HA would be expected to produce signals in the 60 to 65, 70 to 80, and 90 to 104 ppm regions, although other types of aliphatic C bonded to O could also do so. The aromatic region contains a relatively sharp maximum near 130 ppm due to alkyl aromatics. The peak at 155 ppm indicates the presence of O- and N-substituted aromatic C (phenolic OH and/or NH2 bonded to an aromatic C). The broad signal near 180 ppm is due to C in CO2H groups, although amides and esters could also contribute to this resonance. The 13C NMR spectrum of the FA (Fig. 4) consists of a number of aliphatic resonances in the 20- to 50-ppm region, followed by signals from C in OCH3 groups, amino acids, and carbohydrates between 50 and 85 ppm. Broad signals between 130 and 133 ppm indicate the presence of C in alkyl aromatics. The strong signal between 170 and 180 ppm shows the presence of C in CO2H groups. In general, fewer sharp signals are observed in the 13C NMR spectrum of the FA than in that of the HA, possibly because of more H bonding in the FA. 13C NMR data for HA and FA are summarized in Table II in terms of the distribution of C in the different spectral regions. An examination of data in Table II shows a similar C distribution in the two humic fractions. HA is slightly more aro-

Table II Distribution of C (%) in a Haploboroll HA and a Spodosol FA as Determined by 13C NMR % of C Chemical shift range (ppm)

HA

FA

0–40 41–60 61–105 106–150 151–170 171–190 Aliphatic C (0–105 ppm) Aromatic C (106–150 ppm) Phenolic C (151–170 ppm) Aromaticitya

24.0 12.5 13.5 35.0 4.5 10.5 50.0 35.0 4.5 44.1

15.6 12.8 19.3 30.3 3.7 18.3 47.7 30.3 3.7 41.6

((Aromatic C phenolic C) /(Aromatic C phenolic C aliphatic C)) x 100.

a

A LIFETIME PERSPECTIVE

15

matic than FA, but FA is richer in CO2H groups, which appears to be the main difference between the two humic substances. Other differences are that HA is richer in parafﬁnic C but poorer in carbohydrate C than FA. However, on the whole, the main structural features, as well as aromaticity and aliphaticity, are similar so that HA and FA have similar chemical structures. These ﬁndings disagree with those of Sprengel (1826) and other earlier workers who thought that the chemical structures of HA and FA were quite different from each other. Little is known about the chemical structure of humin, which is that portion of SOM that stays behind after extraction of the soil with dilute alkali. In a more recent investigation, Preston et al. (1989) deashed the surface layer of Bainsville soil with aqueous HCl/HF (1.16 M HCl and 2.88 M HF) at room temperature for a prolonged period of time. With progressive deashing, the humin became more soluble in 0.5 M NaOH. After extensive deashing the solid-state 13C NMR spectrum was similar to that of HA extracted from the same soil. This suggests that humin is essentially HA bound strongly to soil minerals. Valuable information on the chemical structure of humic substances can be obtained by combining 13C NMR with chemical methods. In this manner, effects on the chemical structure of humic substances of different extractants, methylation, hydrolysis, oxidation, and reduction can be evaluated. Spectra shown in Fig. 5 illustrate this point. Figure 5a shows the solution-state 13C NMR spectrum of HA extracted from the Ah horizon of a Haploboroll from northern Alberta (Preston and Schnitzer, 1984). The presence of aliphatic C (i.e., C in straight chain, branched and cyclic alkanes, alkanoic acids, and other aliphatic compounds) is indicated by signals in the 10to 40-ppm region of the spectrum. Carbon in proteinaceous materials (amino acids, peptides, and proteins) exhibits resonances between 40 and 60 ppm, whereas C in OCH3 groups shows signals near 56 ppm. Signals between 61 and 105 ppm arise from C in carbohydrates. Resonances between 106 and 150 ppm are due to aromatic C whereas those between 150 and 160 ppm arise from phenolic C. The strong signal between 170 and 180 ppm comes mainly from C in CO2H groups, with possibly some contributions of C in esters and amides. Figure 5b shows the solution-state 13C NMR spectrum of the same HA after hydrolysis for 24 hr with hot 6 M HCl. Most of the resonances in the 40- to 105-ppm region arising from C in proteinaceous materials and carbohydrates are no longer observed because of the hydrolytic removal of these materials by the hot acid. Also, the intensity of the signal between 170 and 180 ppm, due largely to C in CO2H groups, has been reduced because of partial decarboxylation of these groups by the strong acid. However, intensities of the two principal HA components, i.e., aliphatic C (10 –60 ppm) and aromatic C (106 –150 ppm), remain undiminished. Thus, by removing carbohydrates and proteinaceous components, acid hydrolysis “puriﬁes” the HA and allows us to study the remaining aliphatic and aromatic components in greater detail.

16

M. SCHNITZER

Figure 5 Solution-state 13C NMR spectra of a HA extracted from the Ah horizon of a Haploboroll before (a) and after (b) hydrolysis with hot 6 M HCl. From Schnitzer (1991), with permission of the publisher.

D. ELECTRON SPIN RESONANCE SPECTROSCOPY Electron spin Resonance (ESR) spectroscopy measures free radicals (unpaired electrons) in humic substances. It has been known since the early 1960s (Rex, 1960; Steelink and Tollin, 1962) that humic substances contain free radicals that may participate in a wide variety of organic–organic and organic–inorganic interactions. The theory and a number of applications of ESR spectroscopy have been described by Atherton (1973). The ESR spectrum of a typical HA is shown in Fig. 6. The spectrum consists of a single line devoid of hyperﬁne splitting. From the spectrum (by comparison with a standard), the number of free radicals per unit weight as well as the g value (the spectroscopic splitting constant) and also the line width can be calculated. From the magnitudes of the g values for humic materials, with most ranging from 2.0038 to 2.0042 (Senesi and Schnitzer, 1977), it appears that prominent free radicals in humic materials are semiquinones or substituted

A LIFETIME PERSPECTIVE

Figure 6

17

ESR spectrum of a typical HA.

semiquinones. There are two types of free radicals in these materials: (a) permanent free radicals with long lifetimes and (b) transient free radicals with relatively short lives (several hours). Transient free radicals in humic materials can be generated in relatively high concentrations by chemical reduction, irradiation, or increase in pH (Senesi and Schnitzer, 1977). Permanent free radicals, in contrast, appear to be stabilized by the complex chemical structures of humic materials, which can act both as electron donors and as electron acceptors. These oxidation– reduction reactions are reversible and can be assumed to proceed in the following manner:

18

M. SCHNITZER

The semiquinone (shown in the center) can be produced either by the reduction of a quinone or by the oxidation of a phenol. Under alkaline conditions, a semiquinone anion and the semiquinone dianion are formed. Except for indicating the presence of phenols, semiquinones, and quinones in humic materials, ESR spectroscopy has so far contributed little to our understanding of the structural chemistry of these substances. The main reason for this is that it has been difﬁcult to split the signal. However, signiﬁcant new information has been generated by ESR on the symmetry and coordination of metals in metal–humic complexes (Cheshire et al., 1977; Lakatos et al., 1977; Senesi et al., 1977; McBride, 1978; Boyd et al., 1981; Cheshire and Senesi, 1998). HAs and FAs interact with paramagnetic metal ions to form a variety of metal–organic complexes. These metal ions include Cu2+, (VO)2+, Mn2+, Mo(V) and Mo(III), Cr3+, and Fe3+. ESR parameters of complexes formed between these metal ions and HA and FA have been described in considerable detail by Cheshire and Senesi (1998). Especially interesting is the ESR spectrum of a Fe3+ –FA complex (Fig. 7) (Senesi et al., 1977; Cheshire and Senesi, 1998). The signal at g⬃ 2.0 (resonance C) generally arises from Fe3+ ions in close proximity to each other. This signal is removed easily by reduction with hydrazine. From Mössbauer studies, it is suggested that this type of Fe3+ is held in octahedral coordination on external HA and FA surfaces. The signal at g~ 4.3 (resonance A) is assigned to Fe3+ in an organic complex, in which high-spin Fe3+ ions occupy sites of approximately orthorhombic sym-

Figure 7

ESR spectrum of a Fe3+ –FA complex.

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19

metry. Thus, ESR spectroscopy can provide important information on the coordination and symmetry of paramagnetic metal ions complexed by humic substances.

E. ELECTRON MICROSCOPY Particle sizes and shapes of HAs and FAs seen under the electron microscope are affected by pH, electrolyte concentration, and HA and FA concentrations (Chen and Schnitzer, 1989). Freeze dried at pH 2, FA produces a scanning electron microscopy (SEM) micrograph (Fig. 8a) that exhibits elongated ﬁbers and bundles of ﬁbers (Chen and Schnitzer, 1976b; Chen et al., 1976). The ﬁbers are either linear or curved, ranging up to 6–7 m in length and 120–400 nm in thickness. At pH 4 (Fig. 8b), the ﬁbers tend to become thinner and the bundles of ﬁbers more prominent. The lengths of the ﬁbers remain unchanged, but their thicknesses are reduced to 120– 200 nm. At pH 6 (Fig. 8c), the ﬁbers diminish in numbers and thicknesses and a greater proportion of the FA occurs in bundles of closely knit ﬁbers. A pH 7 (Fig. 8d), a ﬁne network of tightly meshed ﬁbers with parallel orientation is observed. At pH 8 (Fig. 8e), the ﬁne network turns into a sheet-like structure of varying thicknesses. At pH 9 (Fig. 8f), the sheets tend to thicken. At pH 10 (Fig. 8e), the SEM micrograph shows homogeneous grain-like particles. Because of solubility constraints, the effect of pH on the structure of HA over the pH range 6 to 10 is similar to that observed on FA (Chen and Schnitzer, 1976b). Effects of increasing concentrations of neutral salts in aqueous solutions of HA and FA on their SEM micrographs following freeze-drying were studied by Ghosh and Schnitzer (1982). Fiber thicknesses increase but particle orientation gradually decreases with increases in salt concentrations. These effects are similar to those observed when the pH is decreased from neutrality to the moderately acidic region. Thus, both pH and elevated salt concentrations affect the dissociation and conformation of HAs and FAs in a similar manner. Effects of varying the concentrations of HA and FA in solution prior to freezedrying on the sizes and shapes of the micromolecules under transmission electron microscopy were examined by Stevenson and Schnitzer (1982). Single drops of dilute aqueous HA and FA solutions, adjusted to different pH values, were spread uniformly on strips of freshly cleaned mica. The solution-bearing strips were tilted and then frozen rapidly in Freon. This allowed the surface ﬁlm to run toward the lower edges of the mica and form a concentration gradient. The smallest individual particles that appear in the dilute areas are spheroidal with diameters ranging from 9 to 27 nm. Spheroids tend to coalesce to form round-shaped aggregates or linear chain-like structures. At higher concentrations, the spheroids and chainlike structures form ﬂat, elongated, multibranched ﬁbers (Fig. 9), similar to those observed earlier by Chen and Schnitzer (1976b). The width of the ﬁbers (or ﬁlaments) ranges from 20 to 100 nm. At higher concentrations, parallel arrays of ﬁbers

Figure 8 Scanning electron micrograph of a freeze-dried FA at various pH values. From Chen and Schnitzer (1976b), with permission of the publisher.

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21

Figure 9 Transmission electron micrograph of a 0.01% HA solution. From Schnitzer (1991), with permission of the publisher.

tend to coalesce to form sheet-like structures. Spheroids observed at low concentrations are larger and range in diameters from 12 to 50 nm. With increasing concentrations of HA and FA, spheroids coalesce into aggregates of spheroids and then into chain-like structures, followed by ﬁbers and ﬂattened sheets. An interesting conclusion drawn by Chen and Schnitzer (1989) is that humic substances in aqueous solutions act like ﬂexible, linear polymers rather than spheroids. When sample concentrations are high and the pH of the solution is low or when appreciable amounts of neutral electrolytes are present, humic particles may assume a spheroidal shape. At low concentrations and neutral to basic pH, the particles stretch, forming slightly coiled ﬁbers. The latter consist of a large number of oriented molecules. At low solute concentration, low ionic strength, and high pH, the coils stretch further, the ﬁlamentous structure disintegrates, and complete dispersion takes place. If, under similar conditions, the concentration of humic substances is high, sheet-like structures may be formed due to coagulation.

IV. CHEMICAL STRUCTURE OF HUMIC SUBSTANCES A. OXIDATIVE DEGRADATION One of the most useful methods for obtaining information on the chemical structure of complex organic substances is oxidative degradation. Over the course of 20 years, my co-workers and I have investigated the oxidative degradation of HAs,

22

M. SCHNITZER

FAs, humins, and whole soils using a variety of methods. These included the oxidation of methylated and unmethylated humic substances with an alkaline KMnO4 solution (Schnitzer and Khan, 1978; Grifﬁth and Schnitzer, 1989). The somewhat milder oxidation with alkaline CuO, as well as the sequential oxidation with CuONaOH KMnO4 and with CuO-NaOH KMnO4 H2O2 solutions, has also been employed (Schnitzer and Khan, 1978; Schnitzer, 1978). Humic substances have also been degraded under acidic conditions with peracetic acid and nitric acid (Schnitzer, 1978). Other oxidants used include alkaline nitrobenzene, sodium hypochlorite, and H2O2 solutions (Schnitzer and Khan, 1978). Degradations with Na2S and phenol have also been carried out (Hayes and O’Callaghan, 1989). Major compounds produced by the oxidation of methylated and unmethylated humic substances from widely differing pedological and geographical origins under alkaline as well as under acidic conditions are aliphatic carboxylic, phenolic, and benzenecarboxylic acids (Schnitzer and Khan, 1978; Schnitzer, 1978; Grifﬁth and Schnitzer, 1989). Among aliphatic oxidation products are mono-, di-, tri-, and tetracarboxylic acids. Major aromatic oxidation products are benzenecarboxylic acids such as the tri, tetra, penta, and hexa forms (Fig. 10), whereas phenolic acids include compounds containing between one and three OH groups and between one and ﬁve CO2H groups per aromatic ring (Fig. 11). From the oxidation products identiﬁed and from 13C NMR spectra of humic substances, it appears that aromatic rings are cross-linked by parafﬁnic chains (Fig. 12). On oxidation, the aliphatic carbons closest to the rings become the C of CO2H groups and remain bonded to the rings whereas the other carbons in the aliphatic chains are oxidized to either aliphatic acids or CO2. The formation of CO2 from

Figure 10

Major benzenecarboxylic oxidation products.

A LIFETIME PERSPECTIVE

Figure 11

23

Major phenolic oxidation products.

the oxidation of side chains may explain the low oxidation yields of aliphatic acids compared to benzenecarboxylic and phenolic acids. Several conclusions can be drawn from the oxidative degradation of humic substances extracted from hundreds of soils of diverse origins: (a) isolated aromatic rings are important structural units of all humic substances, (b) aliphatic chains are linking aromatic rings to form alkyl aromatic networks, (c) the model structure shown in Fig. 12 has an

Figure 12 Chemical structure for humic substances based on oxidation products.

24

M. SCHNITZER

aromaticity of 50% if we exclude functional groups, and (d) the structure in Fig. 12 also contains voids of various dimensions that can trap organic and inorganic soil constituents. These characteristics are typical of soil humic substances (Schulten and Schnitzer, 1997).

B. REDUCTIVE DEGRADATION Reductive degradation is another approach to obtaining structural information on humic substances. Essentially, the methods used most widely for this purpose are Na-amalgam reduction and Zn-dust distillation and fusion (Stevenson, 1994). Reduction with Na-amalgam produces phenols and phenolic acids that are thought to be released through the cleavage of other linkages present in humic substances. Zn-dust distillation and Zn-dust fusion are harsh methods that have been used for the structural analysis of alkaloids and other complex organic molecules. These methods yield polycyclic hydrocarbons and may provide useful information on the “core” of humic substances. Major products formed by the Zn-dust distillation of HA and FA are methyl-substituted naphthalene, anthracene, phenanthrene, pyrene, and perylene (Hansen and Schnitzer, 1969). Methyl groups on the polycyclic rings are probably the remains of longer alkyl chains linking the polycyclics in HA and FA structures.

C. PY-FIMS OF HA, FA, AND HUMIN The Py-FI mass spectrum of a HA extracted from the Armadale horizon (a Spodosol) (Fig. 13a) (Schnitzer and Schulten, 1992) shows the presence of four major components: carbohydrates, phenols, lignins, and n-fatty acids. Noteworthy is the prominence of the n-C24 (m/z 368), n-C26 (m/z 396), n-C27 (m/z 410), n-C28 (m/z 424), and n-C30 (m/z 452) fatty acids. The whole range of n-fatty acids extends from C16 to C34. Other components present in smaller amounts are monomeric lignins, n-C10 to n-C20 diesters, and n-C44 to n-C50 alkyl monoesters, of which the n-C45 monoester (m/z 662) is the most abundant. Relative weak signals characteristic of N components are m/z 59 (acetamide), 79 (pyridine), 81 (methylindole), 93 (methylpyridine), 117 (indole), 131 (methylindole), and 167. The Py-FI mass spectrum of a FA extracted from the Armadale Bh horizon (Fig. 13b) is dominated by carbohydrates, phenols, and lignins. The most intense signals are m/z 58 (acetone) and m/z 60 (acetic acid). Both compounds are emitted thermally from methylethylketones, carbohydrates, and fatty acids at temperatures 300C. The spectrum also shows the presence of smaller amounts of n-fatty acids (m/z 256, 284, 312, and 382), sterols (m/z 414), n-alkyl diesters, and monomeric and dimeric lignins. No distinct signals due to N-containing compounds can be detected. The Py-FI spectrum of a humin separated from the Armadale Ah horizon (Fig.

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25

Figure 13 Py-FI mass spectrum of (a) a HA extracted from the Armadale Ah horizon and (b) a FA extracted from the Armadale Bh horizon. From Schnitzer and Schulten (1992), with permission of the publisher.

14) shows the presence of carbohydrates, phenols, monomeric and dimeric lignins, alkylbenzenes, and alkyl esters. The presence of a homologous series of n-fatty acids, ranging from n-C16 to n-C27, is indicated. Of special interest is the series of n-alkylbenzenes with signals at m/z 316, 330, 344, 358, 372, 386, 400, 414, and 428, which appear to indicate the presence of C6H5C17H35 to C6H5C25H51 nalkylbenzenes, respectively. Molecular ions m/z 206 and 220 appear to arise from di- and trimethyl phenanthrene. Intense signals probably due to n-C10 to n-C20 nalkyl diesters are observed from m/z 202 to 342. Except for weak signals for pyrrole (m/z 67) and methylpyrrole (m/z 81), no signals due to N-containing compounds appear in this spectrum. Table III summarizes the compounds identiﬁed in the Py-FI mass spectra of HA, FA, humin, and soil. The most abundant compounds identiﬁed in the humic fractions are carbohydrates, phenols, lignin monomers, lignin dimers, n-fatty acids, nalkyldiesters, and n-alkylbenzenes. Minor components include n-alkyl mono- and diesters, n-alkylbenzenes, methylnaphthalenes, methylphenanthrenes, and N-containing compounds. HA tends to be enriched in n-fatty acids and the humin in nalkylbenzenes.

26

M. SCHNITZER

Figure 14 Py-FI mass spectrum of a humin separated from the Armadale Ah horizon. From Schnitzer and Schulten (1992), with permission of the publisher.

Assignments of the major signals in the presented mass spectra were made as described in considerable detail by Schnitzer and Schulten (1995).

D. CURIE-POINT PYROLYSIS GAS CHROMATOGRAPHY / MASS SPECTROMETRY (GC/MS) Detailed descriptions of the experimental details of Py-FIMS and of Curie-point pyrolysis GC/MS of humic substances and whole soils have been published pre-

Table III Compounds Identiﬁed in the Initial Armadale Soil and in HA, FA, and Humin Fractions Isolated from It a Compound identiﬁed

Soil

HA

FA

Humin

Carbohydrates Phenols Lingin monomers Lingin dimers n-Fatty acids n-Alkyl monoesters n-Alkyl diesters n-Alkyl benzenes Methylnaphthalenes Methylphenanthrenes N compounds n-Alkanes

b

a From

Schnitzer and Schulten (1995). weak (relative intensity <20%); , intense (relative intensity 20–60%); , very intense (relative intensity >60%). b,

A LIFETIME PERSPECTIVE

27

viously (Schulten and Schnitzer, 1997; Schulten et al., 1998). While in Py-FIMS, the sample was heated at a rate of 10 K min1 from 323 to 973 K; the ﬁnal pyrolysis temperatures of 573, 773, and 973 K were attained with the Curie-point pyrolyzer between 3 and 9.9 sec. This fast transfer of thermal energy to the sample makes this method a valuable tool for structural studies on humic materials. The resulting thermal shock produces small, stable organic pyrolysis products. While Py-FIMS shows that carbohydrates, phenols, lignin monomers, lignin dimers, lipids (alkanes, alkenes, fatty acids, and n-alkyl esters), alkylaromatics, and N-containing compounds are major HA components, Curie-point pyrolysis GC/MS of HAs indicates the presence of relatively large amounts of alkyl-substituted aromatic hydrocarbons (Schulten et al., 1991). Of special signiﬁcance is the identiﬁcation of a series of C1 to C22 n-alkyl benzenes. In addition, ethylmethyl benzene, methylpropyl benzene, methylheptyl benzene, methyloctyl benzene, and methylundecyl benzene were also detected. Other compounds identiﬁed are trimethyl- and tetramethylbenzenes, alkylnaphthalenes, and alkylphenanthrenes. The alkyl substitution of naphthalene ranges from 1 to 5 methyls, whereas that of phenanthrene ranges from 1 to 4 methyls.

E. A TWO-DIMENSIONAL STRUCTURE OF HA On the basis of both Py-FIMS and Curie-point pyrolysis GC/MS data, Schulten et al. (1991) proposed that HA consists of isolated aromatic rings linked covalently by aliphatic chains. In the hand-drawn HA structure in Fig. 15 (Schulten and Schnitzer, 1993), n-alkyl aromatics play a signiﬁcant role. Oxygen is present in the form of carboxyls, phenolic and alcoholic hydroxyls, esters, ethers, and ketones, whereas nitrogen occurs in nitriles and heterocyclic structures. The resulting carbon skeleton shows high microporosity with voids of various dimensions, which can trap and bind other organic and inorganic soil constituents as well as water. The elemental composition of the HA is C308O90N5, its molecular mass is 5540 Da, and its elemental analysis is 66.8% C, 6.0% H, 26.0% O, and 1.3% N. The HA structure in Fig. 15 is supported by chemical (Schnitzer and Khan, 1978; Schnitzer, 1978), oxidative, and reductive degradative (Schnitzer and Khan, 1978; Schnitzer (1978), colloid-chemical (Ghosh and Schnitzer, 1980) electron microscopic (Stevenson and Schnitzer, 1982), and 13C NMR and X-ray (Schnitzer, 1994) data obtained on HAs over many years and by exhaustive consultations of the voluminous literature on humic substances. As far as the model HA structure is concerned, Schnitzer and Schulten (1995) assumed that carbohydrates and proteinaceous materials are adsorbed on external HA surfaces and in internal voids and that hydrogen bonds play an important role in their immobilization. Aside from carbohydrates and proteinaceous materials, the voids can also trap and bind lipids and biocides as well as inorganics such as clay minerals and hydrous oxides.

28

M. SCHNITZER

Figure 15 Two-dimensional HA model structure. From Schulten and Schnitzer (1993), with permission of the publisher.

F. A THREE-DIMENSIONAL STRUCTURE OF HA, SOM, AND WHOLE SOIL The two-dimensional (2D) HA structure (Fig. 15) was converted to a three-dimensional (3D) structure model with the aid of Hyper Chem software. Details of the different steps involved in this conversion are published elsewhere (Schulten and Schnitzer, 1997; Schulten et al., 1998) so only the most signiﬁcant ﬁndings will be discussed here. The ﬁrst 3D HA model structure was published by Schulten and Schnitzer in 1993. Its elementary composition is C308 H335O90N5, with a molecular mass of 5547.9 g mol1 and an elemental analysis of 66.78% C, 5.79% H, 25.99% O, and 1.26% N. There are different views in the literature on SOM as to whether carbohydrates and proteinaceous materials are adsorbed on or loosely retained by HA or whether they are bonded covalently to HA. Regardless of which mechanism is considered, carbohydrates and proteinaceous materials are HA components for analytical purposes because their presence affects the elemental analysis, functional group content, and molecular weight of HA. According to Lowe (1978), carbohydrates constitute about 10% of the HA weight; a similar value has been suggested for proteinaceous materials in HA (Khan and Sowden, 1971). Thus, Schulten and

A LIFETIME PERSPECTIVE

29

Schnitzer (1993) assumed that one molecular weight of HA interacts with 10% carbohydrates and 10% proteinaceous materials. The resulting HA has an elemental composition of C342H388O124N12, with a molecular weight of 6650.8 g mol1 and an elemental analysis of 61.8% C, 5.9% H, 29.8% O, and 2.5% N. When more carbohydrates and proteins are added to the HA, the C content decreases, but the O content increases. For the development of the HA structure, Schulten and Schnitzer (1993) assumed that carbohydrates and proteinaceous materials were not integral HA components but were adsorbed in internal voids and on external surfaces. In 1997, Schulten and Schnitzer modiﬁed the model because it was too small to accommodate all oxygen-containing functional groups. Also, on average, (CH2)n chains were too long because the proposed preliminary C–C skeleton (Schulten et al., 1991) was based mainly on data obtained by Py-GC/MS and Py-FIMS, which quantitatively showed methylene units ranging from n 1 to n 20. The improved HA model, which includes a trapped trisaccharide and a polypeptide in its voids, has the following elemental composition (Schulten and Schnitzer, 1997): C305H299N16O134S1. Its elemental analysis is 57.56% C, 4.73% H, 3.52% N, 33.68% O, and 0.5% S. Its molecular mass is 6365 Da. The sizes of the voids are large enough to occlude polysaccharides, peptides, water, biocides, etc. The improved model contains 5 aliphatic and 21 aromatic carboxyl groups, 17 phenolic hydroxyls, 17 alcoholic hydroxyls, 7 quinonoid and ketonic carbonyls, 3 methoxyls, and 1 sulfur function. The aliphatic links between aromatic units have been shortened to between 1 and 10 CH2 units, with an average of n 5. Schulten and Schnitzer (1997) considered the relationship between HA and SOM. In agricultural soils, the bulk of SOM consists of humic substances (HA, FA, and humin). Several workers (Schnitzer and Khan, 1978; Schnitzer, 1978; Preston et al.,1989) have shown that the chemical structures of HA and humin are similar. According to these data, humin is HA so strongly complexed by clays and hydrous oxides that it can no longer be extracted by dilute base or acid. As far as FA is concerned, 13C NMR spectra of HA and FA are also similar. The major differences are that FA has a lower molecular weight and is richer in CO2H groups, in O, and in carbohydrates than HA, but structurally the two fractions are similar (Schnitzer, 1994). The same type of information also comes from oxidative degradation studies (Schnitzer, 1978) and Py-FI mass spectrometry (Sorge et al., 1994). Also, in many agricultural soils, except Spodosols, FA constitutes less than 10% of the SOM, so that is a minor humic fraction. Thus, for agricultural soils we can deﬁne SOM in the following manner (Schulten and Schnitzer, 1997): SOM HA carbohydrates proteins (1). For example, for a soil containing 3.0% SOM we can write: SOM 2.50% HA 0.25% carbohydrates 0.25% proteins (2). The improved 3D SOM model is based on this deﬁnition. The only major SOM component that has not been considered so far is water. The water content of air-dry HA, FA, and humin is of the order of 3.0% (M. Schnitzer, unpublished data).

30

M. SCHNITZER

As a next step, Schulten and Schnitzer (1997) further developed the improved SOM model structure to include 3% H2O. The elemental composition of this structure (Fig. 16, see color insert) is C349H401N26O173S1. Its elemental analysis is 54.0% C, 5.2% H, 4.7% N, 35.7% O, and 0.4% S, with a molecular mass of 7760 Da. Note that the elemental analysis of this three-dimensional model HA is close to naturally occurring HAs (Schnitzer, 1978). In an attempt to develop a 3D chemical model of a whole soil, Schulten and Schnitzer (1997) proposed the following deﬁnition for an average agricultural soil: Soil 3% SOM 3% H2O 94% inorganics (3). Detailed structural features of a soil particle are shown in Fig. 17 (see color insert). Voids in the model SOM structure are capable of occluding organics, inorganics, and water, and the functional groups are involved in reactions with metals and minerals and provide nutrients to plant roots and microbes. Note that in Fig. 17, SOM is bound to silicates via Fe3+ and Al3+ ions. The SOM in the simulated soil particle is surrounded by a model matrix of silica sheets. Of interest to soil chemists is that the modeled soil particle displays 23 hydrogen bonds, which again emphasizes the importance of this type of linkage. Schulten and Schnitzer (1997) calculated that 13 of the hydrogen bonds are intramolecular, 9 are in the mineral matrix, and only 1 is between SOM and a silica sheet. The spaces in Fig. 17 between the mineral matrix and SOM are several magnitudes larger than the voids in SOM so that the mineral surfaces are not completely covered or shielded by SOM. This allows access to the mineral surfaces of metal ions, small organic molecules, and water. Other SOM characteristics that can be determined by computational chemistry are surface area, volume, refractivity, polarizability, hydrophobicity, and hydration energy (Schulten and Schnitzer, 1997). These characteristics can also be helpful in the development of model SOM structures. We can expect that applications of computational chemistry to the development of model SOM structures will increase and lead to a better understanding of the spatial arrangements of the molecular constituents of SOM, organic mineral complexes, and soil aggregates. A more comprehensive knowledge of chemistry and reactions of SOM will certainly be beneﬁcial to a sustainable agriculture and help us protect the environment.

V. NITROGEN-, PHOSPHORUS- AND SULFURCONTAINING COMPONENTS OF SOM A. ORIGINS AND FUNCTIONS OF SOIL NITROGEN SOM acts as a storehouse and supplier of N to plant roots and microorganisms; almost 95% of the total soil N is closely associated with SOM (Schnitzer, 1978).

A LIFETIME PERSPECTIVE

31

Nitrogen is essential for crop production as it is an important constituent of proteins, nucleic acids, porphyrins, and alkaloids. Nitrogen is the only essential plant nutrient that is not released by the weathering of minerals in the soil. The main source of soil N is the atmosphere, where dinitrogen (N2) is the predominant gas. Only a few microorganisms have the ability to use molecular N2; all remaining living organisms require combined N for carrying out their life activities. Increases in the level of soil N occur through the ﬁxation of N2 by some microorganisms and from the return of ammonia and nitrate in rain water; losses are due to the harvesting of crops, leaching, and volatilization. Atmospheric ammonia originates from the volatilization from soil surfaces, lightening, fossil fuel combustion, and natural ﬁres. While a considerable amount of research has been done over the years on soil N, most of this work has been limited to the quantitative and qualitative determinations of proteinaceous materials, amino acids, amino sugars, ammonia, and nitrates. Reviews on soil N summarize the known organic N forms in soils (Stevenson, 1994) as well as their mineralization and importance in plant nutrition (Mengel, 1996). Because about one-half of total soil N remains unidentified and poorly understood, there is a need for more research and information in this area.

B. NITROGEN DISTRIBUTION IN SOILS AND HUMIC SUBSTANCES Sowden et al. (1977) determined the distribution of major N compounds in samples taken from soils formed under widely different climatic and geologic conditions on the earth’s surface. While the total N contents of the samples analyzed ranged from 0.01 to 1.61%, the proportions of total N that could be hydrolyzed by hot 6 M HCl were quite similar, ranging from 84.2 to 88.9%. Amino acid N varied from 33.1 to 41.7%, amino sugar N from 4.5 to 7.4%, and ammonia N from 18.0 to 32.0%. Proportions of unidentiﬁed hydrolyzable N ranged from 16.5 to 17.8%, whereas those of nonhydrolyzable N ranged from 11.1 to 15.8%. Estimates of non-protein N ranged from 55% in tropical soils to 64% in arctic soils, averaging 61% of the total N in all soils (Sowden et al., 1977). From these data it appears that about 60% of the total soil N is non-protein or, conversely, that 40% of the total soil N is protein N. To establish whether hydrolysis with hot 6 M HCl hydrolyzed all proteinaceous materials in soils and humic substances, Grifﬁth et al.(1976) hydrolyzed a number of soils and humic materials ﬁrst with hot 6 M HCl and then hydrolyzed separate samples of the acid-treated residues with either 0.2 M Ba(OH)2 or 2.5 M NaOH under reﬂux. The results obtained showed that hot 6 M HCl released almost all of the amino acids in the soil and humic substances in 24 hr.

32

M. SCHNITZER

C. AMINO ACIDS IN SOILS AND HUMIC SUBSTANCES The amino acid composition of soils and humic substances is remarkably similar. Sowden et al. (1977) compared the amino acid composition of soils with those of algae, bacteria, fungi, and yeasts and found that the amino acid composition of soils was most similar to that of bacteria. This indicates that microbes play a major role in the synthesis of proteins, peptides, and amino acids in soils. Stevenson (1994) lists the occurrence of the following -amino acids in soils— neutral amino acids: glycine, alanine, leucine, isoleucine, valine, serine, and threonine; secondary amino acids: proline and hydroxyproline; aromatic amino acids: phenylalanine, tyrosine, and tryptophane; acidic amino acids: aspartic and glutamic acid; basic amino acids: arginine, lysine, and histidine. Other amino acids ﬁrst detected by Bremner (1967) are -amino-n-butyric acid, --diaminopimelic acid, -alanine, and -aminobutyric acid. Stevenson (1994) identiﬁed the amino acids ornithine, 3,4-dihydroxyphenylalanine, and taurine in soils. These amino acids are normally not protein constituents. According to Christensen (1996), diaminopimelic acid, which originates from cell wall peptidoglycans of prokaryotes, may account for 0.5% of the total amino acid N.

D. AMINO SUGARS IN SOILS AND HUMIC SUBSTANCES The most prominent amino sugars detected in soils and humic substances are dglucosamine and d-galactosamine, with the former usually occurring in greater amounts (Stevenson, 1994). Other amino sugars found in small amounts are muramic acid, d-mannosamine, N-acetylglucosamine, and d-fucosamine.

E. NUCLEIC ACID BASES IN SOILS AND HUMIC SUBSTANCES Anderson (1957, 1958, 1961) identiﬁed guanine, adenine, cytosine, thymine, and traces of uracil in acid hydrolysates of HAs extracted from Scottish soils. At a later date, Cortez and Schnitzer (1979) determined the distribution of purines (guanine and adenine) and pyrimidines (uracil, thymine, and cytosine) in soils and humic materials. Quantitatively, the distribution in soils was guanine cytosine adenine thymine uracil. HAs were richer in guanine and adenine but poorer in cytosine, thymine, and uracil than FAs. The ratio of guanine cytosine to adenine thymine was >2 for soils and humic substances. The absence of methylcytosine suggested that the nucleic acid bases extracted from soils and humic substances were of microbial DNA origin. An average of 3.1% of total N in agricultural soils, but only 0.3% of total N in organic soils, occurs in nucleic acid bases.

A LIFETIME PERSPECTIVE

F.

15N

33

NMR ANALYSES OF SOILS AND HUMIC SUBSTANCES

Knicker and Luedemann (1995) reported that the major peak in the 15N NMR spectrum of a wheat compost prior and after hydrolysis is due to amide/peptide structures with minor resonances due to amino acid-and amino sugar N and minute signals arising from indoles, pyrroles, and imidazoles. Similar 15N NMR spectra of N compounds in peats, plant composts, whole soils, and humic materials have been published by Preston et al. (1982), Benzing-Purdie et al. (1983, 1986), Almendros et al. (1991), Zhuo et al. (1992, 1995), Zhuo and Wen (1992), Knicker et al. (1993, 1995, 1996), Knicker and Luedemann (1995), and Steelink (1994). Preston (1996) noted that in all studies done so far on soils, humic substances, and composts, 15NMR spectra recorded are very similar and remarkably simple, consisting of one major peak due to amide/peptide and a few minor signals arising from indoles, pyrroles, and amino acid N. Along the same lines, Zhuo and Wen (1992) reported that in the 15N NMR spectrum of 15N-labeled HA, 86.4% of the total area is due to amide/peptide, 4.3% to aliphatic and/or aromatic amines, and only 5.4% to pyrrole N. Similarly, Knicker et al. (1993) reported that 85% of the signal intensity in 15N NMR spectra of 15N-enriched composts and recently formed humic materials is due to amide/ peptide and that no signals in the range typical of heteroatomic N compounds are detected. From 15N spectra recorded periodically on 15N-enriched rye grass–wheat composted for 600 days, Knicker and Luedemann (1995) concluded that most of the detectable N is present in amide/peptide structures and that spectra do not reveal any 15N signals that could be ascribed unequivocally to N heterocyclics. In contrast to the 85% of the total N in soils and humic substances occurring as protein N as revealed by 15N NMR, chemical methods show that only 40% of the total N occurs as protein N in these materials (Sowden et al., 1977). What are the reasons for these wide divergencies? To provide answers to this question, it may be useful to consider the following: because natural 15 N abundance levels in soils and humic materials are low (0.4%), direct analysis by 15N NMR is very difﬁcult. Another problem is the small gyromagnetic ratio of the 15N nucleus. To overcome these difﬁculties, 15N concentrations in soils and humic substances are increased by adding 15N-labeled salts such as (15NH4)2SO4 and incubating. However, as the work of Knicker and Luedemann (1995) shows, even incubation for 600 days does not produce the same array of 15N compounds as those synthesized in the soil in the presence of reactive (catalytic) surfaces over a period of hundreds or thousands of years. It is likely that during the early stages of incubation, the microbial synthesis of proteins is the predominant reaction but that of heterocyclic N compounds may take a much longer time.

34

M. SCHNITZER

G. DETECTION OF NITROGEN COMPOUNDS IN SOILS AND HUMIC SUBSTANCES BY PYROLYSIS GC/MS Using Py-FIMS and Curie-point GC/MS, Schulten and Schnitzer (1998) identiﬁed over 100 N compounds in soils and humic substances. The N compounds identiﬁed included nonsubstituted and substituted pyrroles, pyrrolidines, imidazoles, pyrazoles, pyridines, pyrazines, nitriles, indoles, quinolines, benzothiazole, and pyrimidines. Low-mass N compounds identiﬁed were hydrocyanic acid, dinitrogen, dinitrogen monoxide, isocyanomethane, acetamide, and hydrazoic acid. A number of soil-speciﬁc N derivatives of benzene were also identiﬁed, including benzeneamine, benzonitrile, and isocyanomethylbenzene. None of the latter three compounds has so far been reported to occur in plants and microbial substances. As to the origins of the N compounds identiﬁed, it is possible that some of these compounds are pyrolysis products of amino acids, peptides, or polypeptides (Martin et al., 1979) or originate from the microbial decomposition of plant lignins and other phenolics in the presence of ammonia (Bremner, 1967) or the pyrolysis of porphyrin, a component of chlorophyll (Bracewell et al., 1987). However, there is considerable evidence that N heterocyclics are signiﬁcant components of soil N compounds rather than degradation products of other molecules produced by pyrolysis. Arguments in favor of N heterocyclics as genuine SOM components are: (a) some heterocyclics are formed by microbial synthesis in the soil from plant residues or remains of animals that contain carbohydrates, proteinaceous materials, aromatic compounds, and lipids. (b) In aquatic humic substances and dissolved organic matter (DOM) at pyrolysis temperatures of only 200 –300C, Schulten et al. (1999) have identiﬁed unsubstituted and substituted N heterocyclics such as pyrroles, pyrrolidines, pyridines, pyrans, and pyrazoles. (c) The identiﬁcation of N heterocyclics such as those referred to earlier in soils and humic substances has also been made without pyrolysis by gel chromatography–GC/MS after reductive acetylation (Schnitzer and Spiteller, 1986), by X-ray photoelectron spectroscopy (Patience et al., 1992), and by spectroscopic, chromatographic, chemical, and isotopic methods (Ikan et al., 1992). Further research is needed to identify additional N heterocyclics in soils and humic substances and to determine whether the heterocyclis are present in the soil and humic substances in the forms in which they were identiﬁed or whether they originate from more complex structures. If the latter is correct, we need to isolate these complex N molecules and identify them. It is likely that many of many of the N heterocyclics identiﬁed by Schulten and Schnitzer (1998) occur in soils and humic substances in low concentrations only so that 15N NMR in its current state of development is unable to detect them. It is hoped that with substantial improvements in instrumental design and procedures, the gulf between results obtained by 15N NMR and chemical and mass spectrometric methods will eventually narrow. On the basis of their data, Schulten and Schnitzer (1998) proposed the follow-

A LIFETIME PERSPECTIVE

35

ing distribution of total N in soils and humic substances: proteinaceous materials (proteins, peptides, amino acids), 40%; amino sugars, 5 –6%; heterocyclic N (including purines and pyrimidines), 35%; and NH3-N, 19%. Thus, proteinaceous materials and N heterocyclics are the major N components.

H. PHOSPHORUS IN SOILS AND SOM According to Stevenson (1994), up to 75% of the total P in soils occurs in organically bound forms but less than one-half of this P has been identiﬁed so far. Principal organic P forms include inositol phosphates (the major components), phospholipids, nucleic acids, and traces of phosphoproteins and metabolic phosphates (Stevenson, 1994). Inositol phosphates are esters of hexahydrocyclohexane (inositol). These esters can occur as mono-, di-, tri-, tetra-, penta-, and hexaphosphates. Inositol phosphates form insoluble complexes with metal ions, which stabilize them so that they tend to accumulate in the soil. Phospholipids identiﬁed in soils include glycerophosphatides, phosphatidyl inositol, phosphatidyl choline or lecithin, phosphatidyl serine, and phosphatidyl ethanolamine. Other organic P compounds detected in soils are glucose-1-phosphate and phosphorylated carboxylic acids (Stevenson, 1994). The following organic P compounds have been identiﬁed in soils and soil extracts by 31P NMR: alkylphosphonic ester (RCH2PO3R1R11), alkylphosphonic 2 acid (RCH2PO2 3 ), choline phosphate [(CH3 )3 N(CH2 )2PO4 ], orthophos2 phate monoester (ROPO3 ), inositol phosphates, and orthophosphate diester 31P NMR include hydroxy(RO)(RO1)PO1 2 . Inorganic phosphates identiﬁed by apatite [Ca5(PO4)3OH], crandallite [CaAl3(PO4 )2(OH)2(H2O)], orthophosphate 4 3 (PO3 4 ), pyrophosphate (P2O7 ), polyphosphate and trimetaphosphate (P3O9 ) (Wilson, 1990). More recently, Bedrock et al. (1994) determined organic and inorganic P in a HA separated from a Scottish blanket peat. The following organic P compounds were identiﬁed: (1) phosphonate, (2) inositol hexaphosphate, (3) phosphate monoester (major component), (4) aromatic phosphate diester and nucleic acid P, and (5) phosphate diester. The only inorganic P compound detected was orthophosphate. There is considerable potential for the use of 31P NMR for structural studies of P in humic substances.

I. SULFUR COMPOUNDS IN SOILS AND HUMIC SUBSTANCES Plants require S for the production of proteins, vitamins, chlorophyll, glycosides, and structurally and physiologically important sulﬁde linkages in cell walls

36

M. SCHNITZER

and sulfhydryl groups. Most plant-available S in soils comes from the weathering of minerals (Biederbeck, 1978). Over 90% of the total S in most noncalcareous soils is in organic forms. The latter can be differentiated into (a) organic S that is reduced to H2S on treatment with HI; these S forms include phenolic sulfates, sulfated polysaccharides, choline sulfate, and sulfated lipids, all of which are considered to be the most labile S forms; (b) organic S that is reduced to inorganic sulﬁde by Raney Nickel and which consists mainly of S-containing amino acids (cystine and methionine); and (c) organic S that is not reduced by either HI or Raney Nickel and which is considered to occur in the form of highly stable C–S linkages in organic compounds. The following organic S- containing compounds are known to occur in soils (Stevenson, 1994): cystathionine, choline sulfate, djenkolic acid, taurine, biotine, and thiamine. In poorly drained soils, the decomposition of organic S compounds produces volatile S compounds such as carbon disulﬁde, carbonyl sulﬁde, methyl mercaptan, diethyl sulﬁde, dimethyl sulﬁde, and dimethyl disulﬁde. So far, 34S NMR has been of little help in identifying organic S compounds in SOM. Significant advances in 34S NMR are needed before this method can assist SOM chemists in this respect.

VI. COLLOID CHEMICAL CHARACTERISTICS OF HAS AND FAS A. SURFACE TENSION, SURFACE PRESSURE, AND VISCOSITY MEASUREMENTS ON HAS AND FAS To obtain information on molecular sizes, shapes, and weights of HAs and FAs, Chen and Schnitzer (1978) and Ghosh and Schnitzer (1980) did surface tension, viscosity, and surface pressure measurements at different pHs and at varying concentrations of humic materials and neutral salts. Some of the data obtained by Ghosh and Schnitzer (1980) are shown in Table IV, which demonstrate the effect of pH on the molecular characteristics of FA. Note that at pH 2.0, the molecular ¯ n and M ¯ v) is four times as high as that at pH 6.5 and 9.5. At weight of FA (both M pH 2.0, four molecules of FA appear to combine to form an aggregate. The molec¯ 2)1/2 are also signiﬁcantly greater ular area (Aô) and the end-to-end separation (R at pH 2.0 than at the higher pH values. Ghosh and Schnitzer (1980) concluded that the three parameters that control the molecular characteristics of HAs and FAs are the concentration of the humic material, the pH of the system, and the ionic strength of the medium. From viscosity and surface pressure measurements, Ghosh and Schnitzer (1980) infer that HAs and FAs are rigid uncharged spheroids at (1) high sample concentration; (>3.5 to 5.0 g liter1), (2) low pH (6.5 for HA and 3.5 for FA), and (3) electrolyte concentrations of 0.05 M and higher. How-

37

A LIFETIME PERSPECTIVE Table IV Effects of pH on Molecular Characteristics of a Spodosol FAa pH

– nb M

A0c (m2mg1)

– 2 )1/2d (nm) (R

– ve M

2.0 3.5 6.5 9.5

4270 1180 1020 1080

0.044 0.030 0.024 0.026

3.03 2.34 2.10 2.27

9720 2580 2290 2450

a From

Ghosh and Schnitzer (1980). molecular weight. cMolecular area. dEnd-to-end separation. eViscosity-average molecular weight. bNumber-average

ever, both HAs and FAs are ﬂexible, linear polyelectrolytes at (1) low sample concentrations (3.5 g liter1), (2) pH 6.5 for HA and 3.5 for FA, and (3) electrolyte concentrations 0.05 M. The different molecular conﬁgurations are summarized in Fig. 18. In soil solutions and fresh waters, where normally both humic and salt concentrations would be expected to be low, HA (at pH 6.5) and FA (at

Figure 18 Macromolecular HA and FA conﬁgurations at different pH values and electrolyte concentrations. From Ghosh and Schnitzer (1980), with permission of the publisher.

38

M. SCHNITZER

pH 3.5) should occur as ﬂexible, linear polyelectrolytes. Additional support for this view comes from the transmission electron micrograph of HA shown in Fig. 9, which shows a linear, chain-like structure.

VII. WATER RETENTION BY HUMIC SUBSTANCES Water vapor sorption isotherms of HA and FA, measured at 40C, are shown in Fig. 19 (Chen and Schnitzer, 1976a). Sorption isotherms at 40C are similar to those determined at 25C. Isotherm shapes and extent of adsorption are similar for the two materials up to a relative humidity (RH) of 60%. At higher RHs, the two isotherms begin to diverge. At a RH of 90%, 1.0 g of HA adsorbs 225.0 mg of H2O at both 25 and 40C. In contrast, 1.0 g of FA adsorbs at the same RH; 508.0 mg of H2O at 25C and 625.0 mg of H2O at 40C. To obtain information on sorption mechanisms, the results were analyzed by the BET (Brunauer, Emmet, Teller) method (Chen and Schnitzer, 1976a). BET plots

Figure 19 Isotherms for adsorption of water vapor by HA and FA at 40C. From Chen and Schnitzer (1976a), with permission of the publisher.

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are shown in Fig. 20. The plots are linear in the 0 –55% RH range for both materials. At higher RHs, slopes of the BET plots rise sharply and become nonlinear. From the slopes and intercepts of the linear sections of the BET plots in Fig. 20, the weight of water adsorbed by 1.0 g of HA or FA to form a monolayer (Xm) and the heat of adsorption (E1-EL) are caculated (see Table V). Very similar weights of water (58–61 mg) are required to form monomolecular layers on the two humic surfaces at 35% RH, but weights of water adsorbed at 60% RH, where the plots become nonlinear, are 110.0 mg for HA and 120 mg for FA, suggesting the adsorption of a second layer of water (Table VI). At 90% RH, 1.0 g of HA adsorbs 225.0 mg of water (four layers), whereas 1.0 g of FA adsorbs 508.0 mg of water (eight layers). Heats of adsorption (E1-EL in Table V) are less than 1 kcal/mol for both materials. Increasing the temperature has little effect on these parameters. The surface areas, measured by the BET method, are very similar for the two materials (Table V).

Figure 20 BET plots of the adsorption of water vapor by HA and FA at 40C. From Chen and Schnitzer (1976a) with permission of the publisher.

40

M. SCHNITZER Table V BET Parameters for a Haploboroll HA and a Spodosol FAa

Type of material HA HA FA FA

Temperature (°C)

Xmb (mg H2O/g)

Cc

E1 E Ld (kcal/mol)

Surface area (SBET) (m2/g)

25 40 25 40

58 59 60 61

3.58 2.77 4.88 4.06

0.688 0.635 0.855 0.871

205.6 209.2 212.7 216.3

aFrom

Chen and Schnitzer (1976a). of water adsorbed by 1.0 g of HA or FA to form a monolayer. cexp (E E )/RT. 1 L dHeat of adsorption. bWeight

The BET plots have shapes that are similar to “type III” isotherms (Gregg and Singh, 1967). These isotherms are characteristics of systems in which the adsorption is cooperative, i.e., the more H2O molecules already adsorbed, the easier it is for additional H2O molecules to become adsorbed. Interactions between adsorbates are enhanced if adsorbate molecules are capable of strong hydrogen bonding, which occurs notably with water (Gregg and Singh, 1967). Thus, the sharp increases in slopes at RHs 55% can be interpreted as being due to the adsorption of fresh H2O molecules, hydrogen bonded to H2O molecules already adsorbed so that HA can adsorb four layers of H2O and FA twice that number under the same experimental conditions (see Table VI). As shown in Table V, the net heat of adsorption is low, which means that the attraction of adsorbate (H2O) molecules for each other exceeds their attraction for the adsorbent (HA or FA). One question that still remains to be answered is why does FA adsorb eight H2O layers, and HA only four H2O layers? As shown in Table

Table VI Water Adsorbed by 1.0 g of HA and 1.0 g of FA at Different Relative Humidities (RHs)a H2O (mg) adsorbed by 1.0 g RH (%)

HA

FA

35 60 90

58 110 225

60 120 508

aFrom

Chen and Schnitzer (1976a).

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I, the CO2H content of FA is considerably higher than that of HA. It appears therefore that H2O molecules tend to cluster around the CO2H groups. For a more detailed discussion of HA and FA surface area measurements and amounts of H2O required to form monomolecular layers on these materials, see Schnitzer (1986c), who used ﬁve different methods (water vapor sorption, surface pressure and surface tension measurements, N2He adsorption, and electron microscopy) to determine these parameters.

VIII. REACTIONS OF HUMIC SUBSTANCES WITH METALS AND MINERALS Humic substances can react with metals and minerals by several mechanisms. These include (1) formation of water-soluble metal complexes, (2) formation of water-soluble mixed ligand complexes, (3) sorption on and desorption from water-insoluble HAs and metal–humate complexes, (4) dissolution of minerals, (5) adsorption on mineral surfaces, and (6) adsorption in clay interlayers.

A. FORMATION OF WATER-SOLUBLE COMPLEXES Reactions in water near pH 7 between di- and trivalent metal ions and HAs and FAs are likely to proceed by either one or more of the mechanisms shown in Fig. 21, taking divalent metal ion M2+ as an example. According to Eq. (4), one CO2H group reacts with one metal ion to form an organic salt or monodentate complex. Equation (5) describes a reaction in which one CO2H and one adjacent OH group react simultaneously with the metal ion to form a bidentate complex or chelate. According to Eq. (6), two adjacent CO2H groups interact simultaneously with the metal ion to also form a bidentate chelate. Equation (7) shows the metal ion Mn+ linked to FA not only by electrostatic bonding but also through a water molecule in its primary hydration shell to a CuO group. The complexes described by Eqs. (5) and (6) are stronger than those described by Eqs. (4) and (7). Stability complexes of water-soluble metal–HA and –FA complexes have been determined by several workers (Stevenson, 1994). A number of major problems have been encountered in the analysis and interpretation of data. One serious obstacle to progress in this area is our lack of adequate knowledge of the chemical structures of the ligands, i.e., HA and FA. It is to be hoped that the structural models for HA proposed by Schulten and Schnitzer (1997) will provide much needed background information to those studying the formation and characteristics of metal–HA and metal–FA complexes.

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Figure 21 Major metal–HA and –FA reaction mechanisms. From Schnitzer (1986a), with permission of the publisher.

B. MIXED LIGAND COMPLEXES The formation of metal–FA–phosphate complexes was ﬁrst described by Lévesque and Schnitzer (1967). It is likely that in soils an appreciable portion of the total P exists in the form of such complexes, but it is difﬁcult to demonstrate this because of the low P content of soils. The formation and stability of mixed ligand complexes of the type Cu2+ –FA– secondary ligand (Y) have been studied by Manning and Ramamoorthy (1973). Secondary ligands (Y) investigated were citrate, tartrate, salicylate, phosphate, nitrilotriacetate (NTA), aspartate, and glycinate. In neutral to weakly acid solutions, mixed complexes predominated over simple complexes. Values of equilibrium constants for mixed complexes with citrate, phosphate, and NTA were particularly high compared to simple complexes. If phosphate functions in the same way as other oxyanions, the relatively high concentrations of HCO 3 and HSO4 in some

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soil solutions should lead to the formation of mixed Cu2+ –FA–HCO3 and Cu2+ – FA–HSO4 complexes. The formation of mixed complexes will prevent the precipitation of metal ions by hydrolysis at elevated pHs and will also interfere with the precipitation of insoluble metal phosphates, sulfates, and carbonates.

C. ADSORPTION AND DESORPTION One of the major characteristics of HAs is their ability to adsorb organic and inorganic substances. Kerndorff and Schnitzer (1980) examined the interaction of HA with a solution containing equimolar concentrations of 11 different metal ions. They report that the efﬁciency of adsorption of metal ions on HA increases with rises in pH and HA concentration and decreases in metal concentrations. At pH 2.4, the order of adsorption is Hg2+>Fe3+>Pb>Cu Al>Ni>Cr Zn Cd Co Mn. At pH 3.7, the order is Hg2+>Fe3+>Al>Pb>Cu>Cr>Cd Zn Ni Co Mn. At pH 4.7, the order is Hg2+ Fe3+ Pb Al Cr>Cd>Ni Zn>Co>Mn. At pH 5.8, the order is Hg2+ Fe3+ Pb Al Cr Cu>Cd>Zu.Ni>Co>Mn. Hg2+ and Fe3+ are always adsorbed most strongly by HA, whereas Co and Mn are adsorbed most weakly. The different metal ions compete for active sites (CO2H and phenolic OH groups) on the HA. Not only do the 11 metal ions plus H+ ions (a total of 12 ions) interact with the HA, but they also interact with each other by ion exchange, coprecipitation, and the formation of inner sphere and outer sphere complexes. Afﬁnities of the 12 ions for sorption on the HA do not correlate with their atomic weights, atomic numbers, and crystal and hydrated ionic radii (Kerndorff and Schnitzer, 1980). The metal ions adsorbed on HA can subsequently be desorbed by a dilute aqueous FA solution. Similarly, FA can desorb metal ions sorbed on clays and hydrous oxides so that FA can change the sorption, desorption, and precipitation characteristics of metals.

D. DISSOLUTION OF MINERALS Due to their ability to complex di-, tri-, and tetravalent metal ions, dilute aqueous FA solutions at any pH and aqueous HA solutions at pH 6.5 can attack and degrade minerals to form water-soluble and water-insoluble metal complexes. Thus, the weathering of minerals in soils and sediments is often enhanced by the action of naturally occurring humic substances, especially FA. Because of its abundance in soils, its solubility in water, and its ability to complex with metal ions and interact with silica, the latter may increase the concentrations of these soil constituents in aqueous solutions to levels that far exceed their normal solubilities. In this manner, aqueous FA solutions may not only bring about the dissolution and

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degradation of existing minerals, but also lead to the synthesis of new minerals by permitting the complexed and dissolved metals and silica to form new combinations. Conversely, active surfaces of inorganic soil constituents may catalyze either the degradation or the synthesis of HAs and FAs.

E. ADSORPTION ON EXTERNAL MINERAL SURFACES The extent of adsorption of humic materials on mineral surfaces depends on the characteristic of the surface, the pH of the system, and its water content (Schnitzer, 1978). Kodama and Schnitzer (1974) report high adsorption of FA on sepiolite surfaces. Sepiolite has a channel-like surface formed by the joining of edges of long and slender talc structures. In untreated sepiolite the channels are occupied by bound and/or zeolitic water, which can be displaced by undissociated FA in aqueous solution at pH 3.0.

F. ADSORPTION IN CLAY INTERLAYERS Schnitzer and Kodama (1966) demonstrated that the interlayer adsorption of FA by expanding clay minerals was pH dependent, being greatest on low pH, and no longer occurring at pH 5.0. Adsorbed FA could not be displaced from clay interlayers by leaching with 1 M NaCl; an inflection was observed in the adsorption–pH curve near the pH corresponding to the pK of the acid species of FA so that the adsorption could be classified as a ligand-exchange reaction (Greenland, 1971). In this type of reaction the anion (FA) is thought to penetrate the coordination shell of the dominant cation in the clay and displace water coordinated to be the dominant cation in the clay interlayer. The ease with which water can be displaced will depend on the affinity for water of the dominant cation with which the clay is saturated and also on the degree of dissociation of the FA. Because the latter is very low at low pH, interlayer adsorption of FA is greatest at low pH levels.

IX. INTERACTIONS OF PESTICIDES AND HERBICIDES WITH HUMIC SUBSTANCES The persistence, degradation, bioavailability, leachability, and volatility of pesticides are directly related to the nature and concentration of humic substances in a particular soil (Khan, 1980). Pesticides may sorb on humic substances and be retained by Van der Waal’s forces, hydrophobic bonding, hydrogen bonding, charge

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transfer, ion exchange, and ligand exchange. From a study on the chemical mechanisms governing the aciﬂuorfen (a diphenyl ether herbicide)–HA interaction, Celi et al. (1997a) concluded that the herbicide appeared to be adsorbed unchanged on external HA surfaces and in internal voids of the model HA structure proposed by Schulten and Schnitzer (1997). The rate at which a pesticide should be applied to a soil may vary widely, depending to a large extent on the nature of the SOM (humic substances) the soil contains. Humic substances can promote the nonbiological degradation of pesticides and can also form strong linkages with residues arising from the partial and chemical degradation of the pesticides (Stevenson, 1994). These processes may play important roles in the detoxiﬁcation, protection, and preservation of the environment. Soils tend to accumulate increasing amounts of pesticide residues that are capable of passing into air and water, into plants and microbes, or being degraded to other products (Khan, 1980). These bound residues may also contain intact pesticide molecules that, when released, could exert deleterious biological effects on the environment. However, ﬁrm binding of pesticides to humic substances may be a method of decontamination. Information on how pesticides interact with humic substances provides a rational basis for their effective use and for minimizing undesirable side effects.

X. FUNCTIONS AND USES OF HUMIC SUBSTANCES A. FUNCTIONS IN SOILS 1. Humic substances exert physical, chemical, and biological effects on soil quality by serving as soil conditioners, nutrient sources, and substrates for microorganisms. 2. They contribute to the maintenance of an adequate and stable soil structure by acting as binding agents in the formation of soil aggregates, thus ensuring satisfactory drainage and aeration, and providing protection against erosion, enhancing mechanical soil properties, and playing a major role in water retention. 3. They act as sources and storehouses of N, P, and S and of micronutrients essential for plant growth. They form complexes with many metals and make these available to plant roots and microorganisms. They buffer soil against drastic changes in pH and also interact with herbicides and pesticides and assist in their degradation and detoxiﬁcation. 4. They serve as substrates for macro- and microorganisms in the soil. Soil microorganisms play a major role in the synthesis and degradation of humic substances. Humic substances can also exert direct physiological effects on plants. 5. All of these affect the impact on soil quality and point to a vital role for humic substances in soil fertility.

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B. USES AND POTENTIAL USES Humic substances are good chelating agents, have large surface areas per unit weight, are excellent dispersants, and, depending on the pH, are oxidizing and reducing agents. Many of the uses that have been proposed so far take advantage of these properties. The following are some of the uses that have been proposed: In agriculture: additives to fertilizers and sprays, coatings of seeds, nutrients in hydroponics (Schnitzer, 1986b). In industry: dispersants; corrosion inhibitors, wood preservatives, ﬂotation reagents, additives to well-drilling ﬂuids, additives to cement, agents in the tanning of leather, binding agents in the processing of minerals, extractants of uranium from its ores, pigments in inks, components of livestock feeds, plasticizers for polyvinyl chloride, sources of hydrocarbons and oils, ion exchangers (MacCarthy and Rice, 1994). In medicine: antimicrobial, antiviral, anti-inﬂammatory, and antitumor agents, liver stimulants, heal gastric ulcers, stop bleeding, treatments of skin burns, estrogenic agents, reduce heavy metal toxicity (Schnitzer, 1986b; Klocking, 1994).

XI. CONCLUSIONS AND OUTLOOK FOR THE FUTURE 1. One of the most important ﬁndings reported in this chapter is the development of methods for the analysis of SOM in whole soils, i.e., in situ. It is no longer necessary to extract, separate, and purify SOM fractions. Meaningful chemical information on the SOM composition can be obtained by direct analysis of air-dried whole soil samples. Methods currently available for this purpose are solid-state 13C NMR and Py-FIMS. While the NMR method can only be used if the paramagnetic metal ion content (especially Fe3+) compared to the C content is low, PyFIMS can be employed without restrictions. Research needs to be done on lowering the sensitivity of 13C NMR to paramagnetic metal ions. 2. During the past 50 years, very impressive progress has been made on the chemistry of C- and N-containing components of SOM. Less progress has been made on the chemistry of P- and S-containing SOM components. From long-term investigations on humic substances by chemical, IR and FTIR, 13C NMR and ESR spectroscopic, oxidative and reductive degradative, colloid chemical, electron microscopic, and mass spectrometric studies, a two dimensional structural model for HA was proposed. With the aid of computational chemistry, the two-dimensional HA model was converted to a three-dimensional HA model structure. Other model structures that were proposed include those for SOM and an agricultural soil. A structural model for SOM will assist us in better understanding, at the atomic and molecular levels, interactions of SOM with pesticides, herbicides, and metal ions and clay minerals.

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3. Considerable progress has also been made on the chemistry of N components of SOM by the identiﬁcation of over 100 heterocyclic N compounds in soils and SOM. These N compounds include nonsubstituted and substituted pyrroles, pyrrolidines, imidazoles, pyridines, pyrazines, nitriles, indoles, auinolines, benzothiazoles, and pyrimidines. These data point to N heterocyclics as being signiﬁcant components of total soil N and SOM N. 4. Other information presented in this chapter emphasizes the tendency of SOM components to associate inter- and intramolecularly through hydrogen bonds and to hydrogen-bonded water through oxygen-cotaining functional groups. Also, the conformation of humic substances changes with changes in pH, electrolyte concentration, and binding of functional groups by metals. 5. As to the future, we can expect increasing applications of computational chemistry to throw light on the spatial arrangements of molecular constituents in SOM and in whole soils, on the nature of interactions between SOM and metals and clays, mechanisms of reactions of SOM with other organic molecules such as pesticides and herbicides, and important characteristics of SOM such as surface area, volume, refractivity, polarizability, hydrophobicity, and hydration energy. It is hoped that future SOM chemists will not forget to use “classical” analytical, organic, and physical chemistry in their enthusiasm for computational chemistry. Both types of chemistry need to be employed simultaneously in order to produce information that will stand the test of time. Finally, let us remember that the soil is our most important natural resource. It is the presence of SOM that distinguishes the soil from a mass of rock particles and allows it to become a living system. Our ability to produce sufﬁcient food for an expanding population and, at the same time, protect the environment demands a comprehensive understanding of the physical, chemical, and biological properties of soils and SOM. It is hoped that the contents of this chapter will contribute to these objectives.

XII. PERSONAL ENCOUNTERS WITH OUTSTANDING SCIENTISTS This section recounts personal encounters with outstanding scientists of my time who inﬂuenced my research. Professor Dr. W. Flaig Professor Dr. W. Flaig was probably the best known and most inﬂuential SOM chemist during the 1950s, 1960s, and 1970s. He was Director of the Institute for Soil Biochemistry in Braunschweig, Germany, the only institute anywhere that

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specialized in SOM research. He traveled extensively, spoke eloquently, and was an outstanding ambassador for SOM. I ﬁrst met him at the 7th International Congress of Soil Science in Madison, Wisconsin, in 1960. During the evening of the ﬁrst day of meetings, we had a special session on nitrogen in SOM. Professor Flaig argued that N was an integral part of SOM, chemically bonded to the major HA structures, while I contended that most of the N was physically adsorbed or loosely held by the HA structural network. The debate lasted for several hours, with many of the scientists who were present participating. It was one of the most interesting debates in which I was ever engaged. Professor Flaig was a very skillful debator; he was conciliatory rather than confrontational. At the end of the debate we all had learned a lot and had become good friends. During the next few days, I listened to his papers and also to his comments on other papers. As to the synthesis of HAs, he postulated that lignin was oxidatively degraded to simple phenolic monomers, which then polymerized oxidatively to produce HA. For this reason, he considered HA to be highly aromatic. Essentially the same view was also held by Professor Kononova. Thus, the two most prominent SOM specialists of that time agreed that HA consisted of aromatic (ring) structures and that aliphatic structures were not signiﬁcant. Professor Dr. Flaig also considered heterocyclic nitrogen to be a signiﬁcant component of soil N. This has been conﬁrmed by Py-FIMS (Schulten and Schnitzer, 1998). During the congress I met Professor Dr. Flaig frequently and I found him to be a very sensitive, gentle and civilized person, socially very adept, a good dancer, and who loved good food and good wine. After the congress, we decided to exchange reprints and to stay in touch. In 1967, he invited me to a very interesting meeting on SOM that he had organized in Vienna on behalf of the International Atomic Energy Agency. In 1976, he organized a similar meeting in Braunschweig, sponsored jointly by the International Atomic Energy Agency and FAO, to which he also invited me. During the 1970s and also in 1987, I visited him several times in Braunschweig and in Wurzburg where we had many interesting discussions on the chemistry of SOM. Professor Dr. Flaig was very active in international organizations, especially in the International Society of Soil Science. He was chairman of the Soil Chemistry Commission for three successive terms. At the congress in Edmonton in 1978, he worked very hard on my behalf and helped me to be elected Chairman of the Soil Chemistry Commission. My successor in this position was Michael Hayes of the University of Birmingham, England, who was elected in New Delhi in 1982. Thus, due to Professor Dr. Flaig’s status and hard work, SOM chemists held the chairmanship of the Chemistry Commission of the ISSS for ﬁve successive terms. After his retirement in the late 1970s, the research direction of his institute changed and SOM was no longer the highest priority. All SOM chemists felt that they had lost an outstanding spokesman when he retired. He is now living in Garbrunn near Würzburg. I wish him many happy years of retirement.

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Professor D. H. R. (Sir Derek) Barton, FRS, Nobel Laureate In 1961, the Director of the Soil Research Institute informed me that I was eligible for a sabbatical. I decided to spend a year in an organic chemistry laboratory in England, learning how to elucidate chemical structures of complex organic molecules. After convincing my director that this was the right thing to do, my wife, daughter, and I sailed to England from Montreal in September 1961. Several days after our arrival, I went to see Dr. D. H. R. Barton, Professor of Organic Chemistry, at the Imperial College of Science and Technology in London, England, who had previously accepted me for a 1-year stay in his laboratory. Professor Barton was a tall, broad-shouldered, good looking man, with prominent glasses, in his midforties. He welcomed me to England and showed me his labs and also my lab space. I was surprised by the large number of scientists who worked in these labs. There were 50–60 scientists in the big lab and another 30–40 in several smaller labs. Only 10 of those who worked there were Ph.D. students, the remaining 90 were either postdoctorate fellows or visiting scientists who came from all over the world. A few days later, I reported to work, equipped with a glass bottle containing 200 g of puriﬁed FA that I had extracted in Ottawa from a Spodosol Bh horizon. I had decided to work on FA because I thought at that time that FA had a simpler molecular structure than HA so that FA would be more amenable to structural analysis than HA. After I started my work, I had a discussion with Professor Barton about what I would do. He told me that because it was completely soluble in water and had a high oxygen-containing functional group content, FA was not a very suitable material for organic chemistry. What I should do was to block the CO2H and phenolic OH groups by methylation so that the FA would be soluble in organic solvents such as benzene or chloroform and behave like an organic molecule. After testing a number of methylating procedures, I found the silver oxide-methyl iodide method most suitable for FA (Barton and Schnitzer, 1963). Three successive methylations made 73% of the FA soluble in benzene. Chromatography of the benzene-soluble FA over Al2O3 gave on elution with benzene, ethylacetate, and methanol seven fractions ranging in molecular weights from 260 to 918, in carbon from 59.29 to 69.26%, in oxygen from 32.33 to 37.00%, and in OCH3 content from 25.79 to 47.86%. Several of the fractions were chromatographically homogeneous but not molecularly. A straight line relationship was found between E 212 m and molecular weight. All fractions had very similar IR spectra and were devoid of optical activity. H NMR spectra of all fractions showed resonances at 9.10 and 8.75 , due to the presence of H in aliphatic CH3 and CH2 groups, and at 6.3 and 6.1 , due to OCH3 and CO2CH3, respectively. These were the ﬁrst NMR spectra ever recorded on a humic material (Wilson, 1987) and also the ﬁrst unambiguous evidence that (CH2 )n, i.e., CH2 groups in long chains, were signiﬁcant components of humic substances.

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My work with Professor Barton clearly demonstrated that the chemistry of humic substances could be investigated by the methods of organic chemistry. During my stay at Imperial College, I had many discussions with Professor Barton on different approaches to the chemistry of humic substances. I also had many opportunities to speak to a number of visiting scientists who were also working there and who had many years of experience in structural organic chemistry. Professor Barton also introduced me to many distinguished visitors who gave seminars there and who were willing to discuss HA chemistry with me. My stay with Professor Barton was a very rewarding experience both practically and intellectually. Imperial College was at that time a hotbed of organic chemistry, and I beneﬁted greatly from that atmosphere. I am most grateful to Professor Barton for taking so much interest in my research and for helping me in any way he could. At the end of the 1960s, Professor Barton won the Nobel Prize in Chemistry for his invention of conformational analysis. He was knighted a few months later by the Queen of England. Professor M. M. Kononova Professor Kononova became known internationally after the publication of her book on SOM after World War II. This book was translated into many languages and was required reading for many of us in the 1960s and 1970s. Professor Kononova was Professor of Soil Biochemistry at the Dokuchaiev Institute of Soil Science in Moscow and a member of the Academy of Sciences of the USSR. I followed her research since the mid-1950s and was impressed by her attempts to combine chemistry and microbiology in her studies on SOM. In 1974, I attended the 10th International Congress of Soil Science in Moscow, where I had the opportunity of meeting Professor Kononova in person. I remember her as a lively, kind lady in her early seventies, always surrounded by a large group of co-workers and assistants. She knew me through correspondence we had in the 1960s and through my publications. Being one of the few SOM specialists in North America gave me a special status in the eyes of Russian soil scientists who regarded SOM very highly. After the second session of the Soil Chemistry Commission she signaled to me to sit beside her and asked me about my impressions of the congress and whether I liked Moscow. She then proceeded to give me a small Russian doll as a souvenir. This procedure was repeated after each session so that I ended up collecting a number of small presents, including some of her books. One morning, she invited me to a reception at the Dokuchaiev Institute. As I entered her ofﬁce, I was surprised by the beautiful decorations on the walls and by the unusual abundance of delicious foods, cakes, fruits, and drinks on the table. Aside from Professor Kononova, her assistants, and the director of the Dokuchaiev Institute, a number of nonRussian scientists were present. These included Wolfgang Flaig, Jack Bremner, and several scientists whose names escape me. We had a high-spirited discussion

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on SOM, while consuming a lot of food and drinks. All of us had a very good time. Before leaving, we were given a tour of the institute and shown newly acquired equipment, which included a sophisticated gas chromatograph and an amino acid analyzer. It was obvious that Professor Kononova was well connected and that her research was supported accordingly. One afternoon, all attendants of the congress were scheduled to go on a sightseeing tour of Moscow. Apparently unaware of our schedule, Professor Kononova asked me to join her and co-workers for a discussion on the oxidative degradation of humic substances. My wife was accompanying me at that time because she thought that we would join a sightseeing tour of Moscow, but she also was invited to attend our discussion. After talking about our chosen topic for about an hour, it was brought to Professor Kononova’s attention that my wife and I had missed the sightseeing tour. She was very unhappy about this and assured me that she would arrange a special tour for us. After 30 min, a large bus arrived and the two of us plus an interpreter got on the bus. The driver plus the three of us were the only passengers and we were given a wonderful tour of Moscow. Thus, being a SOM specialist in Moscow at that time had certain advantages. What impressed me most, however, were the power and influence of Professor Kononova. Professor D. S. Orlov In 1957, after the launching of the Sputnik by the Russians, I began to take courses in Russian at a local university. After 2 years of studying, I could read and understand enough Russian to read abstracts of scientiﬁc papers and to get some understanding of the contents of the papers. Over the ensuing years, I noted that D. S. Orlov was one of the most proliﬁc and imaginative Russian SOM researchers. After I contacted him by mail, we decided to exchange reprints. This went on for a number of years, but I had no opportunity of meeting him in person until 1974, when I attended the 10th International Congress of Soil Science. D. S. Orlov at that time was Professor of Soil Biochemistry at Moscow State University. The second day after our arrival in Moscow, my wife and I proceeded to the University to visit Professor Orlov. When we arrived at the soil science building, I asked one of the concierges in Russian where Professor Orlov’s ofﬁce was. He answered me that he did not know. So I asked him very politely to look up Professor Orlov’s room number on his list. His answer was that the list was incomplete. After asking another concierge the same questions and receiving the same answers, my wife and I decided to ﬁnd Professor Orlov’s ofﬁce on our own. We walked along the lengthy gray corridors and noticed that none of the doors showed name plates or numbers. All doors were identical, none of them exhibited any identiﬁcation, so we kept on walking through colorless, endless corridors on the ﬁrst, second, third, and fourth ﬂoors. It was like a scene out of Kafka. We never met

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anybody who could divulge to us the secret of Professor Orlov’s ofﬁce number. After 2 hr of fruitless searching we returned to our hotel. The next morning, as I was sitting in one of the lecture rooms, listening to the ﬁrst paper, a young lady tapped on my right shoulder and whispered into my right ear: “Professor Orlov is waiting for you, please follow me quietly.” I got up and followed her out of the lecture room, and then we proceeded to the building that my wife and I had visited a day earlier. The young lady told me that she was Professor Orlov’s daughter and that she had graduated from the Philological Institute, specializing in English. Again we walked along endless, gray corridors until we entered Professor Orlov’s ofﬁce. He was a man in his forties of medium height, blond hair, and blue eyes. He was very happy to see me, and he immediately took a bottle of vodka out of the cupboard so that we could celebrate our encounter appropriately. Apparently, somebody had told him that we were looking for him, but he did not mention this to me. As we were familiar with each other’s work, he began by showing me his labs. I noticed that practically all of his equipment was homemade. He and his students had built their own gas chromatograph and IR spectrophotometer. It was amazing to me that Professor Orlov and students were able to publish such excellent scientiﬁc work using so much primitive and outdated equipment. He demonstrated to me how well the equipment functioned. At that time, my group and he and his co-workers were making wide use of thermal methods such as thermogravimetry and differential thermal analysis (DTA) in our research on humic substances. He showed me his home-built thermal equipment, which functioned as well as ours, which we had purchased at high prices. During our discussion, I noticed that Professor Orlov was a highly intelligent and very dynamic person who got very excited when he described his work. After the lab visit, he asked me to accompany him to the greenhouses where he was conducting some experiments. Throughout my visit I never saw any “modern” equipment, although his work and contributions to SOM chemistry were outstanding. After 3 hr, he suggested that I return to the meeting, guided by his daughter. I returned to my seat in the lecture room during the presentation of the last paper and sat down quietly. Apparently nobody had noticed that I had been away for 3 hr. I felt very depressed during the rest of the day. Here was a top scientist functioning in obscurity because of poor political connections. I never found out what his political problems were, but they must have been serious because he was not even allowed to attend the congress in his own city. I never saw Professor Orlov again after our meeting. A few weeks after my return to Canada I received a parcel containing four books that he had authored. Three of the books were in Russian but one was in English. The latter was entitled “Humus Acids of Soils.” In this book he discussed each of the reprints that I had sent him over the years. Some of his comments were favorable, whereas others were somewhat critical. After 1975, our contacts ended. I never found out why. I began to realize how difﬁcult it was for an excellent scientist to survive in a totalitarian state, obviously was Pro-

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fessor Orlov’s problem. I hope that he is still alive and well and I wish him the best. Drs. J. M. Bremner and F. J. Stevenson I met Jack Bremner and Frank Stevenson frequently at the annual meetings of the Soil Science Society of America. The three of us also belonged to the NCR-59 Committee, which was set up by USDA for studies on SOM and whose members met annually for 1- to 2-day meetings at universities in the midwest of the United States. I attended these meetings regularly over a 20-year period. There were about 15–20 of us who belonged to this committee. At the meetings, each of us reported on what we had done during the past year and then discussed plans for the future. In this way we became aware of what each member was doing. Probably the most important point of these meetings was that it provided us with a forum where we could talk to each other. This was especially important for those who worked in isolated locations. Jack Bremner was our “guru” at these meetings. He was not only an eminent authority on soil N, but was also very knowledgeable on the chemistry of SOM. He had worked on SOM at Rothamsted prior to immigrating to the United States and was an excellent chemist. We all told him about some of our difﬁculties, and he would suggest ways of how we could overcome our problems. He was always upbeat and positive. I also met Jack at several International Soil Science Society meetings and we always had interesting times. The careers of Frank Stevenson and my own ran along parallel lines. We started at almost the same time and are now in the twilights of our careers. The major difference between us was that Frank started with studies on soil N, whereas I began my research with investigations on complexes formed among fulvic acid, leaf leachates and leaf extracts, and di- and trivalent metal ions. After that, Frank did extensive studies on metal complexation by HA and FA, while I began working on the structure of HAs and FAs, and then turned to soil N. Frank also made excellent contributions to SOM chemistry through the books he authored. His book entitled “Humus Chemistry” is a very important source of information on SOM and is widely consulted by students, teachers, and researchers. It brings together an enormous amount of information on most aspects of SOM. I have referred to it often in this chapter. It was a great pleasure for me to share with Frank the Wolf Prize for Agriculture in 1996.

ACKNOWLEDGMENTS I am most grateful to the postdoctoral fellows, visiting scientists, and technicians who worked with me for different lengths of time and who made outstanding contributions to my research. I also thank a number of colleagues who collaborated with me on different aspects of my work. The following is a list of all postdoctoral fellows and visiting scientists. The list shows their names and countries of domi-

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cile in chronological order: U. C. Gupta (Canada), E. H. Hansen (Denmark), G. Ogner (Norway), K. Matsuda (Japan), R. Riffaldi (Italy), M. Ortiz de Serra (Argentina), G. B. Volk (USA), J. A. Neyroud (Switzerland), S. M. Grifﬁth (Trinidad), Y. Chen (Israel), N. Senesi (Italy), J. Cortez (France), K. Ghosh (India), H. Kerndorff (Germany), M. Spiteller (Germany), G. Calderoni (Italy), G. Catroux (France), M. Saharinen (Finland), E. Van Bochove (Canada), and L. Celi (Italy). I thank the following technicians: J. G. Desjardins, S. M. I. Skinner, E. Vendette, D. A. Hindle, B. S. Rauthan, P. R. Marshall, and P. Schuppli. I am most grateful to the following colleagues: J. R. Wright, I. Hoffman, M. Lévesque, P. A. Poapst, H. Kodama, F. J. Sowden, D. S. Gamble, S. U. Khan, K, Ivarson, S. P. Mathur, I. L. Stevenson, C. M. Preston, M. A. Arshad, J. A. Ripmeester, H.-R. Schulten, H. Dinel, C. A. Campbell, C. M. Monreal, B. C. Liang, and T. Paré. I would like to single out Professor Dr. H.-R. Schulten for special gratitude. He and I have collaborated very closely over the past 12 years on the development and applications of pyrolysis-mass spectrometric methods for analyses of SOM components in organic soil extracts, HAs, FAs, humins, and whole soils and on the generation of HA, FA, and whole soil threedimensional model structures. Last but not least, I thank Agriculture and Agri-Food Canada for supporting my research over a long period of time and T. Paré for valuable assistance with proofreading and typing of this manuscript.

REFERENCES Almendros, G. R., Friend, F. J., Gonzalez-Vila, K. M., Haider, K., Knicker, H., and Luedemann, H.-D. (1991). Analysis of 13C and 15N CP-MAS NMR-spectra of soil organic matter and composts. FEBS Lett. 282, 119 –121. Anderson, G. (1957). Nucleic acid derivatives in soil. Nature (London) 180, 287–288. Anderson, G. (1958). Identiﬁcation of derivatives of deoxyribonucleic acid in humic acid. Soil Sci. 86, 169–174. Anderson, G. (1961). Estimation of purines and pyrimidines in soil humic acid. Soil Sci. 91, 156 –161. Archard, F. K. (1786). Chemische Untersuchung des Torfs. Crell’s Chem. Ann. 2, 391– 403. Arshad, M. A., Ripmeester, J. A., and Schnitzer, M. (1988). Attempts to improve solid-state 13C NMR spectra of whole mineral soils. Can. J. Soil Sci. 68, 593 – 602. Atherton, N. M. (1973). “Electron Spin Resonance.” Wiley, New York. Barton, D. H. R., and Schnitzer, M. (1963). A new experimental approach to the humic acid problem. Nature (London) 190, 217–218. Bedrock, C. N., Cheshire, M. V., Chudek, J. A., Goodman, B. A., and Shand, C. A. (1994). 31P NMR studies of humic acid from a blanket peat. In “Humic Substances in the Global Environment and Implications on Human Health” (N. Senesi and T. M. Miano, eds.), pp. 227–232. Elsevier, Amsterdam. Benzing-Purdie, L., Ripmeester, J. A., and Preston, C. M. (1983). Elucidation of the nitrogen forms in melanoidins and humic acid by 15N cross polarization-magic angle spinning nuclear magnetic ressonance spectroscopy. J. Agric. Food Chem. 31, 913 –915. Benzing-Purdie, L., Cheshire, M. V., Williams, B. L., Sparling, G. P., Ratcliffe, C. I., and Ripmeester, J. A. (1986). Fate of N-15 glycine in peat as determined by 13C and 15N-CPMAS NMR spectroscopy. J. Agric. Food Chem. 34, 170 –176. Biederbeck, V. O. (1978). Soil organic sulfur and fertility. In “Soil Organic Matter” (M. Schnitzer and S. U. Khan, eds.), pp. 273 – 310. Elsevier, Amsterdam. Boyd, S. A., Sommers, L. E., Nelson, D. W., and West, D. X. (1981). The mechanism of a copper (II)— humic acid complex and some adducts with nitrogen donors. Soil Sci. Soc. Am. J. 45, 745 –749. Bracewell, J. M., Pacey, N., and Robertson, G. W. (1987). Organic matter in on-shore Cretaceous chalks and its variations, investigated by pyrolysis-mass spectrometry. J. Anal. Appl. Pyrol. 10, 199 –213.

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Breitmaier, E., and Voelter, W. (1978). 13C NMR spectroscopy. In “Monographs in Modern Chemistry” (H. F. Ebel, ed.), Vol. 15, pp. 247–260. Verlag Chemie, Weinheim. Bremner, J. M. (1967). The nitrogenous constituents of soil organic matter and their role in soil fertility. Pontif. Acad. Sci. Scr. Varia 32, 143 –193. Celi, L., Gennari, M., Khan, S. U., and Schnitzer, M. (1997a). Mechanism of aciﬂuorfen interaction with humic acid. Soil Sci. Soc. Am. J. 61, 1659 –1665. Celi, L., Schnitzer, M., and Nègre, M. (1997b). Analysis of carboxyl groups in soil humic acids by a wet chemical method, Fourier-transform infrared spectrophotometry, and solution-state carbon13 nuclear magnetic resonance. A comparative study. Soil Sci. 162, 189 –197. Chen, Y., and Schnitzer, M. (1976a). Water adsorption on humic substances. Can. J. Soil Sci. 56, 521– 524. Chen, Y., and Schnitzer, M. (1976b). Scanning electron microscopy of a humic acid and a fulvic acid and its metal and clay complexes. Soil Sci. Soc. Am. J. 40, 682– 686. Chen, Y., and Schnitzer, M. (1978). The surface tension of soil humic substances. Soil Sci. 125, 7–15. Chen, Y., and Schnitzer, M. (1989). Sizes and shapes of humic substances by electron microscopy. In “Humic Substances II” (M. H. B. Hayes, P. MacCarthy, R. L. Malcolm, and R. S. Swift, eds.), pp. 621–638. Wiley, New York. Chen, Y., Banin, A., and Schnitzer, M. (1976). Use of the scanning electron microscope for structural studies on soils and soil components. In “Proceedings of the International Symposium on Scanning Electron Microscopy” (O. Johari, ed.), pp. 425 – 432. ITT Research Institute, Chicago. Cheshire, M. V., and Senesi, N. (1998). Electron spin resonance spectroscopy of organic and mineral particles. In “Structure and Surface Reactions of Soil Particles” (P. M. Huang, N. Senesi, and J. Bufﬂe, eds.), pp. 325 – 373. Wiley, New York. Cheshire, M. V., Berrow, M. L., Goodman, B. A., and Mundie, C. M. (1977). Metal distribution and nature of some Cu, Mu, and V complexes in humic and fulvic acid fractions of soil organic matter. Geochim. Cosmochim. Acta 41, 1131–1138. Christensen, B. T. (1996). Carbon in primary and secondary organo-mineral complexes. In “Structure and Organic Matter Storage in Agricultural Soils” (M. R. Carter and B. A. Stewart, eds.), pp. 97– 165. CRC Press, Boca Raton, FL. Cortez, J., and Schnitzer, M. (1979). Purines and pyrimidines in soils and humic substances. Soil Sci. Soc. Am. J. 43, 958 – 961. Ghosh, K., and Schnitzer, M. (1980). Macromolecular structures of humic substances. Soil Sci. 129, 266–276. Ghosh, K., and Schnitzer, M. (1982). A scanning electron microscopic study of effects of adding neutral electrolytes to solutions of humic substances. Geoderma 28, 53 – 56. Greenland, D. J. (1971). Interaction between humic and fulvic acids and clays. Soil Sci. 111, 34 – 41. Gregg, J. S., and Singh, K. S. W. (1967). “Adsorption Surface Area and Porosity.” Academic Press, New York. Grifﬁth, S. M. and Schnitzer, M. (1989). Oxidative degradation of soil humic substances. In “Humic Substances II” (M. H. B. Hayes, P. MacCarthy, R. L. Malcolm, and R. S. Swift, eds.), pp. 69– 98. Wiley, New York. Grifﬁth, S. M., Sowden, F. J., and Schnitzer, M. (1976). The alkaline hydrolysis of acid-resistant soil and humic acid residues. Soil Biol. Biochem. 8, 529 – 531. Hansen, E. H., and Schnitzer, M. (1969). Zinc dust distillation and fusion of a soil humic and fulvic acid. Soil Sci. Soc. Am. Proc. 33, 29 – 36. Hayes, M. B. H., and O’Callaghan, M. R. (1989). Degradations with sodium sulﬁde and phenol. In “Humic Substances II” (M. B. H. Hayes, P. MacCarthy, R. L. Malcolm, and R. S. Swift, eds.), pp. 141–180. Wiley, New York. Ikan, R., Ioselis, P., Rubinsztain, Y., Aizenshtat, Z., Miloslavsky, I., Yariv, S. Pugmire, R., Anderson, L. L., Woolfenden, W. R., Kaplan, I. R., Dorsey, T., Peters, K. E., Boon, J. J., Leeuwe, J. W., Ishi-

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watari, R., Morinaga, S., Yamamoto, S., Macihara, T., Muller-Vonmoos, T., and Rub, A. (1992). Chemical, isotopic, spectroscopic and geochemical aspects of natural and synthetic humic substances. Sci. Total Environ. 117/118, 1–12. Kerndorff, H., and Schnitzer, M. (1980). Sorption of metals on humic acid. Geochim. Cosmochim. Acta 44, 1701–1708. Khan, S. U. (1980). “Pesticides in the Soil Environment.” Elsevier, Amsterdam. Khan, S. U. and Sowden, F. J. (1971). Distribution of nitrogen in the Black Solonetzic and Black Chernozemic soils of Alberta. Can. J. Soil Sci. 51, 185 –193. Klocking, R. (1994). Humic substances as potential therapeutics. In “Humic Substances in the Global Environment and Implications on Human Health” (N. Senesi and T. M. Miano, eds.), pp. 1245 – 1257. Elsevier, Amsterdam. Knicker, H., and Luedemann, H. D. (1995). N-15 and C-13 CPMAS and solution NMR studies of N15 enriched plant material during 600 days of microbial degradation. Org. Geochem. 23, 329 – 341. Knicker, H., Friend, R., and Luedemann, H.-D. (1993). The chemical nature of nitrogen in native soil organic matter. Naturwissenschaften 80, 219 –221. Knicker, H., Almandros, G., Gonzalez-Vila, F. J., Luedemann, H.-D., and Martin, F. (1995). 13C and 15N NMR analysis of some fungal melanins in comparison to soil organic matter. Org. Geochem. 23, 1023–1028. Knicker, H., Almendros, G., Gonzalez-Vila, F. J., Martin, F., and Luedemann, H.-D. (1996). 13C and 15N-NMR spectroscopic examination of the transformation of organic nitrogen in plant biomass during thermal treatment. Soil Biol. Biochem. 28, 1053 –1060. Kodama, H., and Schnitzer, M. (1974). Adsorption of fulvic acid by non-expanding day minerals. Trans. Int. Congr. Soil Sci. 10th, 1974, pp. 51– 56. Lakatos, B., Tibai, T., and Meisel, J. (1977). EPR spectra of humic acids and their metal complexes. Geoderma 19, 319 – 338. Lévesque, M., and Schnitzer, M. (1967). Organo-metallic interactions in soils: 6. Preparation and properties of fulvic acid-metal phosphates. Soil Sci. 103, 183 –190. Lowe, L. E. (1978). Carbohydrates in soils. In “Soil Organic Matter” (M. Schnitzer and S. U. Khan, eds.), pp. 65–94. Elsevier, Amsterdam. MacCarthy, P., and Rice, J. A. (1994). Industrial applications of humus: an overview. In “Humic Substances in the Global Environment and Implications on Human Health” (N. Senesi and T. M. Miano, eds.), pp. 1209 –1223. Elsevier, Amsterdam. Manning, P. G., and Ramamoorthy. (1973). Equilibrium studies of metal ion complexes of interest to natural waters. VII. J. Inorg. Nucl. Chem. 35, 1577–1581. Martin, F., Saiz-Jimenez, C., and Cert, A. (1979). Pyrolysis-gas chromatography/mass spectrometry of humic fractions II. The high boiling point compounds. Soil Sci. Soc. Am. J. 43, 309 – 312. McBride, M. B. (1978). Transition metal binding in humic acid: an ESR study. Soil Sci. 126, 200 –209. Mengel, K. (1996). Turnover of organic nitrogen in soils and its availability to crops. Plant Soil 181, 83–93. Norwood, D. L. (1988). Critical comparison of structural implications from degradative and nondegradative approaches. In “Humic Substances and their Role in the Environment” (H. F. Frimmel and R. F. Christman, eds.), pp. 133 –148. Wiley, New York. Patience, R. I., Baxby, M., Bartle, K. D., Perry, D. L., Rees, A. G. W., and Rowland, S. J. (1992). The functionality of organic nitrogen in some recent sediments from the Peru upswelling region. Org. Geochem. 18, 161–169. Preston, C. M. (1996). Applications of NMR to soil organic matter analysis: History and prospects. Soil Sci. 161, 144–166. Preston, C. M., and Schnitzer, M. (1984). Effect of chemical modiﬁcations and extractants on the carbon-13 NMR spectra of humic materials. Soil Sci. Soc. Am. J. 48, 305 – 311.

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Preston, C. M., Rauthan, B. S., Rodger, C., and Ripmeester, J. A. (1982). A hydrogen-1, carbon-13, and nitrogen-15 nuclear magnetic resonance study of p-benzo-quinone polymers incorporating amino nitrogen compounds (“synthetic humic acids”). Soil Sci. 134, 277–293. Preston, C. M., Schnitzer, M., and Ripmeester, J. A. (1989). A spectroscopic and chemical investigation on the de-ashing of a humin. Soil Sci. Soc. Am. J. 53, 1442–1447. Rex, R. W. (1960). Electron paramagnetic resonance studies of stable free radicals in lignins and humic acid. Nature (London) 188, 1185 –1186. Schnitzer, M. (1978). Humic substances: Chemistry and reactions. In: “Soil Organic Matter” (M. Schnitzer and S. U. Khan, eds.), pp. 1– 64. Elsevier, Amsterdam. Schnitzer, M. (1986a). Binding of humic substances by soil colloids. In “Interactions of Soil Minerals with Natural Organics and Microbes” (P. M. Huang and M. Schnitzer, eds.), Spec. Publ. No. 17, pp. 77–101. Soil Sci. Soc. Am., Madison, WI. Schnitzer, M. (1986b). The synthesis, chemical structure, reactions and functions of humic substances. In “Humic Substances, Effects on Soil and Plants” (EniChem Agricoltura, eds.), pp. 14 –28. Reda, Rome. Schnitzer, M. (1986c). Water retention by humic substances. In “Peat and Water” (C. H. Fuchsman, ed), pp. 159–176. Elsevier Applied Science, New York. Schnitzer, M. (1991). Soil organic matter—the next 75 years. Soil Sci. 151, 41– 58. Schnitzer, M. (1994). A chemical structure for humic acid. Chemical, 13C NMR, colloid chemical, and electron microscopic evidence. In “Humic Substances in the Global Environment and Implications on Human Health” (N. Senesi and T. M. Miano, eds.), pp. 57– 69. Elsevier, Amsterdam. Schnitzer, M., and Khan, S. U. (1978). “Soil Organic Matter.” Elsevier, Amsterdam. Schnitzer, M., and Kodama, H. (1966). Montmorillonite: Effect of pH on its adsorption of a soil humic compound. Science 153, 70 –71. Schnitzer, M., and Preston, C. M. (1986). Analysis of humic acids by solution—and solid-state Carbon-13 nuclear magnetic resonance. Soil Sci. Soc. Am. J. 50, 326 – 331. Schnitzer, M., and Schulten, H.-R. (1992). The analysis of soil organic matter by pyrolysis-ﬁeld ionization mass spectrometry. Soil Sci. Soc. Am. J. 56, 1811–1817. Schnitzer, M., and Schulten, H.-R. (1995). Analysis of organic matter in soil extracts and whole soils by pyrolysis-mass spectrometry. Adv. Agron. 55, 168 –217. Schnitzer, M., and Schuppli, P. (1989). Method for the sequential extraction of organic matter from soils and soil fractions. Soil Sci. Soc. Am. J. 53, 1418 –1424. Schnitzer, M., and Spiteller, M. (1986). The chemistry of the “unknown” soil nitrogen. Trans. Congr. Int. Soil Sci. Soc., 13th, Hamburg, Vol. 3, pp. 473 – 474. Schulten, H.-R., and Schnitzer, M. (1993). A state of the art structural concept for humic substances. Naturwissenschaften 80, 29 – 30. Schulten, H.-R., and Schnitzer, M. (1997). Chemical model structures for soil organic matter and soils. Soil Sci. 162, 115–130. Schulten, H.-R., and Schnitzer, M. (1998). The chemistry of soil organic nitrogen: A review. Biol. Fertil. Soil 26, 1–15. Schulten, H.-R., Plage, B., and Schnitzer, M. (1991). A chemical structure for humic substances. Naturwissenschaften 78, 311– 312. Schulten, H.-R., Monreal, C. M., and Schnitzer, M. (1995). Effect of long-term cultivation on the chemical structure of soil organic matter. Naturwissenschaften 82, 42– 44. Schulten, H.-R., Leinweber, P., and Schnitzer, M. (1998). Analytical pyrolysis and computer modelling of humic and soil particles. In “Structure and Surface Reactions of Soil Particles” (P. M. Huang, N Senesi, and J. Bufﬂe, eds.), pp. 282– 324. Wiley, New York. Schulten, H.-R. Gleixner, G., Schmidt, H.-L., and Muller, R. (1999). Analytical pyrolysis of dissolved organic matter in aquatic systems: structure, properties, and origin. Water Res. (in press).

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Senesi, N., and Schnitzer, M. (1977). Effects of pH, reaction time, chemical reduction, and irradiation on ESR spectra of fulvic acid. Soil Sci. 123, 224 –234. Senesi, N., Grifﬁth, S. M., Schnitzer, M., and Townsend, M. G. (1977). Binding of Fe3+ by humic materials. Geochim. Cosmochim. Acta 41, 969 – 976. Sorge, C., Schnitzer, M., Leinweber, P., and Schulten, H.-R. (1994). Molecular-chemical characteristics of organic matter in whole soil and particle-size fractions of a Spodosol by pyrolysis-ﬁeld ionization mass spectrometry. Soil Sci. 158, 189 –203. Sowden, F. J., Chen, Y., and Schnitzer, M. (1977). The nitrogen distribution in soils formed under widely differing climatic conditions. Geochim. Cosmochim. Acta 41, 1524 –1526. Sprengel, C. (1826). Ueber Pﬂanzenhumus, Huminsaure und Humussaure Salze. Kastner’s Arch. Ges. Naturlehre 8, 145 –220. Steelink, C. (1994). Application of N-15 NMR spectroscopy to the study of organic nitrogen and humic substances in the soil. In “Humic Substances in the Global Environment and Implications on Human Health” (N. Senesi and T. M. Miano, eds.), pp. 405 – 426. Elsevier, Amsterdam. Steelink, C., and Tollin, G. (1962). Stable free radicals in soil humic acid. Biochim. Biophys. Acta 59, 25–34. Stevenson, F. J. (1994). “Humus Chemistry.” Wiley, New York. Stevenson, I. L., and Schnitzer, M. (1982). Transmission electron microscopy of extracted fulvic and humic acids. Soil Sci. 133, 179 –185. Wilson, M. A. (1987). “NMR Techniques and Applications in Geochemistry and Soil Chemistry.” Pergamon, Oxford. Wilson, M. A. (1990). Application of nuclear magnetic resonance spectroscopy to organic matter in whole soils. In “Humic Substances in Soil and Crop Sciences: Selected Readings” (P. MacCarthy, C. E. Clapp, R. L. Malcolm, and P. R. Bloom, eds.), pp. 221–260. Am. Soc. Agron., Madison, WI. Zhuo, S., and Wen, Q. (1992). Nitrogen forms in humic substances. Pedosphere 3, 307– 315. Zhuo, S., Wen, Q., Du, L., and Wu, S. (1992). The nitrogen form of non-hydrolyzable residue in humic acid. Chin. Sci. Bull. 37, 508 – 511. Zhuo, S., Wen, Q., and Cheng, L. (1995). Availability of non-hydrolyzable soil nitrogen. Pedosphere 5, 183–186.

REPRODUCTIVE DEVELOPMENT IN GRAIN CROPS DURING DROUGHT Hargurdeep S. Saini1 and Mark E. Westgate2 1Institut

de recherche en biologie végétale Département de sciences biologiques Université de Montréal Montreal, Quebec, Canada H1X 2B2 2Department of Agronomy Iowa State University Ames, Iowa 50011

I. Introduction II. Sensitivity to Drought at Various Reproductive Stages III. Nature of Injury A. Flower Initiation and Development B. Gametophyte Development C. Pollination, Fertilization, and Grain Initiation D. Kernel Growth and Maturation IV. Water Relations of Reproductive Tissues and their Inﬂuence on Yield A. Stress during Flower Initiation and Development B. Meiotic-Stage Stress C. Stress during Anthesis D. Stress during Grain Filling and Maturation V. Physiological and Metabolic Bases for Reproductive Failure under Drought A. Failure of Pollen Development B. Carbohydrate Availability and Kernel Abortion C. Regulation of Grain Filling and Maturation D. Long-Distance Signals as Triggers of Reproductive Failure VI. Concluding Remarks References

Reproductive development of plants is highly vulnerable to water deﬁcit. Stress early during this phase can delay or completely inhibit ﬂowering, both through an inhibition of ﬂoral induction and development. The stage of meiosis is perhaps the most stress-sensitive period of reproduction in all species studied. A meiotic-stage water deﬁcit causes pollen sterility, but usually affects female fertility only under extreme stress. Sterility occurs even though the reproductive structures of stressed plants maintain a high water status, indicating that the response is probably mediated by a sporocidal signal from elsewhere in the plant. Rice and maize plants are 59 Advances in Agronomy, Volume 68 Copyright © 2000 by Academic Press. All rights of reproduction in any form reserved. 0065-2113/00 $30.00

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HARGURDEEP S. SAINI AND MARK E. WESTGATE also highly vulnerable during ﬂowering (anthesis) and early grain initiation. Stress during this period can cause loss of pollen fertility, failure of pollination, spikelet death, or zygotic abortion. Changes in carbohydrate availability and metabolism appear to be involved in the effects of stress during meiosis and anthesis. Stress during early grain development curtails the kernel sink potential by reducing the number of endosperm cells and amyloplasts formed. Controls underlying these effects are poorly understood, although hormones may be involved. A water deﬁcit during any stage of grain development causes the premature cessation of grain ﬁlling. Kernel moisture content and its direct impact on metabolism appear to be key regulatory factors in shortening the duration of grain ﬁlling. © 2000 Academic Press.

I. INTRODUCTION The sedentary nature of plants exposes them constantly to variations in environmental conditions, which are often unfavorable to the point of being stressful. Such abiotic and biotic stresses generally have a negative impact on the productivity of crop plants (Boyer, 1982; Passioura, 1996) and can, in the extreme, even threaten the survival of an entire crop. No other environmental factor limits global crop productivity more severely than water deﬁcit (Boyer, 1982; Fischer and Turner, 1978). Plant growth and development can be affected by water deﬁcit at any time during the crop life cycle, but the extent and nature of damage, the capacity for recovery, and the impact on yield depend on the developmental stage at which a crop encounters the stress. For example, a transitory episode of drought during the vegetative growth phase could reduce the yield of a forage crop much more severely than that of a grain crop, whereas the latter would be more vulnerable at the time when grain number and weight are determined. Moreover, the sensitivity to water deﬁcit is particularly acute during the reproductive development because reproduction involves several processes that are extremely vulnerable to a change in plant water status (Saini, 1997; Salter and Goode, 1967). The combination of a declining reserve of soil moisture and a high transpiration rate compounds the risk of drought at this late stage. The economic and social consequences of high sensitivity during reproduction are particularly important for grain crops because products of their sexual reproduction constitute the economic yield and provide the primary staple for most of humanity. The life cycle of grain crops is conspicuously phasic. The reproductive phase starts with the transformation of a vegetative meristem into an inﬂorescence and ﬂower primordia; it ends when the seed reaches physiological maturity. The entire phase is a sequential process that can be divided for convenience into a number of substages, including ﬂoral initiation, differentiation of various parts of an inﬂorescence and/or ﬂower, male and female meiosis, development of pollen and em-

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bryo sac, pollination, fertilization, and seed development. Drought occurring during each of these substages has quite speciﬁc effects, all of which lead to a decline in yield.

II. SENSITIVITY TO DROUGHT AT VARIOUS REPRODUCTIVE STAGES Drought at any time during the reproductive phase can reduce grain yield (Claassen and Shaw, 1970; Husain and Aspinall, 1970; Jamieson et al., 1995; NeSmith and Ritchie, 1992; O’Toole and Moya, 1981; Saini and Aspinall, 1981; Salter and Goode, 1967; Westgate and Thomson Grant, 1989). The literature contains many studies comparing the effects of drought during various stages of reproductive development on crop yield. Because it is virtually impossible to compare plants of different developmental stages at an equivalent water status, such studies do not provide a uniform scale for assigning differences in sensitivity in response to comparable tissue water deﬁcits. However, taken together, they allow us to identify the stage-speciﬁc nature of drought effects on components of the reproductive machinery from which we can draw credible conclusions about the relative sensitivity of different stages with respect to the potential for an impact on yield. Although grain crops show some sensitivity to drought during ﬂoral initiation and premeiotic differentiation of ﬂoral parts (Aspinall and Husain, 1970; Barlow et al., 1977; Husain and Aspinall, 1970; Salter and Goode, 1967; Winkel et al., 1997), the most dramatic effects on yield have been recorded when stress coincides with the period bracketed by the onset of meiosis and early grain initiation (O’Toole and Namuco, 1983; Saini, 1997; Salter and Goode, 1967; Westgate and Thomson Grant, 1989). Two distinct peaks of sensitivity are evident within this latter phase: The ﬁrst peak, which is common to all the cereals examined, is centered around pollen mother cell (PMC) meiosis and tetrad break up. The timing of this stage has been determined quite precisely for wheat and rice (Bingham, 1966; Dembinska et al,. 1992; Namuco and O’Toole, 1986; Saini and Aspinall, 1981; Sheoran and Saini, 1996; Udol’skaja, 1936) and is probably identical or very similar in barley, oats, and corn (Dubetz and Bole, 1973; Fischer, 1973; Moss and Downey, 1971; Saini, 1997, and the references cited therein; Salter and Goode, 1967, and references cited therein). In the female ﬂoral organs of wheat, and presumably other cereals, this period corresponds to the meiosis in the megaspore mother cell and the subsequent degeneration of three redundant megaspores in the tetrad (Bennett et al., 1973). The second peak, which is conspicuous in rice and maize, occurs during anthesis and initial stages of grain development (Claassen and Shaw, 1970; Ekanayake et al., 1989, 1990; Hsiao, 1982; O’Toole and Namu-

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co, 1983; Schoper et al,. 1986; Westgate and Boyer, 1986b). A much lower level of sensitivity at this stage, generally evident only under severe stress, has also been observed in wheat, barley, and oats (Aspinall et al., 1964; Brocklehurst et al., 1978; Fischer, 1973; Sandhu and Horton, 1977; Wardlaw, 1971). In rye, millet, and sorghum, which are also known to be sensitive to drought during the reproductive phase, the precise stages of sensitivity have not been determined (Craufurd et al., 1993; Lewis et al., 1974; Mahalakshmi and Bidinger, 1985a; Mahalakshmi et al., 1987; Salter and Goode, 1967). Once the grain has been initiated, there is a gradual decline in stress sensitivity as the grain develops (Aspinall, 1984).

III. NATURE OF INJURY The nature of injury to the structure and function of reproductive organs, and the underlying mechanisms involved, depends on the stage of development at which water stress occurs. This section examines injury during ﬂower initiation, gametophyte development, pollination and grain initiation, and grain growth.

A. FLOWER INITIATION AND DEVELOPMENT Appropriate matching of the timing of ﬂowering and the pattern of inﬂorescence development to the temporal variation in water availability is recognized as one of the most important traits conferring adaptation to drought (Bidinger et al., 1987; Passioura, 1996). However, the effects of drought on ﬂower initiation and early development are among the least understood aspects of crop reproductive development under water-limited conditions. Apical morphogenesis in cereals is quite sensitive to water deﬁcit during vegetative and ﬂoral development (Husain and Aspinall, 1970; Nicholls and May, 1963; Skazkin and Fontalina, 1951). Water stress during vegetative development or during ﬂower induction and inﬂorescence development in cereals slows the rate of inﬂorescence development, leading to a delay or even complete inhibition of ﬂowering (anthesis) (Angus and Moncur, 1977; Craufurd et al., 1993; Derouw and Winkel, 1998; Mahalakshmi and Bidinger, 1985a,b; Mahalakshmi et al., 1987; Winkel et al., 1997; Wopereis et al., 1996). similar effects have also been observed under salinity and may be partially attributable to osmotic stress (Khatun et al., 1995). In Pennisetum and Sorghum, ﬂoral initiation is delayed by water stress (Craufurd and Peacock, 1993; Mahalakshmi and Bidinger, 1985b; Matthews et al., 1990). Very few studies have been done to determine the effects of drought on the process of ﬂoral induction in cereals per

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se, which is difﬁcult to separate from the postinduction ﬂoral development in most cereals. However, experiments with Lolium temulentum, a long-day plant with a simple single-cycle photoperiodic response, and Pharbitis nil, a single-cycle shortday plant, clearly show that ﬂower induction is also inhibited by water deﬁcit. The mechanism by which water stress inhibits ﬂower induction remains obscure, but a role for an increased abscisic acid (ABA) level in L. temulentum has been suggested (King and Evans, 1977). In sweet corn, a female inﬂorescence bud is ﬁrst initiated at node 7 and then successively at the lower nodes prior to the initiation of the terminal male inﬂorescence (Damptey and Aspinall, 1976). Normally, only the bud at node 7 develops into a mature inﬂorescence, but if plants are subjected to water stress during terminal inﬂorescence initiation, the plants produce two to three axillary inﬂorescences at the lower nodes, whereas the growth of the terminal male inﬂorescence is impeded (Damptey and Aspinall, 1976). Because removal of the terminal male inﬂorescence prevents the development of additional female inﬂorescences under water stress and excision of the bud at node 7 has the opposite effect, Damptey et al. (1978a) concluded that the promotion of axillary inﬂorescence formation in stressed plants is mediated through an effect on the terminal male inﬂorescence. Certain parallels between the effects of water deﬁcit and ABA application, including increases in endogenous ABA concentrations of terminal and axillary buds, suggest that ABA may play a role in this stress response (Damptey et al., 1978b).

B. GAMETOPHYTE DEVELOPMENT Gametophyte development is impaired when cereals experience water deﬁcit during meiosis. The most common damage is a loss of pollen fertility. The effect has been observed in wheat (Saini and Aspinall, 1981; Skazkin, 1961), barley (Skazkin and Zavadskaya, 1957; Zavadskaya and Skazkin, 1960), oats (Novikov, 1952), rice (Sheoran and Saini, 1996), and maize (Downey, 1969). In all of the self-pollinated cereals, the increase in pollen sterility leads to a decline in grain set and yield (Salter and Goode, 1967, and references cited therein). In wheat and rice, a large proportion of the water stress-affected anthers are small, shriveled, unable to dehisce, and contain sterile pollen (Saini and Aspinall, 1981; Sheoran and Saini, 1996). A proportion of the pollen in apparently normal-looking anthers of wheat is also sterile (Saini and Aspinall, 1981). Such pollen grains have dilute cytoplasm and are devoid of starch (Dorion et al., 1996; Saini and Aspinall, 1981; Saini et al., 1984; Sheoran and Saini, 1996), which is a conspicuous constituent of fertile cereal pollen (Franchi et al., 1996). Details of microsporogenesis during water stress have been determined only in wheat. Anther development following transitory meiotic-stage stress continues

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normally until about ﬁrst pollen grain mitosis (PGM-1), when microspores lose contact with the tapetum, dislodge from their normal peripheral location, and fail to develop further (Saini et al,. 1984). In about half of such anthers the ﬁlament degenerates at the same time. The disoriented pollen grains do not accumulate starch and have dilute cytoplasm and virtually no intine. However, the exine develops normally. This pattern, observed with an Australian cultivar, differs somewhat from that in a Canadian wheat cultivar, where the ﬁrst symptoms of developmental disruption were observed at or soon after meiosis (Lalonde et al., 1997a). The latter included degeneration of some meiocytes, loss of orientation of microspores, and abnormal vacuolization of the tapetal cells. Comparatively few attempts have been made to determine if female infertility also contributes to the decline in yield in response to meiotic-stage drought. Reciprocal crosses between stressed and unstressed wheat plants showed that female fertility was not affected by a water stress treatment that caused complete male sterility in approximately 40% of the ﬂorets (Saini and Aspinall, 1981). The leaf water potential () of these stressed plants declined to approximately 2.3 MPa (control 0.8 MPa), which verges on being a severe stress for wheat. A similar absence of effect on female fertility in wheat was also reported by Bingham (1966). In oats, female fertility remained unaffected even under severe drought (soil moisture 13% of ﬁeld capacity), unless the stress was prolonged, which caused a degeneration of the antipodal cells and then the entire embryo sac to a withered strand lacking functional elements (Skazkin and Lukomskaya, 1962). Nucellar cells ﬁlled the space left by the degenerating embryo sac. In corn, water stress produced a variety of lesions in the embryo sac, including a complete suppression of development; depending on the severity of stress, 15 to 43% of the embryo sacs were affected (Moss and Downey, 1971). Grain set in these plants was reduced severely, despite hand pollination with fertile pollen, indicating that the structurally abnormal embryo sacs were also sterile. Together, these observations indicate that the development and fertility of the male gametophyte are much more drought sensitive than those of the female gametophyte. This situation is probably common to a variety of stresses because temperature extremes, which also reduce male fertility severely, have no or only minor effect on female fertility (Brooking, 1976; Dupuis and Dumas, 1990; Hayase et al., 1969; Saini and Aspinall, 1982a; Saini et al., 1983; Satake and Yoshida, 1978). The greater stress tolerance of the female gametophyte has an adaptive signiﬁcance because the potential impact of male sterility on grain set in the ﬁeld can be partially mitigated by cross-pollination, whereas a reduction in female fertility cannot be overcome. Meiotic-stage drought also causes other ﬂoral abnormalities, which could affect the number of grains formed. These include the stunting of panicle and the production of immature, small, and discolored “blasted” spikelets in rice (Namuco and O’Toole, 1986; Sheoran and Saini, 1996). Stress of similar intensity does not cause the latter effect in wheat (Dorion et al,. 1996; Saini and Aspinall, 1981), but

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extreme drought can lead to spikelet death (Morgan, 1971; Westgate et al., 1996). Slight shortening of spike length has also been reported in wheat (Bingham, 1966).

C. POLLINATION, FERTILIZATION, AND GRAIN INITIATION This stage of reproductive development is particularly sensitive to drought in rice and corn. In rice, water stress during ﬂowering can reduce the harvest index by as much as 60%, largely as a result of a reduction in grain set (Garrity and O’Toole, 1994; Hsiao, 1982; O’Toole and Moya, 1981). Similar yield reduction also results from desiccation caused by dry wind (Ebata and Ishikawa, 1989). Panicles in stressed plants fail to fully exsert (emerge) from the ﬂag leaf sheath, ﬂowering is delayed, and the percentage of spikelets that open at anthesis is reduced (Ekanayake et al., 1989; O’Toole and Namuco, 1983). The failure of panicle exsertion alone accounts for approximately 25 to 30% of spikelet sterility because the unexserted spikelets cannot complete anthesis and shed pollen, even when development is otherwise normal (Cruz and O’Toole, 1984; O’Toole and Namuco, 1983). Rice spikelets lose water easily at this stage, which can result in bleaching and death of lemma and palea and shriveling of anthers (Ekanayake et al., 1989, 1993; O’Toole et al., 1984; O’Toole and Namuco, 1983). In addition, water stress reduces the number of anthers that dehisce and lowers the amount of pollen shed and in vivo pollen germinability (Ekanayake et al., 1990). Presumably, these abnormalities lead to a failure of fertilization. Grain abortion at the early stages following fertilization also accounts for a part of the reduction in grain number (O’Toole and Namuco, 1983), although the relative contribution of the failure of fertilization and seed abortion is not known. The loss of grain number in maize stressed at this stage can be attributed to any of several causes. The rapid expansion of reproductive structures, particularly the silk (stigma/style), is required for successful seed set. Water stress just prior to anthesis inhibits ear and silk growth more than tassel growth. This difference causes asynchrony between pollen-shedding and silk emergence, and thus a failure of pollination (Du Plessis and Dijkhuis, 1967; Herrero and Johnson, 1981; Kisselbach, 1950; Moss and Downey, 1971; Westgate and Boyer, 1985a). The inhibition of tassel emergence or anther exsertion also prevents pollination (Herrero and Johnson, 1981). Water stress during anthesis does not affect pollen viability or its capacity to affect fertilization (Hall et al., 1982; Herrero and Johnson, 1981; Schoper et al., 1986; Westgate and Boyer, 1986b), but it can cause a decline in silk receptivity if pollination is delayed (Bassetti and Westgate, 1993). Even when gamete and ﬂoral development proceed normally, and pollen is not limiting, grain number can be reduced by only a few days of dehydration at ﬂowering (Schoper et al., 1986; Westgate and Boyer, 1985a, 1986b). The failure to set seed is not due to an inhibition of pollen germination, pollen tube growth, or fertilization, but re-

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sults from the failure of the newly formed zygote to survive beyond 2 or 3 days (Westgate and Boyer, 1986b). Cell division in the proembryo and endosperm is initiated, but the proembryos fails to develop beyond the globular stage and the seed coat does not differentiate (Sass, 1977; Westgate and Boyer, 1968b). This form of zygotic abortion occurs even if droughted plants are rewatered prior to pollination so that pollen germination and fertilization occur at high (Westgate and Boyer, 1986b). Evidently, some aspect of zygote development within the pistillate ﬂowers is disrupted irreversibly by low .

D. KERNEL GROWTH AND MATURATION The general pattern of kernel development can be divided into three Phases (Fig. 1). Phase I, often referred to as the “lag phase,” is an active period of cell division and differentiation and is marked by a rapid increase in kernel fresh weight. Enlargement is primarily the result of water inﬂux driven by a rapid accumulation of solutes (Barlow et al., 1980; Westgate and Boyer, 1986c). In wheat, phase I typically extends from 14 to 20 days after anthesis (Gleadow et al., 1982; Jenner et al., 1991), when potential kernel size is determined by the number of endosperm cells and sites for starch synthesis formed therein (Jenner et al., 1991; Jones et al., 1996). Overlapping and following is phase II, the period of grain ﬁlling, which is marked by a rapid gain in kernel dry weight as a result of the deposition of reserves, predominantly starch (Bewley and Black, 1985). Kernel fresh weight remains relatively stable, as water is displaced by the accumulating reserves within

Figure 1 Typical pattern of cereal kernel development in terms of fresh weight (FW), dry weight (DW), and water content (WC). For convenience, kernel development is divided into three phases: (I) cell division, differentiation, and expansion; (II) rapid reserve accumulation; and (III) maturation. Adopted from Bewley and Black (1994).

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the cells of the embryo and endosperm. During phase III, dry matter accumulation ceases and the kernel undergoes maturation drying and approaches a “quiescent state” (Bewley and Black, 1985). A loss in fresh weight reﬂects a continued, and sometimes more rapid, decline in water content. 1. Inhibition of Cell Division and Expansion Lack of water during phase I (from 5 to 11 days after pollination) exerts its effect primarily on kernel sink potential (Artlip et al., 1995; Mambelli and Setter, 1998). In this case, kernels grow sufﬁciently to allow some accumulation of starch and zein, but they abort development prematurely and remain incompletely ﬁlled (NeSmith and Ritchie, 1992). The sink potential of cereal grains is determined during phase I of development. It is a function of both the number of cells and the number of starch grains initiated in the endosperm, i.e., the number of sites for starch deposition (Gleadow et al., 1982; Jones, 1994). Although maximal kernel sink potential is determined genetically, the actual kernel capacity established is a function of competition for space or assimilate supply and is the growth environment prevailing during the early stages of development ( Jones et al., 1985). The endosperm cell number is sensitive to environmental conditions during cell division, and hence, the reduction in yield under drought or high temperature during early kernel development is due mainly to a lesser number of endosperm cells and/ or amyloplasts initiated. Thus drought reduces the capacity of the endosperm to accumulate starch (Brocklehurst, 1977; Wardlaw, 1971). The decrease in the number of cells and starch granules ultimately affects both the rate and the duration of dry matter accumulation (Brocklehurst et al., 1978; Jones, 1994; Jones et al., 1985; Nicolas et al., 1984). 2. Premature Cessation of Grain Filling Once sink potential has been established and the kernel begins to accumulate starch and protein reserves (phase II), drought can decrease ﬁnal kernel size by limiting the rate and duration of reserve deposition. Tissue desiccation and high temperatures that accompany drought can both have an impact on the process of kernel ﬁlling. In barley (Aspinall, 1965; Brooks et al., 1982), wheat (Brooks et al., 1982) and maize (Jurgens et al., 1978; Ouattar et al., 1987a; Westgate, 1994), drought during seed ﬁlling causes physiological maturity to occur earlier, thus shortening the duration of kernel ﬁlling, which reduces the ﬁnal kernel size. Typically, water deﬁcit has little impact on the rate of kernel growth (Brooks et al., 1982; Ouattar et al., 1987a; Westgate, 1994), whereas high temperature often increases the kernel growth rate (Egli, 1994; Wardlaw et al., 1980). Both factors cause premature cessation of kernel ﬁlling, which may result from a lack of assimilate supply associated with leaf senescence (de Souza et al., 1997; Jurgens et

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al,. 1978), a decreased capacity for assimilation within the embryo or endosperm (Jenner et al., 1991; Savin and Nicolas, 1996), or premature desiccation of the kernel (Brooks et al., 1982; Egli, 1994; Soﬁeld et al., 1977a; Westgate, 1994).

IV. WATER RELATIONS OF REPRODUCTIVE TISSUES AND THEIR INFLUENCE ON YIELD A. STRESS DURING FLOWER INITIATION AND DEVELOPMENT The turgor of the cereal apex is remarkably resistant to the change in response to water stress during the reproductive phase prior to grain growth (Morgan, 1980a, 1984). At the initial stages of floral induction and differentiation, the apex can survive at as low as 6 MPa, which are lethal to leaves (Barlow et al., 1977). The of apex declines in parallel with that of leaves, but the accumulation of solutes, such as sucrose and amino acids, maintains apex turgor (Munns et al., 1979). These solutes are evidently imported from vegetative tissues. Water stress during floral initiation reduces spikelet number in the “indeterminate” type inflorescence of barley (Husain and Aspinall, 1970). Although the extent of osmoregulation in barley inflorescence was not determined, it probably does occur because it is a common feature of water-stressed expanding tissues, including cereal apex (Meyer and Boyer, 1981; Michelena and Boyer, 1982; Morgan, 1984; Munns et al., 1979). The decrease in spikelet number at low w suggests that osmoregulation does not give full protection against water stress during floral initiation.

B. MEIOTIC-STAGE STRESS Even moderate water deﬁcit at this stage can be quite damaging to grain set. In wheat subjected to 3- to 4-day episodes of water stress between 15 and 5 days prior to ear emergence (a period that includes meiosis and gametophyte development), the threshold xylem for a reduction in grain was 1.2 MPa (Fischer, 1973). At lower xylem , the grain set declined linearly with a decline in , reaching zero at values approaching 2.4 MPa. When stress was timed precisely to coincide with pollen meiosis, a decline in leaf relative water content to 67% and to 2.3 MPa, compared to the control values of 93% and 0.8 MPa, respectively, reduced grain set by approximately 35% (Saini and Aspinall, 1981). A more rapid water stress of a similar magnitude was slightly more damaging (Dorion et al., 1996). Meiotic stage stress can reduce grain set by 35 to 75% in different cultivars of rice (Namuco and O’Toole, 1986; Sheoran and Saini, 1996), which dis-

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play wide genotypic differences in susceptibility to water stress during reproductive development (Garrity and O’Toole, 1994). Wheat and rice plants resist changes in the water status of inﬂorescence prior to its emergence. The of ﬂowers and ﬂoral organs of wheat plants stressed during or close to meiosis either remains unaffected (Saini and Aspinall, 1981) or declines much less than that of the leaf (Dorion et al., 1996; Morgan and King, 1984; Westgate et al., 1996). Relative water content of the spikelets also remains constant during stress (Morgan, 1980b). Similarly, of rice panicle changes little diurnally or in response to water deﬁcit during meiosis, but varies markedly with evaporative demand after panicle emergence (Tsuda and Takami, 1993). The resistance of wheat and rice inﬂorescences to water loss during meiosis, which occurs about 7 to 10 days prior to inﬂorescence emergence (Saini and Aspinall, 1981; Sheoran and Saini, 1996), may be due partly to limited transpiration within two or more enclosing leaf sheaths. Moreover, xylem discontinuity between the ﬂoral stalk and the pericarp probably contributes to the apparent hydraulic isolation (Zee and O’Brien, 1970). Even when the of spikelets, anthers, or ovaries does decline in response to water stress, the decline is fully matched by a reduction in osmotic potential (s), and hence, the spikelet turgor does not change despite a drop in leaf turgor to zero (Morgan, 1980b; Morgan and King, 1984; Westgate et al., 1996). Even an increase in the turgor of ﬂoral parts during stress has been reported (Westgate et al,. 1996). Thus, a reduction in grain set in response to meiotic-stage water stress does not correlate with the water status of the reproductive structures. Further, the grain set declines only after the leaf turgor falls to zero (Morgan, 1980b; Morgan and King, 1984). The implications of this dichotomy between the water status of the vegetative and the reproductive structures for regulating ﬂoret fertility is discussed in Section VA.

C. STRESS DURING ANTHESIS Rice panicles have a very low diffusive resistance to transpirational water loss. Therefore, they are generally very poor at preventing water loss after they emerge (Ekanayake et al., 1993; O’Toole et al., 1984; Tsuda and Takami, 1993). Certain upland adapted cultivars of rice, however, are better able to prevent water loss from panicle and suffer less sterility during drought (Ekanayake et al., 1993). This correlation suggests that the deleterious effects of water stress during the ﬂowering of rice could be attributable to the desiccation of the ﬂoral parts. However, limiting transpirational water loss by covering the panicles does not prevent sterility (Garrity et al., 1986). Thus other factors probably act in concert with desiccation to cause sterility. Insufﬁcient turgidity caused by excessive water loss could also be responsible for the inhibition of spikelet opening (Ekanayake et al., 1989), which is driven by the swelling of ﬂoral structures on water uptake (Parmar et al., 1979).

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Pollen in well-watered plants of maize is about 2.0 MPa, but it does not decline further under water stress (Westgate and Boyer, 1986a). In contrast, female reproductive organs of maize are prone to water loss, especially at anthesis (Westgate and Thomson Grant, 1989). As soil water is depleted, declines throughout the plant, but unlike leaf, root, and stem, silks are unable to lower their s and thus maintain turgor (Westgate and Boyer, 1985b). The failure of silks to maintain turgor, which is essential for cell enlargement (Cosgrove, 1981, 1993; Lockhart, 1965), could be responsible for the delay in their growth and emergence under water stress (Herrero and Johnson, 1981; Moss and Downey, 1971). Although asynchrony between male and female development can prevent pollination, the decline in silk per se does not prevent pollen germination because pollen always remains lower than that of silks, allowing the pollen to draw water even from quite dry silks (Westgate and Boyer, 1986a,b). In addition, pollen viability is maintained at low during anthesis (Barnabas and Rajki, 1981; Schoper et al., 1986, 1987; Westgate and Boyer, 1986b). Thus, pollen desiccation is not a factor in limiting seed set under water stress nor does water stress prevent fertilization, but grain development may be aborted (Westgate and Boyer, 1986b). Moreover, drought-induced zygotic abortion occurs even in droughted plants that are completely rehydrated prior to pollination (Westgate and Boyer, 1968b). The fact that reproductive failure in maize cannot be attributed to a direct effect of low at the time of pollination and fertilization implies that the effects of low on metabolism and cellular differentiation may persist long after the stress has been relieved.

D. STRESS DURING GRAIN FILLING AND MATURATION Barley, wheat, and maize exhibit little, if any, change in grain when drought occurs during rapid grain ﬁlling (phase II), whereas other plant structures undergo large decreases in (Barlow et al., 1980; Brooks et al., 1982; Ouattar et al., 1987b; Westgate, 1994; Westgate and Thomson Grant, 1989). Various anatomical, physicochemical, and theoretical models have been proposed to explain this apparent hydraulic isolation of the kernels from other structures of the plant. These include vascular discontinuities within the caryopsis (Brooks et al., 1982; Zee and O’Brien, 1970), large hydraulic discontinuities within the grain and stable xylem within the stem (Ouattar et al., 1987a), osmotic regulation in the apoplast (Westgate and Boyer, 1986c), and the presence of specialized tissues within the vasculature that control osmotic potential of the apoplast (Bradford, 1994). Although each of these possibilities is supported by some experimental observations, none has been tested rigorously. Nonetheless, maintenance of a favorable water status within the kernel presumably permits metabolism to continue despite severe water deﬁcits in the vegetative tissues. If this were true, grain growth during drought would be limited only by the capacity of the plant to supply assimilates. This ap-

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pears to be the case only when plants are stressed severely and rapidly ( Jurgens et al., 1978; Kobata et al., 1992; Nicolas and Turner, 1993; Westgate and Boyer, 1985a). Although several studies have shown that kernel remains fairly constant during rapid kernel ﬁlling (Ouattar et al., 1987b; Westgate, 1994; Westgate and Thomson Grant, 1989), drought during grain ﬁlling causes a decrease in kernel water volume (Westgate, 1994). Drought also causes a premature decline in kernel and s late during ﬁlling (Brooks et al., 1982; Westgate, 1994). The decrease in kernel water content (Westgate, 1994) indicates that drought does indeed alter the water status of the developing kernel. Because endosperm and embryo desiccation may ultimately limit the metabolism of incoming assimilates (see discussion later), the hydraulic response of the kernel to late-season drought could have a direct impact on the duration of kernel ﬁlling. Egli and TeKrony (1997) proposed that the cessation of cell expansion ultimately determines the subsequent timing of seed desiccation and maturation. Cell expansion during phase I is driven by osmotic water uptake, and the maximum seed water content establishes the cellular volume that can be ﬁlled by storage materials. Deposition of storage reserves during phase II replaces cell water, effectively desiccating the seed (Ray, 1978), which eventually triggers seed maturation. Therefore, kernel water volume determines ﬁnal kernel size (dry matter) in two ways: (1) it sets the maximum volume for storage reserve accumulation and (2) it establishes the maximum duration for grain ﬁlling (Brooks et al., 1982; Nicolas et al., 1984). Water stress initiates the process of grain desiccation prematurely so that kernels on water-stressed plants cease to accumulate dry matter sooner after anthesis (Egli and TeKrony, 1997; Soﬁeld et al., 1977a; Westgate, 1994). This analysis presumes that there is a minimum moisture content below which dry matter accumulation ceases. This possibility is discussed further in Section VC.

V. PHYSIOLOGICAL AND METABOLIC BASES FOR REPRODUCTIVE FAILURE UNDER DROUGHT The information on control processes in the effects of drought on ﬂower initiation and early ﬂower morphogenesis is scant. Therefore, we will focus on the events including and following meiotic division in the reproductive organs.

A. FAILURE OF POLLEN DEVELOPMENT Early work aimed at understanding the reasons for the failure of pollen development under water stress found that chromosomal pairing and separation during

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meiosis in PMCs was not always normal in rather severely stressed barley plants (Skazkin and Zavadskaya, 1957). More recently, an increase in meiotic abnormalities was also noticed in water-stressed rice plants (Namuco and O’Toole, 1986). These included an increase in univalents, lagging chromosomes, noncongression of bivalents in metaphase, and micronuclei formation. The abnormalities started to increase at relatively moderate stress (leaf 1.1 to 1.9 MPa) and peaked at around 2.2 MPa. Under severe stress (leaf 3.5 MPa), the entire meiotic process was arrested. Whether any of these abnormalities contributed to pollen sterility is not known, although the cessation of meiosis under severe stress would certainly do so, unless it is reversed on rewatering. In contrast, no such abnormalities were noticed in wheat PMCs (Saini, 1982), which always complete meiotic division under stress (Lalonde et al., 1997a; Saini et al., 1984). Meiosis is apparently also completed in the PMCs of moderately stressed rice because microspores are produced, although their subsequent development fails (Sheoran and Saini, 1996). Microspores of wheat, with a few exceptions (Lalonde et al., 1997a), continue to develop normally for several days before their development is arrested (Lalonde et al., 1997a; Saini et al., 1984). These observations indicate that a more subtle lesion rather than a catastrophic failure of meiosis is the probable cause of male sterility. At least in wheat, the direct desiccation of the sporogenous tissue is not responsible for this developmental arrest (Saini and Aspinall, 1981; Westgate et al., 1996). Developmental anatomy of stress-affected anthers gives some promising clues about the metabolic events that may be linked to the failure of pollen development. Cereal pollen grains are rich in starch (Franchi et al., 1996). Starch accumulates late during pollen development and is then used to support pollen germination and pollen tube growth (Clément et al., 1994; Franchi et al., 1996; Miki-Hirosige and Nakamura, 1983; Pacini and Franchi, 1988). Pollen grains rendered sterile by drought or other stresses fail to accumulate starch (Ito, 1978; Saini and Aspinall, 1981, 1982a; Sheoran and Saini, 1996). Water stress also changes the pattern of starch distribution in anthers and inhibits intine development in pollen grains (Lalonde et al., 1997a; Saini et al., 1984). The timing of the inhibition of starch deposition coincides with the appearance of structural lesions during anther development (Lalonde et al., 1997a; Saini et al., 1984), suggesting that a disturbance in carbohydrate availability and/or metabolism may be involved in this developmental failure. This view is also supported by the observation that increased sucrose uptake increases the grain set in wheat spikes cultured in nutrient solution (Waters et al., 1984). The addition of ABA to the nutrient solution decreases sucrose uptake and grain set, and both effects are reversed by supplementing the medium with additional sucrose (Waters et al., 1984). By extrapolating these observations to intact plants, one could argue that because water stress inhibits the rate of photosynthesis and export of assimilate from leaves (Boyer and McPherson, 1975; Hanson and Hitz, 1982), the supply of sucrose to anthers could be the

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limiting factor under drought. However, the levels of sucrose and other sugars in wheat and rice anthers increase in response to drought stress, suggesting that sugar starvation per se may not be the trigger for pollen sterility (Dorion et al., 1996; Sheoran and Saini, 1996). This accumulation of sugars could, however, be misleading because sucrose utilization in anthers is inhibited by stress. Activities of soluble acid invertase (Dorion et al., 1996; Sheoran and Saini, 1996) and cell wallbound invertase (J. S. Minhas and H. S. Saini, unpublished) decline dramatically in wheat and rice anthers and do not recover even after plants are rehydrated. This is not a generalized effect on anther enzyme activities, particularly in wheat, because the activities of ADP-glucose pyrophosphorylase and soluble and bound forms of starch synthase are not affected (Dorion et al., 1996). Preliminary results show that the expression of genes encoding these invertases is also inhibited (J. S. Minhas and H. S. Saini, unpublished results). The latter effect is not due to a generalized inhibition of transcription because the expression of the ADP-glucose pyrophosphorylase gene is affected little by stress (Lalonde et al., 1997b). In rice anthers, the activities of starch synthases and ADP-glucose pyrophosphorylase decline somewhat during meiotic-stage stress or soon thereafter (Sheoran and Saini, 1996), but the most dramatic effect of stress is on invertase activity. Invertase is the dominant enzyme of sucrose cleavage in the anthers of many species, including the two just mentioned (Bryce and Nelsen, 1979; Dorion et al., 1996; Nakamura et al., 19890, 1992; Sheoran and Saini, 1996). A decline in invertase activity in anthers would interfere with the proper processing of incoming sucrose. The resulting decline in the levels of hexoses needed for biosynthesis and energy generation could jeopardize crucial metabolic and developmental processes. In wheat, the decline in invertase activity precedes or coincides with the ﬁrst anatomical signs of pollen abortion (Dorion et al., 1996; Lalonde et al., 1997a; Saini et al., 1984). The observed accumulation of sucrose in stress-affected anthers, despite the expected decline in the sucrose supply on inhibition of photosynthesis (Boyer and McPherson, 1975; Hanson and Hitz, 1982), could be due to the inhibition of invertase (Dorion et al., 1996). The redistribution of starch, particularly its accumulation in the connective tissue (Lalonde et al., 1997a), is consistent with the inhibition of sucrose utilization by stressed anthers. A similar fate of excess sucrose has been demonstrated in Lilium anthers (Clément and Audran, 1995). The molecular and metabolic regulation of invertase inhibition in relation to stress-induced pollen sterility merits close examination because it is the earliest occurring stress-induced lesion identiﬁed to date, and its timing qualiﬁes it as a potential causal event.

B. CARBOHYDRATE AVAILABILITY AND KERNEL ABORTION The magnitude of decline in leaf that causes kernel abortion in maize also completely inhibits photosynthesis and leads to a reduction in carbohydrate reserves in

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stem (Westgate and Boyer, 1985a). Therefore, it has been suggested that the abortion may be attributable to a curtailed supply of carbohydrates during pollination (Westgate and Boyer, 1986b). This view is also supported by the observation that the CO2 enrichment of wheat plants partially overcomes the yield losses due to water stress (Gifford, 1979). Kernel number is reduced in proportion to the inhibition of photosynthesis by water deﬁcit or varying light intensity (Schussler and Westgate, 1991a). Although carbohydrates continue to accumulate in vegetative sinks such as leaf and stalk, despite an inhibition of photosynthesis, their movement to reproductive sinks is restricted severely (Schussler and Westgate, 1991a,b). These data indicate that early kernel development is dependent on the supply of assimilate from concurrent photosynthesis, which cannot be replaced by the remobilization of reserves stored in other tissues. Because sugar concentrations in the ovaries do not change and because the sugar uptake by ovaries isolated from stressed plants is inhibited, kernel set depends on the rate of movement of the current assimilate to reproductive organs rather than the concentration of sugars per se (Schussler and Westgate, 1991b). Consistent with this conclusion, experimental manipulations to either enhance the accumulation of carbohydrate reserves prior to anthesis or reduce the sink size for a ﬁxed availability of assimilates do not diminish the extent of water stress-induced kernel loss (Schussler and Westgate, 1994; Zinselmeier et al., 1995a). Infusing liquid culture medium into the stem in a quantity sufﬁcient to replace carbohydrates lost by the inhibition of photosynthesis during ﬂoweringstage water deﬁcit, however, can prevent the abortion of most (⬃70%) kernels (Boyle et al., 1991; Zinselmeier et al., 1995b). Stem infusion does not rehydrate the plant, nor is kernel abortion prevented by the infusion of water alone. The ingredient in the medium that prevents abortion is sucrose, the level of which has to be elevated above that in well-watered ovaries to prevent abortion under stress (Zinselmeier et al., 1995b). Supplemental sucrose sustains the ovary growth rate, which is correlated with a high starch content and turgor maintenance by osmotic adjustment (J. S. Boyer and C. Zinselmeier, personal communication). These results further support the conclusion that the maintenance of assimilate supply from current photosynthesis is essential in preventing kernel abortion in stressed plants. Delivery of photosynthate to the developing ovaries also depends on their metabolic activity (i.e., sink demand) (Jenner, 1982). Several observations indicate that the capacity to utilize available assimilate is impaired by water stress. The reduction in the level of starch, which is a product of sucrose metabolism in ovaries, is only partially restored by sucrose infusion (Zinselmeier et al., 1995b,c). Ovaries isolated from stressed plants take up sucrose less rapidly than those from wellwatered plants (Schussler and Westgate, 1991b). Kernel abortion is not prevented completely by feeding culture medium or sucrose (Boyle et al., 1991; Zinselmeier et al., 1995b) or by increasing assimilate supply through cultural or genetic manipulations (Schussler and Westgate, 1991a,b, 1994; Zinselmeier et al., 1995a). Direct evidence for a metabolic lesion has been furnished by Zinselmeier et al.

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(1995c), who showed that acid invertase activity in the ovaries of water-stressed plants was inhibited strongly in parallel with the cessation of ovary growth, an accumulation of sucrose, and a decrease in the level of reducing sugars. Maize ovaries induced to abort in vitro by high temperature also have low acid invertase activity (Hanft and Jones, 1986). These metabolic events are remarkably similar to those observed in stress-affected anthers (Dorion et al., 1996; Sheoran and Saini, 1996) and may point to a common metabolic basis for the failure of these two sinks to develop during drought. The abortion of kernels (and anthers) in waterstressed plants could be caused by the metabolic block resulting from a decline in invertase activity. Further support for this conclusion comes from the maize mutant miniature-1, which lacks soluble and wall-bound acid invertase and fails to produce normal kernels (Miller and Chourey, 1992). How invertase activity could be modulated by sucrose is discussed in Section VC2.

C. REGULATION OF GRAIN FILLING AND MATURATION In cereals, the ﬁrst 6 to 14 days after anthesis set the potential for subsequent development (Jones, 1994). Drought and high temperature during this period reduce the storage capacity of cereal grains by decreasing the number of endosperm cells and/or the number of amyloplasts initiated (Artlip et al., 1995; Brocklehurst et al., 1978; Jones et al., 1985, 1996; Nicolas et al., 1985; Ouattar et al., 1987a). Neither high temperature nor water deﬁcit affects the supply of sucrose to the endosperm, suggesting that the availability of assimilates, per se, is not limiting (Nicolas et al., 1984; Ober and Setter, 1990). Several lines of evidence indicate that hormonal regulation, particularly by ABA and cytokinins, may be involved. In maize, the ABA content of pedicel/placento-chalazal tissues and endosperm is low, whereas the ABA content of the embryo is high when sink potential is established (Jones and Brenner, 1987). Water deﬁcits imposed during the ﬁrst week after pollination increased kernel ABA levels about eight-fold (Ober and Setter, 1990). The ABA apparently is of maternal origin and decreases to control levels on plant rehydration. The temporary increase in ABA has little, if any, effect on invertase, sucrose synthase, or starch synthase activities, measured in vitro, or on subsequent starch accumulation (Ober and Setter, 1990). However, the increase in ABA content in apical kernels of droughted plants was correlated with an inhibition of endosperm cell division and a decrease in capacity for starch synthesis (Ober et al., 1991). Similarly, a rapid rise in endosperm ABA content coincides with the disruption of cell division caused by high temperature (Jones et al., 1985). Exogenous application of ABA has a similar negative impact on endosperm cell division (Mambelli and Setter, 1998; Myers et al., 1990) Thus, an increase in endosperm ABA concentration may serve to downregulate the kernel sink potential.

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In contrast, evidence from maize, rice, barley, and wheat implicates cytokinins as a positive effector in establishing sink potential. Levels of zeatin and zeatinriboside increase as much as 500-fold after pollination (Dietrich et al., 1995; Lur and Setter, 1993; Morris et al., 1993; Schreiber, 1990). While maximum cytokinin levels have been correlated with kernel size, a causal relationship between the two has not been demonstrated. The application of cytokinins can enhance kernel set and sometimes increase the yield of cereals (Dietrich et al., 1995; Hradecka and Petr, 1992). In each case, however, cytokinins were applied prior to the period of rapid cell division and promoted earlier stages of ﬂower and zygote development. To our knowledge, no direct evidence shows that peak cytokinin levels decrease in drought-stressed kernels nor has a beneﬁt from the exogenous application of cytokinins during the rapid cell division phase been demonstrated. Such information would clarify the role of cytokinins (or perhaps the cytokinin/ABA ratio) in maintaining the kernel sink potential during drought. Once cell division in the endosperm is complete and the kernel enters into the linear growth phase (phase II, Fig. 1), the machinery for reserve accumulation is established for the remainder of kernel growth. Water deﬁcits have little impact on the rate of kernel growth, but often shorten the duration of ﬁlling. The hormonal regulation of grain ﬁll duration has been implicated by correlative data linking ABA content with storage protein deposition, late embryo-genesis abundant (LEA) proteins, and acquisition of desiccation tolerance (Dure, 1997; Ingram and Bartels, 1996; Quatrano et al., 1983). However, several studies suggest that ABAresponsive events late in grain ﬁlling are initiated by dehydration itself rather than by a precedent hormonal signal (Bochicchio et al., 1993; Chandler et al., 1993; Iturriaga et al., 1992). Therefore, we need to consider how drought during kernel ﬁlling affects kernel water relations and the desiccation process. Egli and co-workers have shown that the ﬁnal mass of soybean embryos can be manipulated by restricting seed water volume mechanically (Egli et al., 1987) or osmotically (Egli, 1990). Similarly, the physical restriction of seeds during the period of rapid water uptake leads to decreases in kernel size in wheat (Millet and Pinthus, 1984), barley (Graﬁus, 1978), oat (Graﬁus, 1978), and rice (Murata and Matsushima, 1975). Moreover, embryos can grow beyond their normal size in culture if cell expansion is allowed to continue and C and N are available (Egli, 1990). The mechanism by which water uptake (maximum seed volume) is controlled has not been established, but it has been suggested that a decrease in assimilate supply to the seed alters the osmotic gradient driving water ﬂow into the seed (Egli and TeKrony, 1997). This idea, however, is not well supported by evidence. For example, the maximum water content of the maize endosperm is reached during rapid seed ﬁlling (Westgate, 1994; Westgate and Boyer, 1986c), and therefore, it is difﬁcult to envision that there would be sufﬁcient assimilate to maintain rapid dry matter accumulation, but insufﬁcient solute to sustain an osmotic gradient for water ﬂux. There is also no evidence that osmotic conditions within the seed vary

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during this transition period. The and osmotic potential of the maize embryo and endosperm are nearly constant during rapid grain ﬁlling when the maximum osmotic (water) volume is reached (Westgate, 1994; Westgate and Boyer, 1986c). Similarly, in wheat and barley, there is no discernible change in osmotic conditions within the kernel as they achieve maximum water content early in grain ﬁlling (Barlow et al., 1980; Morris et al., 1991; Soﬁeld et al,. 1977b). In fact, a change in the osmotic environment within the kernel would not be expected because water uptake continues rapidly throughout grain ﬁlling (Soﬁeld et al., 1977b). The possibility that the yield threshold for cell wall expansion increases at this time is also unlikely because no change in embryo or endosperm turgor was observed as the maximum water content was reached (Barlow et al., 1980; Westgate, 1994; Westgate and Boyer, 1986c). Nonetheless, data from a number of unrelated studies on kernel development in cereals show a general correspondence (r2 0.86) between ﬁnal kernel dry weight and maximum water content during grain ﬁlling (Fig. 2). Among these studies, however, the individual relationships for maize, rye, and triticale are rather tenuous. Triticale kernels, for example, accumulated much less dry matter than expected from their potential volume. Maize kernels achieved a wide range of

Figure 2 Relationship between maximum water content and ﬁnal kernel dry weight in maize, wheat, rye, and triticale. Data were calculated from kernel growth curves for maize (Egli and TeKrony, 1997; Westgate and Boyer, 1986c; M. E. Westgate, unpublished), wheat (Brooks et al., 1982; Egli and TeKrony, 1997; Soﬁeld et al., 1997a,b), triticale, and rye (Saari et al., 1985). Regression equations include only wheat or maize data. Open symbols, well watered; closed symbols, water stressed.

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weights from a maximum water content of about 150 mg. Within a species, the relationship between ﬁnal kernel weight and water content was not genotype speciﬁc (data not shown). In general, kernels from water-stressed plants followed the same general pattern as those from well-watered plants. The pattern of net water content during kernel development varies considerably among the cereals. The general pattern presented in Fig. 1 is typical of maize (Egli and TeKrony, 1997; Westgate and Boyer, 1986c) and triticale (Saari et al., 1985) in that the water content begins to decrease during rapid grain ﬁlling. Rapid deposition of storage reserves replaces the volume occupied by water, leading to tissue desiccation. In contrast, the water content in wheat, rye, and barley kernels remains fairly constant throughout grain ﬁll and begins to decrease roughly when the maximum dry matter is attained (Barlow et al., 1980; Brooks et al., 1982; Egli and TeKrony, 1997; Saari et al., 1985; Soﬁeld et al., 1977b). Soﬁeld et al. (1977b) provided convincing evidence that the rapid decrease in water content late in ﬁlling of wheat kernels was due to the blockage of water uptake caused by lipid deposition in the apoplast of the pigment strand. Regardless of the mechanism by which cellular desiccation occurs, lack of water late in development could limit the synthesis of storage reserves even when assimilates are available (Adams and Rinne, 1980; Westgate, 1994). If so, the general pattern of kernel dry matter accumulation across species, genotypes, and environments should be fairly similar when expressed on a kernel moisture basis. Figure 3 shows the pattern of kernel dry weight accumulation with decreasing kernel moisture for maize and wheat in several unrelated studies in which the kernel water content was monitored during development. A large variation in the ﬁnal dry weight within species reﬂects the differences in genetic potential as well as environmental effects on kernel growth. The key feature of these data is that dry matter accumulation ceased at about the same moisture content regardless of genotype or environment. Kernel development on water-deﬁcient plants follows the same general pattern (Fig. 4). Whether soil moisture is abundant or severely limiting, dry matter accumulation ceases at about 30 and 40% moisture in maize and wheat kernels, respectively (Figs.3 and 4). The consistency of this relationship implies that a common mechanism may control the cessation of kernel growth and that this mechanism is coupled to the water status of the kernel. It is reasonable to assume that the rapid synthesis of end products such as starch, protein, and oil during grain ﬁlling requires optimum coordination among substrate availability, enzyme activation, translation, and transcription (Bewley and Black, 1994). A decrease in any one of these factors during desiccation could lead to the cessation of dry matter accumulation. In both control and water-deﬁcient plants, the progressive loss of water from the endosperm and the restriction of water uptake by the embryo lead to a rapid decrease in endosperm and embryo s late in grain ﬁlling (Westgate, 1994). Concentrations (per gram of water) of sucrose and amino acids in the endosperm of maize, wheat, barley, and

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Figure 3 Pattern of kernel dry weight accumulation with decreasing kernel moisture. Data for maize are adopted from Egli and TeKrony (1997), Westgate and Boyer (1986c) and M. E. Westgate (unpublished data). Wheat data are adopted from Egli and TeKroney (1997), Soﬁeld et al. (1997a,b), Barlow et al. (1980), and Brooks et al. (1982). Closed symbols are used to highlight differences between identical genotypes.

triticale remain fairly constant during linear grain ﬁll and then increase as the water content declines (Brooks et al., 1982; Chevalier and Lingle, 1983; Nicolas et al., 1984; Saari et al., 1985). Therefore, the initial decrease in s must be due, in large part, to the passive concentration of solutes. Although it has been suggested that specialized structures within the apoplast may exist to maintain high solute concentrations within the embryo and endosperm (Bradford, 1994), the fact that the decrease in kernel s occurs well in advance of physiological maturity strongly suggests that end product formation was not limited by a lack of substrate when

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Figure 4 Pattern of dry weight accumulation with decreasing kernel moisture in maize and wheat kernels sampled from well-watered (closed symbols) and water-deﬁcient (open symbols) plants. Water was withheld at developmental stages indicated by arrows. Data are adopted from Brooks et al. (1982). and M. E. Westgate (unpublished data).

dry matter accumulation ceased in either well-watered or water-deﬁcient plants (Soﬁeld et al., 1977b; Westgate, 1994). Activities of enzymes involved in nitrogen and carbohydrate metabolism in the endosperms of maize, wheat, and barley decrease late in grain ﬁll (Chevalier and Lingle, 1983; Doehlert et al., 1986; Muhitch, 1991; Saari et al., 1985; Singletary et al., 1990). Adams and Rinne (1980) proposed that dehydration inactivated these enzymes, thus terminating the accumulation of reserves. In vitro studies show that desiccation does indeed inhibit enzyme activity dramatically, but only at very low moisture contents typical of dry seeds (Rupley et al., 1983; Stevens and Stevens,

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1977). Seeds are considered fully “wetted” at about 28% moisture, at which point the properties of the water resemble those of water in dilute solutions (Vertucci, 1989). The of such seeds is about 20 MPa, which is much lower than the 1.0 to 2.0 MPa at which grain dry matter accumulation ceases. The complete hydration of macromolecule surfaces occurs at about 95% RH or about 7.5 MPa (Vertucci, 1989), which is well below seed at end of grain ﬁlling. Also, enzymes extracted from physiologically mature seeds retain fairly high levels of activity (Chevalier and Lingle, 1983; Doehlert et al., 1986; Muhitch, 1991; Saari et al., 1985). Although such measurements likely overestimate in vivo activity, as enzymes are assayed under ideal conditions, available data suggest that the osmotic conditions prevalent in the endosperm and embryo late in grain ﬁlling do not directly inhibit enzyme activity. Rather, the loss of enzyme activity probably reﬂects a decreased capacity for protein synthesis as water content declines (Bewley, 1981). Greene (1983) and Kermode et al. (1989) have shown that the decline in protein synthesis late in seed development is coupled to the level of translatable mRNAs. Premature desiccation of castor bean (Ricinus communis L.) seeds caused the same quantitative and qualitative changes in developmental mRNAs that occur during normal maturation drying (Kermode et al., 1989). This result implies that desiccation itself may be a developmental queue for terminating transcription and/or translation of mRNAs required for storage product formation. To date, studies on these mRNAs in cereal kernels have been restricted to the period of rapid storage product formation ( Jones, 1978; Marks et al., 1985; Viotta et al., 1975). Such measurements need to be extended to the later stages of grain ﬁlling to determine the fate of developmental mRNAs during tissue desiccation. Similar mRNA proﬁles in kernels of well-watered and water-deﬁcient plants would support the hypothesis that the duration of dry matter accumulation is controlled at the level of transcription and/ or translation.

D. LONG-DISTANCE SIGNALS AS TRIGGERS OF REPRODUCTIVE FAILURE 1. Role of Hormones in Drought-Induced Male Sterility Initial inquiries into the physiological basis for water stress-induced male sterility in wheat (see Section IVB) revealed that the water status of spikelets changed little, despite a substantial decline in leaf in response to meiotic-stage water stress (Morgan, 1980b; Saini and Aspinall, 1981). Westgate et al. (1996) later demonstrated that individual ﬂoral organs, including anthers, were capable of osmoregulating effectively to maintain or even increase their turgor during stress. It is clear, therefore, that the male sterility in water-stress wheat, and perhaps other

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plants, is not caused by desiccation of reproductive structures, but results from indirect consequences of a drop in or turgor elsewhere in the plant (Morgan, 1980b; Saini and Aspinall, 1981). How the existence of stress in the vegetative parts of the plant is communicated to the reproductive tissue is an important question. The fact that sterility is induced only after leaf turgor falls to zero (Morgan, 1980b) suggests that a turgor-responsive process, such as the accumulation of abscisic acid (Aspinall, 1980; Pierce and Raschke, 1980; Walton, 1980), may be involved. The possibility that ABA produced on turgor loss in the vegetative tissues (e.g., leaf) is translocated to the inﬂorescence where it triggers events leading to male sterility, has been studied in some detail, and evidence favoring this hypothesis is summarized next. a. ABA Accumulation in Reproductive Tissues Meiotic-stage spikelets and anthers accumulate ABA during water stress, despite no change in their turgor (Morgan, 1980b; Saini and Aspinall, 1982b; Westgate et al., 1996), indicating that the hormone is transported from leaves or other vegetative tissues. The long-distance transport of ABA to spikes in wheat, and in other species, has been demonstrated (Goldbach and Goldbach, 1977; Ober and Setter, 1990; Wolf et al., 1990). b. Effects of ABA Application Application of exogenous ABA to spike or leaf causes pollen sterility and loss of grain set in wheat (Morgan, 1980b; Saini and Aspinall, 1982b; Waters et al., 1984; Zeng et al., 1985). Both ABA application and water deﬁcit have their maximal effect during meiosis (Morgan, 1980b; Morgan and King, 1984; Saini and Aspinall, 1981, 1982b; Zeng et al., 1985), and both treatments induce male sterility without affecting female fertility (Saini and Aspinall, 1982b). Stress- and ABAaffected anthers and pollen grains look morphologically similar at maturity (Morgan, 1980b; Saini and Aspinall, 1981, 1982b). c. ABA Concentrations and Sterility When ABA and stress treatments induce similar levels of sterility, the spikelet ABA content resulting from exogenous ABA application is within the same order of magnitude as when the hormone accumulates endogenously in response to water stress (Saini and Aspinall, 1982b). Grain set correlates negatively and tightly with ABA levels in anthers, ovaries, and glumes of water-stressed wheat plants (Westgate et al., 1996). d. Other Indirect Evidence Consistent with the just-mentioned observations, differences in the seed set between well-watered plants of two cultivars of wheat correlate inversely with the ABA content of the spike (Morgan and King, 1984). Spikelets and leaves of a nu-

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clear male sterile mutant of wheat have a greater ABA level and a lower rate of ABA metabolism than the corresponding fertile plants (Zeng and King, 1986). Distal ﬂorets within a wheat spikelet set fewer grains and contain higher ABA levels than the more fertile basal ﬂorets (Lee et al., 1988). Stamens of the male sterile Stamenless-2 mutant of tomato have a markedly higher ABA content than those of wild type plants, and the ABA content of the mutant’s stamens increases concomitant with the ﬁrst signs of developmental abnormalities (Singh and Sawhney, 1998). Low temperature restores fertility in the mutant and causes a drop in ABA levels in anthers and leaves. The just-described correlations notwithstanding, direct evidence is lacking to support the inductive role of ABA in pollen sterility caused by water deﬁcit. A major difﬁculty in settling this question is that no speciﬁc inhibitor of ABA biosynthesis or action is known, and ABA-deﬁcient or -insensitive mutants suffer many pleiotropic effects when water stressed. Dembinska et al. (1992) took an alternative approach and used wheat plants grown with their root system split into two equal halves. In these plants, half the roots could be subjected to water stress while keeping the other half wet. Water uptake by the wet half was sufﬁcient to keep the aerial parts of the plant hydrated to normal levels of leaf as the soil in the other half dried. The ABA content of the spike at high leaf increased, presumably via the transport from dry roots, to the level measured in plants with the entire root system stressed. Sterility was induced, however, only when leaf was allowed to decline. Thus, the increased ABA level in the spike could not be the sole regulator of fertility. This view is also supported by an earlier observation of developmental differences between anthers affected by ABA and water stress (Saini et al., 1984). The results of Dembinska et al. (1992), however, leave open the possibility that ABA arriving from the roots and leaves could be compartmentalized differentially in the reproductive tissues. It is also possible that ABA affects fertility only in concern with some other consequence(s) of reduced leaf , as indeed seems to happen in the regulation of stomatal aperture (Tardieu and Davies, 1992). It is important to note that the putative role of ABA as a pollen sterilent in stressed plants is based largely on the initial observation that ABA application induces male sterility (Morgan, 1980b). However, the sporocidic effect of ABA is not unique, as a variety of synthetic and natural substances induce sterility in plants (Cross and Ladyman, 1991; McRae, 1985; Sawhney and Shukla, 1994). Moreover, stresses that have no effect on tissue ABA levels can also cause nearly complete pollen sterility (Saini and Aspinall, 1982b). Hence, apparent similarities among the effects of chemicals and water stress on pollen fertility may simply be a reﬂection of the limited range of vulnerable events during the reproductive development rather than a similarity in the underlying mechanisms. Other phytohormones may also play a role in this response to stress. Cytokinins are involved in the regulation of a number of developmental processes, and water stress can alter cytokinin levels in plants (Aspinall, 1980; Davies et al., 1986;

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Davies and Zhang, 1991). Application of cytokinins prevents heat-induced kernel abortion in maize and improves ﬂower production and fruit set in other species (Atkins and Pigeaire, 1993; Carlson et al., 1987; Cheikh and Jones, 1994; Mosjidis et al., 1993). Spikelet fertility in wheat was improved by pressurizing the roots of water-stressed plants (Westgate et al., 1996). This treatment also prevents the browning of apical spikelets, an indication that pressurization may have increased the ﬂow of cytokinins to the spikelets. Thus, the possible involvement of cytokinins in the control of fertility in water-stressed plants merits further investigation. A role for ethylene in the induction of sterility in stressed plants was also proposed (Morgan, 1980b). This role, however, is unlikely because pharmacological concentrations are required to cause male sterility (Bennett and Hughes, 1972; Fairey and Stoskopf, 1975; Saini, 1982). Moreover, water stress does not promote ethylene evolution from intact wheat and other plants (Morgan et al., 1990; Narayana et al., 1991). Plant roots are recognized as the source of signals that inﬂuence physiological responses of the aerial parts of a plant (Davies and Zhang, 1991). Using wheat plants with a split-root system, the experiments of Dembrinska et al. (1992) indicate that roots are probably not the source of the putative signal that affects male fertility. Because roots are the primary source of cytokinins (Davies and Zhang, 1991), the results of these experiments also argue against a major role of cytokinins in regulating fertility. Consistent with this is the observation that root pressurization to restore high level in water-stressed plants improved grain set, indicating that shoot is a more important determinant of grain set than root (Westgate et al., 1996). 2. Sugars as Long-Distance Signals Inﬂuencing Reproductive Development in Stressed Plants An increasing body of evidence indicates that the concentration or ﬂux of sugars can serve as long-distance signals involved in the control of expression of various genes in carbohydrate metabolism (Jang and Sheen, 1994; Koch, 1996; Koch et al., 1995). Genes encoding invertase and sucrose synthase, the enzymes that catalyze the critical ﬁrst step in sucrose metabolism in different tissues, are regulated in this fashion; the expression of different members of these gene families can either be enhanced or be repressed by sugars, often in a tissue-speciﬁc manner (Ehness et al., 1997; Godt and Roitsch, 1997; Koch et al., 1992; Roitsch et al., 1995; Xu et al., 1996). It has been proposed that differential sucrose modulation of the expression of these genes in vegetative and reproductive tissues could adjust carbon allocation to priority areas during development and under stressful conditions (Ehness et al., 1997; Koch, 1996; Xu et al., 1996). In this context, the effects of water stress on invertase activity and gene expression in anthers and ovules present exciting possibilities for inquiry into the mechanisms of reproductive fail-

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ure (Dorion et al., 1996; Sheoran and Saini, 1996). Sugar could ﬁt the description of the pollen sterility-inducing stress signal hypothesized in the preceding section; a decline in photosynthesis at low leaf would reduce sucrose ﬂux to the inﬂorescence, inhibiting the expression of certain invertase genes in anthers, either directly or through one of the several regulatory molecules downstream of sucrose cleavage (Koch, 1996; Smeekens and Rook, 1997). The attendant impact on sugar processing could jeopardize pollen development by restricting the routing of sugars to biosynthesis and energy generation. The observed sugar accumulation and its abnormal diversion to starch in cells outside the anther locule is consistent with such a mechanism (Dorion et al., 1996; Lalonde et al., 1997a). Sugars may exert a similar regulatory inﬂuence on the water stress-induced kernel abortion in maize (Zinselmeier et al., 1995c), a process that has many important metabolic parallels with the failure of anther development in wheat (see Sections VA and VB). This raises a strong possibility that the basic mechanisms controlling the failure of anther and zygote development under water stress may be very similar.

VI. CONCLUDING REMARKS Although the vulnerability of cereals to water deﬁcit during reproductive development and its direct impact on yield have been well documented over the last century, the progress toward overcoming this problem has been very slow. However, efforts over the last two decades to understand the physiological, biochemical, and molecular bases of reproductive failure are beginning to bear fruit. The current state of knowledge can be summarized as follows. Meiotic stage is highly sensitive to water deﬁcit in all the species examined. Stress at this stage reduces grain set, primarily through the induction of pollen sterility. Water stress during ﬂowering causes pollen sterility and the failure of pollination in rice and zygotic abortion in maize. Drought during endosperm cell division decreases sink potential by inhibiting cell division and DNA endoreduplication, and stress later during grain ﬁlling shortens the duration of ﬁlling by causing premature desiccation of the endosperm and by limiting embryo volume. The induction of pollen sterility by meiotic-stage water stress probably involves an endogenous sporocide transported from vegetative to reproductive organs. Correlative evidence favors the hypothesis that ABA could be such a sporocide, but direct demonstration of a causal link between plant-produced ABA and sterility is still lacking. In fact, the most recent evidence tends to reject this hypothesis. A reduced assimilate supply and the metabolic ability of the reproductive structures to process the incoming sucrose appear to be important factors in kernel abortion in maize and in the arrest of pollen development in wheat and rice. Similarities between these two systems could imply similar regulatory mechanisms.

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Because the resistance of panicle to water loss is apparently a major cause of damage by ﬂowering-stage water stress in rice, any attempts to understand the mechanisms underlying the damage at this stage must consider the characters that affect panicle transpiration and osmoregulation, as well as root morphology and depth, which inﬂuence water delivery to the shoot (Garrity et al., 1986; Hanson et al., 1990). Overcoming the negative effects of drought after pollination on kernel sink potential requires a greater understanding of the hormonal regulation of endosperm cell division and amyloplast formation. Although cytokinins and ABA are considered integral regulators of these processes, their levels and interactions are largely unexplored in young kernels of droughted plants. Drought events early in kernel development that decrease kernel sink potential, events during linear ﬁll that inhibit enzyme activity directly, or those that occur late in development and cause premature desiccation all appear to decrease ﬁnal kernel mass by shortening the duration of ﬁlling. A common denominator in all cases, however, is the eventual depletion of kernel moisture to a level below which metabolism cannot continue. Therefore, identifying the plant and kernel mechanisms that control kernel moisture content is imperative. Incorporating more desiccation-tolerant enzymes for carbohydrate metabolism is also an intriguing possibility for stabilizing kernel size during drought.

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ADVANCES IN CHLORIDE NUTRITION OF PLANTS Guohua Xu,1 Hillel Magen,2 Jorge Tarchitzky,3 and Uzi Kafkaﬁ4 1College

of Resources and Environmental Sciences Nanjing Agricultural University Nanjing 210095, People’s Republic of China 2Dead Sea Works Ltd., Potash House Beer-Sheva 84100, Israel 3Extension Service Ministry of Agriculture Tel-Aviv 61070, Israel 4Department of Field Crops, Vegetables, and Genetics The Hebrew University of Jerusalem Rehovot 76100, Israel

I. Introduction II. Behavior of Chloride in Soil A. Sources and Accumulation of Chloride B. Chloride in Soil C. Chloride as a Nitriﬁcation Inhibitor III. Chloride in Plants A. Yield and Quality Response to Chloride B. Uptake and Distribution of Chloride C. Biochemical Functions D. Physiological Functions E. Interaction of Chloride Uptake with Uptake of Other Nutrients F. Disease Suppression IV. Chloride in Crops A. Small Grains B. Vegetables C. Fruit Trees D. Oil Seeds E. Other Crops V. Chloride Management in Fertilization and Irrigation A. Salt Accumulation in Soil B. Monitoring of Chloride Concentration in Soil C. Fertilization Under Conditions of Chloride Salinity D. Irrigation and Leaching of Chloride Salts in the Root Zone E. Sprinkler Irrigation and Chloride-Induced Foliar Injury VI. Summary References 97 Advances in Agronomy, Volume 68 Copyright © 2000 by Academic Press. All rights of reproduction in any form reserved. 0065-2113/00 $30.00

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The dependence of modern agriculture on irrigation and chemical fertilization emphasizes the problem of chloride accumulation in soils and its adverse effect on plants rather than on its deﬁciency. This review evaluates chloride behavior in the soil environment and its functions in plants as an essential nutrient, as a disease suppressor, and as an osmoticum in plant cells. The management practice of chloride content in the root zone of irrigated agricultural soil is reviewed. Both the positive and the negative effects of chloride on plant growth and marketable quality under normal and particularly under saline conditions are discussed. Some mechanisms of crop requirement and tolerance to chloride are also evaluated. The close relationship between potassium and chloride in speciﬁc plant cells is described. Potassium and nitrate roles in increasing plant tolerance to salinity and in reducing the hazard of using saline water are discussed on the cell and whole plant levels. Chloride inﬂuence on reducing the nitriﬁcation rate of ammonium fertilizers in the soils is discussed in relation to rice ﬁeld management. The practical applications of irrigation methods to prevent excessive chloride accumulation in the root zone are also discussed. © 2000 Academic Press.

I. INTRODUCTION The chloride ion is present in abundance almost everywhere in the world. It is required as a micronutrient for optimal plant growth, at a rate of only 0.3 –1 mg/ g dry matter (DM) in most plants (Marschner, 1986). The inﬂuence of the chloride ion on plant growth depends on the plant variety (Tottingham, 1919). Lipman (1938) stressed the beneﬁcial effect of chloride on buckwheat growth. Warburg (1949) claimed that chloride is an essential micronutrient for plant growth. He showed that chloride is required for the water-splitting system at the oxidizing site of photosystem II (PSII). The work of Broyer et al. (1954) led to the general recognition of chloride as an essential plant nutrient. Chloride deﬁciency was demonstrated in sugar beet (Ulrich and Ohki, 1956) and in eight other plant species ( Johnson et al., 1957). Chloride toxicity is associated with saline conditions (Mengel and Kirkby, 1987). The function of chloride in yield formation (Fixen, 1993) has been largely neglected, as it becomes a limiting factor for plant growth only in areas of high precipitation far from the sea. Therefore, the negative effects of high chloride concentrations in the soil and in irrigation water on crop production are observed in coastal, arid, and semiarid areas, where freshwater sources are often scarce and the available groundwater is saline. The dependence of modern agriculture on irrigation and chemical fertilization causes more concern about the toxicity of chloride than about Cl deﬁciency (Marschner, 1986). The amount of chloride found in plants varies with habitat. Both the external chloride concentration and the balance of other available anions inﬂuence the chloride content of the plant. There are great differences in tolerance to chloride salts among crops and plants of the same species. As an example, high salt-tolerant bar-

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ley cultivars cultured with 150 mmol/liter of NaCl produced 42–86% of the yields of the same cultivars grown under nonsaline conditions (Sopandie et al., 1993). Cl1 has a number of essential biochemical functions in plants (Fixen, 1993). It operates as a counterion for cation transport and as an osmoticum (Flowers, 1988). Potassium and Cl together play a role in the regulation of stomatal movement (Talbott and Zeiger, 1996). In the literature on salinity, however, chloride is regarded mainly as an osmoticum and its effects on plant nutrition and on nitriﬁcation in soils are overlooked for the most part. This review deals with both the positive and the negative effects of chloride on plant growth under normal and saline conditions. The uses of nitrate to reduce Cl accumulation in the plant and the function of potassium as the main coupled cation of Cl in plants are also discussed. We attempt to evaluate the behavior of chloride in plants and soils and provide suggestions for further studies of its role in plants.

II. BEHAVIOR OF CHLORIDE IN SOIL A. SOURCES AND ACCUMULATION OF CHLORIDE The sources and concentrations of Cl in nature are listed in Table I. Four basic factors determine the amount of Cl available to crops growing in well-drained soils: (1) Cl concentration in the soil solution; (2) atmospheric deposition of Cl; (3) Cl concentration in the irrigation water; and (4) the content of Cl in fertilizers and manure (Goos, 1987). As a result of their genetic characteristics, crops have different requirements for and tolerance to chloride. Therefore, when attempting to establish exact levels of permitted Cl in soils and irrigation water, plant factors should be taken into account. 1. Soil Reserves The chloride content of the lithosphere is similar to that of sulfur (about 500 mg/ kg) and is slightly less than half of that of phosphorus (Flowers, 1988). Salt marsh soils may contain Cl at a rate exceeding 800 mmol/liter (Flowers, 1988). Because of its mobility in the soil with the moving water in the soil, the chloride content of the soil is not an intrinsic property of the soil but rather a result of soil management. 2. Rainwater The importance of airborne chloride for the evolution of soils, especially in coastal regions, has been recognized for many years (Flowers, 1988). Near the 1

Because “chloride” always refers to the anion, we designate it as Cl, not Cl.

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GUOHUA XU ET AL. Table I Chloride Concentrations in Some Natural Sourcesa Source

Chloride (g/kg)

Earth crust Lithosphere Basalt rocks Syenite Igneous rocks Shale Sandstone Limestone Dolomite Soils Ocean Plants Low to medium saline water High to very high saline water Table salt (NaCl) Potassium chloride (KCl)

1.50 0.48 0.50 0.98 0.23 0.16 0.02 0.37 0.50 0.10 19.0 1.0 –10.0 0.10 – 0.30b 0.30 –1.20b 607 450 –570

aCompiled bUnit:

from Yaalon (1963) and Flowers (1988). g/liter.

coast, many soils and crops receive a more than adequate supply of chloride from wind-borne sprays of rain and snow (McWilliams and Sealy, 1987). The amount of chloride derived from the atmosphere ranges from 17.6 to 36.0 kg/ha per year (Reynolds et al., 1997). The salt concentration in the air decreases exponentially with increasing distance from the shore, becoming uniform at about 50–150 km from the shore. The salt concentration in the air depends on topography, wind direction, and storm distribution (Yaalon, 1963). Cl concentrations of 20–50 mg/liter have been found in rainwater close to the shore, diminishing rapidly with distance from the ocean. In inner continental areas the corresponding concentrations are 2–6 mg/liter. The quantity of Cl deposited annually is about 175 kg/ha near the sea, but only 50 kg/ha at a distance of 6 km from the sea (Yaalon, 1963). Midcontinental areas such as the Great Plains of North America receive less than 1.0 kg Cl/ha annually through precipitation (Junge, 1963). Atmospheric Cl inputs often increase near heavily industrialized areas where large quantities of coal are burned (Fixen, 1993). 3. Addition of Chloride via Irrigation and Fertilization The amount of Cl added to a ﬁeld via irrigation water and municipal efﬂuents depends on farm activities. Water of low to medium salinity contains 100–300 g Cl/m3 (Table I), whereas saline water contains 300–1200 g Cl/m3. It is estimated that by the year 2000, the total annual amount of Cl used in fer-

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tilizers in China, mainly as NH4Cl and KCl, will be 5 109 kg (Pan et al., 1991a; Yin et al., 1989). For each hectare of soil, the amount of Cl introduced by 500 mm of irrigation water containing only 200 g Cl/m3 is 1000 kg. This is four times more than the amount of Cl applied by fertilization with KCl at 500 kg/ha.

B. CHLORIDE IN SOIL The chloride anion is not adsorbed on soil particles at neutral and basic pH values and is therefore leached easily. In Cl-deﬁcient soils, the optimal depth for soil sampling to estimate Cl availability to plants depends on the rooting characteristics of the crop, as well as the cropping system, soil type, precipitation or frequency of irrigation, and drainage. In paddy soil, a large amount of Cl was washed to a depth of 40 –60 cm by water after one season of rice growing (Huang et al., 1995). In the North American Great Plains, sampling down to 60 cm was recommended for spring wheat and barley (Fixen et al., 1987). 1. Reaction of Chloride with Clay Surfaces The assumption that chloride ions, such as nitrate and perchlorate ions, are adsorbed onto positive sites on clay particles through electrostatic attraction (Borggaard, 1984) is the theoretical basis for determination of the positive surface charge of soils, as suggested by Schoﬁeld (1949). The real behavior of Cl in variable charge soils differs, however, from that of NO 3 or CIO4 (Wang and Yu, 1998). The diffusion coefﬁcient of Cl is smaller than that of NO 3 . X-ray photoelectron spectroscopic studies showed that Cl but not NO reacted with the soil surface ( Ji, 3 1997). The pH values of the suspensions in a HCl-treated system and a HNO3treated system differed in the variable-charge soil, but not in the permanent charge soil (Table II). Therefore, at least for Cl at low pH in variable-charge soils, a speciﬁc adsorption mechanism is involved. As Cl concentration increases, Cl replaced more OH than H2O (Wang and Yu, 1998). The release of OH ions during the speciﬁc adsorption of chloride decreased on removal of the free iron oxides. This hydroxyl ion release, caused by speciﬁc adsorption of Cl, increases the soil pH value in chloride solutions (Zhang et al., 1989). 2. Plant Uptake Crop foliage can remove substantial amounts of Cl, especially when soil levels of available Cl are high. At peak accumulation, the Cl content of spring wheat was 18 and 61 kg/ha on sites testing low and high in Cl, respectively (Schumacher, 1988, cited by Fixen, 1993). By the time of crop maturity, Cl in the portion of the plant above ground had dropped to 50 and 43% of these values, respectively, meaning that it had returned to the soil. Similar behavior was reported for potassium in

102

GUOHUA XU ET AL. Table II Changes in pH of Soil Suspensions of a Rhodic Ferralsol and a Cambisol after the Addition of Different Quantities of HCl or HNO3a pH value Rhodic Ferralsol

Cambisol

Acid added (mmol/kg)

HCl

HNO3

HCl

HNO3

0 4 8 10 12 14 16 18 20

5.60 5.17 4.79 4.56 4.36 4.26 4.05 3.98 3.83

5.60 5.09 4.64 4.37 4.23 4.04 3.81 3.65 3.37

7.00 6.12 5.17 5.06 4.99 4.67 4.31 4.33 4.23

7.00 6.08 5.13 5.02 4.94 4.70 4.35 4.29 4.20

aBased

on Wang and Yu (1998).

wheat (Kafkaﬁ et al., 1978). The removal of Cl in grains is very limited. In spring wheat, soybean, and rice, the amount of Cl distributed in the grains was only 2.15, 1.34, and 1.62%, respectively, of the crop’s total Cl uptake (Pan et al., 1991b). The concentration of Cl in dry matter wheat grain is only 0.05% (Knowles and Watkin, 1931). Removal of Cl in soybean seed amounts to 0.45 kg/ha, which is less than the amount deposited annually in rainfall (Parker et al., 1986).

C. CHLORIDE AS A NITRIFICATION INHIBITOR Ammonia fertilizers differ in their rates of nitriﬁcation (Meelu et al., 1990). Nitriﬁcation of 300 mg NH4Cl/kg in an acid soil (pH 5.6) at 30C was only 6% after 21 days as compared to 75% in the case of urea (Hauck, 1984). However, differences in nitriﬁcation rates among N sources decreased markedly in alkaline soil (pH 8.2), except when high N rates above 400 mg/kg were applied. Whereas all N added as urea was nitriﬁed in 10 days, it took 35 days for the nitriﬁcation of only 91% of 300 mg N/kg as ammonium chloride (Meelu et al,. 1990). While 80% of ammonium sulfate was nitriﬁed in 12–18 days, it took 30 –35 days for the nitriﬁcation of ammonium chloride to the same extent (Babriwara, 1959, cited by VedeNarayanan, 1990). The relative inhibition of ammonium chloride nitriﬁcation was attributed to differences in osmotic potential under different saline conditions or to a direct effect of Cl (Roseberg et al., 1986). Results of ﬁeld and laboratory studies indicate that nitriﬁcation in moderately

103

ADVANCES IN CHLORIDE NUTRITION

acid soils (pH 5.0–5.5) is reduced both by Cl and by low osmotic potential of the soil solution (Christensen et al., 1986). In soils in which the osmotic potential was increased four-fold by the addition of ammonium salts of chloride or sulfate, the nitriﬁcation of applied NH 4 was slowed both by Cl and by the decreasing osmotic potential of the N solution (Table III). Addition of a 93-kPa solution of Cl salt to the soil resulted in a lower nitriﬁcation rate than that obtained with a 680-kPa SO42 solution. The nitriﬁcation essentially stopped when a Cl salt solution of 338 or 680 kPa was added. The chloride ion functions as a nitriﬁcation inhibitor in the soil at pH 5.5 but not at pH 6.6 (Christensen et al., 1986). In slightly acid soils (pH 6.5–7.0), however, the impact of the Cl ion on nitriﬁcation is much lower. The slow rate of nitriﬁcation inhibition induced by chloride fertilization, particularly in slightly acid soils, might help to increase N use efﬁciency in rice ﬁelds by preventing N losses due to denitriﬁcation in the event of ﬂooding.

III. CHLORIDE IN PLANTS A. YIELD AND QUALITY RESPONSE TO CHLORIDE 1. Positive Yield Response to Chloride Chloride deﬁciencies in plants generally occur in inland soils (Fixen, 1987). Substantial responses to Cl-containing fertilizers have been reported for different

Table III Effects of Soil pH and Osmotic Potential of Added Ammonium Chloride and Ammonium a Sulfate Solutions (100 mg NHⴙ 4 -N/kg soil) on Nitriﬁcation Rate

Soil

Soil pH

Woodburn

5.3 5.3 5.3 5.3

Nekia silt Clay loam

4.9 5.5 6.2

aBased

Osmotic potential of added NH 4 solution (kPa) 93 171 338 680 LSD ( p 0.01) 93 93 93 LSD ( p 0.01)

on Christensen et al. (1986).

Nitriﬁcation rate (mgNO 3 -N/kg day) NH4Cl

(NH4)2SO4

1.4 0.82 0.1 0.07

2.4 2.12 1.88 1.75 0.24

5.8 11.2 13.8

8.8 13.2 14.4 0.48

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crops in many parts of the world; e.g., coconut (von Uexkull and Sanders, 1986), corn (Heckman, 1995), kiwifruit (Smith et al., 1987), oil palm (von Uexkull, 1990), potato (Gausman et al., 1958b), spring wheat and barley (Fixen et al., 1986; Engel et al., 1994), tobacco (Li et al., 1994), and sugar beet (Zhou and Zhang, 1992). Typical symptoms of Cl deﬁciency include wilting of leaves, curling of leaﬂets, bronzing and chlorosis similar to those seen with Mn deﬁciency, and severe inhibition of root growth (Ozanne et al., 1957; Smith et al,. 1987). The concentration range of chloride deﬁciency in plants varies between 0.13 and 5.7 mg/g for spinach and sugar beet, respectively (Table IV). In wheat, the Cl concentration of leaf tissue at heading is a good predictor of the response to Cl fertilization (Engel et al,. 1998); the critical range is between 1.5 and 4 mg/g DM, above which no further response is expected. In pot experiments, positive responses to chloride at 100 –200 mg/kg soil were reported for white potato, peanut, tomato, and young may trees and at 100 –1600 mg/kg soil for sugar beet (Jing et al., 1992). On a sandy loam soil, chloride applications of up to 400 kg/ha yielded 500 –1500 kg/ha more corn grain than was obtained in the control (Heckman, 1995). Grain yields of corn were correlated positively with increases in Cl concentrations in the leaf ears. In wheat, there was no yield response to Cl fertilization when the Cl content was above 70 kg/ha in the top 12 cm of soil (Fixen et al., 1987). Yield increases due to Cl fertilization from sources such as KCl, CaCl2, NH4Cl, and NaCl have also been associated with the suppression of foliar or root diseases of wheat (Christensen et al., 1981; Engel et al., 1997). Ammonium chloride produced yields of rice that were equal to or higher than those obtained with urea and ammonium sulfate. In a greenhouse study, rice yields with ammonium chloride were signiﬁcantly inferior to ammonium sulfate, especially at high salinity levels (Meelu et al,. 1990). Yields of sugarcane fertilized with ammonium chloride exceeded or equaled those of ammonium sulfate at 67–225 kg N/ha (about 170 –570 kg Cl/ha) (Veda-Narayanan, 1990). 2. Crop Sensitivity or Tolerance to Chloride Sensitivity to high Cl concentrations varies widely between plant species and cultivars. Generally, most nonwoody crops tolerate excessive levels of Cl, whereas many woody plant species and beans are susceptible to Cl toxicity (Maas, 1986). The critical toxicity concentration is about 4–7 and 15–50 mg/g for Cl-sensitive and Cl-tolerant plant species, respectively (Table IV). For a ‘Washington’ navel orange cultivar grafted on the poor chloride excluder rootstock ‘Rough Lemon,’ when the Cl of the leaf is higher than 2 mg/g the fruit yield declines linearly with leaf Cl content (Fig. 1). However, mature leaves of citrus were able to tolerate Cl concentrations of up to 350 mmol/liter in leaf tissue water or approximately 25 mg/g DM under glasshouse conditions without sustaining permanent damage to the photosynthetic system (Walker et al., 1982).

Table IV Chloride Concentrations in Plants Concentration ranges of tissue Cl (mg/g DM) Crop Alfalfa Apple Avocado Barley Citrus Coconut palm Corn Corn Cotton Grapevine Kiwifruit Lettuce Pear Peach Peanut Potato Potato Red clover Rice Rice Soybean Spinach Spring wheat Strawberry Subterranean clover Sugar beet Sugar beet Tobacco Tomato Wheat aThe

Latin name

Plant part

Deﬁcient

Normal

Toxicitya

References

Medicago sativa L. Malus domestica Persea americana Mill. Hordeum vulgare L. Citrus sp. L. Cocos nucifera L. Zea mays L. Z. mays L. Gossypium hirsutum L. Vitis vinifera L. ssp. vinifera Actinidia deliciosa Lactuca sativa L. Pyrus communis Prunus persica Arachis hypogaea L. Solanum tuberosum L. S. tuberosum L. Trifolium pratense L. Oryza sativa L. O. sativa L. Glycine max L. Merr. Spinacia oleracea L. Triticum aestivum L. Fragaria vesca Trifolium subterraneum L. Beta vulgaris L. B. vulgaris L. Nicotiana tabacum L. Lycopersicon esculentum Mill. Triticum aestivum L.

Shoot Leaves Leaves Heading shoot Leaves Leaves Ear leaves Shoots Leaves Petioles Leaves Leaves Leaves Leaves Shoot Mature shoot Petioles Shoot Shoot Mature straw Leaves Shoot Heading shoot Shoot Shoot Leaves Petioles Leaves Shoot Heading shoot

0.65 0.1

0.9–2.7

6.1 2.1 ⬃7.0

Ozanne et al. (1957); Eaton (1966) Eaton (1966) Bar et al. (1997); Lahav et al. (1992) Engel et al. (1994, 1997) Bell et al. (1997); Bar et al. (1997) von Uexkull and Sanders (1986) Parker et al. (1985) Johnson et al. (1957) Tan and Shen (1993) Downton (1985); Eaton (1966) Smith et al. (1987); Prasad et al. (1993) Johnson et al. (1957); Wei et al. (1989) Robinson (1986) Robinson (1986); Eaton (1966) Wang et al. (1989) Corbett and Gausman (1960) James et al. (1970); Bernstein et al. (1951) Whitehead (1985) Yin et al. (1989) Huang et al. (1995); Zhu and Yu (1991) Parker et al. (1986); Yang and Blanchar (1993) Robinson and Downton (1984) Fixen et al. (1986) Wang et al. (1989) Wang et al. (1989); Robinson (1986) Ozanne et al. (1957) Ulrich and Ohki (1956); Terry (1977) Ulrich and Ohki (1956) Zhou and Zhang (1992) Li et al. (1994); Eaton (1966) Broyer et al. (1954); Kafkaﬁ et al. (1982) Engel et al. (1994, 1997)

plant yields decline or the plant shows visible scorching symptoms in leaves.

1.2–4.0 2.5–4.5

⬃1.5–4.0 4.0 ⬃2.0 6.0–7.0 1.1–10.0

⬃4.0–7.0

10.0–25.0 0.7–8.0 6.0–13.0 2.8–19.8 0.50 0.9–3.9 3.9 2.0–3.3 18.0

25.0–33.1 10.0–11.0 15.0 23.0 10.0 10.0 –16.0 4.6 12.2 44.8

5.1–10.0 0.3–1.5

7.0–8.0 13.6 16.7–24.3

3.7–4.7 1.0–5.0

7.0 5.3

7.1–7.2 1.2–10.0

50.8 10.0 ⬃30.0

32.7

0.05– 0.11

2.1 >0.14

1.0 0.71–1.42 0.15–0.21 3.0

0.13 1.5 1.0 0.71–1.78 5.7 0.25 1.2–4.0

4.0

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GUOHUA XU ET AL.

Figure 1 Relationship between citrus fruit yield and leaf chloride content. Recalculated from Cole (1985).

The tolerance order of common agricultural crops to chloride (Table V) is very similar to the order of critical electrical conductivity (EC) values of saturated soil extracts. This is to be expected given the relationship between the chloride salt content and the ECs of salt solutions (Richards, 1954). The crop with the greatest tolerance to chloride is sugar beet, which may contain up to 50.8 mg Cl/g in the leaves (Zhou and Zhang, 1992). Chinese cabbage is sensitive to Cl; when the Cl level in irrigation water reached 80 mg/liter, its dry matter percentage was decreased signiﬁcantly (Yin et al., 1989). Corn is tolerant to high levels of soil Cl, but soybean is sensitive (Parker et al., 1983, 1985). At soil chloride levels of 100– 200 mg/kg, even sensitive crops such as sweet potato, white potato, sugarcane, and tobacco showed no negative effects in yield or quality ( Jing et al., 1992). The critical tolerance values of rice and wheat to chloride were found to be 780–800 and 1000–1350 mg/kg in a clay soil, respectively, and 380–400 and 600–650 mg/ kg in a loam soil, respectively (Zhu and Yu, 1991). Because the water-holding capacity in clay soil is much higher than in loam soil, the critical Cl concentration in saturated soil solutions is expected to be similar for the same crop in different soils. The tolerance of a crop to Cl is not related directly to its concentration in plant

Table V Critical Toxicity Concentrations of Chloride and ECe Values in Soil and in Saturated Soil Extracts, Listed in Order of Increasing Tolerance to Chloride Critical toxicity concentration Crop Strawberry Bean Onion Carrot Radish Lettuce Turnip Pepper Apple Sweet potato Grape Corn Flax Potato a,b,c

Critical toxicity concentration

mmol Cl/litera

ECe (dS/m)b

mgCl/kg soilc

10 10 10 10 10 10 10 15

1.0 1.0 1.2 1.0 1.2 1.3 0.9 1.5 1.7 1.5 1.8 1.7 1.7 1.7

250

15 15 15 15

100

250 300 400 800 500 500

Crop Broadbean Sugarcane Cabbage Spinach Cucumber Tomato Broccoli Sugar beet Cowpea Wheat Sorghum Sugar beet Cotton Barley

mmol Cl/liter

ECe (dS/m)

15 15 15 20 25 25 25 40 50 60 70 70 75 80

1.5 1.7 1.8 2.0 2.5 2.5 2.8 4.0 1.3

Selected and recompiled from Maas (1986), Ayers and Wescott (1985), and Jing et al. (1992), respectively. Maximum Cl concentration in saturated soil extracts without loss in yield. b Maximum ECe value in saturated soil extracts without loss in yield. c Maximum soil Cl concentration above which yield decline to 95% of the maximum yield is observed. d From Tan and Shen (1993). a

6.8 7.0 7.7

mgCl/kg soil

500 600 600 3200 600 700 1600 1600d

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GUOHUA XU ET AL.

tissues as is shown for different varieties of grapevine (Fig. 2). Scions on ‘Dogridge’ rootstock contained the highest leaf chloride concentration but exhibited the greatest growth and were the least affected by salinity. There was no relationship between the amount of chloride in plant parts and cane weight. Similar ﬁndings were reported by Skene and Barlass (1988) for two rootstocks of grapevine. Dry matter yields of the whole plant and chloride levels in leaves of the salt-tolerant avocado cultivar Degania-113 were higher than in the salt-sensitive cultivar Smith (Bar et al., 1997). The salt-tolerant alfalfa variety accumulated considerably higher concentrations of Na and Cl than the salt-sensitive variety (Ashraf and O’Leary, 1994). The salt-tolerant and salt-sensitive accessions of safﬂower did not differ in tissues Cl, K, or Ca2 (Ashraf and Fatima, 1995). It seems likely that factors associated with vigorous growth or Cl compartmentation within the cell could offset the inhibitory effects of chloride accumulation. The level of accumulated Cl in the plant should therefore not be considered the sole criterion of crop tolerance to chloride. Plants are generally more tolerant to soil salinity during cooler seasons than in

Figure 2 Effects of chloride in irrigation water on leaf lamina Cl content and cane dry matter of Sultana grapevine scion grafted on three rootstocks. Recalculated and redrawn from Downton (1985).

ADVANCES IN CHLORIDE NUTRITION

109

warmer ones (Pasternak and De-Malach, 1995). The salt tolerance of citrus rootstock varies with the stage of seedling development (Zekri, 1993). Cucumber is more salt tolerant during germination than during vegetative or fruiting stages (Chartzoulakis, 1991). Chloride toxicity in plants is often hard to diagnose, for two reasons: (1) it is difﬁcult to separate the effects of chloride from those of any accompanying cation, commonly sodium; and (2) it is difﬁcult to distinguish between the speciﬁc toxic effects of ions and the cellular dehydration caused by their excessive external concentrations. Visual symptoms of marginal leaf necrosis due to chloride accumulation such as those seen in avocado (Fig. 3) (see also color insert) might be misleading, as similar symptoms in mango (Fig. 4) (see also color insert) are a result of iron deﬁciency (U. Kafkaﬁ, unpublished data). Citrus-sensitive plants shed their leaves when exposed to salts but do not exhibit leaf necrosis (Bar et al., 1997). 3. Yield Quality and Chloride Content in Harvested Plant Parts Salinity improves both fruit taste and appearance quality of tomato and melon (Mizrahi, 1982; Mizrahi and Pasternak, 1985; Faiz et al., 1994). This phenomenon was attributed to the signiﬁcantly higher content of total soluble solids and of aromatic and other components found in these fruits under saline condition (Davies and Hobson, 1981). Most of the reported salinity effects are to the integrated

Figure 3 Relieving chloride toxicity in avocado leaves by increasing nitrate concentration in irrigation water containing 16 mM Cl (Y. Bar, M.Sc. 1986 Rehovot, Israel) (see also color insert).

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GUOHUA XU ET AL.

Figure 4 Marginal leaf necrosis symptoms in mango due to iron deﬁciency. Visual symptoms are similar to chloride toxicity (U. Kafkaﬁ, unpublished results) (see also color insert).

action of both Na and Cl. The speciﬁc inﬂuences of chloride on the quality of agricultural products are not clear. Wang et al. (1989) found that the soluble sugar and vitamin C contents in the fruit of strawberry grown in soil containing Cl at 100–200 mg/kg soil were signiﬁcantly higher than in soil containing 37 mg Cl/kg soil.

ADVANCES IN CHLORIDE NUTRITION

111

Chloride generally accumulates in the vegetative parts, mainly in the leaves (Fig. 5) of cotton, lettuce (Wei et al., 1989), wheat, soybean, and rice (Pan et al., 1991b). The Cl content in grain, fruits, and seeds is very low and is hardly affected by the Cl concentration of the soil solution. The concentration of Cl in sugar beet leaves increased from 9.8 to 54.1 mg/g with Cl fertilization, whereas the concentration in the roots was only 1.5–1.6 mg/g. The sugar content of the roots was not affected by the Cl application (Zhou and Zhang, 1992). Seeds of cotton maintained a concentration range of 0.48–0.59 mg Cl/g DM and the lint length was kept constant in the range of 28 –29 mm when Cl application was increased to 3200 mg/kg soil (Tan and Shen, 1993). There were no negative and even some positive effects of chloride salinity on the grain quality of corn, sorghum, rice, spiked millet, and wheat when Cl was applied at a rate of up to 800 mg/kg soil (Wang et al,. 1989; Jing et al., 1992). The quality of leafy vegetables is relatively sensitive to chloride. Whereas the total fresh weight of Chinese cabbage was not affected when the Cl content of irrigation water was less than 150 mg/liter, the dry weight and its vitamin C content were reduced markedly (Yin et al., 1989). Both the soluble sugar and the vitamin C content of lettuce decreased signiﬁcantly when soil Cl application was above 100 mg/kg (Wei et al., 1989). In tobacco, good leaf quality was maintained as long as the amount of Cl in the soil was less than 72–107 mg/kg and the leaf Cl content was below 10 mg/g (Li et al., 1994). The effects of chloride on crop quality

Figure 5 Distribution of Cl in the different organs of cotton at the boll-opening stage. Redrawn from Tan and Shen (1993).

112

GUOHUA XU ET AL.

depend mainly on the marketed plant part. Fruit quality is generally more tolerant to high chloride than fresh leaf yield.

B. UPTAKE AND DISTRIBUTION OF CHLORIDE 1. Absorption Mechanism Uptake of Cl by the plant roots is an active process that requires energy. Earlier studies suggested that the Cl transport through the cell membrane involves the 2H: Cl symporter (Sanders, 1984) or occurs via antiport with hydroxyl ions energized by ATP (Jacoby and Rudich, 1980). The H /Cl symporter has been studied using electrophysiological methods (Felle, 1994) that demonstrated that nH / Cl would be better described as the ratio between proton and chloride uptake. He concluded that the kinetics of chloride transport depended on the pH gradient across the plasma membrane rather than on the membrane potential. Speciﬁc protein channels energized by ATP for chloride transport were suggested both for the plasmalemma (Lin, 1981) and the tonoplast (Martinoia et al., 1986). Cl ions always keep their negative charge, whereas SO2 4 and NO 3 are partly or completely reduced during metabolism in the plant. It was suggested that the physiological mechanisms for the control of chloride accumulation in plant cells operate at the cell or organ level (Cram, 1988). Changes in root temperature and external Cl concentration affect Cl inﬂux and accumulation (Cram, 1983, 1988). It is difﬁcult to determine the balance of individual ions across the plasmalemma and the tonoplast (Glass and Siddiqi, 1985). The situation is complicated further by interaction between the shoot and the root. Ion inﬂux is regulated by the ﬂux to the xylem and involves recycling in the phloem (Marschner, 1995). Fluxes of chloride in intact plants are very different from those measured in plants with excised roots (Collins and Abbas, 1985). Glass and Siddiqi (1985) proposed a homeostatis mechanism that senses vacuolar nitrite plus chloride or total anion concentration. A variety of other schemes are discussed by Deane-Drummond (1986). All of the suggested mechanisms must be balanced with growth, and as yet there is no generally accepted view of the control of chloride uptake and transport in plants (Flowers, 1988). The composition of the root cell membrane not only affects ion selectivity, but is also of particular importance in preventing Cl from entering the root. The salinity tolerance in grapes was correlated positively with the solubility of chloride in the lipids that constitute the root membranes (Kuiper, 1968). Enrichment of root cell membranes with phospholipids relative to their monogalactose diglyceride content limits chloride uptake (Kuiper, 1968). There were no apparent differences in the chemical composition of root microsomal membrane lipids between varieties of corn with low and high Cl uptake, and these membrane lipids composition were not affected by chloride salt (Hajibagheri et al., 1989).

ADVANCES IN CHLORIDE NUTRITION

113

2. Translocation and Distribution of Chloride In potato, Cl is partially stored in the vacuoles of leaf cells and its charge there is neutralized by Ca2 and Mg2 (Beringer et al., 1990). Differences in salinity tolerance between species of citrus were invoked by compartmentation of the chloride under saline conditions (Lloyd et al., 1989). In leaf cells of NaCl-treated halophytes, Na tended to concentrate in the cytoplasm, whereas Cl was distributed evenly in the cytoplasm and the vacuole (Eshel and Waisel, 1979). The substantially higher sensitivity of nitrate reductase activity to NaCl in bean leaves than in cotton leaves seems to be due to a decrease in ion compartmentation rather than a difference in salt tolerance of the enzyme itself (Gouia et al., 1994). Anderson and Steveninck (1987) suggested that the salt tolerance of alfalfa is due to the restricted entry of Na and Cl into the roots as a result of retention of these ions in the vacuoles of the epidermis and upper cortex. Chloride compartmentation appears to be highly regulated. In the chloroplast, the Cl concentration remains relatively constant regardless of whether the plant growth medium is characterized by deﬁcient or excessive levels of Cl (Maas, 1986). Chloride has high mobility in both short-distance and long-distance transport. The leaf chloride content is controlled not only by root uptake but also by the restricted translocation of Cl from the roots to the leaves. The type of rootstock controls leaf chloride accumulation in many crops (Downton, 1985; Velagaleti et al., 1990; Lahav et al., 1992; Stevens and Harvey, 1995). Tomato seedlings were able to prevent Na and Cl accumulation in the root and shoot as compared with the chloride-sensitive cucumber grown in the same solution (AlHarbi, 1995). Chloride-tolerant cultivars restrict Cl transport to the shoots by a mechanism that resides in the roots (Grattan and Maas, 1985). Plants can withstand high Cl concentrations by restricting Cl uptake and transport to the leaves and/or as a result of an increase in the ability of their leaf tissue to tolerate high Cl concentrations (Bar et al., 1997). Boursier et al. (1987) observed that salt-sensitive plants generally accumulate Na and Cl in the sensitive leaf tissue instead of retaining them in the root and stem. Shoot Cl concentration gives a more accurate evaluation than root Cl concentration of the extent of the injurious effect of Cl in citrus rootstocks (Zekri, 1993; Banuls et al., 1990). In the tolerant wild accessions and F1 (Le x Lpen) of tomato, Cl concentrations in the leaves and the ratio of leaf Cl to stem Cl were lower than in the sensitive Le cultivar (Saranga et al., 1993). Leaf concentrations of Cl, Ca2, Mg2, and K in salt-tolerant soybean cultivars were low, whereas Na levels in the leaves of both sensitive and tolerant cultivars remained low under salt stress (Velagaleti et al., 1990). Reduced Na or Cl accumulation in the shoots was used as a physiological index of salt tolerance in rice, but there was no direct relationship between salt tolerance at the cellular level and at the whole plant level (Yan et al., 1992).

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3. Scion and Rootstock Selection for the Control of Chloride Uptake In the selection and breeding of salt-tolerant crop varieties, attention should be given to the control of Cl content. Rootstocks that exclude salt are often deﬁcient with regard to other desirable characteristics, and their use is therefore limited (Sykes, 1993). The ability of a particular crop to exclude Cl ions is independent of its ability to exclude Na ions; a good Cl ion excluder is not necessarily a good Na excluder, and vice versa (Sykes, 1993). Downton (1985) found that irrigation with water containing 0–75 mmol/liter chloride salt resulted in lower leaf Cl concentrations in grafted grapevine (except ‘Dogridge’) than in self-rooted vines. Under saline conditions, the use of a Cl-excluding rootstock of grapevine reduced leaf Cl concentration by 60% in grapevine with free-draining root zones but only by 18% in vines with water-logged root zones (Stevens and Harvey, 1995). The Cl concentration of avocado leaves from trees on West Indian salt-resistant rootstocks was almost one-third of that in leaves from trees on the Mexican rootstock (Lahav et al., 1992). Soybean scions of sensitive cultivars grafted onto the rootstocks of tolerant cultivars showed typical tolerant responses (Velagaleti et al., 1990). In general, the sensitivity of citrus to Cl was found to be determined by its scion, whereas Na levels in the leaves were affected greatly by both scion and rootstock (Lloyd et al., 1989). Harmful effects of high leaf Cl concentrations can be avoided if salt-excluding varieties are used as rootstocks. Turgor maintenance in leaves of the scion variety appears to be an important factor inﬂuencing the response to salinity of selected root–scion combinations (Walker et al., 1982).

C. BIOCHEMICAL FUNCTIONS More than 120 chlorinated organic compounds have been identiﬁed in higher plants (Engvild, 1986). The importance of these compounds in terms of the functional requirement of chloride for higher plants is not known. In addition to wilting symptoms, the principle effects of chloride deﬁciencies in most plants are manifested by a reduction in the leaf surface area relative to the rate of leaf cell division (Terry, 1977). A speciﬁc effect of chloride on cell extension may be assumed in some legume species, such as peas and faba bean, which contain substantial amounts of chlorinated indole-3-ascetic acid (IAA) in their seeds. The chlorinated compound enhances hypocotyl elongation at 10 times the rate of IAA itself, probably because of the resistance of the former compound to degradation by peroxides (Hoﬁnger and Bottger, 1979). The contents of certain amino acids and amides are exceptionally high in chloride-deﬁcient cabbage and cauliﬂower plants (Freney et al., 1959) as a result of either inhibition of protein synthesis or enhanced protein degradation.

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At least three plant enzymes appear to require Cl for optimal activity: asparagine synthethase (Rognes, 1980), amylase (Metzler, 1979), and ATPase (Churchill and Sze, 1984). That chloride plays a role in nitrogen metabolism is indicated by its stimulatory effect on asparagine synthethase, which uses glutamate as a substrate. Chloride increases the afﬁnity of this enzyme for its substrate (Rognes, 1980). In plant species in which asparagine is the major compound in the long-distance transport of soluble nitrogen, chloride may also play a role in nitrogen metabolism (Marschner, 1995). The proton-pumping ATPase at the tonoplast is not affected by monovalent cations but is stimulated directly by chloride (Churchill and Sze, 1984). ATPase activity increased asymptotically with increasing Cl concentration, approaching a maximum at 50 mmol/liter. The close relationship between KCl supply and ADP-glucose starch synthethase activity in roots of maize (Nitsos and Evans, 1969) is probably a reﬂection of two different stimulatory functions of K and Cl on ATPase activity located at the plasma membrane and the tonoplast, respectively (Marschner, 1995). There are also striking similarities between the chloridestimulated H-ATPase and the mechanisms regulating the elongation of coleoptiles (Hager and Helmle, 1981). Chloride is an essential cofactor in photosynthetic O2 evolution in the watersplitting system of photosystem II, as suggested by Arnon and Whalley, (1949) and demonstrated by Izawa et al. (1969). Binding of Cl to membranes is needed for activation of the O2-evolving enzyme (Baianu et al., 1984). The preference for chloride over bromide, nitrate, or iodide with regard to its ability to promote oxygen evolution is due to its speciﬁc ionic volume (Critchley, 1985). A number of proteins are involved in modulating the requirement for chloride in the evolution of oxygen (Andersson et al., 1984). It was suggested that chloride might act as a bridging ligand between manganese atoms during the transfer of electrons from water to PSII (Critchley, 1985) or as a structural component of the associated (extrinsic) polypeptides (Coleman et al., 1987). The various polypeptides (33, 24, and 18 kDa) attached to PSII provide the charges for binding, whereas chloride protects the polypeptides from dissociation (Homann, 1988). The mechanism of Cl action in PSII still remains controversial. Evidence from studies utilizing electron paramagnetic resonance of the manganese involved in PSII suggests that chloride is not bound to the manganese (Yachandra et al., 1986). The site of action in the thylakoid membrane appears to be positively charged and to attract ions by electrostatic force (Itoh and Uwano, 1986). As shown in sugar beet (Terry, 1977) and spinach (Robinson and Downton, 1984), even in severely stunted or growth-reduced plants the Cl levels in chloroplasts remained high enough to maintain photosynthesis. The chloroplast chloride concentration in the leaf water was highly stable at 88–99 mmol/liter in spinach plants, regardless of the total plant chloride status (Robinson and Downton, 1984). Even in Suaeda maritima, a halophytic member of the Chenopodiaceae in which the chloride con-

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centration in leaf cells can exceed 500 mmol/liter, the chloride concentration of the chloroplast remained at around 100 mmol/liter (Robinson and Downton, 1985). In Cl-deﬁcient leaves, however, it appears that nearly all of the chloride accumulates in the chloroplast. Therefore, the amount of Cl required by the plant for photosynthesis is very small.

D. PHYSIOLOGICAL FUNCTIONS 1. Osmotic and Anionic Balance The ability of Cl to move rapidly across cell membranes against an electrochemical gradient and its relatively low biochemical activity are two important properties that make Cl particularly well suited to serve as a key osmotic solute in plants (Maas, 1986). Chloride concentration in wheat ﬂag leaf tissue is closely related to its osmotic potential (Christensen et al., 1981), suggesting that an increased Cl concentration in the symplasm is probably the reason for the measured change in osmotic potential. The accumulation of chloride by plants contributes greatly to an increase in cell hydration and turgor pressure, both of which are essential for cell elongation (Maas, 1986). A similar osmotic function, as well as a close association between K and Cl, was reported for the guard cells of leaf stomata of Vicia faba (Talbott and Zeiger, 1996). The osmoregulatory function of chloride in plants seems to operate at different levels. The Cl concentration range usually found in plants (50–150 mmol/liter of tissue water) exceeds its critical deﬁciency level by 1 to 2 orders of magnitude. Chloride serves as a main osmoticum in the vacuoles of plant tissue. Together with K, Cl has a role in maintaining xylem volume ﬂow and root pressure (Marschner, 1986). In phloem sap, the Cl concentrations might reach 120 mmol/liter and seems to play a role in phloem loading and unloading of sugars (Fromm and Eschrich, 1989). At the low whole tissue Cl content of 1 mmol/liter or less, these osmoregulatory functions of chloride are presumably conﬁned to specialized compartments in tissues or cells, such as the extension zones of roots and shoots, pulvini, stigmata, and guard cells, where the concentrations of Cl might be much higher than the average Cl concentration in the bulk tissue. Under saline conditions, the maintenance of low Cl concentrations in the leaves does not necessarily safeguard against a reduction in photosynthetic activity if the osmotic adjustment is not sufﬁcient to offset the reduction in water potential. Conversely, osmotic adjustment and maintenance of turgor during salt treatment do not necessarily provide a safeguard against photosynthetic reduction in leaves that have accumulated high concentrations of Cl (Walker et al., 1982). The reduction of photosynthesis of the Cl-sensitive leaves of the citrus variety Etrog citron was associated with high leaf Cl concentrations, whereas in leaves of the chloride-tolerant citrus variety Rangpur lime it was related to a loss of leaf turgor (Walker et al., 1982).

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The observed beneﬁts from Cl applied in ﬁeld studies are most probably due to the osmoregulatory role of Cl in the plants (Flowers, 1988). The importance of this function for plant growth and grain yield is highly dependent on growing conditions such as water, temperature, and the presence of other ions that might potentially act as substitutes for Cl in its osmoregulatory role. An increase in Cl concentrations in the leaves of kiwifruit merely resulted in an equivalent decrease of nitrate and did not change the concentrations of P, S, or organic acids or the total anion concentration (Smith et al., 1987). Further research is needed in order to determine the speciﬁc factors that involve chloride in the response intensity of saltstressed plants (Banuls and Primo-Millo, 1992). 2. Stomatal Activity and Regulation Activation of a H pump in the plasma membrane initiates K and Cl inﬂux, accompanied by malate synthesis, resulting in osmotic water ﬂow into the guard cells, a bowing apart of the guard cell pair, and consequent stomatal opening (Lee and Assmann, 1991). Using a Cl-sensitive microelectrode, Penny et a. (1976) found a close correlation in Commelina communis between stomatal activity and the movement of Cl between epidermal cells. Chloride concentrations in the vacuoles of guard cells increased when the stomata opened and decreased when they closed. The relative contributions of Cl and malate may vary among species and may depend on the availability of external Cl (Raschke and Schnabl, 1978) and on the plant growth environment (Talbott and Zeiger, 1996). In plants grown in a greenhouse, a correlation between Cl and K in the guard cells was found during a single daily light cycle of stomatal movements (Talbott and Zeiger, 1996, Fig. 6). The malate that accumulates during stomatal opening is synthesized in the guard cells (Du et al., 1997) and both its synthesis and its accumulation are affected by the Cl concentration of the growth medium (Raschke and Schnabl, 1978). The pH dependence of PEP carboxylase (Du et al., 1997) may provide an explanation for the coordination between Cl inﬂux and malate synthesis. Chloride uptake will cause internal acidiﬁcation, with a consequent inhibition of PEP carboxylase. In the absence of external Cl, K /H exchange will cause alkalization of the guard cells, thereby promoting malate synthesis. In plant species such as onion (Allium cepa L.), which lack the functional chloroplasts for malate synthesis in guard cells, chloride is essential for stomatal functioning (Schnabl and Raschke, 1980). Onion guard cells contain equivalent amounts of K and Cl. Stomatal opening is inhibited in the absence of chloride. Members of the Palmaceae, such as coconut (Cocos nucifera L.) and oil palm (Elaeis guineensis Jacq.), which might possess starch-containing chloroplasts in their guard cells (Braconnier and d’Auzac, 1990), also require chloride for stomatal functioning (von Uexkull and Sanders, 1986; von Uexkull, 1990). In chloridedeﬁcient coconut plants, stomatal opening is delayed by about 3 hr. Impairment of stomatal regulation in palm trees is thought to be a major factor responsible for

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Figure 6 Relationship between K and Cl contents of guard cells over a single daily light cycle of stomatal movements of a Vicia faba plant grown in a greenhouse. Recalculated from Talbott and Zeiger (1996).

growth depression and wilting symptoms in chloride-deﬁcient plants (von Uexkull and Sanders, 1986; Braconnier and d’Auzac, 1990).

E. INTERACTION OF CHLORIDE UPTAKE WITH UPTAKE OF OTHER NUTRIENTS 1. Nitrate The form in which nitrogen is taken up by a plant (ammonium or nitrate) affects the sensitivity of the plant to chloride salt stress. Increasing nitrate concentration from 2 to 16 mmol/liter in the irrigation solution that contained 16 mmol/liter of chloride relieved the chloride toxicity symptoms from avocado leaves (Figs. 3 and 7). The antagonism between nitrate and chloride uptake was demonstrated in avocado (Wiesman, 1995; Bar et al., 1997), barley (Glass and Siddiqi, 1985; Smith, 1973), broccoli (Liu and Shelp, 1996), citrus (Chapman and Liebig, 1940; Banuls et al., 1990, 1997; Bar et al., 1997; Cerezo et al., 1997), corn (Imas, 1991), kiwifruit (Smith et al., 1987), melon and lettuce (Feigin, 1985; Wei et al., 1989), peanut

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(Wang et al., 1989; Leidi et al., 19920, potato (James et al., 1970), strawberry (Wang et al., 1989), tobacco (Fuqua et al., 1976), tomato (Kafkaﬁ et al., 1982; Zabala, 1984; Feigin et al., 1987), and wheat (Silberbush and Lips, 1991; Wang et al., 1989). Increasing concentrations of NO 3 decrease Cl concentrations in plants in a linear fashion (Table VI). An increase of 1 mmol NO 3 /g DM prevented the accumulation of 2.38 mmol Cl/g DM in the tomato plant (Kafkaﬁ et al., 1982). The inhibition of nitrate uptake by Cl depends on the plant species and the concentrations of both nitrate and Cl in the uptake medium (Cerezo et al., 1997). In root cells, the high-afﬁnity saturable system for nitrate uptake that operates at low nitrate concentrations (Siddiqi et al., 1990) is inhibited by high external Cl, whereas the lowafﬁnity linear system that operates at high nitrate concentrations seems to be inhibited by high internal Cl (Cerezo et al., 1997). The competition of Cl vs NO3 was found to be stronger in salt-sensitive plants, such as peanut, than in salt-tolerant plants, such as cotton (Leidi et al., 1992). The Cl content of citrus leaves was 27–39 mg/g DM of nitrate-deﬁcient plants and only 5.3 mg/g DM in plants with an ample nitrate supply (Adler and Wilcox, 1995). Higher rates of KCl application may be needed in systems where nitrate levels are elevated (Fixen et al., 1986). In kiwifruit, the severity of leaf necrosis following KCl application was attributed not to Cl toxicity but rather to N deﬁciency enhanced by competition between Cl and NO 3 (Buwalda and Smith, 1991). However, chlorosis resulting from nitrate-induced iron deﬁciency in avocado rootstocks could be prevented by increasing the chloride levels (Bar and Kafkaﬁ, 1992). Deane-Drummond (1986) suggested that there are two populations of Cl carri ers and NO 3 carriers, each differing in their sensitivity to external NO3 and Cl. In

Figure 7 Effects of chloride and nitrate in the irrigation water on leaf chloride content and leaf scorching index in avocado (cultivar: Degania). Drawn on the basis of data from Bar et al. (1997).

Table VI Antagonistic Uptake Effects between Nitrate and Chloride Concentration range (mmol/g DM)

120

Plant

Plant part

Avocado Barley Barley Broccoli Citrus seedlings Kiwifruit Melon Tomato Tomato Peanut Strawberry Spring wheat

Leaves Root Shoot Shoot Leaves Leaves Shoot Shoot Shoot Shoot Shoot Shoot

aTotal

Y : Cl

X: NO -N 3

0.12– 0.74 0.008– 0.052 0.021– 0.093 0.05–1.0 0.15– 0.5 0.02– 0.10 0.08–1.0 0.25–1.0 0.16–2.3 0.094– 0.18 0.03– 0.26 0.10– 0.20

1.29–1.86a 0.003– 0.084 0.017– 0.107 0.10 – 0.65 1.7–2.7a 0.14– 0.26 0.6– 0.93 0.18– 0.89 0.5–1.4 0.003– 0.035 0.016 – 0.058 0.014– 0.14

N concentration of plant growth with nitrate-N as sole N source. Recalculated from original data. c Field-grown ‘Emperor’ broccoli. b

Equation Y 2.51 1.35X Y 0.05 0.52X Y 0.098 0.55X Y 1.50 2.97X Y 0.82 0.34X Y 0.29 1.47X Y 2.87 3.22X Y 1.25 1.25X Y 3.21 2.38X Y 0.18 2.86X Y 0.36 6.25X Y 0.21 0.91X

r2 0.70, n 11 r2 0.96, n 13 r2 0.89, n 13 r2 0.70, n 12 r2 0.97, n 3 r2 0.99, n 3 r2 0.80, n 4 r2 0.89, n 8 r2 0.79, n 36 r2 0.80, n 6 r2 0.93, n 6 r2 0.84, n 7

Source Bar et al. (1997)b Glass and Siddiqi (1985) Glass and Siddiqi (1985) Liu and Shelp (1996)c Chapman and Liebig (1940)b Smith et al. (1987)b Feigin et al. (1987)b Feigin et al. (1987)b Kafakﬁ et al. (1982) Wang et al. (1989)b Wang et al. (1989)b Wang et al. (1989)b

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barley plants grown previously in solutions lacking NO 3 , Glass and Siddiqi (1985) found that the inﬂux of 36Cl from outside of the plasmalemma into the cytoplasm was initially insensitive to external NO 3 , but became sensitive after a lag period of 3–6 hr. The results of kinetic analysis suggested that the inhibition of 36Cl inﬂux by external NO 3 was complex. Cl efﬂux, however, was found to be insensitive to external NO 3 . A time-course study and other experiments led to the proposal of a model (Glass and Siddiqi, 1985) for the regulation of Cl inﬂux, involving both of the negative feedback effects from vacuolar (NO 3 Cl) or total anion concentration and the inhibition by external NO of Cl inﬂux at the plasmalemma. 3 This model suggests additional aspects to those suggested by Cram (1973). The combined effects suggest an explanation for the discrimination against Cl and in favor of NO 3 accumulation, when the latter ion is available. Bar et al. (1997) suggested that while both chloride and nitrate anions are taken up by the root against their electrochemical gradient, nitrate is reduced after uptake whereas chloride maintains its negative charge. As a result, the active uptake of chloride is reduced as the chloride electrochemical potential gradient builds up during its accumulation. Chloride-stimulated ATPase activity is more sensitive than basal ATPase activity to nitrate (Grifﬁth et al., 1986). As the total N content of plants was found in some experiments not to decrease in response to Cl treatment (Ourry et al., 1992; Liu and Shelp, 1996), Liu and Shelp (1996) believe that Cl absorption does not compete directly with nitrate absorption. The addition of moderate amounts of Cl to the growing medium of broccoli plants decreased the nitrate content by increasing the extent of nitrate reduction. Nevertheless, they still suggested that Cl application could be used as a strategy to decrease the nitrate content of vegetables, particularly in plants such as spinach, lettuce, and cabbage, which are classiﬁed as nitrate accumulators (Maynard et al., 1976). In light of the observed nature of the competition between nitrate and chloride and the preference for nitrate absorption, Bar et al. (1997) suggested that in the presence of chloride up to 16 mmol/liter in the irrigation water the molar nitrate concentration in the soil solution be maintained at about 50% of that of chloride to ensure the reduction of chloride uptake. Increased Cl availability could increase the optimal application rate of N fertilizer for potatoes ( James et al., 1970). These ﬁndings suggest that the interpretation of soil and plant tissue analyses for N-NO 3 would have to be modiﬁed to take into account the uptake competition between NO 3 and Cl. 2. Ammonium Ammonium is taken up as a cation, and therefore relatively more anions have to be taken up to maintain the electrical neutrality of the process (Harward et al., 1956; Teyker et al., 1992). As a result, when Cl is present in the external solution, ammonium uptake increases the salt sensitivity of plants such as pea (Speer et al.,

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1994), maize, and wheat (Lewis et al., 1989). Plants fertilized with NH 4 usually contain much more Cl in the tissue than plants fertilized with NO or NH 3 4 NO3 , irrespective of the Cl level in the nutrient solution. Results with marigold, petunia, salvia (Jeong and Lee, 1992), potato (Cao and Tibbitts, 1993), and oilseed rape (Ali et al., 1998) also showed that the concentrations of the anions of Cl and P in the shoots and roots were increased when NH 4 rather than NO3 was present in the solutions. When the ratios of applied NH4 to NO3 were relatively high, both the vegetative and the grain yields of wheat were more susceptible to NaCl than when plants were fertilized with NO 3 only (Silberbush and Lips, 1991). Speer and Kaiser (1994) suggested that the intracellular compartmentation capacity of ammonium-grown plants was considerably lower than that of nitrategrown plants. Why this should be the case remains unclear. 3. Phosphorus The interaction of chloride with P appears to be complex. It seems unlikely that competition in the uptake between H2PO 4 and Cl ions is important because of the great differences in their physical and physiological properties (Champagnol, 1979). In hydroponic experiments, both chloride and sulfate in the culture solution impaired the uptake of phosphorus by potato root (Hang, 1993). In tart cherry leaves, an excess of applied KCl suppressed P and increased Mn (Callan and Westcott, 1996). The results of pot experiments with potatoes on Caribou loam soil using 32P led Gausman et al. (1958a) to suggest the existence of an optimal or critical level of Cl for maximal P uptake. The optimal Cl level appeared to be 300–450 mg/kg soil. P uptake was stimulated by lower amounts and suppressed by higher amounts of chloride (Wang et al., 1989). The optimal soil concentration of chloride for maximum P uptake differs among crops; it is about 237–437 mg/kg for strawberry, 437 mg/kg for peanut, and 837 mg/kg for spring wheat (Fig. 8). Chloride had very little effect on the P content of tomato plants grown in nutrient solution (Kafkaﬁ et al., 1982), whereas many reports describe a decline in P uptake when plants were grown in soil under conditions of chloride salinity (Kafkaﬁ, 1987). Several mechanisms that operate simultaneously are responsible for the apparently conﬂicting evidence. (1) An increase in nitrate uptake results in an increase in pH near the root (Marschner and Romheld, 1983). The increased pH in the rhizosphere increases the divalent phosphate and trivalent iron content of the soil solution near the root and reduces the phosphate uptake. (2) An increase in chloride concentration reduces the rate of nitriﬁcation. In case of ammonium fertilizer addition, this leads to a high ammonium concentration in the soil and of ammonium available to the roots. (3) An increase in chloride concentration reduces nitrate uptake, thereby attenuating the increase in the rhizospheric pH, with all the attendant consequences. (4) An increase in chloride concentration induces an increase in EC near the root. As a result, the rate of root elongation is reduced and a decline in total plant growth commonly occurs under saline conditions. P and Fe uptake are

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Figure 8 Effects of soil Cl application level on crop P nutrition status. Based on data from Wang et al. (1989).

also inﬂuenced by the reduction in root elongation (Kafkaﬁ and Bernstein, 1996). Because all of these mechanisms operate simultaneously, while each also depends on the concentration of all the other nutrients, it is difﬁcult to make a quantitative prediction with respect to the direction of the effects on chloride uptake under variable soil conditions. 4. Potassium Accumulation of Cl in the leaf blade did not reduce leaf K concentration in muskmelon (Adler and Wilcox, 1995). Increasing concentrations of chloride in the nutrient solution had no consistent effect on potassium concentrations in the leaves of kiwifruit (Smith et al., 1987). An increase in soil chloride of up to 600 mg/kg soil did not affect plant potassium contents in peanut or wheat (Wang et al., 1989). Copra yields of palm trees were increased with increasing leaf Cl content, whereas potassium concentrations maintained relatively constant levels (von Uexkull and Sanders, 1986). K and Mg2 in citrus leaves were not affected by the Cl content of the nutrient solution (Bar et al., 1996). The effect of chloride on plant K uptake does not follow the nearly synchronous pattern that exists between K and

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Cl observed in guard cells of intact attached leaves of V. faba (Talbott and Zeiger, 1996). A cytoplasmic K concentration (100–140 mmol/liter) is independent of the external K supply even in the presence of 100 mmol/liter of NaCl (Jeschke and Wolf, 1988). In some plant varieties, chloride tended to inhibit K uptake at high Cl concentrations. Callan and Westcott (1996) found that K concentrations in tart cherry plants declined in the third year when KCl was applied as the sole K source at a rate of 0.58 kg per tree, indicating an inhibitory effect of accumulated Cl on K uptake (Fig. 9). An increase in Cl application from 90 to 250–400 mg/liter caused a signiﬁcant increase in the K content of avocado leaves (Lahav et al., 1992). Chloride applications tended to increase the K content and decrease the Ca2 content of citrus seedlings, whereas an increase in nitrate concentration in the culture solution increased K /Ca2 ratios and reduced Cl uptake (Table VII). Application of Cl at a rate of 100 mg/kg soil had a stimulatory effect on the K content of strawberry plants (Wang et al., 1989). K concentrations in the leaves of kiwifruit were signiﬁcantly higher for vines receiving KCl than for vines receiving K2SO4. The kiwifruit uses Cl rather than organic anions for charge balance and thus maintains a high K uptake (Buwalda and Smith, 1991).

Figure 9 Relationship between leaf K and Cl content of tart cherry tree as inﬂuenced by the amount and form of K fertilizer (third-year results). Redrawn from Callan and Westcott (1996).

125

ADVANCES IN CHLORIDE NUTRITION Table VII

Inﬂuence of Nitrate on Cation Content (mg/g) of Citrus Leaves at Two Chloride Levelsa Chloride concentration in culture solution 0.28 mmol/liter Nitrate (mmol/liter) 0.05 0.10 0.50 5.00 aRecalculated

20.28 mmol/liter

K

Ca

K/Ca

Mg

Cl

K

Ca

K/Ca

Mg

30.6 26.5 25.6 27.6

29.9 30.3 26.7 24.8

1.0 0.9 1.0 1.1

2.3 2.4 2.7 2.4

27.8 17.9 8.7 5.3

40.2 31.6 29.0 32.1

29.0 26.8 22.3 20.9

1.4 1.2 1.3 1.5

2.8 2.5 2.3 2.2

from Chapman and Liebig (1940).

5. Calcium Calcium ions are required for the maintenance of membrane integrity and ion transport regulation (Marschner, 1995). The ability of Ca2 to mitigate the adverse effects of NaCl by inhibiting Na uptake has been tested in many experiments (Banuls et al., 1991; Chien et al,. 1991). The higher the concentration of NaCl in the medium, the more Ca2 is required (up to a maximum of 10 mmol/liter) to achieve the maximal gain in fresh weight (Kafkaﬁ and Bernstein, 1996). The accumulation of chloride in orange leaves grafted on either Cl-tolerant or Cl-sensitive rootstocks was reduced when external Ca2 concentrations were increased. At the same time, the Cl concentration in the roots remained constant or was slightly increased (Fig. 10). The distribution of Cl in the plants suggests that a high external Ca2 level increased Cl accumulation in the basal stem and roots, reduced the transport of Cl from the roots to the leaves, and increased photosynthesis and stomatal conductance (Banuls et al., 1991; Banuls and Primo-Millo, 1992). This effect appears to be associated with an ability of Ca2 to withdraw Cl from the xylem stream, particularly in the basal stem and roots. Comparison of the effects of the same concentration of KCl and CaCl2 (Kafkaﬁ et al., 1992) showed that inhibition of the inﬂux of 13NO in tomato and melon was caused by high Ca2 in 3 the solution rather than by Cl. Decreasing the Na /Ca2 ratio under saline conditions had no effect on Cl concentrations in tomato or cucumber, but Cl concentrations were decreased in the roots and increased in the shoots (Al-Harbi, 1995). Cl concentrations in cotton tended to decrease initially and then increase as the CaCl2 concentration increased, but were largely unaffected by changes in external CaCl2 (Gorham and Bridges, 1995).

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Figure 10 Effects of Ca2 concentration in the external solution on Cl content of citrus leaves of two scion/rootstock combinations. Recalculated and drawn from Banuls et al. (1991).

F. DISEASE SUPPRESSION The effects of soil chloride and plant nutrition on plant diseases have been the subject of a number of investigations over the past two decades. In studies of the inﬂuence of potassium on disease reduction (Huber and Arny, 1985), the test compound used was KCl and the observed effects were ascribed to the macronutrient K, whereas the role of Cl was not taken into account. Christensen et al. (1981) showed that the Cl anion is responsible for suppression of take-all root rot in wheat, provided that the K is sufﬁcient for optimal host nutrition. More recently, Heckman (1998) conﬁrmed the speciﬁc effect of chloride in controlling the incidence of corn stalk rot. In some studies, the effects of Cl on stripe rust and take-all disease of wheat have been distinguished from those of its accompanying cation (Taylor et al., 1981; Christensen et al., 1981). Chloride in macronutrient fertilizers was found to partially control a number of diseases in different crop species (Fixen, 1993). Whether the mechanisms underlying these responses involve a direct effect on the plant pathogen or increased host tolerance has not always been clear. The effect of chloride appears to be distinct from that of its accompanying cations. Huber and Arny (1985) explained the early reported effects of Cl on stalk rot in terms

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of a competitive effect of Cl on NO 3 absorption and the resulting inﬂuence on rhizosphere pH. An important effect of Cl, when added to the soil in sufﬁcient quantity, is the suppression of nitriﬁcation, as mentioned earlier. Huber and Wilhelm (1988) argue that the inhibition of nitriﬁcation suppresses take-all and other diseases via a decrease in rhizosphere pH as a result of the increased uptake of N NH 4 and the decreased uptake of N-NO3 . This in turn increases the availability and uptake of Mn, a beneﬁcial effect of disease control. NH 4 may also affect host physiology in other ways, leading to disease resistance. Christensen et al. (1986) provided clear evidence on the change in the ratio of soil NH 4 to NO3 induced by Cl and linked it to suppression of take-all disease. An intriguing possibility is the direct Cl-mediated release of Mn from soils (Krishnamurti and Huang, 1987). Microbial intervention could easily trigger a process of Cl-reduced MnO2 reduction and increases in available Mn following the application of Cl salts (Norvell, 1988). Enhancement by Cl of the rhizosphere populations of ﬂuorescent pseudomonads, potential Mn-reducing organisms, was observed by Halsey and Powerlson as reported by Christensen et al. (1981). The free Cl2 produced by such a reaction would not last long in soils, so the involvement of Cl would essentially be catalytic, although the amounts required are not small. Environmental conditions such as temperature, humidity, and light intensity contribute strongly to the occurrence and severity of disease and also affect uptake and physiological functions of the major nutrients. Thier et al. (1986) concluded that chloride fertilization in wheat grown under their experimental conditions did not offer any measurable protection against powdery mildew. Therefore, the growing season and location inﬂuence the relationship between the potassium and the chloride nutrition status of crops.

IV. CHLORIDE IN CROPS A. SMALL GRAINS 1. Wheat (Triticum aestivum L.) and Barley (Hordeum vulgare L.) Large yield increases of wheat and barley as a result of KCl application in the Great Plains of North America (Fixen, 1993), where the levels of available potassium in the soil were quite high, suggested that the response was due to chloride rather than potassium. Comparing two ammonium fertilizers with the same amount of N, yield increase ranged from 470 to 2150 kg grains/ha, with an average of 1076 kg/ha due to NH4Cl as compared to (NH4)2SO4. Fixen et al. (1986), Gaspar et al. (1994), and Engel et al. (1994) described the responses of wheat and barley to chloride in some detail.

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Some of these responses to chloride were attributed to either suppression of root or foliar diseases or to enhancement of host tolerance to the disease (Fixen, 1993). Leaf spots and tissue necrosis are caused by inadequate Cl nutrition rather than pathogen infection (Engel et al., 1997). The so called “Cl-deﬁcient leaf spot syndrome” of wheat is due to cultivar sensitivity. It was found to increase exponentially as the chloride content of the whole plant at head emergence dropped below 1 mg/g. 2. Rice (Oryza sativa L.) Rice is relatively tolerant to chloride. When the content of chloride in rice shoots was lower than 3 mg/g DM, irrigation with water containing 50–150 g Cl/m3 increased rice yield (Fig. 11). There were no negative effects of chloride on the yield or quality of rice grains when the Cl content in mature straw was about 12–13 mg/ g (Zhu and Yu, 1991; Huang et al., 1995). Rice could tolerate chloride as high as 400–800 mg/kg in soil (Zhu and Yu, 1991) or 300–500 g/m3 in irrigation water (Yin et al., 1989).

Figure 11 Effect of chloride in irrigation water on yield and on Cl content in the shoot of rice. Based on Yin et al. (1989).

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B. VEGETABLES 1. Potato (Solanum tuberosum L.) Relative to K2SO4, KCl delayed potato tuber development signiﬁcantly as a result of lower osmotic potentials, higher water contents, and greater shoot growth in KCl-treated plants (Beringer et al., 1990). The decline of starch accumulation at K concentrations greater than 20 –25 mg/g DM in tubers (Marschner and Krauss, 1980) might be explained in terms of an osmotic optimum for starch synthesis (Wright and Oparka, 1990). The favorable effect on tuber starch content with K2SO4 as compared to KCl has been attributed to a higher translocation of assimilates into the tubers with K2SO4 (Haeder, 1976). Jackson and McBride (1986) found that application of KCl as compared to K2SO4 increased both the yield (by over 170 g/tuber) and the grade percentage and reduced the percentage of hollow hearts and brown centers. The higher concentrations of Cl and counteractions in the tubers reduced their susceptibility to heat and moisture. Westermann et al. (1994) suggested that N or K fertilizers can be applied to potato according to their soil test concentrations and crop requirements, without considering the K source. 2. Tomato (Lycopersicon esculentum Mill.) Tomatoes are especially sensitive to salinity at the young seedling stage (Satti et al., 1994, 1995; Satti and Al-Yahyai, 1995). Chloride salts reduced plant dry weight, increased defoliation and accumulation of Cl in the leaves, and caused a sharp reduction in photosynthesis, leaf water potential, and stomatal conductance (Pasternak and De-Malach, 1995). In contrast, these parameters were not affected by leaf Na concentrations of up to 478 mmol/liter in the tissue water. Different tomato cultivars responded differently to the treatment applied. Salinity had no signiﬁcant effect on the number of fruits or on the fruit set of tomato (Satti et al., 1995). Tomato growth was not affected as long as the Cl concentration in the plant tissue was lower than about 30 mg/g DM (Kafkaﬁ et al., 1982). Chloride salt affected fruit quality, as well as other growth and yield parameters, by reducing the water content of the fruit. During fruit development, chloride salinity during the cell enlargement stage reduced cell growth by lowering the amount of water transport to the fruit (Ho et al., 1987). Irrigating tomato with saline water for 2 weeks before the expected time of ripening increased total soluble solids and acid contents of the fruits (Mizrahi and Pasternak, 1985). However, NaCl is not recommended as a soil additive due to the dispersive action of Na on soil clay particles. Applications of KCl 2 weeks before harvest might improve tomato quality.

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C. FRUIT TREES 1. Citrus (Citrus sp. L.) Citrus trees are generally sensitive to salt (Zekri, 1993). Large amounts of chloride were found to accumulate in the leaves and in the fruit juice when trees were irrigated with water containing high Cl concentrations (Syvertsen et al., 1993). The Cl content of old mature leaves is much higher than that of young leaves (Bell et al., 1997). The adverse effects of salinity in citrus leaves were caused by the accumulation of Cl and not by Na levels or water potential (Walker et al., 1982; Banuls and Primo-Millo, 1992). For beneﬁcial effects on fruit yield of ‘Washington’ navel oranges grafted on Rough Lemon rootstock, the Cl concentration of irrigation water should not be higher than about 4.3 mmol/liter (Cole, 1985). Necrotic burn symptoms usually appeared when Cl levels in mature leaves of orange citrus exceeded 15 mg/g DM (Romero and Syvertsen, 1996). Irrigation in a citrus orchard (annual application about 1100 mm) with water containing Cl at a concentration of 1.0 –3.7 mmol/liter had almost no effect on soil salinity or leaf Cl concentration (Cole, 1985). The degree of Cl damage to citrus trees varies with rootstock characteristics (Zekri, 1993). In mature leaves, the critical Cl level for the appearance of leaf toxic symptoms is between 2 mg/g (Cole, 1985) and 7 mg/g (Embleton et al., 1978). Analysis of the distribution of Cl in the whole plant showed that Cl-tolerant rootstock prevents the transport of Cl from the roots to the leaves (Skene and Barlass, 1988). Bar et al. (1997) suggested that the relative tolerance of citrus to chloride might be attributable both to its ability to restrict Cl uptake and transport to the leaves and to the ability of the leaf tissue to withstand high Cl concentrations. The reduction in citrus growth caused by NaCl treatment depended more on the identity of the scion than on that of the rootstock, whereas the opposite was true for defoliation (Bannuls and Primo-Millo, 1995). Changes in the ionic contents of leaves and roots indicated that the Cleopatra mandarin scion (Cl-tolerant) accumulated less Cl in the leaves than did Troyer citrange scions (Cl-sensitive), even when both were grafted on the same rootstock. Plants of ‘Ramsey,’ irrespective of their origins, accumulated quite high levels of Cl in their petioles, despite the fact that this cultivar is noted for its ability to restrict Cl uptake under saline conditions. Leaf injury and defoliation were correlated closely with leaf Cl concentrations (Skene and Barlass, 1988). 2. Kiwifruit (Actinidia deliciosa) Kiwifruit requires exceptionally high levels of chloride. Typically, the Cl concentration in the leaves of high-yielding kiwifruit vines exceeds 8 mg/g DM and should be at least 2.1 mg/g in order to maintain healthy growth (Smith et al., 1987). The symptom of chloride deﬁciency is the appearance of discrete patches of pale

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green chlorotic tissue between the main veins near the leaf tip (Smith et al., 1987). At a leaf Cl concentration higher than 15 mg/g DM, application of KCl induced leaf breakdown (marginal leaf scorch), followed by necrosis and leaf drop (Prasad et al., 1993). Kiwifruit is well adapted to the use of Cl rather than organic acid anions for charge balance, and hence for the maintenance of K uptake. Therefore, the anion accompanying K in the solution around plant roots can inﬂuence K uptake signiﬁcantly. K inﬂux in kiwifruit appears to be limited when SO 2 4 rather than Cl is the anion accompanying K in the fertilizer. Both fruit yield and plant dry weight were signiﬁcantly higher in cases where Cl rather than SO 2 4 was the accompanying anion (Buwalda and Smith, 1991). 3. Avocado (Persea americana Mill.) Avocado is extremely sensitive to soil salinity. Chloride concentrations in irrigation water, considered tolerable for many crops, are detrimental to avocado (Bingham et al., 1968). Increasing the chloride concentration in the nutrient solution resulted in an increase in chloride levels in the leaves, roots, cotyledons, bark, and wood of both salt-tolerant Degania-113 and salt-sensitive Smith rootstocks (Bar et al., 1997). Foliar accumulation of chloride induces necrosis, early leaf shedding, and decreased yield (Bingham et al., 1968). Different rootstocks of avocado exhibit marked differences in leaf chloride concentrations as well as in tolerance to chloride (Lahav et al., 1992). The chloride tolerance of the rootstock Dagania-113 is not a result of reduced chloride accumulation in the leaves, but rather of three other properties: (1) the ability of the leaf tissue to withstand high chloride levels, (2) the capacity to shed its chloride-loaded leaves prematurely, and (3) its higher growth rate, enabling accelerated growth after leaf shedding. Leaf shedding may be a way in which avocado adapts to high chloride (Bar et al., 1997). 4. Grapevine (Vitis vinifera L. ssp. vinifera) Irrigation of grapevine with saline water increases the chloride concentration in the leaf and induces leaf damage. The severity of such damage and the decline in photosynthesis were found to be proportional to the leaf Cl concentration (Stevens and Harvey, 1995). Certain rootstocks exclude chloride from the leaf, which was associated with better growth under saline conditions (Downton, 1985). Chloride concentrations in grapevine rootstocks were reduced greatly in scions grown under a controlled chloride concentration at constant osmotic potential (Bernstein et al., 1969). The average shoot dry weight of Sultana scions grafted onto ﬁve rootstocks decreased by 10 and 20% when treated with water containing NaCl at 10 and 20 mmol/liter, respectively (Downton, 1985). Exclusion of the Cl ion in grapevines is inherited either as a polygenetic or as a monogenetic trait (Sykes, 1993). Rootstocks of grapes such as Ramsey can restrict the uptake and/

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or transport of Cl to the shoots, conferring salt tolerance on scions such as Sultana and resulting in higher yields than own-rooted vines at high salinity. The use of a Cl-excluding rootstock reduced leaf chloride by about 60% (Stevens and Harvey, 1995). Chloride salinity applied before bud burst (Downton, 1985) or before ﬂowering (Hawker and Walker, 1978) had much more drastic effects on shoot development and cane weight than when applied later. With applications of up to 465 kg K/ha to grapevine, KCl and K2SO4 produced the same yield response and quality of fruit juice (Cline and Bradt, 1980).

D. OIL SEEDS 1. Peanut (Arachis hypogaea L.) The vegetative growth of peanut is rather tolerant to chloride salinity, but the pod yield was reduced severely when irrigated with water with an EC of 4.5 dS/ m (Silberbush and Lips, 1988). They found that the sensitivity of peanut plants to salt under ﬁeld conditions is mainly the result of salt damage to the gynophore when it touches the upper soil layer. The application of Cl stimulated growth and increased seed yield when the total Cl concentration in the soil was below 137 mg/ kg. Higher Cl levels in the range of 400–600 mg/kg soil reduced almost every physiological index (Wang et al., 1989). 2. Soybean (Glycine max L. Merr.) In ﬁeld experiments, seed yields and chloride levels in leaves of ﬁve soybean cultivars were not affected by K application rates, but the yields decreased with increasing leaf Cl levels (Snyder et al., 1993). Soybean leaf scorch was always associated with excessive amounts of Cl in the leaves (Parker et al., 1986). Incipient damage was seen at leaf Cl levels of 186 mg/kg soil and plant death occurred at 370 mg/kg. The average Cl level in the leaves of susceptible cultivars was 16.7 mg/g, i.e., 18 times higher than the 0.9 mg/g found in the leaves of tolerant cultivars. In grafted soybean cultivars grown in a saline solution of 49.3 mmol Cl/liter, the onset, severity, and visual symptoms of foliar injury were controlled by the genotype of the rootstock and were correlated with leaf Cl concentration (Grattan and Maas, 1985). When grafted on rootstocks that translocated Cl to the shoots, scions of all four genotypes were sensitive to Cl. The scions do not appear to restrict the translocation of Cl from the roots to the leaves (Grattan and Maas, 1985). The water content of soybean tissues was hardly inﬂuenced by KCl fertilization, but the effect differed among different soybean cultivars and even toxicity was reported (Parker et al., 1983).

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E. OTHER CROPS 1. Coconut Palms (Cocos nucifera L.) KCl is the most widely needed and widely used fertilizer for both coconut and oil palms and has an inﬂuence on nut size and copra yield, as well as on Cl and Ca2 concentrations (von Uexkull and Sanders, 1986). The use of NaCl and seawater is an ancient and very common practice among coconut growers in many parts of the world (Bonneaux et al., 1997). Chloride plays a vital role in stomatal movement in the palm. Healthy coconut palms along seashores usually contain Cl at a concentration of 7–10 mg/g DM in their foliage. The optimal Cl concentration is usually in the range of 4.5–5.5 mg/g. At Cl concentrations lower than 2.5 mg/g, coconut palms may exhibit some visual symptoms of yellowing and/or orange mottling of the older leaves and the leaf tips and edges (von Uexkull and Sanders, 1986). The threshold value of EC in soil extract was found to be 4.5 dS/m, and above this level tree growth and copra yield began to decline (Hassan and El-Samnoudi, 1993). No copra yield was obtained when the EC value of the soil extract exceeded 23.2 dS/m, and salinity symptoms appeared on the leaves, but the trees survived. Soil salinity leads to the accumulation of Cl, Na, and K in the leaves. The accumulation of Cl was higher than that of Na and was highly correlated with salinity symptoms (Hassan and El-Samnoudi, 1993). 2. Corn (Zea mays L.) Grain yield was positively correlated with an increase in the Cl concentration of ear leaves as chloride fertilization increased up to 400 kg/ha (Heckman, 1995), whereas the stover yield was not affected by chloride. When produced in high-yield environments, corn may respond to enhanced levels of Cl nutrition (Heckman, 1995). Corn beneﬁts less than other cereal crops from Cl applications (Fixen, 1993). Three N sources (ammonium chloride, ammonium sulfate, and ammonium nitrate) produced almost identical yields of corn in a silt loam soil (pH 6.7) as long as the Cl content was below 600 kg/ha (Meelu et al., 1990). Applications of chloride had no deleterious effects on growth or yields of corn, even at rates of 728 kg Cl/ha (Parker et al., 1985). The incidence of stalk rot in corn decreased with increasing rates of KCl application, whereas K2SO4 or KH2PO4 had little or no effect (Younts and Musgrave, 1958; Heckman, 1995). 3. Cotton (Gossypium hirsutum) Cotton can tolerate high salt levels in the soil. No changes in cotton yield or quality are observed when Cl concentrations are below 1600 mg/kg, whereas cot-

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ton seedlings are very sensitive to chloride above 100–200 mg/kg (Tan and Shen, 1993). Therefore, fertilizers such as NH4Cl and KCl are not recommended for placement in the seed row. Over 80% of total Cl in cotton occurs in the leaves and stems and only a small part in the roots and seeds (Fig. 5). 4. Sugarbeet (Beta vulgaris L.) Sugar beet requires large amounts of chloride. A chloride concentration of 0.18– 0.29 mg/g DM in the petioles was found to be indicative of extreme Cl deﬁciency for beets. The critical Cl concentration was about 1.4 mg/g in the leaves and 5.7 mg/g in the petiole; an adequate concentration in the petioles was 7.1 mg/g (Ulrich and Ohki, 1956). The Cl content of sugar beet tops at harvest varied from 28 to 148 kg/ha depending on the soil Cl level (Moraghan, 1987). Chloride applications of up to 1600 mg/kg produced positive effects on sugar beet, and in clay soils, applications as high as 3200 mg/kg were tolerated before any yield reduction was observed (Jing et al., 1992).

V. CHLORIDE MANAGEMENT IN FERTILIZATION AND IRRIGATION A. SALT ACCUMULATION IN SOIL The concentrations of total salts and of each speciﬁc ion present in the soil solution are a consequence of several factors: irrigation method, fertilization practices, rate of water uptake by the plants, evaporation from the soil surface, soil type, and rainfall rate and distribution. Furrow-irrigated ﬁelds gradually accumulate soluble salts on the soil surface between the furrows. Bernstein et al. (1955) demonstrated differences in saline water qualities between a single seed line placed in the middle of the bed and two seed lines each placed on one side of the bed. When the bed has a rounded shape, sowing the seeds on the bed slopes keeps the seedlings away from the zones of salt accumulation. The irrigation method can be used to control the concentration of soluble salts in the soil solution (Rhoades, 1993). In level basin irrigation the salt accumulation is minimal, although some problems of aeration and crust formation may appear. Furrow irrigation is used for row crops and for plots in which the slope is unsuitable for level basin irrigation. The uniformity of water distribution is important to avoid the formation of areas with low water inﬁltration rates in which salt spots may be formed. Reduction of furrow length (Worstell, 1979) or surge irrigation (Bishop et al., 1981) may increase the uniformity of water distribution. In places with a shallow

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underground water table, the draining water may create a continuous wet zone with the groundwater level In that case, the upward ﬂow of capillary water leads to salinization of the soil surface as a result of water evaporation with chloride precipitation on the soil surface. In drip irrigation, the salt distribution pattern depends on the rate of evaporation from the soil surface (Yaron et al., 1973), the water uptake by the plant, the location of the wetting front, the total amount of applied irrigation water, and the distance between drip lines. As the amount of applied water increases, more salts are leached below the drip line and there is a larger salt accumulation on the soil surface between the irrigation lines. Chloride concentration in the soil solution increases as more water is taken up by the plant and the soil moisture level approaches the permanent wilting point. Plants can tolerate water with a high salt concentration when the soil moisture level is high (Rhoades, 1993) and when the high salt concentration zone is located in deeper soil layers with lower root density. Therefore, classiﬁcations of water quality should also consider the effects of irrigation practices and plant root distribution.

B. MONITORING OF CHLORIDE CONCENTRATION IN SOIL The correct management of irrigation requires periodic monitoring of the concentration of the soil solution in the root zone. The salt concentration must remain below a given threshold value, according to the sensitivity of the crop to salinity and particularly to chloride (see Table V). The methods used for monitoring soil salinity were reviewed by Rhoades and Oster (1986). The total concentration of soluble salts in the soil solution is generally estimated by determining the EC of the saturated paste extract or by sampling the soil solution. In most cases, chloride is the main anion present in the soil solution. The chloride concentration can be determined directly in the saturated soil extract or in the soil solution extracted with suction cups or with gypsum block sensors. Chloride testing methods have been reviewed by Johnson and Fixen (1990). The reliability of the results is inﬂuenced by the soil variability, the position of the soil solution suction cups relative to the pattern of water and salt accumulation in the soil proﬁle, and sensors type (Rhoades and Miyamoto, 1990). The number of samples and their spatial distribution will affect the reliability of the ﬁeld salinity level determination.

C. FERTILIZATION UNDER CONDITIONS OF CHLORIDE SALINITY With the increasing use of saline and recycled sewage water for agriculture, fertilizer application under saline conditions has become a subject of considerable in-

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terest (Feigin, 1985). Sodium chloride salinity disrupts mineral nutrition acquisition by plants in two ways (Grattan and Grieve, 1992): (1) total ionic strength of the soil solution, regardless of its composition, can reduce nutrient uptake and translocation; and (2) uptake competition with Na and Cl ions can reduce nutrient availability. These interactions may lead to Na-induced Ca2 and/or K deﬁciencies (Volkmar et al., 1998) and Cl-induced inhibition of nitrate uptake (Kafkaﬁ et al., 1982). It was postulated that the salinity tolerance of crops can be improved by the suitable use of nutrients (Kafkaﬁ, 1987). Most of the reported studies on NaCl salinity effects do not separate the effects of Na from those of Cl (Volkmar et al., 1998). A low concentration of Cl salts is beneﬁcial (Fixen et al., 1986). In the wet volume of irrigated soils used to grow regular ﬁeld crops such as tomatoes and melons, EC values of about 3.0 dS/m in the soil solution, with chloride salts as the dominant component, are common (Kafkaﬁ et al., 1992). Chloride concentrations above about 10 mmol/liter in the irrigation water are generally considered problematic for plant growth (Ayers and Westcott, 1985). Salinization with NaCl or KCl inhibits the net uptake of nitrate in citrus (Cerezo et al., 1997) and causes nitrogen deﬁciency (Embleton et al., 1978). Nitrate competes with chloride for uptake by the plant, as discussed in Section IIIE; therefore, when the irrigation water contains nitrate at about 8–16 mmol/liter, even sensitive plants like avocado can survive at chloride concentrations of 8–16 mmol/liter (Fig. 3). High potassium fertilization might enhance the capacity for osmotic adjustment of plants growing in saline habitats (Cerda et al., 1995), as potassium is the most abundant cation in the cytoplasm of glycophytes (Marschner, 1986). In spinach, higher K requirements are needed for shoot growth under high salinity than under low salinity conditions (Chow et al., 1990). Differences in salt tolerance among maize varieties appear to be related to higher K ﬂuxes and cytoplasmic concentrations on the one hand and lower Na and Cl ﬂuxes an cytoplasmic concentrations on the other (Hajibagheri et al., 1989). Potassium uptake is greater in the high salt-tolerant group of barley cultivars than in salt-sensitive ones (Sopandie et al., 1993). However, an external K supply is not required for root growth of castor bean (Ricinus communis L.) under saline conditions ( Jeschke and Wolf, 1988). Increasing the K supply in the rooting media of maize did not alleviate the growth reduction imposed by treatment with NaCl at 50 mmol/liter (Cerda et al., 1995). Tip-burn symptoms in Chinese cabbage, induced by salinity with NaCl and CaCl2, was not alleviated by the addition of KNO3 (Feigin et al., 1991). The addition of adequate P can also be helpful in alleviating salt stress (Champagnol, 1979; Awad et al., 1990). A positive effect on P on the yield of foxtail millet and clover grown in a saline soil was reported (Ravikovitch and Yoles, 1971). As crops can differ greatly in their response to nutrition management under different combinations of environmental salinity (Feigin et al., 1991), speciﬁc information on the behavior of crops under different situations of Cl salinity is needed for the optimal fertilization management of speciﬁc crops.

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D. IRRIGATION AND LEACHING OF CHLORIDE SALTS IN THE ROOT ZONE The use of water containing chloride must be accompanied by appropriate practices to keep Cl levels in the soil within the limits of crop tolerance. The amount of supplementary Cl salt added to the soil depends on the salt concentration in the water, the evaporation level and the amount of irrigation water (which depends on the physiological development of the plant). With 500 mm of irrigation water containing Cl at 100 –200 mg/liter (low to medium level of salinity, see Table I), the applied Cl reaches 500 –1000 kg/ha. This amount of Cl is equivalent to KCl fertilization of 1000 –2000 kg/ha. In ﬁeld practice, the recommended range of KCl fertilization is 75–150 kg/ha for ﬁeld crops and 300–500 kg/ha for horticultural crops. This suggests that the addition of Cl in KCl fertilizer is relatively safe for most agricultural crops, especially when the rainfall during the rainy season is capable of leaching the excess Cl accumulated during fertilization and irrigation. Irrigation with saline water is managed by an excess of irrigation to meet the leaching requirement for avoiding salt accumulation in the crop root zone (Richards, 1954). When plants are present, however, there is a risk that the advantages gained from salt leaching may be lost with the onset of temporary oxygen shortage due to water-logging (Stevens and Harvey, 1995). The ability of grapevine roots to exclude sodium and chloride from the leaf was strongly reduced by a short period of waterlogging (West and Taylor, 1984; Stevens and Harvey, 1995). In a river land of southern Australia, the EC of irrigation water during the period 1985–1990 was less than 0.5 dS/m; however, grapevines suffered salinity damage because of excessive irrigation. The excess water drained to an aquitard just below the root zone and formed a temporary water table that mobilized the previously leached salts back into the root zone as a result of the capillary rise of the water (Stevens and Harvey, 1995). The irrigation system inﬂuences the distribution of salt in the soil’s proﬁle and surface. Keller and Bliesner (1990) presented a detailed calculation of the efﬁciency of chloride leaching by different irrigation methods. In drip irrigation, the water is applied at short intervals so that the application of minimum leaching doses and the relatively small change in the soil’s water content keep the salinity of the soil close to that of the irrigation water. In drip irrigation the leaching dose (LRt) required to wash salts out of the root zone is deﬁned as the ratio between the water height applied for leaching and the irrigation water height applied for satisfying crop and leaching demands. The equation is simply expressed as: LRt ECw /2ECemx, where ECw is the EC of the irrigation water and ECemx is the maximum EC value of the saturated soil extract at which the crop can survive. In sprinkler or surface irrigation the water is applied at longer intervals, during which the salt concentration of the soil solution gradually increases. Before the next irrigation, salt concentrations on the soil surface reach relatively high values.

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The equation for calculating the leaching requirement dose is: LRt ECw /(5ECe ECw), where ECe is the mean EC of the saturated soil extract at which no yield reduction occurs (Keller and Bliesner, 1990).

E. SPRINKLER IRRIGATION AND CHLORIDE-INDUCED FOLIAR INJURY Chloride can be absorbed directly by crop leaves and can cause foliar injury when its concentration in the sprinkler water is high (Maas et al., 1982). Leaf scorching due to excessive Cl accumulation in the leaves varies among different species and depends on leaf properties and on the rate of Cl absorption by the leaves. Temperature, relative humidity, and water stress all have marked effects on the leaf injury. Absorption of Cl continues as long as the leaf is wet. Evaporation from the leaf surface increases the salt concentration on the leaf and consequently also the leaf scorching level. Deciduous trees, such as almond, apricot, and plum, absorb Na and Cl readily through the leaves, and partial leaf abscission occurs after a 50-hr sprinkler irrigation with water containing CaCl2 or NaCl at a concentration of 10 mmol/liter (Ehlig and Bernstein, 1959). The crop leaf Cl concentrations causing leaf injury in plum, almond, and orange are 4.3–7.1, 6.4–10.6, and 7.1–10.6 mg/g DM, respectively. No visual foliar injury was observed in grapes sprinkled with water containing Cl at 5 or 10 mmol/liter (Francois and Clark, 1979). In avocado, where the thick waxy layer of the leaves limits the absorption of ions present in the sprinkling water, chloride accumulation in the leaves is very low and no visual injuries are observed (Ehlig and Bernstein, 1959). Therefore, although avocado is known to be sensitive to salt concentration in the growing medium, there is no risk of direct foliar absorption of chloride because of the leaf surface characteristics. Chloride accumulation in crop leaves depends mainly on the time of watering (Francois and Clark, 1979). Therefore, rootstocks known to limit chloride absorption by the roots are not suitable when sprinkler irrigation is used and they do not avoid chloride accumulation in the leaves. Field and vegetable crops are not especially sensitive to salt accumulation in the leaves (Ehlig and Bernstein, 1959). Strawberry is highly sensitive to chloride in the soil solution, but is less affected by salt absorption through leaves (Ehlig, 1961). The rate of foliar absorption of chloride increases in the following order: sorghum cotton, sunﬂowercauliﬂowersesame, alfalfa, sugar beetbarley, tomatopotato, safﬂower (Maas et al., 1982). However, this order does not apply to foliar injury. The relative values of crop sensitivity to foliar injury due to chloride in the sprinkling water are summarized in Table VIII. Because both crop and environmental conditions inﬂuence the injury level, these data constitute only a guideline to irrigation during daytime hours.

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Table VIII Crop Sensitivity to Foliar Injury due to Chloride in Sprinkler Irrigation Watera Cl concentration (mmol/liter) 5 5–10 10–20 20 aBased

Crops exhibiting foliar injury Almond, apricot, citrus, plum Grape, pepper, potato, tomato Alfalfa, barley, corn, cucumber, safﬂower, sesame, sorghum Cauliﬂower, cotton, sugar beet, sunﬂower

on Maas et al. (1982).

Sprinkler irrigation of crops that are less sensitive to chloride is possible provided that steps are taken to avoid or minimize foliar injuries. Such measures might include the use of mobile sprinklers, uniform water distribution, night irrigation, and the scheduling of longer intervals between irrigations (Maas, 1985).

VI. SUMMARY Chloride anions are hardly sorbed to soil particles and are easily leached in soil proﬁles. In acid soils containing variable-charge clays, a slight speciﬁc sorption of chloride is observed. The crop response to Cl varies among genera, species, and cultivars. The lowest critical Cl concentration for plants below which response to Cl addition is observed ranges between 0.1 and 6 mg/g DM or between about 0.03 and 17 mmol/ liter of chloride on the basis of the plant tissue water content. The normally nontoxic Cl concentrations in plants range from 1 to 20 mg/g. The Cl concentration in the plant depends in part on the concentration of Cl in the external solution and its ratio to other anions, particularly nitrate. Chloride compartmentation appears to be highly regulated. In the chloroplast, the Cl concentration remains relatively constant regardless of whether Cl in the soil solution is deﬁcient or excessive. Chloride is required for photosynthesis, charge compensation, and osmoregulation of the whole plant, as well on a single cell basis as in stomatal guard cells. Palms and coconuts use Cl for charge balance in the guard cells. Relatively large amounts of Cl are essential for some crops, such as kiwifruit and sugar beet. Diagnosis of salt toxicity in plants must distinguish between the effects of chloride and those of the accompanying sodium cation. The overall tolerance to high

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concentrations of external chloride is due to the ability of the plant to limit Cl uptake by the roots and its transport to the shoots. Accumulation of Cl in the leaves depends both on its rate of uptake and translocation from the roots to the leaves. In most crops, the accumulation of chloride in the leaves is controlled by the rootstock. Chloride-sensitive cultivars accumulate an excessive amount of Cl in the shoots and tolerant cultivars restrict Cl transport to the shoots by a mechanism that resides in the root. The level of accumulated Cl in the plant should not be regarded as the sole criterion of crop tolerance to chloride. Ammonium stimulates Cl accumulation in plants. Nitrate can prevent Cl toxicity at a concentration of up to 16 mmol Cl/liter in the soil solution. A model for the regulation of Cl inﬂux suggests that both negative feedback effects from vacuolar (NO 3 /Cl) or total anion concentrations and external NO3 inhibition of Cl inﬂux at the plasmalemma may be operating. These combined effects serve to discriminate against Cl accumulation, favoring NO 3 uptake and its subsequent metabolism. The uptake interaction between chloride and phosphorus appears to be complex. Phosphate uptake is stimulated when chloride concentrations in the external solution are low and suppressed when they are high. High levels of NaCl reduce Ca2 and K in the roots and leaves. The interaction of chloride with other plant nutrients needs further study. The mechanisms of the effects of Cl on foliar disease infection are not well understood. The possibility that both climatic and biological factors may interact with the plant response to Cl makes it difﬁcult to interpret plant and soil diagnostic data, except in cases of extreme Cl deﬁciency in the soil. Irrigation water containing Cl at less than 150 mg/liter, with ECs in the range of 1–3 dS/m, can be used for most crops, provided that management practices are taken into consideration. The main fertilizers containing chloride are potassium chloride and ammonium chloride. When the annual application rates of these fertilizers supply less than 140 kg Cl/ha, no negative effects on crop growth or yield are expected.

REFERENCES Adler, P. R., and Wilcox, G. E. (1995). Ammonium increases the net rate of sodium inﬂux and partitioning to the leaf of muskmelon. J. Plant Nutr. 18, 1951–1962. Al-Harbi, A. R. (1995). Growth and nutrient composition of tomato and cucumber seedlings as affected by sodium chloride salinity and supplemental calcium. J. Plant Nutr. 18(7), 1403 –1416. Ali, I. A., Kafkaﬁ, U., Yamaguchi, S., and Inanaga, S. (1998). Response of oilseed rape plant to low root temperature and nitrate:ammonium ratios. J. Plant Nutr. 21(7), 1463 –1481. Anderson, C. A., and Steveninck, R. F. M. (1987). Accumulation and subcellular distribution of sodium and chloride and potassium ions in lucerne populations differing in salt tolerance. Aust. Salinity Newsl. 15, 74 –75. Andersson, B., Critchley, C., Ryrie, I. J., Jansson, C., Larsson, C., and Anderson, J. M. (1984). Modiﬁcation of the chloride requirement for photosynthetic O2 evolution. FEBS Lett. 168, 113 –117.

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OXISOLS S. W. Buol1 and H. Eswaran2 1Department of Soil Science North Carolina State University Raleigh, North Carolina 27695 2Natural Resources Conservation Service U.S. Department of Agriculture Washington, DC

I. Introduction II. Historical Background A. Early European Contributions B. Modern Pedology III. Geography of Oxisols IV. Deﬁnition and Kinds of Oxisols V. Processes and Formation of Oxisols VI. Soil-Landscape Relations A. Sur Americana and Associated Surfaces in Brazil B. Lower Amazon Basin C. Central Zaire Basin D. Tertiary Surfaces of Africa E. Localized Formations on Basic Rocks F. Occurrence on Recent and Subrecent Alluvium VII. Features and Properties A. Mineralogy and Micromorphology B. Structure and Consistence C. Chemistry and Physics D. Soil Color E. Hydrologic Properties F. Nutrient Retention Characteristics G. Fertility Characteristics H. Micronutrients and Heavy Metals VIII. Ecosystem Management A. Forest Ecosystems B. Forestry and Pasture Management C. Modern Agricultural Management IX. Summary References

Soils now known in Soil Taxonomy as Oxisols have been historically identiﬁed as Laterites, Latosols, and various Lateritic soils. Other soil classiﬁcation systems 151 Advances in Agronomy, Volume 68 Copyright © 2000 by Academic Press. All rights of reproduction in any form reserved. 0065-2113/00 $30.00

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S. W. BUOL AND H. ESWARAN identify them as Ferrisols, Kaolisols, and Ferrasols. The geographic distribution of Oxisols is primarily related to geologic materials and stable geomorphic surfaces in intertropical regions. Some oxisols form from in situ rock weathering or easily weathered basic rock, but many form in polycyclic sediments that have undergone weathering prior to deposition. Oxisols are present in all soil moisture regimes, but most occur in perudic, udic, or ustic soil moisture regimes. Chemical properties are dictated by an abundance of low charge clays and sesquioxides. Sand and silt fractions contain few weatherable minerals. All Oxisols have relatively low cation exchange values and some have a net positive charge in the subsoil. Most have lowbase saturation but some, formed from basic parent materials, are highly base saturated. Physical characteristics include low bulk density, high permeability, friable consistence, and low plant available water-holding capacity. There is little micromorphological evidence of clay translocation, and particle-size distribution is nearly uniform with depth in most pedons. Pedogenic scenarios are varied but relate to a relative loss of silica and concentration of iron and aluminum oxides. Many areas of Oxisols remain in natural vegetation. In areas devoid of modern agronomic technology, “slash and burn” agronomic practices provide meager subsistence for sparse human populations. Where modern agronomic infrastructure provides access to markets, natural limitations of acidity and low nutrient content have been overcome economically with lime and fertilizer applications. Robust agronomic production is now a reality on Oxisols. © 2000 Academic Press.

I. INTRODUCTION Geological, landscape-forming and pedological processes govern the composition and characteristics of the earth’s mantle. The net effect of some or many of these processes has created a wide variety of soils on the earth’s surface (Buol et al., 1997). The absence or minimal role of a process is sometimes as important as the prime soil-forming process (Tavernier and Eswaran, 1972). In intertropical areas there has been no large-scale glaciation. Geologic material at the earth’s surface was subjected to alteration and redistribution by water and wind, but mineralogical rejuvenation of the material has been minimal in intertropical areas. As a result, material at the land surface could weather and transform over long periods of time so that many areas now consist only of minerals resistant to subaerial weathering. Soils formed in such materials have distinctive features and are the conceptual basis of Oxisols. Forms of rejuvenation in nonglaciated areas include additions of pyroclastic materials from volcanic activity, aerosolic materials such as loess, and exposure of unweathered materials by a relatively rapid erosion of surface materials. These rejuvenation processes often provide weatherable mineral-rich material that precludes the formation of Oxisols. Many soil-forming processes active in weatherable mineral-rich parent materials are precluded in material lacking such minerals. Processes not unique to the tropical environment but which attain their maximum expression in such environments result in differ-

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entiating soil features. Information on the extent and distribution of Oxisols in the world has increased in recent years but still lags behind our knowledge of several other soils due in large part to the low population density and lack of infrastructure in major areas of Oxisol occurrence where few detailed soil surveys have been conducted. Once considered of low potential for food and ﬁber production, advances in soil management technology have now made it possible to consider Oxisols as viable and sustainable sites for intensive crop and pasture production (Sanchez and Salinas, 1981).

II. HISTORICAL BACKGROUND The evolution in the thinking that led to the current concepts in the understanding of soils and to the deﬁnition and rationalization of the group of soils called Oxisols dates back to the early colonizers of the tropical countries. The quest for producing better raw materials in European countries provided the sense of urgency to enhance agricultural production, and thus an understanding of the resource base for the production on these materials. In modern times, developments in other sciences, such as in physics and chemistry and later in mineralogy, became the basis for quantiﬁcation of pedology. Particularly with respect to the understanding of Oxisols, developments in the study of the colloidal fraction of soils, such as X-ray diffraction analysis, electron microscopy, and thermal and infrared analysis, were essential in establishing the Oxisols as a distinct group of soils (Uehara and Gillman, 1980). Some of the highlights in the historical developments are presented herewith.

A. EARLY EUROPEAN CONTRIBUTIONS It is traditional to associate the beginning studies of soils in the tropics with Buchanan (1807), who described and coined the term “Laterites.” Buchanan, a naturalist, was searching for building materials within the expanding empire of the British East India Company. His contribution is noteworthy due to his vivid observations, his descriptions, and perhaps to the impetus he gave to the study of such materials. Early studies of the highly weathered soils of the tropics were the result of geological investigations to locate sources of bauxite and iron ores. However, such studies were the basis for the present day classiﬁcation for the group of soils called Oxisols. One of the ﬁrst to classify Laterites was Lacroix (1923), who based his classiﬁcation on iron hydroxide content: • true Laterites contain more than 90% hydroxides • silicate Laterites, 50–90% hydroxides • Lateritic class, 10–50% hydroxides

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Harrassowitz (1930) used elemental analysis to differentiate the different kinds of materials and coined the term “Allites” for aluminum-rich materials and “Siallites” for materials with a SiO2 /Al2O3 molar ratio higher than 1.33. Rock weathering and mineral transformation leading to the overlying soil received considerable attention from the early 1930s (Harrassowitz, 1930; Harrison, 1933). The monograph entitled “The Katamorphism of Igneous Rocks” by Harrison (1933) is considered an early major contribution to tropical weathering and soil formation. Robinson (1951) introduced the two terms, Ferrallitic and Siallitic soils, in 1932 and these remained in soil literature until today. The silica/sesquioxide ratio of the soil material was used to differentiate the two kinds of soils by Robinson. These terms would be the precursors of other terms used in international soil classiﬁcation systems, such as Ferralsols (FAO/UNESCO, 1971–1976), Sol Ferralitique (Commission de Pédologie et de Cartographie des Sols, 1967), Ferrisols (Tavernier and Sys, 1965), and Sol Ferrigineaux Tropicaux (CPCS, 1967). The German classiﬁcation of the period used other terms such as Rotlehm and Roterde (Vageler, 1930). The German approach did not emphasize the elemental composition of the material, inherited from the geochemical inﬂuence, but focused on the nature of the soil material. It was also inﬂuenced by the Russian school of Dokuchaev (1898) using the concept of soils as natural bodies, which can be grouped through a classiﬁcation system. In the United States, Marbut (1928), also inﬂuenced by the Russian school, presented his approach to soil classiﬁcation. At the highest level in his system, he considered the universe of soils as belonging to two groups: the Pedocals where calcium dominated the system and the Pedalfers where aluminum and iron inﬂuenced the system. Within the Pedalfers, he identiﬁed Lateritic soils, which he divided as Yellow Earths, Red Earths, Laterites, and Ferruginous Laterites. The period of the second and third decades of the 20th century, inﬂuenced by Russian pedology, subscribed to the N. M. Sibertsev (1901) concept of soil zonality, i.e., groups of soils occurred in a zonal pattern while recognizing that some soils that did not respect the zonal concept as intrazonal and a few soils such as local alluvium, as azonal. Marbut (1935) elaborated on this zonal concept and developed his classiﬁcation in this framework. The “genetic” approach to the understanding of soils was well entrenched by the 1940s (Jenny, 1941). Total elemental composition of the soil material was deemphasized and employed only at lower categorical levels in the many classiﬁcation systems that were subsequently proposed.

B. MODERN PEDOLOGY In response to a request from the Secretary of the U.S. Department of Agriculture in 1936, Baldwin et al. (1938) presented the U.S. classiﬁcation of soils. The system adhered to the zonal concept of soils and was a major improvement in com-

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parison to previous contributions in that it attempted to provide strict deﬁnitions for the classes. However, the system was still subjective and the user did not have guidelines or a “key” for the placement of soils. An approach to develop a key was already attempted by Del Villar (1937), but it appears that scientists of the time had ignored this approach. The approach of Baldwin et al. (1938), popularly known as the 1938 classiﬁcation system, was to inﬂuence all future modiﬁcations, including the current soil taxonomy. The onset of World War II interrupted further developments, but immediately after the war, the Soil Conservation Service (SCS) reinitiated the national soil survey program. After World War II, there was a new interest in the classiﬁcation of soils. The interest resulted from the recognition of the values obtained from documenting the soil resources of the country through detailed soil surveys. In addition, developments in other sciences provided tools for the soil scientists to use. The trend to move away from the use of geochemical (total) elemental analysis and to focus on intrinsic properties of the soil had already begun and new methods of soil analysis were being developed. In the United States, the SCS, the predecessor to the present Natural Resources Conservation Service (NRCS), formalized the national cooperative soil survey program with a goal of documenting the soil resources of the country at a detailed level. There was already recognition that such an organization of all national, state, and local soil surveys was essential for the development of an appropriate resource inventory. The visionary Dr. Charles Kellogg, administrator of the soil survey in SCS, ensured that this vision was implemented (Smith, 1965). Soil scientists in the United States reinitiated the discussions on soil classiﬁcation and a benchmark publication was the special issue of the journal, Soil Science in 1949, devoted to classiﬁcation. In this issue, Cline (1949, and later in 1963) introduced the concepts of “differentiating, accessory, and accidental characteristics” and the notion of “ceiling of independence of a property.” These two concepts were the basis for deﬁning the diagnostic horizons, properties, and features in Soil Taxonomy. Another marked deviation from the past practice was presented by Thorp and Smith (1949) when they declared their intent to disengage themselves from the concept of zonality, “it is hoped that terms indicating zonality or lack of it may be abandoned in favor of terms based on soil characteristics.” Riecken and Smith (1949) introduced the concept of a “soil individual” but related it to a mapping unit on a large-scale map. In the same journal, Cline (1949) also developed the concept of a unit of sampling and characterization that would later be deﬁned as the “pedon.” There was an undeclared recognition in all these papers that to be useful and meaningful, a classiﬁcation of soils should consider all soils of the world (Smith, 1963) and this approach would inﬂuence all future attempts. In the immediate postwar period, attention was given to soils in the tropics. Cline (1975) refers to a meeting held at Washington, DC, on December 15 and 16, 1947. The term Latosol was proposed at this meeting for “all soils having their

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dominant characteristics associated with strong laterization, including such features as low silica–sesquioxide ratios, low base exchange capacity, low activity of the clays, low content of primary minerals, low content of soluble constituents, and with some red colors.” Kellogg (1949) proposed a formal deﬁnition for Latosols with the following characteristics. 1. Low silica–sesquioxide ratios of the clay 2. Medium- to low-cation exchange capacity of the mineral fraction in relation to clay 3. Low content of primary minerals, except for the highly resistant ones 4. Low content of soluble materials 5. Relatively high degree of aggregate stability 6. Red color or reddish hues of other colors 7. No essential horizons of accumulations through additions 8. Relatively thin organic layers above A1 9. Generally low content of silt relative to other separates. Criteria 2, 3, and 7 remain a part of the deﬁnition of the oxic horizon in soil taxonomy. Emphasis was then placed on the morphology of the soil and Kellogg (1949) subdivided the Latosols into Red Latosols, Earthy Red Latosols, Reddish Brown Latosols, Black-Red Latosols, Red-Yellow Latosols, Yellow Latosols, and Groundwater Laterites. The European school continued to build on their previous approaches. Botelho da Costa (1954) split the Ferralitic soils of Robinson into Fersiallitic and Ferrallitic using the silica–alumina ratio of 2 as a limit. Later Botelho da Costa (1959) split the Ferrallitic soils further into Paraferrallitic and Leviferrallitic soils (equivalent to the present day “Hapl” and “Acr” great groups of Oxisols). He also introduced the Psammoferrallitic group (current Psammentic subgroups) for highly weathered soils developed on Kalahari sands in Angola. The French school (Aubert, 1954, 1958) developed a class of “Sols a Hydroxides,” which was subdivided into Mediterranean soils, Ferruginous Tropical soils, and Laterite soils. In 1956, the term Laterite soils was replaced by the term “Ferralitic soils,” which remained until the ﬁnal publication of the French system (CPCS, 1967). The subclasses recognized in the French system (with soil taxonomy equivalents) are Sol Ferralitique Typique (Oxisols), Sol Ferralitique Lessive (Ultisols), and Sol Ferralitique Remanies (no equivalent). The “remanies” or “reworking” concept relates to geomorphology and the accumulation of the material and has not been used in any other soil classiﬁcation system. It, however, is a very useful concept for understanding the genesis of Oxisols. In the 1950s, systematic soil surveys were conducted in the three former Belgian colonies of Zaire (Congo), Rwanda, and Burundi. Their detailed investigations made major contributions to the knowledge of soils in the tropics, particu-

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larly with respect to soils on old stable geomorphic surfaces. The classiﬁcation system developed by the Belgian soil survey (Sys et al., 1961; Tavernier and Sys, 1965; Sys, 1968, 1972, 1983) inﬂuenced other systems of the time. Soils with lowactivity clays (CEC 25 cmol kg1 at pH 7) were considered Kaolisols. They introduced soil moisture and temperature regimes, degree of weathering, and intensity of proﬁle development as criteria for differentiating soil groups. They used the terms “Ferrisols” and “Ferralsols,” where the former will be equivalent to Oxic subgroups of Ultisols and Alﬁsols whereas the latter are equivalent to the order of Oxisols (Smith et al., 1975). The early deﬁnition of the oxic horizon and Oxisols used the criteria developed by the Belgian and French classiﬁcation systems. Attempts to develop a new classiﬁcation system in the United States commenced in 1951 under the leadership of Dr. Guy D. Smith of the Soil Conservation Service. In 1954, as part of the process of developing diagnostic horizons, the “latosolic B” horizon was proposed as the residual concentration of clay size minerals consisting of sesquioxides with or without 1:1 lattice clays, lacking evidence of silicate clay illuviation, with diffuse boundaries, with a silica/sesquioxide ratio of less than 2, and generally with a bulk density of less than 2. By 1954, the concept of the textural B horizon was well understood and so the latosolic B horizon excluded clay illuviation processes. Emphasis on the bulk density resulted from the soil survey of Hawaii in which the present-day Andisols were mapped as latosols (and so a low bulk density). In the Fifth Approximation that was released in 1956, the latosolic B horizon was eliminated as Guy Smith determined that the term Latosols, like Laterites (Maignen, 1966), had too broad a meaning. The Latosolic B horizon was substituted with the “sesquioxide horizon” and the term “plinthite” was introduced and studied later by Alexander and Cady (1962). The sesquioxidic horizon speciﬁcally excluded the illuviation features (clay skins) and stressed the general homogeneity of proﬁle morphology and the friable consistence of the material. Commencement of soil survey activities in Brazil under a program of FAO brought more information toward the understanding of these soils. Bennema et al. (1959) deﬁned the Brazilian latosolic B horizon, and elements of this were incorporated in the Sixth Approximation released in 1958. Working in west Africa, Charter (1958) coined the terms Oxysols and Ochrisols. In the Seventh Approximation released in 1960, the class of Laterisols was replaced by Oxisols, a term which remains in Soil Taxonomy. The Seventh Approximation was the last of the numbered approximations, although other revisions were brought out in the decade of the 1960s. Major changes in the classiﬁcation of Oxisols were introduced in the supplement released in 1967, which contained almost the ﬁnal version of Oxisols as published in soil taxonomy in 1975 (Soil Survey Staff, 1975). One very signiﬁcant change made in the key to orders between the Seventh Approximation (Soil Survey Staff, 1960) and Soil Taxonomy (Soil Survey Staff, 1975) precipitated inter-

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national confusion regarding the identiﬁcation of Oxisols from Ultisols and Alﬁsols. In 1960, Oxisols preceded Ultisols and Alﬁsols in the key, identifying them as “other mineral soils having an oxic horizon . . .” with no mention of the absence of an argillic horizon. In the 1975 key the Oxisol order, which still preceded Ultisols and Alﬁsols, stated “ . . . and do not have either an argillic or a natric horizon that overlies the oxic horizon.” By that time it was well known in the United States that several soils in the southeastern part of the United States had CEC values and weatherable mineral suites that would qualify them as oxic horizons, although classiﬁcation as ultisols had been ﬁrmly established. It was thought that a distinction could be made on the presence of clay skins in the argillic horizons and their absence in oxic horizons. These criteria proved to be extremely difﬁcult and other criteria were needed (Buol and Eswaran, 1988). International committees (ICOMs) were formed (Table I) to address the quest for better criteria to classify soils with low-activity clay (ICOMLAC) and Oxisols (ICOMOX). The reader is referred to the proceedings of workshops conducted by these committees for more detailed information (Beinroth and Paramanathan, 1978; Beinroth and Panichapong, 1978; Beinroth et al., 1983, 1988; Camargo and Beinroth, 1978; Moormann, 1985).

III. GEOGRAPHY OF OXISOLS The speciﬁc mineralochemical composition of the material that produces the deﬁning attributes of an Oxisol requires speciﬁc conditions to form. A number of soil-forming processes must act on the rock or its weathering products and simultaneously a number of processes must be absent or operate at much lower intensities (Tavernier and Eswaran, 1972). These considerations preclude Oxisols from many parts of the world and in many physiographic environments. Areas that, over geological periods, receive materials containing high amounts of weatherable minerals consequently rejuvenate the soil system. Materials such as loess or volcanic deposits or materials that have been geologically reworked and transported without surface weathering, such as glacially derived deposits, are not suitable for Oxisol formation. However, if there was a period of stability after deposition of the material and the climatic conditions are conducive, a low-activity clay system may result. Soil climate is the other major determinant. A perudic or udic soil moisture regime (SMR) and an isohyperthermic soil temperature regime (STR) are thought to provide conditions most suitable for Oxisol formation. Cooler, isothermic soil temperature regimes and some drier, ustic SMRs also have sufﬁciently moist rainy seasons that, with time, Oxisols can form. Once formed, the inert nature of oxic soil material precludes many other pedogenic processes (Buol and Eswaran, 1978).

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Table I Sequence of Events in the History of Oxisol Classiﬁcation Year

Event /Activity

1807 1900 1913 1926 1927 1928 1930 1932 1933 1935 1938 1947 1949

Buchanan introduces concept of Laterite Dokuchaiev introduces concept of soil as natural body Classiﬁcation of Laterites by Lacroix Harrosowitz introduces “Allites and Siallites” Sibertsev introduces zonality concept Marbut presents U.S. classiﬁcation with Pedocals and Pedalfers Vagelar introduces concept of Rotlehm and Roterde Robinson introduces Ferrallitic and Fersiallitic soils Harrison evaluates the weathering proﬁle Del Villar is the ﬁrst to introduce a “key” for classifying soils Baldwin, Kellogg, and Thorp reﬁne the U.S. classiﬁcation The term Latosol is coined in Washington, DC “Soil Science” publishes a special issue on soil classiﬁcation with papers that will shape future U.S. soil classiﬁcation systems Kellogg deﬁnes Latosols; U.S.D.A. Soil Conservation Service embarks on developing a new classiﬁcation system First approximation of the new classiﬁcation distributed in July Second approximation distributed Third approximation distributed Fourth approximation distributed; G. D. Smith attends European meeting on soil classiﬁcation; latosolic horizon concept developed Fifth approximation released; sesquioxidic horizon replaces latosolic horizon; “Laterisols” class introduced; G. D. Smith and R. Tavernier meet with A. L. Leemans (Department of Classics, University of Gent, Belgium) and J. L. Heller (Department of Classics, University of Illinois) to discuss nomenclature Sixth approximation issued C. F. charter introduces terms Oxysols and Ochrisols Bennema and colleagues deﬁne latosolic B horizon in Brazil; J. V. Botelho da Costa deﬁnes paraferralitic and leviferralitic (basis for Haplo and acri) Seventh approximation presented to International Congress of Soil Science U.S. National Cooperative Soil Survey adopts system for U.S. soil survey; Tavernier and Sys introduce “kaolisols” and the 24 mEq/100-g limit Supplement to Seventh Approximation distributed; “Oxisol classiﬁcation lags” Soil taxonomy is published; Frank Moormann appointed as Chairman of International Committee on Soils with Low Activity Clay (ICOMLAC) Hari Eswaran appointed as Chairman of International Committee on Classiﬁcation of Oxisols (ICOMOX) ICOMOX deﬁnes oxic horizon to give priority to oxic over argillic; introduces “kur” great groups, which later become “Kandi” great groups Stan Buol replaces Eswaran as chairman of ICOMOX Kandic horizon deﬁned by ICOMLAC Draft of ICOMOX proposal discussed at Brazil soil classiﬁcation workshop SCS accpets ICOMOX proposal

1950 1951 1952 1953 1954 1956

1957 1958 1959 1960 1965 1966 1975 1979 1980 1982 1984 1986 1987

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The geographic distribution of oxic soil materials, composing Oxisols and associated Kandic great groups of Ultisols and Alﬁsols, is primarily limited to three distinct settings. The ﬁrst is where conducive physiographic and soil climate conditions prevail in the humid tropics and subtropics. There, on stable slopes, slow transformation of the acid igneous rock into soil material with low-activity clay takes place uninterrupted by glaciers over long geologic time periods (Bennema et al., 1970; Beinroth et al., 1974). In such soils, the classical vertical sequence of soil underlain by a saprolite zone, which rests on hard rock, may be seen (Bonifas, 1959; Calvert et al., 1980a,b). The epipedon and subsurface horizons in such soils are generally formed of weathering products of the rocks with few evidences of lateral additions of soil material. Stone lines and other evidences of lithological discontinuities are absent or conﬁned to the surface horizons. The rocks may be geologically old, such as many of the granites and granodiorites of southeast Asia and Brazil. These soils frequently occur as associations of Kandiudults and Kandiudalfs, both of which have low activity clays but differ from Oxisols in that clay translocation and accumulation (lessivage) have been additional active processes. The second setting is where easily weatherable materials, such as basic and ultrabasic rocks, are exposed under a conducive soil climatic environment (Buurman and Soepraptohardjo, 1980). Such Oxisols are present on the stable surfaces of inactive volcanic islands and other old volcanic areas of the tropics (Beinroth, 1982). Occurrences are generally local and their extent is shown only on largescale soil maps. They are usually in association with shallow Inceptisols on steeper slopes and Andisols in more recent volcanic ash. The materials from which the Oxisols are formed may have signiﬁcant alluvial additions and most often occur on the lower parts of the landscape (Eswaran, 1972). Large contiguous areas of Oxisols in South America and central Africa are related to the third mode of formation. These areas are presently on mid- to late-Tertiary surfaces and the material has been subject not only to long periods of weathering and mineral alteration, but it has also been transported and redeposited (Eswaran and Anantharaman 1975; Lepsch and Buol, 1974; Lepsch et al., 1977a,b). Although difﬁcult to establish, the soil to a depth of several meters is an accumulation of several layers of deposits. Each deposit is composed of weathered materials with little mineral diversity, which makes it difﬁcult to establish mineralogical discontinuities. Some of the deposits may have stonelines (Fig. 1) with the stones composed of quartz or other resistant minerals, petroplinthic gravel, or a mixture of both (Van Wambeke, 1974, 1992). The evolution of a soil is strongly controlled by the evolution of the landscape, and soil genetic studies in the absence of a clear understanding of the geomorphic evolution of the landscape lead to erroneous conclusions, as in the early literature. Because of the very strong linkage between landscape and soil, Oxisols are associated with other soils that have a similar colloid composition but have additional features resulting from other signiﬁcant pedogenic processes. Local topographic

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Figure 1 Acrustox with stone line. Photograph was taken in a road cut near Brazilia, Brazil. Vertical cracks are due to exteme drying and are not characteristic of Oxisols in situ.

variations may be the cause of the soil being an Oxisol or Ultisol as seen on the cerrados of Brazil (Moniz and Buol, 1982; Moniz et al., 1982; Lepsch and Buol, 1974; Lepsch et al., 1977a,b). Oxisols also form on weathering products of limestones mixed intimately with alluvial and colluvial additions, in karst landscapes. Such Oxisols have been studied in Puerto Rico (Beinroth, 1982). The impurity of the limestone materials and the hydrological conditions prevailing during the dissolution period are strong determinants of the ﬁnal mineralogical composition of such Oxisols (Jones et al., 1982; Fox, 1982). In Jamaica, gibbsite-rich Oxisols are derived from alumina-rich, silica-poor limestones. Bauxite may be considered as one of the ultimate stages of Oxisol formation; bauxite is considered a rock and not a soil. Table II provides an estimate of the global extent of the 12 orders of soils recognized in Soil Taxonomy. Because of the changes in deﬁnition of soil classes in Soil Taxonomy and the introduction of new orders, Andisols and Gelisols, the areas presented in Table III will differ from previous estimates, such as those of Eswaran et al. (1986, 1995). Figure 2 shows the global distribution of areas dominated by Oxisols. The map is derived from the soil map of the world (FAO/UNESCO, 1971–1976) modiﬁed to incorporate more recent data. As the deﬁnition and concepts of Oxisols have changed since the publication of the soil map of the world, Fig. 2 and Table II

162

S. W. BUOL AND H. ESWARAN Table II Global Distribution of Soil Orders Area ice-free land Soil order

km2

Percentage

Alﬁsols Andisols Aridisols Entisols Gelisols Histosols Inceptisols Mollisols Oxisols Spodosols Ultisols Vertisols

12,725,464 974,511 14,780,086 23,364,480 17,689,941 2,539,790 15,655,393 8,712,394 9,810,846 4,712,633 11,053,834 3,084,855

9.7 0.8 11.3 17.9 13.5 1.9 12.0 6.7 7.5 3.6 8.5 2.4

slightly overestimate the Oxisol. This overestimate results partly from the introduction of the kandic horizon in soil taxonomy and the change in deﬁnition of Oxisols since 1992, which excludes soils with less than 40% clay in the surface horizons and with an underlying kandic horizon from the Oxisols. Oxisols occupy about 7.5% of the ice-free land surface of the world. The Udoxs and Utoxs occupy about 4 and 2.4%, respectively. The largest contiguous extent of Udoxs is in central Africa with a smaller extent in South America. The largest contiguous extent of ustoxs is in South America. The Sahara in the north and the Kalahari deserts in the south in Africa are sources of sands that have drifted into more moist parts of the landscapes and buried former Oxisols. It is not uncommon as in Zambia, Angola, Nigeria, and some countries of the Sahel to ﬁnd “B” hori-

Table III Global Distribution of Oxisol Suborders Area ice-free land Suborder Aquox Perox Torrox Udox Ustox

km2

Percentage

320,065 1,161,980 31,233 5,201,102 3,096,466

0.24 0.89 0.02 3.98 2.37

OXISOLS

Figure 2

163

Global distribution of areas dominated by Oxisols.

zons of soils classiﬁed as “kandi” great groups of Ultisols or Alﬁsols to meet all the charge and weatherable requirements of oxic horizons. Such soils are present in association with Psamments. Quartzipsamments are extensive in parts of the central plateau of Brazil in association with Oxisols. Many of the soils labeled as Torroxs on the map and in Table III are located in the transition areas of ustic and aridic soil moisture regimes (Van Wambeke, 1981, 1982, 1985). The coefﬁcient of variation of annual rainfall is generally between 40 and 60% and so the actual soil moisture regime also varies. In some years, the SMR may be ustic. Statistical analysis of data from a few stations in these transitional zones indicates that there is generally 60% or more probability over a 30year period that the SMR is aridic.

IV. DEFINITION AND KINDS OF OXISOLS Oxisols (Soil Survey Staff, 1994; 1996) are soils with an oxic horizon that has its upper boundary within 150 cm of the mineral soil surface. If the surface 18 cm (after mixing) contains less than 40% clay, the amount of clay content increase within a 15 cm vertical depth must be less than 4% (absolute) if the clay content of the surface horizon is less than 20% or less than 20% (relative) if the surface horizon contains between 20 and 40% clay. If the surface horizon contains 40% or more clay, a rate of clay content increase with depth is not a criteria for the order Oxisols but is criteria for “Kandi” great groups of Oxisols. Kandic horizons are permitted in Oxisols that have 40% or more clay in the surface 18 cm, but must be at least 30 cm thick, have less than 10% weatherable minerals in the 50to 200-m fraction, have a CEC7 of 16 cmol kg1 clay or less, and an ECEC of 12 cmol kg1 clay or less. Speciﬁcally excluded from Oxisols are soils that have spodic horizons, andic soil properties, or organic soil materials (Soil Survey Staff, 1996).

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Oxisols are divided into ﬁve suborders according to their soil moisture regimes. Aquoxs are Oxisols with aquic conditions. Aquoxs are identiﬁed as having one of the following: 1. A histic epipedon 2. An epipedon with a moist color value of 3 or less directly overlying a horizon with a chroma of 2 or less 3. Distinct or prominent redox concentrations within 50 cm of the surface and a. a horizon that is either 50% or more 2.5 Y or yellower in hue or b. contain enough ferrous iron to react positively with alpha,alphadipyridyl within 50 cm of the surface Aquoxs are present in depressions and seepage areas at toe slopes in association with other Oxisols. Torroxs are Oxisols with aridic SMRs indicating less than 90 consecutive days of plant available moisture each year. Of limited extent, Torroxs appear to have formed from previously weathered material or paleoclimates. Ustoxs have ustic SMRs. They have a minimum of 90 consecutive days of plant available moisture in most years so that one crop is possible with natural rainfall. Some Ustoxs have as much as 270 consecutive days of reliable moisture in most years and two crops are possible in most years. Some Ustoxs have a bimodal rainfall pattern where the two growing seasons each year are separated by dry seasons; in some, the two growing seasons are consecutive. Ustoxs have at least 90 consecutive days in most years during which rainfall is too low to permit crop growth. Udoxs are Oxisols with udic SMRs. Less than 90 consecutive days are too dry for reliable crop growth. Two and sometime three consecutive crops can be grown each year. Local cultivators time many of their operations to take advantage of the drier months for burning, in slash and burn management, and to mature grain crops that are subject to disease and spoilage if conditions are too wet as they mature. Peroxs have perudic SMRs in which every month of the year has rainfall that exceeds potential evapotranspiration. These areas present severe problems for the management of many crops. With no dry season it is difﬁcult to harvest grain crops and road transportation is constrained. Also, there is difﬁculty in slash and burn management because of a lack of a reliable time to burn. Although such management is practiced, the completeness of burn is often less than desired.

V. PROCESSES AND FORMATION OF OXISOLS Processes that cause a concentration of aluminum and a depletion of silica favor the formation of Oxisols (Uehara and Keng, 1975). These processes may have their origin in the magma of the earth’s crust as in the formation of minerals with low Si/Al ratios that when exposed to the soil environment persist. Soils subject-

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ed to water that is undersaturated with silica experience a loss of silica. Silica loss is to be expected in the surface of almost all soils as rainwater inﬁltrates and moves through the surface layer. Dissolution of silica from any of the silicate minerals is time dependent (Wilding et al., 1977). As inﬁltrated water moves downward in the soil, silica is dissolved. The amount of silica removed depends on the residence time around the silicate mineral and the type of mineral. If the inﬁltrated water is taken up by actively growing plants, some silica is biocycled but any layer of soil subject to the downward movement of water that is undersaturated with respect to the solubility of the silicate mineral present will experience a net loss of silica. Net silica loss results in a lowering of the Si/Al ratio and kaolinite enrichment of the clay fraction (Carvalho et al., 1983). Desilication and the formation of gibbsite, halloysite, and kaolinite take place on the initial weathering of granite rock and clay suites with the mineralogical composition of oxic horizons are formed in saprolite 5 to 15 m below the soil surface in udic SMRs (Eswaran and Bin, 1978a,b,c; Calvert et al., 1980a,b). Such saprolite material needs only to be disturbed by pedoturbation processes to be recognized as kandic horizons or, if lacking weatherable sand and silt, oxic horizons. Quartz solubility is between 3 and 7 mg liter1, with the rate at which it dissolves increasing as the particle size decreases accounting for the almost universal lack of clay-sized quartz in soil materials subject to leaching. Sand-sized quartz, with less surface area per unit weight, is dissolved less rapidly, accounting for quartz sand stability in leached soil material. Noting this behavior of silica, a mechanism of kaolinite-rich clay and quartz-rich sand material can accumulate in gently sloping landscapes where the surface material is subjected to minimal erosion and redeposition after the transport of short distances. If the soil environment is not subjected to a reducing condition, iron released from iron silicates is concentrated as iron oxides by the dissolution and removal of silica. Almost all of the iron in Oxisols is present as iron oxides, with the iron-bearing silicates having been weathered. Oxisols found on stable landforms and formed in apparently translocated sediments often have relatively high iron oxide contents. The rapid accumulation of eroded surface sediments desilicated previously perhaps accounts for the presence of Oxisols in ﬂood plains as reported in Sierra Leone (Odell et al., 1974). Oxisols are often best known because of the many red, red-yellow, and yellow hues various pedons exhibit. Dark red hues are indicative of hematite mineral, whereas yellow colors indicate geothite. Mixtures of these minerals (Bigham et al., 1978) obtain intermediate colors. Macedo and Bryant (1987, 1989) found that hematite is reduced more easily, and thus dissolved, than geothite. Even short periods of reduction around decaying roots will, over time, cause the removal of hematite, leaving geothite as the dominate iron oxide in the more yellow Oxisols. If an Oxisol or oxic material is subjected to prolonged periods of reduction, the iron oxides are removed as in gray or gley Aquoxs, which are present in poorly drained depressions with high water tables.

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Although Oxisol pedons contain more organic carbon than most other mineral soils (Sanchez et al., 1982; Sanchez, 1987; Eswaran et al., 1993, 1995; Lepsch et al., 1994), much of the organic carbon in the subsoil appears to be relatively unavailable to soil microbes. Couto et al. (1985) observed Ustoxs that developed no low chroma (2 or less) whereas saturated conditions were observed throughout much of the year. They subjected samples to saturated conditions and observed that surface (0 to 40 cm) horizon material was reduced, but subsurface (40 to 80 cm) samples, some containing as much as 12.5 g kg1 organic carbon, did not become reduced until sucrose was added to provide an energy (available carbon) source for the microbes. Areas of Oxisol formation must have minimal rejuvenation of weatherable minerals in the pedon. Rapid burial of Oxisol pedons by volcanic ash or eolian deposits obviously result in a different soil. Slow recharge of an area by aerosols is often detected less easily because pedoturbation processes, especially the activity of ants and termites, tend to homogenize accumulating sediments. Areas of Oxisols are frequently rejuvenated by erosion, which removes oxic material and exposes relatively unweathered material at the land surface (Ruhe, 1956). Relatively level and stable surfaces of Oxisols, surrounded by erosional geomorphic surfaces of Inceptisols, Ultisols or Alﬁsols, are common features of many Oxisol-dominated areas (Camargo et al., 1981; Lepsch and Buol, 1988). The nature of the soils on the associated erosional surfaces depends in large part on the nature of the material being exposed. While Inceptisols and Ultisols are perhaps the most common soils within areas dominated by Oxisols, signiﬁcant areas of Mollisols and Alﬁsols are present where the exposed material is calcareous or basic rock (Lepsch et al., 1977b). The presence of kandic horizons on side slopes below nearly level landscapes of Oxisols is frequently observed in thick formations of oxic materials. On sloping landscapes, usually where slopes exceed 8 to 10%, pedons are present that have clay content increases with depth and kandic horizons, whereas pedons on more level land have little clay content increase with depth. Clay skins are frequently present in these kandic horizons but are absent in the Oxisols. Beinroth et al. (1974) in Hawaii attributed this development to shear processes associated with creep movement on the slopes that disrupted interparticle bonds of the stable oxic material, allowing the clay to be mobilized for lessivage. In Brazil, Moniz and Buol (1982) observed the same relationship but attributed it to seasonal interﬂow of water from the above surface that caused saturation, accompanied by the complete relaxation of interaggregate tension and the formation of blocky structure of lower hydraulic conductivity than the granular structure. The anisotropic hydraulic conductivity within the soil proﬁles on the lower slopes would be responsible for microsite reduction of iron oxides, thus freeing silicate clays for lessivage and clay skin formation on the surface of the blocky peds and increased clay content in the subsoil.

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167

VI. SOIL–LANDSCAPE RELATIONS A. SUR AMERICANA AND ASSOCIATED SURFACES IN BRAZIL Central Brazil, centered at the capital Brasilia, is a classic example of landscape resulting from multiple erosion cycles (Feuer, 1956). The area forms the major divide between the watersheds of the Amazon and Parana rivers. Precambrian crystalline metamorphic rocks underlie the area, but the soils are formed in younger Neocenozoic superﬁcial deposits. Layers of gravelly stone lines are occasionally present in pedons (Fig. 1). Stone lines are most frequent below escarpments and decrease in thickness with distance from the escarpment (Cline and Buol, 1973). Gravel in the stone lines is quartz, ironstone (laterite), or mixtures of the two. Ironstone gravel is more abundant as distance from the center of the divide increases and quartz is more abundant near the divide center. The broad plateau features have plinthite formations near their perimeters that harden into ironstone as erosion cause the scarps to retreat. Ironstone gravel is derived from this ironstone and is deposited on lower planation surfaces. Near the center of the divide the quartz gravel can be attributed to a few Gondwana remnants that rise above the highest planation surface (Lepsch and Buol, 1988). The edaphic savanna or cerrado vegetation in central Brazil is maintained by the nutrient-poor, acid Oxisols, as attested to by the presence of semideciduous forests on rare occurrences of basalt or other basic rock that give rise to higher base-saturated soils (Camargo et al., 1981). Many of these naturally forested areas are no longer visible because local inhabitants, mainly gold miners prior to present farmers, sought these more chemically fertile areas and cleared the trees to grow food crops. Although the central Brazilian planation surfaces are characteristic of the Oxisol–geomorphic associations, smaller areas are present throughout central Brazil, with Oxisols formed in polycyclic (remanie) reworked superﬁcial deposits and studied by several authors cited by Lepsch and Buol (1988). Where the reworked deposits have been derived from basalt or other basic rock Eutrustoxs and Eutrudoxs, often associated with Mollisols and Alﬁsols, are present (Carvalho et al., 1983). In these areas, Eutrustoxs or Eutrudoxs in udic SMRs are most often present on the more gentle slopes below Acrustoxs (acrudoxs). Mollisols and Alﬁsols are often present frequently on the steeper slopes where they may be forming in saprolite from basalt or other basic rock rather than from the superﬁcial deposits.

B. LOWER AMAZON BASIN Oxisols dominate the landscape in the lower Amazon basin but few are present west of Manaus between the Negro and Madeira rivers (Camargo et al., 1981). Most

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S. W. BUOL AND H. ESWARAN

of these Oxisols are identiﬁed as yellow and red-yellow latosols (Latosolo Amarelo and Vermelho-Amarelo Latossolo) of acid reaction (distroﬁco). In Soil Taxonomy, Xanthic subgroups were established to identify their distinctive pale yellow color. Although limited research has been conducted, these Oxisols have lower contents of iron than present in many Oxisols. This probably accounts for the lower phosphorus ﬁxation experience when fertilizer P is applied. It is probable that the deposition of kaolinitic clay in the basin occurred during the time the Andean mountains were being formed and the basin was forced to reverse its discharge from a westward direction and eventually drain into the Atlantic ocean. Saturated conditions in this delta-like environment dictated that iron oxides were reduced and removed. Xanthic Oxisols are formed in clayey, kaolinitic sediments on nearly level surfaces dissected by modern erosional stream valleys. Limited observations have found that the kaolinitic sediments often overlie white sand deposits. Where the sand deposits are exposed in the river valleys, Spodosols and Quartzipsamments, depending on the depth to a spodic horizon that forms at the water table, have formed.

C. CENTRAL ZAIRE BASIN Extensive areas dominated by Oxisols and other soils with low activity clay mineralogy, primarily Ultisols, are present in central Africa. Ruhe (1956) determined that many of the soils formed in transported material most probably of Tertiary age. Eswaran et al. (1975) conﬁrmed the transported nature of these materials with micromorphologic studies and the presence of gravel deposits, stone lines, below the sediment, and the underlying mica schist. Sys (1972) compiled data from 230 pedons in the region, not all Oxisols, and Smith et al. (1975) classiﬁed the Sys pedons according to Soil Taxonomy (Soil Survey Staff, 1975). This work provides a valuable link to the Institut National pour les Etudes Agronomiques au Congo (INEAC) soil classiﬁcation system (Tavernier and Sys, 1965) used by many authors and Soil Taxonomy. Many volcanic mountains bound the Zaire basin on the east. Therefore, many of the soils bordering that area have been enriched by either direct ash fall or alluvium containing volcanic materials (Matungulu, 1992). It is probable that much of the fertility accorded higher elevations in the eastern part of the basin can be attributed to these more recent basaltic materials. The Kalahari desert lies to the south of the basin, and sand from that area is suspected to have inﬂuenced soil textures in the southwest part of the basin (Sys, 1983; Kalima and Spaargaren, 1988).

D. TERTIARY SURFACES OF AFRICA Old stable land surfaces are characteristic of Oxisol landscapes. It is often not possible to determine with certainty that these surfaces are of Tertiary age, but most

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169

authors place them as older than Pliocene or Pleistocene. In Kenya, Oxisols are found in udic and ustic soil moisture regimes, but many have aridic soil moisture regimes (Muchena and Sombroek, 1983). Where formed from materials derived from schist, base saturation is low, but a higher base saturation (“Eutr” great groups) is often present where the parent material has been derived from basic rocks. Oxisols are identiﬁed well into the isomesic soil temperature regime, with sites in Burundi having mean annual temperatures of 11C (Opdecamp and Sottiaux, 1983). The cooler Oxisols often have umbric epipedons and some, with a higher base saturation, have mollic epipedons. Sombric horizons are also commonly present (Frankart, 1983).

E. LOCALIZED FORMATIONS ON BASIC ROCKS Buurman and Soepraptohardjo (1980) described Acrudoxs on stable landscapes on lower foot slopes in Sulawesi, Indonesia. Associated soils on steeper slopes were Rhodudalfs and Inceptisols. These soils formed on materials with an abundance of iron-containing primary minerals are often bright red to dusky red in color. Local farmers call them “tanah Merah” (red soils). Their ease of cultivation and general resistance to erosion are properties liked by farmers. However, their low fertility status, particularly their propensity to ﬁx added phosphorous, makes them difﬁcult to manage. Local farmers use heavy application of farmyard manure for annual crops or interplant with tree crops. Similar occurrences of Oxisols have been reported on the volcanic islands of the Paciﬁc and the Caribbean. In Fiji and western Samoa, Hapludoxs are present with acrudoxs on the toe slopes of volcanic terrain. In New Caledonia, Acrudoxs are formed on ultrabasic rocks; these rocks also yield copper and nickel. In Malaysia, Oxisols are formed on basaltic or andesitic parent materials. The basaltic deposits are about 8 million years old as shown by K-Ar dating (Eswaran and Paramanathan, 1977). A characteristic feature of these Oxisols formed on basic and ultrabasic rocks is the absence of a distinct saprolite zone. At contact with the underlying rock is a rim, a few centimeters thick, that is weathered and frequently rich in smectite. The overlying solum is composed of kaolinite, gibbsite, and goethite clays. Many of these soils on the basic rocks are chocolate colored and were referred to as “chocolate soils” by local inhabitants. In many of the islands of the Paciﬁc, coincidentally, cocoa is the dominant crop on such soils.

F. OCCURRENCE ON RECENT AND SUBRECENT ALLUVIUM Oxisols are present on recent river alluvium where the material deposited is rich in kaolinite and contains few weatherable minerals. Odell et al. (1974) found this in the river systems of Sierra Leone, west Africa, where the upland soils were pri-

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marily Alﬁsols and Ultisols, and attributed the lack of weatherable minerals and the low-cation exchange capacity to preweathering of parent material. Riverine alluvium is generally the parent material for many of the Aquoxs. Upland soils associated with these Aquoxs are usually well-drained Oxisols or Alﬁsols and Ultisols belonging to Kandi great groups. In bauxitic areas, as in Malaysia, Aquoxs are formed on the alluvium from the bauxite deposits. In these soils, rounded gibbsitic nodules are present and are probably deposited as alluvium (Eswaran et al., 1977).

VII. FEATURES AND PROPERTIES A. MINERALOGY AND MICROMORPHOLOGY Weathering and transformation of primary rock-forming minerals and the subsequent alteration of the secondary clay minerals eventually leads to a mineral suite that is, for most practical purposes, “resistant” (M. P. F. Fontes, 1988; M. R. Fontes, 1990). The resistant minerals are frequently the oxide or oxyhydrate forms of the metal elements such as iron, aluminum, and silica. The silicate clay mineral associated with these is usually kaolinite. The common iron minerals are goethite and hematite with maghemite being present in soils derived from basic or ultrabasic rocks (Herbillon, 1980). The aluminum mineral is generally gibbsite; boehmite has been reported but is not normally present in Oxisols (Eswaran et al., 1977). Anatase, rutile, and other very resistant minerals are present in small amounts in the heavy sand fraction but much of the sand is quartz (Herbillon, 1988). These relatively resistant minerals also undergo slow weathering, as shown by the etch surfaces of the minerals when observed under the high magniﬁcations of the scanning electron microscope (Eswaran et al., 1977). The clay fraction (2 m) is composed largely of kaolinite, which is generally disordered and has a higher amount of lattice iron (Fripiat and Gastuche, 1952) than those from geologic deposits (Herbillon et al., 1976). Inclusion of iron or aluminum in the lattice of secondary minerals takes place during the mineral formation stage and results in properties different from the purer form of the mineral (Fey and Le Roux, 1976). Aluminum substitution in goethite has been established since the ﬁrst observations of Norrish and Taylor (1961). Aluminum substitution was found to vary from 17 to 36 mol % for goethite, 6–26 mol % for hematite, and 16–26 mol % in maghemite (Fontes, 1988). Partly resulting from substitutions and also due to the lower crystallinity of the minerals, Oxisol clays have a higher speciﬁc surface area with an associated higher pH-dependent negative and positive charge when compared to their purer and better crystallized forms (Fey and Le Roux, 1976). The surface area of Oxisol kaolinite is about 60 m2g1, approximately four times greater than pure kaolinite.

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171

The relative proportions of kaolinite to gibbsite in Oxisols are quite variable. Kaolinite:gibbsite ratios of 0.5 to 3.14 are present in Oxisols derived from maﬁc (basalt) rocks, schist, and clayey sediments, but pedons with ratios above 9.2 and pedons with no gibbsite are present in Bauru sandstone within the Triangulo Mineiro region of Minas Gerais State, Brazil (Fontes, 1988). Fontes (1990) found that humic acid extracted from Humic and Typic Hapludoxs contained crystalline, highly Al-substituted goethite particles 10–15 nm in diameter. While the surface area of the humic acid/goethite complex is 1 m2g1 or less, the surface area of the goethite is 100 m2g1 or more. She concluded that ligand-exchange mechanisms and Coulombic attraction were responsible. Phosphate adsorption on the goethite was reduced greatly in humic acid/goethite complexes. Another mineral that has been reported in the surface horizons of Oxisols is aluminuous chlorite or hydroxy interlayered mineral (HIM). Some suspect that it is inherited from the rock and preserved in the soil (Flach et al., 1969), whereas Lelong and Millot (1966) subscribed to a process they term as retrogenesis, whereby a mica mineral is formed through soil processes. In a review of the literature, Barnhisel (1977) attributed these very resistant clays to either incomplete degradation of chlorite or depositions of hydroxy material within the interlayers of expansible layer silicates. Greater HIM abundance near the surface and lack of chlorite in subsoils would indicate that the hydroxy interlayering process is more probable in most Oxisols. The presence of amorphous or short range ordered minerals has been speculated for a long time due to the high speciﬁc surface areas and the presence of a pHdependent charge. Opaque masses in transmission electron microscopy were considered as the amorphous component, but Segalen (1968) and Gallez et al. (1976) showed that this form is a minor component. More recent nuclear magnetic resonance studies and other supporting data show the presence of cryptocrystalline goethite and hematite in the clay fraction. Unlike iron minerals, aluminum minerals are usually very well crystallized and present dominantly in the ﬁne-silt fraction (Eswaran et al., 1977). The characteristic mineral association—disordered kaolinite with sesquioxides—imparts many of the deﬁning characteristics of Oxisols. These include stable aggregate structure, nutrient-holding characteristics, water-holding characteristics, and general response to management. These features are elaborated later. Other soils may have one or more of these characteristics, but the combination of all these characteristics throughout the soil proﬁle distinguishes Oxisols from other soils. The microfabric characteristics also show major differences from other soils and have been well documented by Stoops (1968), Buol and Eswaran (1978), and others. Clayey Oxisols with a low sand content have a homogeneous fabric with few modiﬁcations of plasma. Stress-induced features are absent and cutanic features resulting from clay translocation, such as ferriargillans, are rare. This uniform ma-

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trix may extend to depths of several meters in Oxisols formed from sediments. Relics of pedogenesis, from which the transformations may be interpreted, may be present. Such relics include weathered rock fragments, nodules and concretions of iron with or without manganese, embedded cutans in which the clay aggregates have lost their optical properties, and preserved biological features. There are Oxisols where pedogenesis results in speciﬁc horizons, and in thin sections of these soils, special features may be observed. Many Oxisols on old geomorphic surfaces are formed on materials with laterite or petroplinthite fragments. The petroplinthites are generally composed of well-crystallized goethite (Eswaran and Raghumohan, 1973) and may also have inclusions of manganese and iron minerals (Eswaran et al., 1978). The petroplinthite may incorporate fragments of ferriargillans that are fossilized features, indicating that the petroplinthite was formed in another environment and during another period (Eswaran et al., 1978). Gibbsitic nodules are also frequent in such soils and the gibbsite crystals are usually well crystallized (Eswaran et al., 1977). A range of other features may be present in Oxisols and these are well documented (Brewer, 1964; Stoops, 1968; Bennema et al., 1970; Comerma and Chirinos, 1976; Eswaran and Tavernier, 1980). Oxisols with isothermic soil temperature regimes often have organic-enriched horizons extending to a meter or more in depth. Apart from making the fabric opaque, the high amounts of humus do not impart other features to the soil. When a water table is present, the redoximorphic condition results in a net removal or reorganization of the iron. In many Aquoxs, the lower horizons are bleached as the soil material is devoid of staining iron and have low contents of dithionite–citrate–bicarbonate (DCB) extractable iron (FeDCB) (Table V). Under certain conditions, the reorganization of iron results in the formation of plinthite, with the fabric showing iron-rich and iron-impoverished zones. In the zone of water table ﬂuctuation, nodules and concretions of iron with or without manganese also form. The plinthite, nodules, and concretions are all precursors of petroplinthite (Eswaran and Raghumohan, 1973) that form on the permanent lowering of the water table as rivers entrench and hydrology of the landscape is altered. Hardening of plinthite to petroplinthite takes place in situ as the water table deepens. As erosion of the landscape progresses, the petroplinthite is exposed and the most resistant nodules and concretions behave as gravel when transported and deposited in stone lines at a new site (Alexander and Cady, 1962; Cline and Buol, 1973). By deﬁnition, the weatherable mineral content of the 50- to 200-m fraction in the oxic horizon must contain less than 10% weatherable minerals. Feldspars, feldspathoids, ferromagnesian minerals, glass, micas, zeolites, and apatite are considered weatherable minerals (Soil Survey Staff, 1996). The determination of weatherable mineral content is most often made by optical grain counts (National Soil Survey Center, 1996). Also, a total elemental analysis ﬁnding less than 25 cmol of Ca2+, Mg2+, K+, and Na+ kg1 soil offers a viable estimate of weatherable mineral content less than 10% (Herbillon, 1988).

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173

B. STRUCTURE AND CONSISTENCE Oxic horizons, being sandy loam or ﬁner by deﬁnition, usually have a minimum of 15% clay and may contain 80% or more clay (Table IV). Despite the wide range of clay content that Oxisols may have, their physical properties are determined by the sesquioxides and the poorly ordered kaolinite mineralogy. In a slightly dry state, the soil has a very friable to friable consistence (Soil Survey Division Staff, 1993). The ﬂoury ﬂow of the soil material between the ﬁngers was an early observed property of the soil. Some form of subangular blocky structure may be discerned in the oxic horizon in the ﬁeld, but the grade of blocky structure is very weak. The blocky structural elements break abruptly when slight pressure is applied by the thumb and foreﬁnger, revealing a strong ﬁne and very ﬁne granular structure (Soil Survey Division Staff, 1993). The microfabric of the ﬁne and very ﬁne granular materials is very porous and is well established by water retention studies. The high porosity leads to a low bulk density, which is generally between 1.0 and 1.3 Mg m3 (El Swaify, 1980). The high sesquioxide content also imparts other features. Oxic material with a high oxide content is generally not very sticky. Even the quartz grains appear clean with little or no soil material sticking to them. However, Xanthic Hapludoxs with low oxide content and more than 60% clay, most of which is less than 1 m, are very sticky and very plastic. Oxide-rich material is also hydrophobic to some extent. Vigorous rubbing for several minutes is required to destroy the very ﬁne granules that feel like sand when particle-size estimates are made by “hand texturing” in the ﬁeld (Cline and Buol, 1973). Water moves rapidly through the large pores between the ﬁne and very ﬁne granules. The combination of high porosity and low wettability makes the soil resistant to erosion. Oxisol landscapes are stable unless poor management initiates erosion and soil loss. Due to the low shrink–swell capacity of the material, biologic features in the soil are generally well preserved. Evidence of worm and other soil faunal activities is preserved and may be seen in the ﬁeld or in thin sections. In some Oxisols, the preponderance of such relict features has caused some to suggest that such soils are continuously biologically reworked (Stoops, 1968), speciﬁcally by termites. They have also used this to explain the absence of other pedological features, such as cutans.

C. CHEMISTRY AND PHYSICS The mineralogy of Oxisols, particularly those with high amounts of sesquioxides, speciﬁcally iron oxides and oxyhydrates in the ﬁne earth fraction, imparts some unique properties to these soils. These properties are used as deﬁning characteristics of Acric (Acr) great groups. Aluminum oxyhydrate minerals, such as gibbsite, are inert and do not appear to contribute to these special features.

Table IV Physical Properties of Selected Pedons Water retention (%) Classiﬁcation Typic Acraquox (Brazil)

Humic Rhodic Eutrustox (Brazil)

Typic Kandiperox (Indonesia)

Anionic Acrudox (Puerto Rico)

Depth (cm)

Horizon

0 –10 10 –30 30 – 48 48–77 77–90 0 –25 25– 40 40 – 64 64 –110 110 –210 0 –10 10 –21 21–51 51–81 0 –28 28– 46 46 –71 71–97 97–120 120 –155

A1 Ag Bog1 Bog2 Bov Ap AB Bo1 Bo2 Bo3 Ap1 Ap2 Bo1 Bo2 A1 B1 Bo1 Bo2 Bo3 Bo4

Particle size (%)

B density (Mg m3)

0.03 MPa

1.5 MPa

Sand

Silt

Clay

Soil (color)

1.3 1.4 1.3 1.3 1.4

32.5 21.8 28.7 29.4 27.3

1.2 1.2 1.1 1.1 0.9 0.9 1.0 0.9 1.1 1.2 1.1 1.3 1.4 1.3

32.4 30.8 32.2 31.4 42.0 39.5 48.0 50.9 35.4 26.7 34.4 35.7 31.6 29.8

26.9 17.2 21.1 23.1 21.9 23.8 23.3 24.0 24.6 24.6 26.5 26.6 31.8 32.6 26.5 22.8 24.8 25.9 26.4 24.5

33.1 34.5 25.1 44.5 62.3 18.0 16.8 10.9 13.3 18.5 13.2 11.4 7.0 6.1 9.2 7.4 9.8 23.3 17.0 19.2

10.5 11.6 8.8 13.5 11.8 41.2 37.9 25.1 28.4 38.1 28.9 28.7 20.7 18.5 36.3 34.9 30.6 21.0 23.3 27.2

56.4 53.9 66.1 42.0 25.9 40.8 45.3 64.0 58.3 43.4 57.9 59.9 72.3 75.4 53.8 54.5 57.7 59.6 55.7 59.7

10YR 6/1 10YR 7/1 10YR 7/1 10YR 8/2 10YR 5/8 2.5YR 3/2 2.5YR 3/2 2.5YR 3/2 2.5YR 3/2 2.5YR 3/2 5YR 3/3 5YR 4/3 5YR 3/4 5YR 3/4 2.5YR 2/4 2.5YR 2/4 2.5YR 2/4 7.5YR 3/8 7.5YR 3/4 7.5YR 3/4

OXISOLS

175

With increasing iron oxyhydrate content, usually about 10% Fe2O3, and when the organic matter content is low, the soils attain a net positive charge. Organic matter has a very high negative charge and a very low zero point of net charge (ZPNC) and, when present in high amounts, imparts a net negative charge to the soil. For this reason, the surface horizons of Oxisols always have a net negative charge and if there are sufﬁcient positive charge minerals in the soil, the net positive charge is expressed in deeper layers (Van Raij and Peech, 1972). pH, which is the difference between pH in 1 M KCl solution and pH in H2O, is used to express the sign of the charge. When pH is negative, the net charge is negative and vice versa. The pH at ZPNC, termed pHo, is a more reliable measure but is tedious to measure and so pH is used as a surrogate value (Uehara and Gillman, 1980; Uehara, 1995). Table V shows pH values and pH, and Fig. 3 shows a plot of pH values in H2O, KCl, and pHo with depth in four kinds of Oxisols. When pH is zero, the system has a net zero charge, and this point differentiates the soil into two distinct parts. The upper part of the soil, where organic matter contents are greater, as in the case of the Anionic Acrudox and Typic Acraquox in Fig. 3 and Table V, the soil has a net negative charge and can be considered as a cation exchanger. Fertilizer elements such as Ca2+ and K+ are exchanged by the H+ ion on the exchange surfaces and are retained for plant uptake. However, in horizons where pH is positive, the soil behaves as an anion exchanger. Here the cations may leach readily and anions such as nitrates and sulfates are captured. In some Oxisols, anion exchange may extend to a depth of several meters. The positive charge of subsoils minimizes the leaching of NO 3 and the potential contamination of groundwater. This is an important feature of Oxisol landscapes. In other soils, nitrate leaching and pollution of the aquatic system are environmental concerns. Erosion and loss of organic matter may result in the positive charge being expressed closer to the surface.

D. SOIL COLOR Oxisols come in all colors. Although hues of red, red-yellow, and yellow are most common, the poorly drained Oxisols subjected to gleization are gray, reﬂecting the color of kaolinite, quartz, and gibbsite. Some Oxisols are nearly black due to the high contents of organic matter. Bigham et al. (1978) found that DCB iron was most concentrated in the ﬁne clay fraction (2 m) of oxic and kandic horizons. Proportions of hematite and goethite in the ﬁne clay fraction were indicated by color hue. No hematite was found in samples of 7.5 or 10 YR hue. Hues between 2.5 and 5 YR contained between 25 and 75% of both hematite and goethite. Soil hue of 5 R contained more than 75% of its iron as hematite and less than 25% as goethite. Resende (1976) found that the 10 YR hue of an Oxisol changed to a 5 YR hue when 1% by weight of ﬁnely powdered hematite was added. Eswaran and

Table V Chemical Properties of Selected Pedons Charge (cmol/kg1)

pH Classiﬁcation Typic Acraquox (Brazil)

176

Humic Rhodic Eutrustox (Brazil)

Typic Kandiperox (Indonesia)

Anionic Acrudox (Puerto Rico)

Horizon

pH0

H2O

KCl

O. C. (%)

A1 Ag Bog1 Bog2 Bov Ap AB Bo1 Bo2 Bo3 Ap1 Ap2 Bo1 Bo2 A1 B1 Bo1 Bo2 Bo3 Bo4

4.2 4.3 5.3 6.8 6.8 5.4 5.9 5.4 5.7 5.9 4.1 3.8 4.4 4.9 3.5 3.8 4.4 6.2 6.7 7.1

4.8 4.9 5.5 6.0 6.0 6.6 6.5 6.8 6.9 7.1 4.9 4.8 5.2 5.3 5.1 5.0 5.0 5.2 5.5 5.7

4.5 4.6 5.4 6.4 6.4 5.8 5.8 5.9 6.1 6.4 4.5 4.3 4.8 5.1 4.3 4.4 4.7 5.7 6.1 6.4

0.3 0.3 0.1 0.4 0.4 0.8 0.7 0.9 0.8 0.7 0.4 0.5 0.4 0.2 0.8 0.6 0.3 0.5 0.6 0.7

2.4 1.6 0.9 0.6 0.4 2.8 2.2 1.2 0.9 0.5 2.0 1.5 1.9 0.5 6.0 2.0 1.3 0.9 0.7 0.6

Base saturation (%)

FeDCB (%)

ECEC

CEC7

CEC8.2

CEC7

CEC8.2

Al saturation (%)

0.3 0.2 0.4 1.3 1.4 14.3 14.7 14.3 14.9 14.8 5.6 5.7 5.6 5.8 13.0 12.9 16.5 19.2 23.1 25.7

1.9 1.2 0.1 0.2 0.1 15.8 13.7 8.0 6.1 4.1 5.9 4.4 5.4 6.0 7.9 1.7 0.0 0.0 0.2 0.0

6.8 4.8 2.0 1.4 1.0 17.6 15.4 9.0 6.6 4.3 16.9 15.4 15.1 14.3 25.4 12.1 8.2 6.4 5.3 3.8

13.6 10.1 6.3 5.3 4.9 26.7 23.6 15.9 13.5 11.1 25.5 22.9 23.4 21.3 34.8 21.5 15.7 12.8 12.1 12.8

3.0 4.0 5.0 14.0 2.0 90.0 89.0 89.0 92.0 95.0 30.0 15.0 34.0 42.0 11.0 1.0 0.0 0.0 2.0 0.0

1.0 2.0 2.0 4.0 1.0 59.0 58.0 50.0 45.0 35.0 20.0 10.0 22.0 28.0 8.0 0.0 0.0 0.0 0.0 0.0

89.0 83.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 15.0 48.0 6.0 0.0 17.7 52.9 0.0 0.0 0.0 0.0

Figure 3 Salt and water pH values, zero point of charge, and charge characteristics with depth in different Oxisols. 177

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S. W. BUOL AND H. ESWARAN

Sys (1970) found a general positive correlation between iron oxide content and increased red hue, but this was not true in the samples analyzed by Bigham et al. (1978).

E. HYDROLOGIC PROPERTIES A high amount of iron oxyhydrates, indicated earlier, also results in marked effects on physical properties of the soil. Aggregation of the colloidal fraction into silt-sized, microaggregates is a common phenomenon and is easily observed in micromorphological studies. Due to aggregation into strong, ﬁne, and very ﬁne granular structures, pore sizes are predominately large and small with few pores of intermediate size. The pattern of water release with tension is very much like that of sand, with the bulk of the water released at tensions less than 0.1 MPa (Table VI). The amount of available water retained between ﬁeld capacity, 0.033 or 0.010 MPa negative pressure, and permanent wilting, 1.5 MPa negative pressure, is low (Table VI). Plant-available water, calculated as the water retention difference between 0.033 and 1.5 MPa in some 20 pedons studied by ICOMOX, was between 0.05 and 0.10 cm cm1 in most horizons with some surface horizons being as high as 0.15 cm cm1. The large pores formed by the strong granular structure provide for rapid water movement at saturation. Field observations conﬁrm that although the soil is clayey, it behaves like sand when it comes to retaining water. Double-ring inﬁltrometer measurements in eutrustoxs containing 70% clay were reported between 50 and 108 cm hr1 in undisturbed forested sites, with the higher rates measured at a depth of 12 cm after removing the surface (Moura Filho and Buol, 1972). Measurements on sites that had been cultivated for 15 years found that the rates were between 10 and 16 cm hr1, indicating a decreased size of the large pores as a result of compaction. Wolf (1975) found inﬁltration rates to range from 17 to 22 cm hr1 on irrigated Haplustoxs containing 45% clay in the surface horizon.

Table VI Water Retention of Oxisols (Weight Percentages)a

Soil ISCW-18 80–156 cm ISCW-17 23–40 cm ISCW-3 4–22 cm ISCW-10 12–30 cm aSelected

Clay (%)

Total pores (%)

0.01 MPa (%)

0.03 MPa (%)

0.1 MPa (%)

0.2 MPa (%)

1.5 MPa (%)

80 52 31 16

63 55 44 39

34.7 26.1 16.7 11.3

30.6 24.2 14.1 8.5

26.6 20.0 12.6 6.5

25.9 19.1 11.5 6.2

24.6 16.5 10.3 5.0

from the International Committee on Classiﬁcation of Oxisols (ICOMOX) (1988).

OXISOLS

179

In addition, iron oxyhydrates are also water repellent. Rain or irrigation water on the soil (a) moves rapidly through the soil due to the high amount of macropores, (b) is not absorbed by the aggregates due to the hydrophobic nature of the colloids, and (c) if any is absorbed, it is not retained due to the very low waterholding capacity. Although Oxisols have a very low available water-holding capacity per volume of soil, the low CEC facilitates the rather rapid downward translocation of calcium, and the rooting depth of aluminum-sensitive crop plants can be increased by gypsum applications. Ritchey et al. (1980) found signiﬁcant decreases in Al saturation to depths of 75 –90 cm in Haplustoxs following gypsum applications, which is now a common practice in central Brazil because the deeper root system decreases the risk of drought damage during rainless periods during the growing season (Lopes, 1996). The preceding discussion might suggest that “Acr” great group Oxisols behave like pure sands. Observations in the ﬁeld indicate that the onset of drought is not immediate. Detailed studies and accurate monitoring of the moisture content in the macropores (Sharma and Uehara, 1968a,b; Tsuji et al., 1975) have shown that, despite the rapid dewatering, the atmosphere in the macropores remains humid for prolonged periods. This is attributed to a distillation process taking place and maintaining a high humidity in the pores. Perhaps for this reason, roots show a preferential accumulation in the macropores and few ﬁne roots penetrate the microaggregates. The physics of water movement in the “Acr” great group Oxisols is yet to be clariﬁed. There are many processes operating and the system is not an easy candidate for modeling. It sufﬁces to state that the hydrologic behavior of clayey “Acr” great group Oxisols is distinctly different from soils with high negative charge aluminasilicate minerals.

F. NUTRIENT RETENTION CHARACTERISTICS Oxisols have a low capacity to retain cations. Because the CEC arises from kaolinite clays and organic matter, it is pH dependent, and effective cation exchange capacity (ECEC) values in ambient acid soil are considerably less than CEC7 and CEC8.2 values (Table IV). Exchangeable Ca2+, Mg2+, and K+ contents may total from as much as 15 cmol kg1 in the surface horizon of some Eutrustroxs to less than 1 cmol kg1 in the surface horizon of some Acrustoxs and always decrease with depth (Soil Management Support Services, 1988). Oxisols with substantial contents of iron oxide have a high ﬁxation capacity for P. Oxisols are notorious for ﬁxing fertilizer P because they have higher iron oxide contents in their surface horizons than any other kind of soil (Smyth and Cravo, 1992). The phosphorus ﬁxation capacity in Oxisols is directly related to the surface area and clay content of the soil material (Bigham et al., 1978; Fontes, 1988) and inversely to SiO2 /R2O3 and SiO2 /Al2O3 ratios (Curi and de Camargo, 1988).

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G. FERTILITY CHARACTERISTICS Oxisols have low quantities of essential nutrient elements (Melgar et al., 1992; Moraghan and Mascagni, 1991). This does not infer that they are infertile for an individual crop but rather their inability to sustain continued crop production without fertilization. Deﬁnition of the oxic horizon limits Oxisols to less than 10% weatherable minerals and, due to the low CEC, they have low total quantities of Ca, Mg, and K. Most Oxisols have low total quantities of P, but some with high quantities of iron have rather substantial total amounts, most of which are quite insoluble and unavailable for plant uptake. Because almost all Oxisols are located in isothermic and isohyperthermic STR, they have no seasonal limitations for crop growth. Seasonal availability of moisture ranges from too little to too much as identiﬁed by the suborders.

H. MICRONUTRIENTS AND HEAVY METALS Data are not extensive enough to characterize micronutrient and heavy metal contents in all Oxisols. Lopes and Cox (1977a) analyzed 518 topsoil samples collected under natural vegetation in central Brazil. Although the soils were not classiﬁed, almost all were thought to be Oxisols. They found soil test values below “critical levels” considered necessary for crop production with respect to Zn, Cu, and Mn in 81, 70, and 37% of the samples, respectively. Iron contents were considered adequate. From the same samples they determined a positive relationship between Zn, Mn, and Cu contents and the density and height of woody vegetation (Lopes and Cox, 1977b). There are sufﬁcient data from other studies to alert scientists that Zn, Cu, Mn, B, and Mo deﬁciencies can be expected. As in all soils the plant availability of these elements is related to soil pH values, and liming Oxisols to pH 7 or higher decreases the availability of Zn, Cu, and B (Moraghan and Mascagni, 1991). Limited data are available on the total heavy metal content of oxic horizons, but representative data selected from Ker (1995) are given in Table VII. Most contents are above means reported from other soils (Holmgren et al., 1993).

VIII. ECOSYSTEM MANAGEMENT A. FOREST ECOSYSTEMS Slash and burn subsistence farming has been and continues to be practiced by indigenous peoples in forested areas throughout the world. Indigenous settlements

181

OXISOLS Table VII Heavy Metal Content (mg/kgⴚ1) of Oxic Horizonsa Clay (%)

Co

Ni

Cu

Zn

Mn

Cr

Cd

Pb

77 69 76 76 75 66 60 83 68

95 91 5 26 101 18 22 37 19

69 66 13 75 71 11 36 57 11

174 365 43 80 183 21 45 265 16

102 120 40 52 62 19 42 86 43

791 929 164 141 526 78 209 472 96

80 — 93 118 76 — 169 63 18

8 40 7 8 10 40 9 7 10

95 17 103 111 104 12 50 75 92

aAfter

Ker (1995).

tend to concentrate along rivers that afford transportation. Should roads be constructed, there is an almost immediate proliferation of settlements as people seek to avail themselves of the transportation afforded. In areas of unrestricted land availability, most settlements are small. They usually utilize ﬂood plains for part of their food requirements, but clearing and planting ﬂood-free uplands mitigate the ever present danger of crop loss from ﬂooding. If the upland areas are Oxisols, or equally infertile Ultisols, the following scenario is practiced. An area of land is selected near the housing site on which there is an ample amount of vegetative growth. All of the trees and undergrowth are cut and allowed to dry. The season of the year with the least rainfall is selected for burning. The drier the felled vegetation, the hotter the ﬁre and thus the less logs to interfere with planting. Selection of a site for clearing is a matter of individual judgement and care is practiced by experienced cultivators to select areas with the greatest volume of vegetation. During clearing, some of the largest trees may be avoided if they have limited access to chain saws and must utilize hand cutting only. It is not an easy job to fell dense jungle. What the experienced slash and burn cultivator is looking for is the greatest amount of ash that will result from the burn. Greater quantities of nutrient elements are frequently obtained from smaller dense stands of trees than from stands of huge trees. The burning is frequently an almost ceremonial occasion. At approximately noon, preferably on a day of low humidity and wind, the entire area of dried biomass is ﬁred as rapidly as possible. The heat of the ﬁre creates an intense column of smoke and air rushes in from all sides, creating intense winds in and surrounding the area of ﬁre. If conditions are good, the intense ﬁre is over in less than an hour and does not spread to the surrounding growing vegetation. Some logs may continue to burn slowly for several days or until there is sufﬁcient rainfall to extinguish them. Within a few days the planter will enter the area, often with

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nothing more than a wooden planting stick, to create a small hole in the soil surface. Seeds of corn, rice, or other grain are placed in the hole and covered with a deft compacting human foot, thereby creating a good seed-to-soil contact. Unburned logs are simply avoided, although some may be moved to create paths. In ustic soil moisture regimes the planting is at the beginning of the rainy season, and even in udic soil moisture regimes, planting precedes the season with the most reliable rainfall. What is planted the ﬁrst growing season after the burn differs among slash and burn cultivators but usually it is the most desirable food crop, most often rice (Oryza sativa) or corn (Zea mays). Weed growth is minimal in part because the ﬁre destroys most of the surface seeds, and by limiting surface disturbance few more deeply placed weed seeds will geminate (Mt Pleasant et al., 1992). Upon maturity the crop is harvested and a second crop is planted. This may follow immediately in udic soil moisture regimes or be delayed until the onset of the next rainy season in ustic soil moisture regimes. The second crop will most often be a legume such as peanut (Arachis hypogea), winged bean (Psophocarpus tetraglobulus), or cowpea (Vigna unguiculata). The planting procedure is much the same and often slower growing food crops such as yam (Dioscorea species), cassava (Manihot esculenta), or plantain (Musa paradisiaca) are interplanted. The slower growing crops tend to compete with the weeds that by this time are becoming more of a problem and also are able to utilize any nitrogen contained in the legume crop residue left on harvest. Also, the slower growing crops have a slower rate of nutrient uptake from the soil where, after the harvest of two crops, available nutrient contents are at a low level. In parts of southern Africa where the volume of aboveground biomass is not great, subsistence farmers practice a variation of slash and burn management called “chitemene.” On Haplustoxs, Acrustoxs, and associated soils, farmers pile branches or entire trees cut from adjacent areas into an area of about 0.25 ha until they have sufﬁcient volume to acquire the needed ash on burning. Just prior to the rainy season, when the material is dry, they burn to acquire the inorganic forms of nutrients necessary for their crop (Kalima and Spaargaren, 1988). Because the needed biomass is gathered from the area immediately surrounding the area to be planted, to minimize the labor required, an interesting pattern of “bulls eyes” is created with the cropped circle surrounded by a larger circle from which the woody biomass has been removed. Smyth and Bastos (1984) on Xanthic Hapludoxs near Manaus, Brazil determined the amount of nutrients in the ash following the burning of virgin forest and 12-year old secondary forest (Table VIII). Note that the amount of nutrients in the ash is not proportional to the amount of ash. The primary forest consisted of a large amount of large trees, the wood of which has a lower nutrient element concentration than younger, smaller trees (T. J. Smyth, personal communication, 1997). Smyth and Bastos (1994) compared the nutrients in the ash with prior analysis of

183

OXISOLS Table VIII

Nutrient Levels (kg haⴚ1) in Ash after Burning Two Forest Types near Manaus, Brazila

Vegetation Virgin forest 12-year forest aAfter

Ash dry (T ha1)

N

Ca

Mg

K

P

Zn

Cu

Fe

Mn

9.2 4.8

80 41

82 76

22 26

19 83

6 8

0.2 0.3

0.2 0.1

58 22

2.3 1.3

Smyth and Bastos (1984).

the vegetation they burned. They found that approximately 90% of the nitrogen and lesser amounts of other elements were lost in the ﬁre, but increases in soil pH and soil test values with reductions in aluminum saturation resulted from the ﬁre (Table IX). The nutrient content of forest biomass is reported to have typical ranges of 100–600 kg N ha1, 10–40 kg P ha1, 200–400 kg K ha1, 150–1150 kg Ca ha1, and 30–170 kg Mg ha1. During burning, 88–95% of the N, 42–51% of the P, 30–44% of the K, 33–52% of the Ca, and 31–40% of the Mg remain in unburned biomass or are lost (Fernandes and Souza Matos, 1995; Sanchez, 1976; Andriesse, 1987). Although these losses are great, the increased pH, weed suppression, and immediate return of inorganic elemental forms readily available for crop use, coupled with the enormous human labor that would be required to reduce the biomass via composting or mulching techniques, clearly indicate why burning is used by indigenous cultivators.

B. FORESTRY AND PASTURE MANAGEMENT Humans have and are harvesting high-grade timber products from forests growing on Oxisols. Rosewood (Dalbergia spruceana) and mahogany (Swietenia macrophilla) are among the most sought after species (Peck, 1982). Most plantations have introduced exotic species such as Pinus caribaea, Gmilina arborea, and Eucalyptus deglupta. Introduction of legumes at the time of planting can serve to enrich the soil with N, but tree crops quickly shade most legume species. Sanchez et al. (1985) reviewed tree crop production in the tropics, including several studies on Oxisols, and concluded that most tree crops were able to maintain soil nutrient levels during growth but that nutrient losses took place when the trees were harvested. Only limited data exist but indications are the second rotations of tree crops probably require fertilization for satisfactory growth on nutrient-poor soils containing few weatherable minerals. Pastures on Oxisols frequently form an intermediate stage in land use. Cattle grazing is often a way to establish ownership of land with minimum management. Extensive areas of Oxisols have been cleared of forest to make way for cattle,

Table IX Chemical Properties of Hapludox Topsoil before and after Burning Two Forest Typesa

Vegetation Virgin forest 12-year forest

a

Time

pH

Cab (cmol liter1)

Before After Before After

4.2 5.3 4.7 5.2

0.1 2.1 1.7 2.3

Mgb (cmol liter1)

Alb (cmol liter1)

ECEC (cmol liter1)

Al saturated (%)

C (g kg1)

Pc (mg kg1)

Kc (mg kg1)

0.3 0.8 0.9 0.8

1.8 0.6 1.0 0.3

2.3 3.7 3.8 3.8

78 16 26 8

33 31 34 28

2 6 3 6

22 106 65 151

After Smyth and Bastos (1984). Extracted with 1 N KCl. c Extracted with 0.025 N H2SO4 and 0.05 N HCl. b

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185

which offer the absentee investor some income while awaiting the introduction of infrastructure needed for more intensive agriculture. Native savannas on Oxisols have nutrient deﬁciencies, most often Ca and P, that often result in animal health problems. These are frequently corrected by direct mineral supplements in the animal diet. The most common pastures are Panicum maximum without legumes or fertilization, and an annual live weight gain of only 100 kg ha1 can be expected (Sanchez, 1987). Within 5 years productivity declines due to nitrogen and phosphorus deﬁciency. Increased carrying capacity can be obtained by woody species cutting, burning, broadcasting 25 kg ha1 P, and planting aluminum-tolerant species such as Brachiaria humidicola and Pueraria phaseoloides (Serrao et al., 1979). The use of grass–legume pastures enhances nitrogen nutrition, but maintenance is difﬁcult and probably requires annual fertilization and liming (Sanchez, 1987).

C. MODERN AGRICULTURAL MANAGEMENT The cerrado (savanna) area of central Brazil occupies approximately 204 million hectares of which 50% is considered arable. Approximately 46% of the area is occupied by Acrustoxs and Haplustoxs (Lopes, 1996). Wright and Bennema (1965) reported almost no intensive agriculture and further stated that farmers attempting to exploit nutrients stored in the organic residues of cerrado vegetation by traditional methods were discouraged by negligible yields. By 1992 the cerrado area, with only 10 million hectares cultivated, contributed 20 million tons of grains (28% of Brazilian production) and accounted for 43% of the soybeans, 3% of the wheat, 14% of the dry beans, 24% of the rice, 9% of the sugarcane, and 21% of the coffee produced in Brazil (Fundacao Instituto Brasileiro de Geograﬁa e Estatistica, 1993, as cited in Lopes, 1996). Macedo (1995) estimated that with the use of available technology it is feasible for farmers to obtain 3.2 t ha1 year1 grain or with improved pastures 200 kg ha1 year1 meat in the cerrado. This remarkable utilization of Acrustoxs and Haplustoxs is possible when Ca is deeply incorporated or surface applied, preferably as CaSO4 (gypsum), which is translocated rapidly downward in low CEC soils (Ritchey et al., 1980; Ritchey and Sousa, 1997). The Ca2+ replaces Al3+ on the exchange sites and sulfate replaces OH from the hydrated oxides of Fe and Al, resulting in the formation of hydroxy Al, which reduces Al saturation percentage and enhances root growth in the subsoil. Smyth and Cravo (1992) determined that Al saturation less than 27% of the ECEC was critical for corn and that Al saturation less than 54% was critical for peanuts on Xanthic Hapludoxs near Manaus, Brazil. Deeper rooting means more available water for the crop simply because a greater volume of soil is utilized. Deep rooted crops are less vulnerable to drought during the growing season. The amount of lime needed is not great due to the low ECEC and low amount of extractable Al.

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The Allic families were established in Soil Taxonomy to identify Oxisols with more than 2 cmol kg1 of KCL extractable Al and therefore require higher lime application rates than most Oxisols (Lopes and Curi, 1988). Approximately 1 ton of lime (CaCO3) Ha1 is required for each cmol kg1 of extractable Al (Kamprath, 1984). Phosphorus application is imperative, with initial applications of 100 to 200 kg P ha1 or more needed to overcome initial P ﬁxation in some Oxisols (Goedert and Lobato, 1988). Although the initial cost of P is high, it is a onetime capital investment. Once the ﬁxation capacity is satisﬁed, annual rates of P fertilizer are no greater than in other soils, i.e., replacement of P harvested (Lopes, 1996). Several scenarios have been used to meet the P requirements (Goedert and Lobato, 1988; Curi and Lopes, 1988; Van Raij, 1988; Lopes, 1996; Smyth and Cravo, 1990). Potassium fertilization is needed at rates related to yield on Oxisols as on all soils (Gill and Kamprath, 1990). Nitrogen fertilization differs little from other soils; however, NO 3 leaching may be slowed by a net positive charge in some Oxisols (Melgar et al., 1992). Several micronutrients, most often Zn, B, Cu, and Mo, are required in small quantities on some Oxisols. Sulfur is frequently deﬁcient but is easily supplied often as gypsum (CaSO4). Compaction by cultivation has been found to create dense, root-restricting, conditions and abet erosion, especially in very ﬁne textured Oxisols (Stoner et al., 1991). In some Oxisols, bulk density as low as 1.1 Mg m3 restricted the development of some crop roots. Alleviation of soil compaction through subsoiling or chiseling is effective, but expensive. Lepsch et al. (1994) monitored the long-term effects of moderate to high input farming on 77 sites, paired to represent natural and farmed Peroxs, Ustoxs, and Udoxs in Sao Paulo, Brazil. They found no signiﬁcant decreases in the 0- to 20cm organic carbon contents in Peroxs, Hapludoxs, and Haplustoxs, but slight decreases, signiﬁcant at the 0.05% level, in Acrudoxs and Acrustoxs. Signiﬁcant increases in exchangeable Ca2+ were found in all Oxisol subsoils (60–100 cm) where high input farming was practiced. Signiﬁcant increases in base saturation and decreases in organic carbon content and exchangeable Al3+ and K+ were detected in Udox and Ustox subsoils from farmed sites. Acrudox and Acrustox subsoils had signiﬁcant increases in exchangeable Mg2+ under farmed sites. Farming did not signiﬁcantly alter subsoil pH values. Fertility management of any soil for intensive crop production must be site speciﬁc. On Xanthic Hapludoxs near Manaus, Brazil, total grain yields obtained from 1 hectare, during 8 years with soil analysis-based fertilizer and lime management, corresponded to yields from 24 hectares under shifting cultivation (Cravo and Smyth, 1997). Utilization of Oxisols for intensive food crop production is no longer limited by a lack of sustainable technology but rather by a lack of roads, railroads, storage facilities, electricity, economic infrastructure, and readily available technical services such as soil testing within regions of Oxisols (Lopes, 1996; Wade et al., 1988).

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187

IX. SUMMARY Oxisols are loamy and clayey soils containing few weatherable minerals. Formed in materials long subjected to subaerial environments, they are composed mainly of kaolinite, quartz, and oxides of iron and aluminum. Their chemical and physical properties represent one extreme of the soil spectra. Most Oxisols are located in warm intertropical areas with annual rainfall adequate for abundant plant growth. Natural forest or grassland vegetation belies their limited supply of essential nutrients and they have a very limited ability to replenish essential nutrient elements exported in the harvest of food crops. For this reason, only sparse human populations have inhabited areas where Oxisols dominate the landscape. Access to transportation and scientiﬁc inquiry is limited, and modern soil management technology is only recently available in limited areas. Experience to date indicates that the challenges Oxisol properties present to human endeavors can be overcome with the application of sound, scientiﬁcally based management. The deep, warm, moist, friable, and chemically poor environment of Oxisols, now largely unexploited by humans, presents humankind with alternatives of preservation, conservation, utilization, and exploitation in the future.

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CROP RESIDUES AND MANAGEMENT PRACTICES: EFFECTS ON SOIL QUALITY, SOIL NITROGEN DYNAMICS, CROP YIELD, AND NITROGEN RECOVERY K. Kumar and K. M. Goh Soil, Plant, and Ecological Sciences Division Lincoln University Canterbury, New Zealand

I. Introduction II. Crop Residues and Their Uses A. Deﬁnition of Crop Residues B. World Crop Residue Production and Utilization III. Decomposition of Crop Residues A. Factors Affecting Crop Residue Decomposition B. Methods of Studying Crop Residue Decomposition C. Modeling Decomposition of Crop Residues IV. Crop Residues and Management Practices A. Effects of Residues and Management on Soil Quality B. Responses of Crop Growth and Yield C. Effect of Residue Management on the Environment V. Soil Nitrogen Dynamics and Crop Nitrogen Recovery A. Nitrogen Mineralization/Immobilization Turnover B. Crop Nitrogen Recovery VI. Nitrogen Beneﬁts to Subsequent Crops A. Grain Yield and Nitrogen Responses B. Fertilizer Nitrogen Responses C. Responses of Cereals to the Antecedent Legume D. Relative Contribution of Fixed and Nonﬁxed Nitrogen to Crop Nitrogen Responses E. Role of Legume in the Gain or Drain of Soil Nitrogen VII. Conclusions References

This review reveals that crop residues of common cultivated crops are an important resource not only as a source of signiﬁcant quantities of nutrients for crop production but 197 Advances in Agronomy, Volume 68 Copyright © 2000 by Academic Press. All rights of reproduction in any form reserved. 0065-2113/00 $30.00

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K. KUMAR AND K. M. GOH also affecting soil physical, chemical, and biological functions and properties and water and soil quality. When crop residues are returned to the soils, their decomposition can have both positive and negative effects on crop production and the environment. Our aim as agricultural scientists is to increase the positive effects. This can only be achieved with the better understanding of residue, soil, and management factors and their interactions, which affect the decomposition and nutrient release processes. Data on nitrogen beneﬁts and nitrogen recoveries from residues show that a considerable potential exists from residues, especially leguminous residues, not only in meeting the N demands of the succeeding crops, but also in increasing the long-term fertility of the soils. In addition, crop residues and their proper management affects the soil quality either directly or indirectly. Intensive cropping systems are very diverse and complex, so no one residue management system is superior under all situations. Ideally, crop residue management practices should be selected to enhance crop yields with a minimum adverse effect on the environment. It is suggested that in each cropping system, the constraints to production and sustainability should be identiﬁed and conceptualized to guide toward the best option. Multidisciplinary and integrated efforts by soil scientists, agronomists, ecologists, environmentalists, and economists are needed to design a system approach for the best choice of crop residue management system to enhance both agricultural productivity and sustainability. © 2000 Academic Press.

I. INTRODUCTION Present-day agriculture evolved as we controlled nature to meet our food and ﬁber needs and to support the increasing population and urbanization of society. In the last thousands of years, the struggle for survival in the seemingly hostile natural environment led to the dominant role of human beings in exploiting soils for increasing food and ﬁber production. Ancient writings from early civilizations in Mesopotamia, Greece, and India suggested that nutrient supply to the soil, to replenish its lost fertility or to augment its productivity, is a time-honored practice that started after the settled agriculture struck roots. The unabated use of chemical fertilizers in the last four to five decades has led to the paralleled corresponding decline in the use of cover crops and organic manures (Power and Papendick, 1985). Increased monoculture production of cash grain crops and greater reliance on the importation of chemical fertilizers and pesticides to maintain crop growth have resulted in greatly increased grain yields and labor efficiency. However, these conventional management practices have led to the decline in soil organic matter (SOM), increased soil erosion, and surface and groundwater contamination (Reganold et al., 1987). Until recently, we failed to recognize the consequences of management on the balances and cycling of energy and matter and soil productivity (Goh and Nguyen, 1992). Awareness of the environmental aspects of soil quality and crop production has been increasing in recent years, which has led to the renewed interest in crop residues,

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green manures, and other organic manures as sources of SOM and nutrients for crops. Crop residues are a tremendous natural resource—not a waste. Residue management is receiving a great deal of attention because of its diverse effects on soil physical, chemical, and biological properties. The quantities of nutrients that can be returned annually to soils as residues of common cultivated crops are considerable, requiring worthwhile consideration. Biological nitrogen ﬁxation (BNF) by leguminous crops and the recycling of ﬁxed N when leguminous crop residues are returned to the soil can be a signiﬁcant source of N to the soil organic N pool as well as for subsequent plant uptake. The amount of N that recycles into agricultural ﬁelds through residues may add 25 –100 Tg of N year1 into agricultural soils (Mosier and Kroeze, 1998). Thus, there is a need to determine sink sizes and turnover rates of different quality residues and to increase the efﬁciency of nutrient cycling from residues through different soil sinks, and eventually to growing plants, with minimum loss from the system. This may involve development of practices to enhance the immobilization of nutrients when plants are not growing, as well as practices to increase nutrient availability when plants are actively growing. Information on the kinetics of decomposition of the crop residues and mineralization–immobilization turnover of different quality residues (leguminous and nonleguminous) is required to ascertain the actual amount of crop residues needed to maintain the soil productivity and to ensure environmental protection by minimizing nutrient losses and soil erosion. Addition of organic matter (OM) to the soil through the return of crop residues also improves soil structure, inﬂuences soil water, air, and temperature relations, helps control runoff and erosion, and makes tillage easier.

II. CROP RESIDUES AND THEIR USES A. DEFINITION OF CROP RESIDUES Crop residues, in general, are parts of the plants left in the ﬁeld after crops have been harvested and thrashed or left after pastures are grazed. These materials have at times been regarded as waste materials that require disposal but it has become increasingly realized that they are important natural resources and not wastes.

B. WORLD CROP RESIDUE PRODUCTION AND UTILIZATION According to Lal (1995) in the United States, annual crop residues produced by 19 principal crops are estimated as 400 million tons year1, compared with 2962

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million tons year1 produced in the world. The total amount of principal nutrients in crop residues ranges from 40 to 100 kg tons1. Major nutrients (N P K) contained in crop residues amount to 9 million tons year1 in the United States and 74 million tons year1 in the world. Although large amounts of nutrients are removed in harvested grain, signiﬁcant amounts are still contained in the plant residues after harvest (Table I). The management of crop residues has important implications for the total amounts of nutrients removed from the soil. A considerable quantity of fertilizers can be saved by returning crop residues produced to the soil. In addition, approximately 1.5 Pg (1 Pg 1015 g) of carbon (C) is stored in the crop residues produced in the world, which can be an important source of OM added to the soils. Crop residues can be used for improving soil health and productivity and are a major source of lignocellulose entering the soil. Lignocellulolysis can have positive or negative effects on crop productivity, and the challenge is to enhance the positive value of plant residue decomposition at the expense of the negative value (Smith et al., 1992). In developing countries, most crop residues are used as animal feed and housing material and in industrial uses such as paper making (Stumborg et al., 1996; Latham, 1997; Powell and Unger, 1997). Only a part of crop residues is used as soil amendment.

III. DECOMPOSITION OF CROP RESIDUES The decomposition of crop residues is a microbial-mediated progressive breakdown of organic materials with ultimate end products C and nutrients released into the biological circulation in the ecosystem at both a local and a global scale. Crop Table I Amounts of Chemical Constituents Found in 1 Tonne of Residues of Three Cereal Straws, Maize Residues, and Pasture Herbage a Approximate content (kg ton1 residue)

Nitrogen Phosphorous Potassium Sulfur Calcium Magnesium aFrom bNot

Barley

Oats

Wheat

Pasture

Maize

4.6 0.4 14.3 1.4 2.6 0.8

5.9 0.6 23.3 1.1 1.4 0.5

6.9 0.8 13.5 1.3 1.8 0.8

21.2 2.7 24.6 2.0 5.6 1.4

8.4 2.0 16.5 ndb nd nd

Fraser and Francis (1996). determined.

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residues represent only a fraction of total C in soil but these decompose very rapidly. It has been estimated that about half of the global CO2 output from soil originates from the decomposition of annual litter fall (Coûteaux et al., 1995). However, there is a vast pool of stable OM in the soil, but this decomposes very slowly—over centuries or millennia (Campbell et al., 1967; Jenkinson and Rayner, 1977; Goh et al., 1984; Parton et al., 1987). Coûteaux et al. (1995) stated that plant residue decomposition involves two simultaneous and fundamental processes: the concomitant mineralization and humiﬁcation of C compounds by microorganisms and the leaching downward in the soil of soluble compounds, whose C and N are progressively mineralized and immobilized. According to Gregorich and Janzen (1998), in natural ecosystems the decomposition synchronizes with plant growth and C and other nutrients are utilized in the system with maximum efﬁciency. However, disturbance of these ecosystems may retard or accelerate the decomposition process relative to the other ecosystem processes and may lead to the deterioration of some of the components of the ecosystems. An understanding of this process will help to ensure the proper management of this important resource.

A. FACTORS AFFECTING CROP RESIDUE DECOMPOSITION Residue decomposition processes are controlled by three main factors: (i) kind of plant residues, (ii) edaphic factors, and (iii) residue management factors. Edaphic factors are dominant in areas subjected to unfavorable weather conditions, whereas plant residue factors largely play a role as regulator under favorable environmental conditions. Many of these factors are not independent as a change in one factor may affect other factors. For example, high soil moisture may result in lower soil temperature and aeration and surface residue application may affect soil moisture and temperature simultaneously. Because of these strong interactions, it is often difﬁcult to isolate the effects of speciﬁc environmental factors on residue decomposition. 1. Crop Residue Factors a. Residue Particle Size Ground plant material has often been used for convenience in the study of plant residue decomposition because of their uniform substrate. There is controversy regarding the effect of plant residue particle size on the rate of residue decomposition, and mineralization–immobilization turnover (MIT) of N in the soil (Angers and Recous, 1997; Jensen, 1994b; Sorensen et al., 1996). Small particles may decompose faster than larger particles because of the increased surface area and greater disper-

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sion in soil increasing the susceptibility to microbial attack due to lack of ligniﬁed barrier tissue (Summerell and Burgess, 1989), especially if residues are not penetrated readily by fungi and bacteria (Amato et al., 1984; Jensen, 1994b; Nelson et al., 1996; Angers and Recous, 1997). Although soil fauna are responsible for only a small proportion (10 %) of soil respiration (Juma and McGill, 1986; Anderson, 1991), they play an important role in increasing the rate of decomposition by comminuting and redistributing OM, making it more accessible to microbial attack. For example, Curry and Byrne (1992) reported a 26–47% faster decomposition of straw in mesh bags allowing earthworms than in bags that excluded earthworms during a 8- to 10-month period of decomposition. However, the microbial biomass and products formed during the initial decomposition of small particles may be better protected against further decomposition due to a more intimate mixing of the mineral soil (Van Schrevan, 1964; Saggar et al., 1996; Skene et al., 1996). In addition, an enhanced exposure to nonsoluble carbohydrates materials can lead to N immobilization (Van Schrevan, 1964), and an enhanced surface area can lead to exposure to more phenolic substances that are known to inhibit decomposition (Vallis and Jones, 1973; Fox et al., 1990). Ambus and Jensen (1997) reported that the higher microbial activity associated with the initial decomposition of ground plant material was due to a more intimate plant residue–soil contact, but in the long term, grinding of plant residues had no signiﬁcant effect on N dynamics. The effect of plant residue particle size on MIT may thus be an interaction between clay and silt content, secondary metabolic products, plant residue chemical composition, period of decomposition, and faunal activity. The management of residue particle size and the degree of mechanical destruction may thus be important for the conservation of N in agricultural systems. b. Age of Residue The chemical composition of most crop plants changes dramatically during their growth period (Luna-Orea et al., 1996). As the plant matures, its content of protein and water-soluble constituents decreases steadily, whereas the amount of hemicellulose, cellulose, and lignin increases. In general, water-soluble fractions (sugars, organic acids, proteins, and part of structural carbohydrates) are degraded ﬁrst (Reber and Scharer, 1971; Knapp et al., 1983a,b) followed by structural polysaccharides (cellulose and hemicellulose) (Harper and Lynch, 1981) and then lignin (Harman et al., 1977; Stout et al., 1981; Collins et al., 1990b). Consequently, the residue of immature plants generally decomposes more readily than those of older plants (Wise and Schaefar, 1994; Cortez et al., 1996) and, as a result, releases more nutrients (Luna-Orea et al., 1996). c. Leaf Toughness Physical leaf toughness affecting residue decomposition has received little attention. Gallardo and Merino (1993) developed a toughness index of residue and

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proposed leaf toughness as an index of substrate quality. Silica content is responsible for leaf toughness and has been reported to affect the digestibility of plant material and their decomposition (Goering and Van Soest, 1970; Ma and Takahashi, 1989). In general, the greater the silica content, the slower the decomposition. d. Desiccation The drying of crop residues before incorporation is a common procedure in N mineralization studies. Heat drying plant materials even at low temperatures between 50 and 60C can produce analytically signiﬁcant increases in lignin concentration because of the production of artifact lignin via a nonenzymatic browning reaction that involves plant N (Goering and Van Soest, 1970; Moore et al., 1988). This resulted in a signiﬁcant reduction in N mineralization from the residues compared to fresh or freeze-dried residues (Moore et al., 1988). Likewise, greater N mineralized from fresh compared to freeze-dried clover residues has been reported (Breland, 1994). 2. Crop Residue Quality Plants contain 15–60% cellulose, 10–30% hemicellulose, 5–30% lignin, 2– 15% protein, and soluble substances, such as sugars, amino acids, amino sugars, and organic acids, which may contribute 10% of dry weight (Paul and Clark, 1989). Plants also contain cutin (Gallardo and Merino, 1993), polyphenols (Tian et al., 1995b), and silica (Goering and Van Soest, 1970). The rate of organic matter breakdown depends on the relative proportions of each of these fractions, such as soluble sugars, cellulose, hemicellulose, and lignin (Stout et al., 1981). Hagin and Amberger (1974) reported that the half-lives of sugars, hemicellulose, cellulose, and lignin were 0.6, 6.7, 14.0, and 364.5 days, respectively. It has long been recognized that the fractional loss rate declines with time (Minderman, 1968; Jenkinson, 1977; Mellilo et al., 1989; Andren et al., 1990; Bending et al., 1998), and this decline reﬂects the decline in the quality of the remaining substrate. a. C/N Ratio and Nitrogen Content Crop residues contain about 40 – 50% C on dry weight basis, but their N content varies considerably, causing the variation in C/N ratios. It is generally accepted that residues with a wide C/N ratio decompose more slowly than those with a narrow C/N ratio (Parr and Papendick, 1978), and plant residues with high N content show high decomposition rates and nutrient release ( Janzen and Kucey, 1988; Douglas and Rickman, 1992). Highly signiﬁcant correlations among N content, N release, and biomass loss have been reported by many workers (Frankenberger and Abdelmagid, 1985; Mellilo et al., 1982; Neely et al., 1991; Giller and Cadisch, 1997). Other studies have also reported the importance of initial N con-

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tent for determining residue decomposition (Aber and Mellilo, 1980; Berendse et al., 1987; Janzen and Kucey, 1988; Vigil and Kissel, 1991). A high N content of residues reduces competition of available N by microorganisms and consequently enhancing the decomposition by maintaining high microbial activity. Bangar and Patil (1980) noted that an addition of N to lower the C/N ratio of wheat straw (75:1) signiﬁcantly resulted in liberation of more CO2 than control. Oh (1979) also reported an enhanced rate of crop residue decomposition on the addition of farmyard manure. Christensen (1986) showed that 44% of straw (0.92 % N) decomposed during the ﬁrst month but that only 7% of the straw (containing 0.4% N) decomposed during the same period of incubation. DeHaan (1977) found no correlation between percentage N of added plant tissue and the rate of decomposition. Jensen (1989) noted that green tops and fresh roots of pea plants (3.9% N) decomposed quickly. Douglas et al. (1980) reported 15% weight loss in 60 days for wheat residues with initial N contents 5.5 g kg1 and 30% when N contents were 5.5 g kg1, whereas Reinstern et al. (1984) reported that 1.13% N straw decomposed 2.3 and 1.6 times faster than 0.18 and 0.79 % N straws, respectively. The threshold C/N ratio, above which decomposition is suppressed, is often about 20 to 30. However, C/N ratios and N contents have not always correlated well with decomposition rates and better explanations are needed. Reinstern et al. (1984) postulated from their studies using extraction and leached straw samples that microbial biomass production and wheat straw decomposition rates in the early stages were largely dependent on the size of the water-soluble C pool. Crop residue decomposition based on available C and N seems to relate more closely to ﬁeld observations than decomposition based on total C and N. Available C for microbial decomposition has been estimated for different plant residues, which correlated with decomposition (Mtambanengwe and Kirchmann, 1995). Although the N content and C/N ratio of crop residues are useful in predicting residue decomposition rates, these should be used with some caution as the C/N ratio reveals little on the availability of C and N to microorganisms. Any factor that increases the rate of decomposition, and hence the N demand, tends to increase the threshold N concentration (lower the threshold C/N ratio). For example, a more favorable climate and higher rates of residue application with a greater amount of readily available C in the substrate would stimulate greater microbial activity, increase N demand, and increase the threshold N concentration. The information generated from laboratory studies conducted in a more favorable environment would therefore provide misleading estimates of threshold C/N ratios or may even overstate the impact of N content on ﬁeld residue decomposition rates (Dendooven et al., 1990). b. Lignin The role of lignin as an inhibitor in the decomposition process has been elucidated in several studies (Meentemeyer, 1978; Berendse et al., 1987; Fox et al.,

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1990; Vanlauwe et al., 1996; Hammel, 1997; Giller and Cadisch, 1997). Lignin is known to be a recalcitrant substance, highly resistant to microbial decomposition (Mellilo et al., 1982), and only relatively few microorganisms can degrade lignin and these are exclusively aerobic (Jenkinson, 1988). Many workers have found that increasing the lignin concentration reduces the decomposition rate and nutrient release from plant residues and also enhances nutrient immobilization, especially N (Aber and Mellilo, 1982; Aber et al., 1990; Tian et al., 1992a). Fogel and Cromack (1977) have shown that the lignin concentration of the substrate was an excellent index for predicting rates of decomposition and weight losses of forest litter samples. Muller et al. (1988) and Rutigliano et al. (1996) found that the lignin concentration was a much better predictor of the residue decomposition rate than N concentration. c. Polyphenols Vallis and Jones (1973) suggested that polyphenols bind to protein and form complexes resistant to decomposition. Polyphenols can also bind to organic N compounds (amino acids and proteins) in leaves, making N unavailable, or bind to soluble organic N released from leaves, forming resistant complexes in the soil (Northup et al., 1995). Polyphenols also inhibit enzyme action (Swain, 1979). Sivapalan et al. (1985) found lower net N mineralization from tea leaves with high soluble N and high polyphenol content in comparison with those with high soluble N but low polyphenol content. Jensen (1989) reported that the top growth of legumes was among the most rapidly degradable plant materials because of being high in protein and low in lignin and other inhibitors such as polyphenol compounds. The importance of polyphenols in residue decomposition and the mineralization process has been debated frequently (Swift et al., 1979). In some studies, ployphenols and ployphenol/N and (lignin polyphenol)/N ratios have been correlated with residue decomposition and nutrient release, whereas in other studies, N content, lignin content, and lignin/N ratios were better correlated with residue decomposition and N release (Haynes, 1986; Fox et al., 1990; Palm and Sanchez, 1991; Vigil and Kissel, 1991; Thomas and Asakawa, 1993; Constantinides and Fownes, 1994a). d. Combined Chemical Composition Herman et al. (1977) and Tian et al. (1995b) found that the decomposition rate of plant residues could not be predicted from individual property of the organic material such as C/N ratio, lignin content, or carbohydrate content, but when combined these properties could accurately predict relative rates of decomposition for a broad range of plant residues. According to Berg and Agren (1984) and Janzen and Kucey (1988), residue decomposition occurred in two phases. Phase I is relatively rapid and is dependent

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on the initial residue N content ( Jama and Nair, 1996), whereas phase II decomposition is relatively slower and is regulated by lignin and polyphenol decomposition (Berg, 1986; Jama and Nair, 1996), which shows little differences in the residue decomposition rate regardless of initial N content because soluble, easily decomposable components have already been utilized by microbes or lost by leaching (Reinstern et al., 1984; Christensen, 1986; Smith and Peckenpaugh, 1986; Collins et al., 1990a; Douglas et al., 1990). Summerell and Burgess (1989) found no relationship between the amount of a chemical component and the rate of straw decomposition other than the higher percentage of water-soluble compounds in barley straw, which may be due to phase II decomposition. The distinction between these two phases occurred when the “lignocellulose index” [LCI ratio of lignin/(lignin cellulose)] reached a value of about 0.7 (Mellilo et al., 1989). When this LCI value has been reached, the composition of decaying material remained unchanged and the decay was determined by environmental factors. Both C and N dynamics were broadly described by this two-phase model (Mellilo et al., 1989). These sequential patterns of residue utilization can result in a shift in the relative variable controlling decomposition and nutrient mineralization (Mellilo et al., 1989). Berg and Staff (1980) showed a shift from nutrient and soluble C control in the early stages of decomposition of Pinus sylvestris needles to the dominance of lignin as the controlling factor in later stages. Similarly, it has been shown that the polyphenol/N ratio may serve as a short-term index for green manures, whereas (lignin polyphenol)/N provides an index of long-term release for more woody and naturally senescent material (Palm, 1995). Microbial, particularly fungal, succession on decomposing litter reﬂects changes in litter composition, as do fauna with recognition of phases in palatability and interaction with microﬂora (e.g., Scheu and Wolters, 1991; Van Wensem et al., 1993; Hammel, 1997). A consequence of this is that correlations between rate of mineralization or nutrient loss and simple expressions of the initial composition of litter will have limitations (Heal et al., 1997). These changes are not only restricted to chemistry. For example, Gallardo and Merino (1993) distinguished an initial leaching phase in which the leaf toughness and toughness-to-P concentration of the original litter provided the best prediction of mass loss; in contrast, the cutinto-N ratio and cutin concentration were the best predictors in the postleaching phase (Palm, 1995). 3. Methodological Problems Associated with Residue Quality Characterization In recent years, several attempts have been made to quantify residue quality and its relationships with residue decomposition, mostly in terms of N mineralization (Palm and Sanchez, 1991; Oglesby and Fownes, 1992; Kachaka et al., 1993; Bend-

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ing et al., 1998). Although general trends have been observed, no unique relationship has been developed (Vanlauwe et al., 1997). This is partly due to different methodologies and approaches used by different workers. a. Extraction Methods for Lignin and Polyphenols Different methods are used for extracting polyphenols from plant tissues. Amounts of total ployphenols extracted from plant tissues varied from 30 to 90% according to a method used (Swain, 1979; Anderson and Ingram, 1989; Quarmby and Allen, 1989; Constantinides and Fownes, 1994b; Vanlauwe et al., 1997). Tedious and inaccurate methods of proximate analysis obscured the biochemical composition. The advent of more sophisticated techniques (e.g., variants of mass spectrometry) allows the rapid and sensitive characterization of organic materials, thus enabling their degradation and synthesis to be followed (Sanger et al., 1996; Heal et al., 1997). b. Age of Plant Residues and Molecular Size of Polyphenols The concentration of polyphenol is generally greater in mature residues than in green leaves (Fox et al., 1990; Palm and Sanchez, 1991; Thomas and Asakawa, 1993), and polyphenols have different properties with respect to binding N-containing compounds, depending on their molecular weights (Scalbert, 1991). These explained why ployphenols correlated with decomposition and N release in some studies but not in others (Fog, 1988; Fox et al., 1990; Palm and Sanchez, 1991; Thomas and Asakawa, 1993; Vanlauwe et al., 1996). c. Methods of Determining Decomposition Rates Different particle sizes of crop residues and methods of determining residue decomposition (viz. direct application to soil or application in mesh bags) are known to affect residue decomposition rates (Summerell and Burgess, 1989; Fox et al., 1990; Constantinides and Fownes, 1994a; Magid et al., 1997a). Variations in residue weight loss determinations using mesh bags, which is the most common method of estimating decomposition, will be discussed later (Section IIIB). d. Variation in Composition of Same Plant Species at Different Sites and Different Plant Parts Several workers have reported differences in residue decomposition due to differences in N, C/N, lignin/N, and polyphenol/N ratios even for the same species. Variation of site, plant part, and environment conditions also affected the litter chemistry of residues from the same plant species (Harper and Lynch, 1981; Berg and Tammy, 1991). For example, Pinus sylvestris needles varied in carbohydrate composition and lignin polymerization in relation to nutrient status and pH of different soils on which they were grown (Sanger et al., 1996). Likewise, Vitrusek et al. (1994) found that the litter of tropical tree Metrosideros polymorpha grown on

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dry Hawaiian lava ﬂows decomposed twice as rapidly as litter of the same species on wet sites. These workers concluded that higher substrate quality from dry sites could be due to trade-offs in nutrient and water use efﬁciency and C gain by plants when grown under different climates. An unconventional approach was adopted by Cornelissen (1996) using the strategy theory of Grime (1988), where the weight loss of more than 100 species of leaf, needles, and shoot litter was measured under standard ﬁeld conditions. The weight loss was related to a variety of plant characteristics such as growth habits, evergreen vs deciduous, autumn correlations, and evolutionary advancement. There was a clear evidence of the adaptive strategy of tissues defense (accumulate different chemicals in their tissues). Cornelissen’s (1996) results suggested that environmentally stressed habitats produced relatively slow decomposing leaves. 4. Residue Decomposition Index/Quality Index Attempts have been made to predict the rate and pattern of decomposition of organic substrates from a range of organic materials based on their chemical components (e.g., C/N ratio, lignin content, lignin-to-N ratio, polyphenol). Herman et al. (1977) proposed the decomposability index as Decomposability index

(C/N) (% lignin) a(% carbohydrates)

(1)

It was found that this decomposition index correlated inversely with total CO2 evolution from the decomposing roots of three grass species (Herman et al., 1977) and predicted accurately the decomposition of other plant materials, including legume residues. Cortez et al. (1996) found that parameters integrating lignin were highly correlated to the decomposition of a wide variety of litter. These workers developed the HLQ index as HLQ

hemicellulose cellulose hemicellulose cellulose lignin

(2)

Tian et al. (1995b) developed the plant residue quality index (PRQI) for assessing the quality of plant residues as follows: PRQI

1 100, (a C/N ratio b lignin c polyphenols)

(3)

where a, b, and c are coefﬁcients of relative contribution of C/N ratio, lignin content (%), and polyphenol content (%) to plant residue quality. The PRQI was found to be correlated with the decomposition rate of plant residues using litter bags. These workers concluded that PRQI can be used for selecting plant residue and projecting their agronomic value.

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Janssen (1996) proposed a resistance index (RI) that depends on the decomposability of different residues. It was found from a desk study using some of the earlier published work that good linear relationships existed between the fraction of organic N mineralized and initial C/N ratio of the substrate for organic materials with similar decomposability. It is thus obvious that a combination of lignin and polyphenol concentration offers perspectives for the quantitative evaluation of decomposability. However, this needs further evaluation on a wide variety of organic materials with a standard proximate analysis of lignin and polyphenol concentration before a universal plant residue quality index could be developed. 5. Edaphic Factors a. Soil pH Soil pH is one of the most important factors influencing residue decomposition as it affects both the nature and size of population of microorganisms and the multiplicity of enzymes at the microbial level, which subsequently affect decomposition (Paul and Clark, 1989). In general, the decomposition of crop residues proceeds more rapidly in neutral than in acid soils. Consequently, liming acid soils accelerate the decay of plant tissues, simple carbonaceous compounds, and SOM (Alexander, 1977; Condron et al., 1993). Under field conditions in the United Kingdom, Jenkinson (1977) reported that 42% of the ryegrass-derived C still remained after 1 year in a soil of pH 3.7, whereas only 31% remained in soils of pH between 4.4 and 6.9. This may be due to alterations in soil microbial populations and activity as soil pH changes. Characteristically, the population shifts from bacteria to actinomycete to fungi as soil pH declines (Alexander, 1980). b. Soil Temperature Parr and Papendick (1978) and Stott et al. (1986) reported that microbial decomposition processes are more important than physical and chemical processes in causing the loss of residues from the ﬁeld, thus releasing nutrients. Temperature affects the physiological reaction rates of organisms and the activity of microbial cells by the laws of thermodynamics and hence microbial activity (Paul and Clark, 1989) and residue decomposition (Westcott and Mikkelson, 1987; De-Neve et al., 1996). The inﬂuence of temperature on crop residue decomposition has been described quantitatively as the temperature quotient Q10. Values of Q10 for the N mineralization rate of native SOM in the temperature range between 5 and 35C have been reported to be approximately 2 (Stanford et al., 1975; Campbell et al., 1981; Kladivko and Keeney, 1987; Scholes et al., 1994). Higher Q10 values have been reported by other workers between different temperature ranges (Pal et al., 1975; Addiscot,

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1983; Campbell et al., 1984; Vigil and Kissel, 1995), indicating some interactions between temperature and quality of crop residues. Microorganisms function at maximum growth and activity in the temperature range of 20–40C and showed maximum decomposition in this range (Roper, 1985; Stott and Martin, 1989). However, signiﬁcant straw decomposition can occur even at 0C (Stott et al., 1986, 1990; Kanal, 1995). At the extreme ends of the temperature scale (e.g., 0 and 40C), it is generally believed that temperature regulates the activity of microorganisms more than their mass. Under ﬁeld conditions, marked diurnal and seasonal ﬂuctuations in surface soil temperature are common (Biederbeck and Campbell, 1973). Although it has generally been found that microbial growth is inhibited by ﬂuctuating temperatures (Biederbeck and Campbell, 1971), some studies showed that N mineralization remains virtually unaffected in the mesophilic (15 –45C but optimum between 25 and 35C) temperature range (Stanford et al., 1975). c. Soil Moisture The growth and activity of soil microorganisms rely on soil moisture, which, in turn, produces signiﬁcant effects on plant residue decomposition and nutrient cycling (Stanford and Epstein, 1974; Sommers et al., 1981; Schomberg et al., 1994). Sommers et al. (1981) observed that soil dried to a water potential of 10 Mpa evolved CO2 at about half the rate of soils incubated at optimal water content (20 to 50 kPa). Pal and Broadbent (1975) showed that the maximum rate of decomposition for plant residues occurred at 60% water holding capacity (WHC) and the rates decreased at either 30 or 150% of WHC. Summerell and Burgess (1989) reported that the rate of straw decomposition as measured by dry weight loss was highest at 0.1 MPa and decreased as the external soil water potential was lowered. Das et al. (1993) observed signiﬁcantly more N release from decomposing crop residues at ﬁeld capacity than at 50% of ﬁeld capacity. Thomsen (1993) reported more soil microbial biomass on straw addition under moist soil conditions (54– 82%) than under wet conditions (4 –27%), probably due to limited aeration for microbial activity under wet conditions. Thus, both very dry and very wet conditions of soil inhibit decomposition by limiting either moisture content or soil aeration for microbial activity. d. Freezing and Thawing The thawing of previously frozen surface detritus resulted in the immediate release of large amounts of soluble materials (Witkamp, 1969; Bunnell et al., 1975). This is thought to represent the release of materials previously immobilized in microbial tissue (Witkamp, 1969). Such a release of soluble materials contributed to the burst of decomposer activity that occurred at the onset of snow melt (Bunnell et al., 1975), which may lead to a substantial increase in the decomposition rate.

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e. Drying and Rewetting The effect of drying and rewetting on the decomposition of plant residues is unclear. For example, Van Schrevan (1968) found that although drying stimulated the subsequent mineralization of C and N from soil humus, it retarded the mineralization of fresh plant materials. In another study, soil drying and wetting were found to promote the turnover of C derived from added 14C-labeled plant material, and the increase in C was mainly due to an enhanced turnover of microbial products (van Gestel et al., 1993). The decay rate of biomass 14C increased relatively greater by soil desiccation and remoistening than decay rates of nonbiomass 14C. Haider and Martin (1981) found that drying and rewetting produced no effect on the decomposition of 14C-labeled lignin when incorporated into soil, but the decomposition of added cellulose in soils was found to increase (Sorensen, 1974). Repeated drying and wetting of the soil appeared to increase the resistance of certain N compounds of the plant to microbial decomposition. Franzluebbers et al. (1994) reported that repeated drying and rewetting did not reduce the C mineralization of cowpea [Vigna unguiculata (L.) Walp.] signiﬁcantly; N mineralization from cowpea, however, was reduced signiﬁcantly. Repeatedly drying and wetting can inhibit microbial growth and/or activity severely. In the ﬁeld, it could reduce N mineralization from legume green manure compared to decomposition in continuously moist soil. This may contribute to long-term soil N fertility by increasing the soil organic N content. f. Aerobic and Anaerobic Conditions Decomposition and mineralization are slower and less complete under anaerobic than aerobic conditions (Pal and Broadbent, 1975; Murthy et al., 1991; Kretzschmar and Ladd, 1993). When soils become so wet that larger pores are ﬁlled with water, the decomposition of OM is limited by the rate at which oxygen can diffuse to the site of microbial activity, as the diffusion coefﬁcient of oxygen in water is 10,000 times slower than in air. Thus, even a modest oxygen demand cannot be met if larger soil pores are ﬁlled with water (Jenkinson, 1988). Reddy et al. (1980) showed that the ﬁrst-order rate constant for rice straw decomposition was 0.0054 day1 for phase I (easily decomposable fraction) and 0.0013 day1 for phase II (slowly decomposable fraction) under aerobic conditions, and corresponding values for anaerobic conditions were 0.0024 and 0.0003 day1, respectively. g. Soil Salinity This is generally attributed to a direct inﬂuence of the osmotic potential on microbial activity ( Johnston and Guenzi, 1963; Singh et al., 1969) or through the alteration of pH, soil structure, aeration, and other factors (Nelson et al., 1996). Results showed that plant residue composition, as well as increased salinity, affected the decomposition and CO2 surface ﬂux dissolved organic C and may be an important factor for C storage in saline systems (Hemminga et al., 1991; Hemminga

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and Buth, 1991; Olsen et al., 1996). In the absence of pH and aeration effects, Nelson et al. (1996) concluded that sodicity increased and salinity decreased the decomposition of ﬁnely ground plant residues, with no signiﬁcant interaction. h. Available Nutrients The primary limiting factor in most soils for microbial growth is C. An abundance of C is added to the soil through crop residues. New cells of microorganisms require not only C, H, O, and N, but also P, K, S, Mg, Ca, and micronutrients. When an organic material is added to the soil, microorganisms decomposing it can obtain the necessary inorganic nutrients (N, P, K, S, Ca, etc.) for growth from two sources: (i) those already present in the soil in plant available forms and (ii) those in the added organic material itself. However, Jenkinson (1981) stated that it is unusual for nutrient elements other than N to limit the decomposition of plant or animal material in soil, even though they can restrict microbial activity in vitro. It has been widely recognized that the element required in the greatest abundance is N. Nitrogen is required by all decomposers because it is a constituent of extracellular and intracellular enzymes, nucleic acids, and lipoprotein membranes, thus making it the most limiting nutrient for microbial activity. However, the effects of added N on the decomposition of OM are variable (Jenkinson, 1981) and will be discussed in detail later. Apart from N, other elements have also been shown to inﬂuence residue decomposition. For example, Cheshire and Chapman (1996) concluded from adding N and P to 14C-labeled ryegrass that P, whether intrinsic or added, increased the rate of decomposition of organic residues but there was a strong interaction with N, which had a predominant inﬂuence. In addition, Enriquez et al. (1993) demonstrated that the decomposition rate increased with both intrinsic N and P. The effect of N depends on its form. Increased intrinsic tissue N increased the rate of C loss, whereas added inorganic N decreased the rate of C loss during decomposition (Cheshire and Chapman, 1996). i. Inorganic Nitrogen Swift et al. (1979) showed that inorganic native N in soil enhanced the mineralization of OM. Woods et al. (1987) reported that the application of NH+4 under Nlimited conditions increased microbial respiration, microbial population, and N mineralization. Negative or no effects of added N on decomposition and microbial activity have also been reported (Fog, 1988; Hassink, 1994a; Cheshire and Chapman, 1996) and have been attributed to (a) the outcome affecting the competition between potent and less potent decomposers through “ammonia metabolite repression” (Keyser et al., 1978); (b) N blocking the production of certain enzymes and enhancing the breakdown of the more available cellulose (Lueken et al., 1962), whereby recalcitrant lignocellulose accumulated and amino compounds condensed with polyphenols; and (c) ammonia toxicity (Vines and Wedding, 1960; Clay et al.,1990).

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Thus N fertilizer has the potential to inﬂuence residue decomposition and mineralization and types of N compounds found in the soil. In soils with low pH, N mineralization rates of plant residues were reduced when N fertilizer was applied to soils (Clay and Clapp, 1990). Soils with a low pH had reduced nitriﬁcation rates resulting in NH+4 concentration. The N fertilizer application increased nonhydrolyzable N and decreased the residue-derived amino acid N when compared to unfertilized treatment. Clay and Clapp (1990) explained these results as due to (a) N fertilizer inﬂuencing the mineralization rate of different substrates, with some depolymerization products reacting with amino compounds, reducing hydrolyzable amino acid N to form condensation products and resulting in increased amounts of nonhydrolyzable N, and (b) NH+4 inhibition of the microbial population. Cerri and Jenkinson (1981) demonstrated a lesser mineralization of plant residue added to a soil rich in easily decomposable OM. j. Soil Texture, Clay Content, and Soil Structure Jenkinson (1977) and Ladd and Foster (1988) showed that the decomposition of plant materials was more rapid in soil with less clay content because the clay protected the OM from decomposition. As clay content increases, soil surface area also increases, which results in an increased SOM stabilization potential (Sorenson, 1981; Merckx et al., 1985; Jenkinson, 1988; Ladd et al., 1996; Saggar et al., 1996). This stabilized OM has turnover time ranging from 10 to 1000 years (Parton et al., 1987). The role of clay in stabilizing OM appears to be more important in warmer soils where higher decomposition rates can be expected. In cold soils, cool temperature may be the main factor slowing decomposition, and clay content may be less important (Anderson, 1995). However, comparing different substrates, Skene et al. (1997), showed that for high-quality substrates, physical protection by inorganic matrices was a major limiting factor to decomposition, whereas for lowquality substrates, chemical protection is the major limiting factor. Texture also inﬂuences the soil physical environment (Elliott et al., 1980; Hassink et al., 1993), which further affects the microbial activity (Stott et al., 1986; Skopp et al., 1990; Hassink et al., 1993; Killham et al., 1993). Other effects of soil texture may be through N and P availability (Mackay et al., 1987) by inﬂuencing total OM accumulation or microbial activity (Schimel et al., 1985; Hassink et al., 1993). Soil structure also exerts a dominant control over the stabilization of SOM (Van Veen and Kuikman, 1990; Ladd et al., 1996) and OM is protected in microaggregates (Tisdall and Oades, 1982; Skjemstad et al., 1993; Golchin et al., 1994). The degree of physical protection of 14C-labeled residues against decomposition in the microaggregate fraction (20 mm) was negatively correlated with the degree of saturation of this particle-size fraction with SOM (Hassink and Dalenberg, 1996). The higher the level of elements retained in the soil, the greater they can be taken up by plants. Jordan et al. (1996) found greater residue N uptake by sorghum

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and soil microbial biomass C and N in clayey soil (34% clay) compared with sandy soil (5% clay). k. Indigenous Macro- and Microorganisms Decomposition and mineralization are largely biological processes, mainly accounted for by the activity of the soil microﬂora. However, abiotic processes cannot be discounted entirely, especially in harsh environments, where appreciable plant litter mass may be lost by abiotic mechanisms, such as fragmentation, physical abrasion, photochemical breakdown, and leaching (Moorhead and Reynolds, 1989; Dormaar, 1991; Coûteaux et al., 1995). Even when these abiotic processes of mass loss predominate, the eventual oxidation to CO2 and other inorganic constituents is probably mediated by soil organisms. Decomposer organisms generally consist of a complex community of soil biota, including microﬂora and soil fauna. Fungi and bacteria are ultimately responsible for the biochemical processes in the decomposition of organic residues (Juma and McGill, 1986). Soil fauna enhance the biodegradation and humiﬁcation of organic residues in several ways (Tian et al., 1997), such as by (a) comminuting organic residues and increasing surface area for microbial activity, (b) producing enzymes that break down complex biomolecules to simple compounds and polymerize compounds to form humus, and (c) improving the environment for microbial growth and interactions. The direct contribution of the soil mesofauna to these processes, derived primarily from their share in the soil biomass, has been estimated as small (e.g., Andren and Schnurer, 1985; De Ruiter et al., 1993; Didden et al., 1994), but indirect contributions, affecting the functioning of other groups of organisms or even the structure and functioning of the food web as a whole, have been postulated to be important (Moore et al., 1988; Bouwman et al., 1994; Brussaard et al., 1995; Beare, 1997). Soil fauna contribute to litter breakdown by (i) grinding plant residues and (ii) channeling and improving the soil structure (Shaw and Pawluk, 1986; Cheshire and Grifﬁths, 1989; Scheu and Wolters, 1991; Coûteaux et al., 1995). Tian et al. (1995a) found that for plant materials with contrasting chemical composition in the ﬁeld, an increased breakdown rate occurred following the addition of earthworms or millipedes. These workers suggested that the role of soil fauna was relatively greater in the decomposition of materials with a high C/N ratio, lignin, and polyphenol content and less important on low C/N ratio residues as these residues were decomposed easily by microorganisms. Termites and ants are also known to be efﬁcient in digesting cellulose-containing substances and, in some cases, ligniﬁed substances (Lee and Wood, 1971; Wood, 1988). Millipedes break down plant litter and mix it with mineral soil, which they ingest (Kevan, 1968; Tian et al., 1995a). House et al. (1984) and Stinner et al. (1988) proposed that with conservation tillage, surface-maintained crop residue can provide a continuous resource in

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space and time for many decomposer organisms. Friebe and Henke (1991) reported that a higher tillage intensity was associated with a lower faunal abundance, whereas a distinct increase in plant–residue decomposition was associated with a reduced tillage intensity. In another study, Reddy et al. (1994) found that the residue mass loss was signiﬁcantly correlated to moisture content and arthropod abundance. However, there is little information on the combined effects of abiotic and biotic factors on the mass loss of plant material during the process of decomposition. The microbial decomposer community is extremely diverse and capable of surviving a wide range of environmental and food-related stresses. Bacteria and fungi, the major decomposers, differ in their mode of growth and activity (Coleman and Crossley, 1996). Bacteria are clustered in colonies occupying a small soil volume and their movement in soil is episodic and related to factors such as rainfall, root growth, tillage, and ingestion by soil fauna (Gregorich and Janzen, 1998). In contrast, fungi have hyphae that can grow over relatively larger distances and penetrate into small spaces where they can decompose OM by secreting enzymes and translocate the nutrients back through hyphae. The species of microorganisms responsible for residue breakdown also depends on the temperature of the substrate (Parr and Papendick, 1978) and the type of substrate (Wani and Shinde, 1977; Wardle and Lavelle, 1997). Actinomycetes are responsible for residue breakdown mainly at high temperatures, whereas species of bacteria and fungi are dominant at lower temperatures (Parr and Papendick, 1978). Wani and Shinde (1977) observed that fungi were responsible for the favorable effect on the mineralization of C during the decomposition process, with Aspergillus species playing a more important role in the breakdown of cellulose. Several studies (Hendrix et al., 1986; Holland and Coleman, 1987; Neely et al., 1991; Wagner and Broder, 1993; Baldy et al., 1995) have reported that fungi tended to be the dominant microorganism controlling litter decomposition. Ingham and Horton (1987) showed that for high C/N ratio soils and materials, the fungal/bacterial ratio increased markedly while the decomposition proceeded, the bacteria presumed to have peaked early in the process, probably had a diminishing role after the ﬁrst week. Thus, when the more easily decomposable constituents of the residue have been exhausted, bacteria may continue at a lower level, perhaps feeding on fungal carbohydrate products (Wagner and Broder, 1993), and the decomposition was predator controlled (Vreeken-Buijs and Brussaard, 1996). 6. Management Factors a. Loading Rate and Quantity Added Evidence from laboratory and ﬁeld studies has generally demonstrated that the rates of decomposition of plant materials added to soil are proportional to amounts

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added initially (Pinck and Allison, 1951; Jenkinson, 1965, 1977; Larson et al., 1972). According to Pinck and Allison (1951), the percentage decomposition of plant C in soils was nearly independent of input levels, provided that C additions did not exceed the amount equivalent to 1.5% of the dry soil. In addition, Ladd et al. (1983) found that the proportions of plant 14C and 15N Medicago littoralis retained in soil were not independent of the amounts of plant 14C and 15N added. These workers concluded that the greater the amount of labeled plant material added, the smaller the proportion of residual organic 14C and 15N. Some workers reported that small amounts of crop residues decompose more rapidly in soil than large amounts. This was supported in some, but not all, experiments with 14C-labeled plant materials (Jenkinson, 1971). Sorensen (1963), however, concluded that the percentage decomposition of organic materials was nearly always independent of the quantity added if the C addition did not exceed 1.5 – 2.0%. Summerell and Burgess (1989) also reported no difference in the rate of straw decomposition at two loading rates of 1.6 and 3.2 tons ha1. Stott et al. (1990) reported that out of three loading rates of 1680, 3360, and 6720 kg ha1 of surface residues added to a soil, the lowest loading rate showed the fastest loss rate. Likewise, Broadbent and Bartholomew (1948) reported the inverse relation between decomposition and quantity of residue added. However, these workers could not ﬁnd any reason for slow decomposition at a higher loading rate. Perhaps these differences may be related to N availability in soil. If, in certain conditions, N is limiting, it may be sufﬁcient for hastening the decomposition of a small amount of low-quality residues but may slow down the decomposition if large amounts of residues are added. b. Method, Mode, and Accessibility The accessibility of plant residues to soil microbes is of primary importance in its rate of decomposition. The method of application of plant residues, such as residue particle size and placement, can provide a different degree of accessibility, which, in turn, affects residue breakdown rates as well as the mineralization– immobilization process. Many studies indicate that burying residues in soils increases the decomposition rate compared to placing residues on the soil surface (Douglas et al., 1980; Harper and Lynch, 1981; Summerell and Burgess, 1989; Aulakh et al., 1991a; Douglas and Rickman, 1992; Buchanan and King, 1993; Schomberg et al., 1994). The effect of placement decreases with time (Christensen, 1986; Cogle et al., 1987). For example, Cogle et al. (1989) observed that incorporated straw decomposed faster than surface straw during the ﬁrst 15 days but the differences abated thereafter. The initial lag in the decomposition of residues on the soil surface is probably due to litter on the surface being more subjected to unfavorable conditions for decomposition, particularly with respect to ﬂuctuations in temperature-

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and moisture-limiting microbial activity than in material buried in soil. The incorporation of straw allowed microorganisms decomposing the residues greater access to nutrients in the soil and factors favoring their activity (Harper and Lynch, 1981). Also rainfall leaches soluble nutrients out of the straw, reducing the microbial activity further. Douglas et al. (1980) reported that the rate of decomposition for surface residues was much more linear with time than buried residues and that surface residues tended to decompose entirely, such as phase II of buried residues as proposed by Berg and Agren (1984). Schomberg et al. (1994) compared the decomposition and N dynamics of surface placed and buried lucerne, wheat, and sorghum residues under different moisture regimes. Their results showed that large differences in residue decomposition occurred between crop residues, with lucerne residue decomposing much faster than wheat and grain sorghum with net N mineralization occurring throughout the study period. Net N immobilization was longer than 1 year for surface wheat and sorghum residues, but was about 0.33 a year when buried. Both Nmax (g N immobilized kg1 of original biomass) and Neq (g N immobilized kg1 of biomass loss) were inﬂuenced by crop and placement. The Nmax value was similar for surface wheat and sorghum residues, but was 50% lower for buried wheat than for sorghum. Neq indicated that the N requirement of microorganisms was less for buried than for surface residues. Results showing a rapid decomposition of incorporated residues compared to surface residues have also been reported by other workers (e.g., Lafond et al., 1996; Beare, 1997). Kanal (1995) found that increasing the depth of residue incorporation from 50 to 200 mm resulted in a decrease in the breakdown rate due to less biological activity. In contrast, Breland (1994) found that increasing the incorporation depth up to 300 mm increased the decomposition rate due to a more favorable moisture regime in lower layers. Residue left over from burning generally decomposed slowly because the C remaining after burn is less active biologically and has a longer turnover time (Rasmussen and Parton, 1994) and is partly due to the decrease in the decomposer population after burning (Prasad and Power, 1991). A lower microbial biomass with burning (Collins et al., 1992) tends to support this contention. Shindo (1991) reported that charred residues evolved much less CO2 than noncharred residues when incubated with soil. c. Irrigation Irrigation in drier regions assists the rapid decomposition of plant residues. Douglas et al. (1980) found that 65% of the buried wheat straw was decomposed under dryland conditions in eastern Oregon in 14 months compared with 70–80% in 13 months reported by Smith and Douglas (1971) in Idaho under irrigated conditions.

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d. Crop Cover Numerous decomposition studies (mostly laboratory incubation and some ﬁeld experiments) have been performed without the presence of plants in the soil. The presence of plants in the soil system interacts with the decomposition and soil N processes by modifying the soil environment physically, chemically, and biologically (Clarholm, 1985). A plant cover reduced straw weight loss rates (Christensen, 1985b), which agreed with ﬁeld results using 14C-labeled plant materials (Jenkinson, 1977; Shields and Paul, 1973). The plant effect was attributed to the drier conditions in planted soils due to plant water uptake, but alternative explanations have been offered (Reid and Goss, 1982). Christensen (1985b) found that both summer soil temperatures and soil moisture were reduced in planted plots. Growing plants may reduce moisture contents by water uptake and by intercepting part of the precipitation. During the relatively drought summer conditions, both these processes may cause planted soils to be drier than fallowed soils. The plant effect on soil temperature arises from their higher reﬂectance, interception of incoming solar radiation, and cooling provided by water transpiration. The reduced moisture and temperature levels of planted soils may reduce microbial activity, extractable N and N mineralization, and slow residue decomposition (Faber and Verhoef, 1991; Parmelee et al., 1993). The possibility that roots exert some control over residue decomposition has been investigated for many years (Gadgil and Gadgil, 1971; Dighton, 1991; Zhu and Ehrenfeld, 1996). Results showed that in soils with low total C and N contents, roots stimulated greater activity of the soil biota (Parmelee et al., 1993, 1995), which contributed, in turn to faster litter decomposition and nutrient release (Zhu and Ehrenfeld, 1996). In other studies involving crop plants in agricultural soils, plant residue decomposition in soil was enhanced by living roots or, in turn, the presence of a crop (Dighton et al., 1987; Sallih and Bottner, 1988; Cheng and Coleman, 1990; Dormaar, 1990). This may be due to root exudates inducing increased microbial activity, although Whitley and Pettit (1994) found that the addition of lignite humic acid was responsible for increases in decomposition. Roots, rhizodeposits, and exudates have attracted little decomposition research effort, especially compared to plant inputs to soil from aboveground parts. This is despite the importance of plant–soil interface and their input in C ﬂow; 16–33% of the total C assimilated by plants is released directly into soil by roots, which contributes to 30–60% of the organic C pool in soil (Boone, 1994). This has particular importance in arable agriculture where root residues may be the only organic inputs and major contributors to SOM replenishment. The advent of C isotope methods has signiﬁcantly improved the knowledge of decay rates and allowed discrimination between root and microbial respiration (Cheng et al., 1994; Swinnen et al., 1995). In general, the decomposition of root products conforms with the

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general pattern of aboveground plant parts, being determined by the chemical composition (Andren et al., 1990; Swinnen et al., 1995). 7. Climate Factors Temperature and rainfall are important factors affecting the decomposition of OM and plant residues (Schomberg et al., 1996). The loss of OM in warm regions where OM decomposes rapidly is of a serious concern. Using 14C-labeled ryegrass, the rate of decomposition was found to be about four times faster in Nigeria (humid tropics) as compared to England (humid temperate) (Jenkinson and Ayanaba, 1977; Barnard and Kristoferson, 1985). Temperature and moisture regimes have been found to be good predictors of the microbial C/total C ratio (Insam et al., 1989). A low temperature in winter and a dry soil in summer limit microbial decomposition, whereas microbial decomposition is greatest during the moist warm spring and autumn seasons (Douglas et al., 1990; Collins et al., 1990b). At a regional level, the effects of climate on decomposition are reﬂected in the accumulation of OM in the soil. Data on SOM from semiarid, semihumid, and humid regions of the world show that soils from warm climates contain less OM than that in cool regions, partly because of faster decomposition (Jenny, 1941). 8. Other Factors a. Elevated Ozone and Carbon Dioxide Plant residues grown under elevated ozone (O3 ) and/or CO2 have been shown to affect plant structure and nutrient status and, as a result, their decomposition (Boerner and Rebbeck, 1995; Cotrufo et al., 1994). Elevated atmospheric CO2 could affect decomposition in one of four ways (Ball, 1997) through • • • •

a direct effect on microbial activity changes in plant species composition of an ecosystem changes in the quantity of plant residues produced changes in the chemical composition of residues

There is more likely to be a general decline in litter quality than other effects in response to increased CO2 (Korner and Miglietta, 1994; Arp et al., 1997; Torbert et al., 1998), as enhanced atmospheric CO2 is widely known to increase photosynthesis, but with an uncertain compensatory uptake of nutrients. This results in a reduction in nutrient concentration in plant tissues (Heal et al., 1997; Arp et al., 1997). Henning et al. (1996) found that although elevated CO2 increased the C/N ratio and lignin content of sorghum stem and soybean leaves, it had no impact on soil C turnover, relative N mineralization, cumulative C and N mineralization, and C/N

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mineralized, thus suggesting that increasing atmospheric CO2 will have little effect on composition or decomposition of ﬁeld crop residues. Because CO2 enrichment results in increased photosynthetic C ﬁxation, the possibility exists for increased soil C storage under ﬁeld crops in an elevated CO2 world (Henning et al., 1996). For details regarding the effects on processes and magnitude of decomposition as affected by elevated CO2 levels, readers are advised to read the review by Ball (1997). Brieﬂy, Ball concluded that the growth of C3 plants in elevated atmospheric CO2 (600–700 mol mol1) may lead to a signiﬁcant increase in either or both C/N and lignin/N ratios of plant residues (Table II). Short-term decomposition of litter from plants showing this response in elevated CO2 has conﬁrmed that decomposition occurs at a signiﬁcantly lower rate (Table II). However, growth responses and degradability of C4 plants suggested no difference in litter quality or degradability (Table II). Further long-term studies are needed to conﬁrm these effects in complex ecosystems; if these results are repeated at the ecosystem level, then signiﬁcant changes in the cycling of C and N may occur as a result of elevated CO2. b. Ultraviolet (UV) Radiations The impact of UV radiations on litter quality and decomposition presents an analogous situation to CO2 elevation as increasing levels of UV radiation at higher latitudes were found to cause changes in plant biochemical composition, with increased pigmentation and secondary compounds (Heal et al., 1997). Increased tannins and decreased ␣-cellulose in the leaf of Vaccinium spp. have been recorded in experimentally increased UV radiation studies (Gehrke et al., 1995; Johnson et al., 1995). There was evidence of a small direct effect of UV in reducing decomposition and changes in fungal community structure. Because direct UV exposure affects only superﬁcial litter, the main effects are anticipated to be through an alteration in litter quality (Heal et al., 1997). However, the combination of direct and indirect effects could cause a reduction in the decomposition rate of the order of 5–10% at higher latitudes.

B. METHODS OF STUDYING CROP RESIDUE DECOMPOSITION The range and objectives within studies of crop residue decomposition have generated a wide variety of methods used to study residue decomposition, each appropriate for a speciﬁc purpose (Harper, 1989; Conteh et al., 1997). Some of the commonly used methods are reviewed here. 1. Measurement of Carbon Dioxide Evolution In this method, the loss of C (as CO2) is measured following the decomposition of soil and crop residues incubated together minus the loss of C from soil in-

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Nutrient Ratios of Residues and Soil Respiratory Activity in Soils Amended with Plant Litter from C3 and C4 Plants Grown in Elevated CO2a Species

Ambient CO2

Elevated CO2

C/N ratio of plant residues C3 plants Acer pseudoplatanus L. Betula pubescens Fraxinus excelsior Lolium perenne Picea sitchensis Triticum aestivum C4 plants Andropogen gerardii Poa pratensis Sorghastrum nutans Sorghum bicolor Spartina patens C3 plants A. pseudoplatanus L. B. pubescens F. excelsior L. perenne P. sitchensis T. aestivum C4 plants A. gerardii P. pratensis S. nutans S. bicolor S. patens Soil only (control) C3 plants Scirpus olneyi Triticu aestivum Lolium perenne C4 plants S. patens S. bicolor aColated

106 53 56 30 18 79

102 33 104 14 81

96 39 99 14 77 Lignin/N ratio of plant residues

16 10 5 16 5 5

22 17 9 25 6 6

38 36 10 11 33 31 6 6 28 28 Soil respiration (mg Co2-C g soil1 day1) with crop residues 75 75

from reviewed data by Ball (1997). different at P 0.05.

bSigniﬁcantly

81 35 42 17 18 42

183 180 216

147b 129 b 162b

165 156

150 171

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cubated in the absence of added residues (Stevenson, 1986; Scheu, 1993; Bremner et al., 1991; Hassink, 1994b; Ladd et al., 1995). These laboratory incubations were conducted under favorable temperature and moisture regimes and often excluded processes such as faunal activity and nutrient leaching, which inﬂuence the decomposition in the ﬁeld. Nevertheless, the techniques used are useful for characterizing the inﬂuence of individual factors on decomposition such as inorganic matrices (Skene et al., 1997), sodicity and salinity (Nelson et al., 1996), elevated CO2 (Gorissen et al., 1995), residue composition (Vanlauwe et al., 1996), N addition (Green et al., 1995), and the presence of living roots (Cheng and Coleman, 1990; Nicolardot et al., 1995). This approach, however, has limitations, including analytical errors in measuring the long-term release of CO2, particularly when the amount of plant residues added is kept small, relative to the amount of native SOM. Also, the assumption is made that the addition of plant residues to the soil does not alter the decomposition rate of the native SOM (Stevenson, 1986). In the CO2 evolution method, evolved CO2 from microbial respiration is absorbed in an alkali solution, precipitated with BaCl2 and subsequently estimated by titrating against standard HCl, assuming that 1 ml of 1 M HCl releases 22 mg CO2 (Ladd et al., 1995). Reactions of the process are as follows: 2OH CO2

r CO 3 H2O

Ba2 CO 3

r BaCO3 (solid)

The rate of CO2 production is used to describe the rate of plant residue decomposition (Hassink, 1994b; Ladd et al., 1995; Saggar et al., 1996). Nordgren (1988) developed an apparatus for the continuous, long-term monitoring of the soil respiration rate in a large number of samples. The method is based on the principle that a decrease in electrical conductivity is directly proportional to the amount of CO2 absorbed in the alkali solution. The method gives accurate measurements and is rapid. The amount of CO2 absorbed in the alkali solution is calculated as in Eq. (4): Ct a (1 R0 /Rt).

(4)

Ct denotes absorbed CO2 at time t and a is the proportionality constant relating decrease in conductance to absorbed CO2. The solution electrical conductance at time t is Rt and R0 at t 0. Apart from the automatic operation, the conductivity method has several advantages per se. The system is closed and involves no tubing and air ﬂows and each recorded observation is an average of about 1000 readings. Many variations exist in the methods used for measuring CO2 (Alef and Nannipieri, 1995) such as the use of biometer ﬂasks [Organization for Economic Cooperation and Development (OECD), 1981; Haigh, 1993) and, more recently, infrared (IR) gas analyzers (Alef and Nannipieri, 1995).

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These CO2 evolution methods have also been used to characterize the decomposition in ﬁeld conditions using both static chambers and dynamic chamber methods (Jensen et al., 1996). The advent of portable IR gas analyzers made it possible to measure the rate of CO2 evolution within a short time in the ﬁeld (Alef and Nannipieri, 1995; Magid et al., 1997b). Jensen et al. (1996) compared the static chamber method using alkali trapping of CO2 for 24 hr and a dynamic method using IR gas analysis for 2 min at each point for estimating ﬁeld scale CO2 ﬂuxes from unplanted soils. They concluded that the static method provided the best integrative measure, but it would be necessary to measure continuously in order to obtain a better estimate of CO2 evolution using the dynamic method. However, a major limitation of the static chamber method is that it does not accurately reﬂect high rates of CO2 evolution from the soil due to a diffusional limitation of the trapping of CO2 into the alkali. Thus the initial CO2 evolution from the decomposition of freshly added plant material will often be considerably underestimated by this method (Magid et al., 1997b). 2. Recording Weight Loss Using Mesh Bags The mesh bag method was developed to elucidate decomposition in undisturbed soil systems and, because of its simplicity, was extended to arable systems, in which plant residues are normally admixed with the soil by tillage practices (Christensen, 1985a). Mesh bags containing straw or a soil–straw mixture have been used most frequently to study plant residue decomposition (Brown and Dickey, 1970; Smith and Douglas, 1971; Harper and Lynch, 1981; Douglas and Rickman, 1992; Thomas and Asakawa, 1993; Schomberg et al., 1994; Cotrufo et al., 1995; Cortez et al., 1996; Jama and Nair, 1996; Vreeken-Buijs and Brussaard, 1996). The total weight loss of straw enclosed in mesh bags is attributed to three loss components: leaching, microbial decomposition, and loss of straw particles through mesh openings. The initial weight loss is probably determined by leaching and microbial decomposition of a readily available substrate. In the following stages, decomposition proceeds and straw becomes more fragile, eventually disintegrating and causing a loss of straw particles from the mesh bags. Mesh bags can be criticized for providing conditions potentially dissimilar from that in bulk soil and have a number of limitations (Heal et al., 1997): only net changes are measured and the fate of material leaving the bag is ignored, thus overestimating the decomposition. Also, this material may be more labile and contain more nutrients than the residues remaining and may play an important role in nutrient cycling processes. In order to minimize these difﬁculties, small mesh sizes are normally used, but apart from the analytical procedures connected with the contamination of the plant residues by soil, the conﬁnement or isolation from soil can generate a microenvironment that differs considerably from that of residues in

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a more intimate contact with soil (Malkomes, 1980). Residues within the litterbags have been reported to have more moisture compared to unbagged residues and are thus more favorable for microbial activity (Vossbrinck et al., 1979). Also, when bags are used in arable soils, straw does not suffer the physical damage likely to result from contact with agricultural machinery during incorporation (Harper, 1989). Tian et al. (1992a) found a positive correlation between the decomposition rate and the mesh size of litterbags that was interpreted as an effect of increasing accessibility to soil fauna. Fine meshes exclude soil animals that may inﬂuence decomposition (Jensen, 1985; Vreeken-Buijs and Brussaard, 1996), although House et al. (1987) reported that the decomposition rate was independent of mesh size. Another problem arises from the signiﬁcant quantities of soil adhering to the decomposing residues. Various cleanup procedures, such as brushing and washing with water (Brown and Dickey, 1970; Douglas et al., 1980; Harper and Lynch, 1981; Schomberg et al., 1994; Cotrufo et al., 1995; Vreeken-Buijs and Brussaard, 1996) have been used, but methods are not satisfactory as they most likely provide selective and incomplete removal of adhering soil components (Brown and Dickey, 1970). Moreover, washing procedures undoubtedly remove water-soluble OM and nutrients from the residues (Douglas et al., 1980; Christensen, 1985a). In some studies using mesh bags, the recovery of straw samples has been corrected for its ash content, with the weight loss being expressed on an ash-free dry weight basis (Harper and Lynch, 1981; Thomas and Asakawa, 1993; Schomberg et al., 1994; Jama and Nair, 1996; Vreeken-Buijs and Brussaard, 1996). This approach does not account for SOM introduced into the straw sample by soil entering the litterbags. In most cases, mesh bags probably underestimate the actual breakdown rates. The exclusion of certain macrofauna components is a justiﬁed criticism when the absolute rate of weight loss and N immobilization–mineralization is required. The continuing popularity of the mesh bag method results primarily from the relative ease with which samples of known initial weight can be recovered and used to study nutrient dynamics under ﬁeld conditions. 3. In Vitro Profusion Methods This method uses a closed, continuous air ﬂow system in which the plant material is allowed to decompose in a medium of constant ionic strength (Nyami, 1992). The evolved CO2 is absorbed in an alkali solution and reactions involved are similar to those discussed earlier. The rate of CO2 production is used to describe the rate of plant residue decomposition. However, the profusion apparatus of Nyami (1992), which was made from glass, is expensive to make and difﬁcult to use. Using simple components used in health care, Lefroy et al. (1995) developed an apparatus at the University of New England, Australia, that provides comparable results and is simple to use.

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Although this technique has not been compared with actual decomposition data obtained from the ﬁeld, it allows the screening of a wide range of crop residues under a standardized set of conditions, without the complications of differences in soil biota, and allows a ranking of relative breakdown and nutrient release. 4. Lignin Analysis Because lignin decays relatively very slowly (Scheu, 1993; Rutigliano et al., 1996), the weight loss of decomposing residues may be estimated from the increase in lignin content. Although initial weight loss could be estimated by lignin content, there was increasing deviation from unity as decay proceeded, probably resulting from lignin decay at later stages (Harper, 1989). Also, lignin decomposition can be enhanced by earthworms (Scheu, 1993), which may lead to the underestimation of residue decomposition. Thus, these methods need calibration relating the periodic increase or decrease in lignin content with decomposition under a wide variety of soils, environments, and other biotic factors (Harper, 1989). 5. Size Density Fractionation for in Situ Measurements of Decomposition Magid et al. (1996) proposed that decomposition can be estimated by determining the temporal distribution of plant residues in different density and size-density fractions of OM. This approach was used successfully to study the decomposition of rape straw, and estimates of decomposition from particulate organic matter (POM) were in qualitative agreement with those based on ﬁeld-scale CO2 ﬂuxes (Magid et al., 1997a). This method shows considerable potential but needs to be tested for different kinds of residues. It can be applied without much problem to residues that are predominantly insoluble in water, as recovery of the POM fraction necessitates a decantation or ﬂotation step (Magid et al., 1997b). However, the technique leads to a mixing of added residues with native SOM, thus requiring an unamended control treatment. A major advantage of this technique is that added residues are completely exposed to the soil environment and thus the full range of faunal and other soil interactions. 6. Isotopic Techniques A method of studying residual decomposition in the ﬁeld is to add labeled (15N, C, 13C, etc.) plant residues to the soil. Labeled residues are placed in small cylinders to prevent contamination by unlabeled materials. At periodic intervals, soil is removed from the cylinder and analyzed to measure the quantity of label remaining. This approach permits the direct measurement of the fate of plant litter, but is 14

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usually limited to small-scale studies because of the cost of establishment and analysis. a. Nitrogen-15 Techniques Introduction of the 15N technique has provided a means for reexamining earlier concepts. The 15N technique has been used in both laboratory and ﬁeld experiments in the following types of studies: • Gross rates of N mineralization associated with the decomposition of plant residues (Watkins and Barraclough, 1996). • As an indicator of SOM turnover (Ladd, 1981; Tiessen et al., 1984; Muller and Sundman, 1988; Cadisch et al., 1993; Green and Blackmer, 1995). • Using 15N nuclear magnetic resonance (NMR) in decomposition studies (Knicker and Lüdemann, 1995). b. Carbon-14-Labeled Residues 14C-labeled organic compounds have been used extensively to study the rate of decomposition and stability of microbial products (Voroney et al., 1991; Amato and Ladd, 1992; Alvarez et al., 1995; Cheshire and Chapman, 1996; Hassink and Dalenberg, 1996; Nelson et al., 1996; Saggar et al., 1996). The use of 14C-labeled residues makes it possible to follow the decomposition of added residues with considerable accuracy, even in the presence of large amounts of native SOM. It has also been possible to identify plant C as it becomes incorporated into fractions of the soil humus (Stevenson, 1986). 14CO evolved from 14C labeled crop residues is absorbed in alkali solution as 2 described earlier, and the radioactivity of absorbed 14CO2 is determined with a scintillation counter (Amato et al., 1984; Ladd et al., 1995; Saggar et al., 1996). 13C NMR (Inbar et al., 1989; Baldock and Preston, 1995; Preston, 1996) and bomb 14C techniques have also been used in studying the annual inputs of C, the rate of residue decomposition, and turnover time (O’Brien and Stout, 1978; O’Brien, 1984; Goh, 1991). c. Variation in Natural Stable Carbon Isotope Ratio (13C/ 12C) The stable C isotope ratio (13C/ 12C) of SOM is very similar to that of the vegetation growing on it. Plants with a C3 photosynthesis pathway have a low 13C/ 12C ratio compared to those with C4 pathway, and thus at sites where the vegetation has changed from one photosynthetic pathway type to the other, the 13C/ 12C ratio of SOM changes and this change can be measured (Gregorich et al., 1996). This approach has been used to evaluate the decomposition in ﬁeld studies (Balesdent and Mariotti, 1996), but is limited to sites where information on the time of change of vegetation is known. However, the method has considerable potential

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as it characterizes C produced in situ, which can be used to evaluate soil C dynamics over short (1 year) or long (>100 years) time periods (Gregorich et al., 1996; Gregorich and Janzen, 1998). d. Sulfur-35 and Phosphorus-32 The short half-lives of 32P (14 days), 33P (25 days), and 35S (87 days) make them less appropriate than 14C (half life 5730 years) for studies of SOM dynamics, particularly in long-term studies in the ﬁeld (Lefroy et al., 1995). Despite these limitations, these techniques have been used successfully in studies of the breakdown of the residues labeled with 33P (Friesen and Blair, 1988), 31P NMR (Gressel and McColl, 1997) and 35S (Lefroy et al., 1994) and in the dynamics of 35S in fractions of SOM (Eriksen et al., 1995). 7. Other Methods The strength of decomposing wheat internode walls as measured by needle penetrometers (Harper, 1989), breaking strength of residue (McCalla, 1943; Armburst, 1980), water, and alkali soluble color of decomposing residues (Pauli, 1970; Harper and Lynch, 1981; Harper, 1989) has been determined and related to the decomposition of residues, but these methods generally work well only during initial stages of decomposition.

C. MODELING DECOMPOSITION OF CROP RESIDUES Theoretical and predictive models have been developed to describe the decomposition process mathematically. Most of these models were based on the concepts of microbial activities. These concepts have been abstracted and simpliﬁed by Paustian et al. (1997) as shown in Table III. The simple ﬁrst-order model has been used most frequently to characterize the decomposition of crop residues (Meentemeyer, 1978; Aber and Mellilo, 1982; Table III), where the decomposition rate was related to substrate remaining and thus mass loss followed the exponential decline. Single component ﬁrst-order decay models [Eq. (5)] was used by Jenny (1941) to describe N loss in cultivated soil as dX/dt A k X,

(5)

where X is the soil organic C or N content, A is the addition rate, and k is the ﬁrstorder rate constant (i.e., the fraction of soil C or N decomposed each year). This model was ﬁtted to residue decomposition data from several ﬁeld experiments and the calculated turnover times (1/k) varied from 18 to 36 years (Gregorich and Janzen, 1998). The use of ﬁrst-order kinetics to describe decomposition implied

Table III Overview of Some Litter Decomposition Models and Their Representation of Litter Quality Effects on Decompositiona

Model type/author

No. of litter compartments

Elements

Decomposer biomass

Litter quality representations/other comments

Single pool-models Aber and Mellilo (1982) Berg and Ekbohm (1991) Janssen (1984) Meentemeyer (1978) Middleburg (1989)

1 1 1 1 1

C, N C C C C

No No No No No

Lignin to N Finite asymptote included Decomposability deﬁned by “apparent initial age” Lignin; used for regional comparisons Decomposability deﬁned by “apparent initial age,” speciﬁc rate constant decreases with time

Multiple litter pool models Berg and Agren (1984) Jenkinson (1977) Minderman (1968) Moorhead and Reynolds (1991)

2 2 6 4

C C C C, N

No No No Yes

Parnas (1975)

2

C, N

Andren and Paustian (1987)

4

C

Growth rate only No

Flux from resistant to labile component to represent solubilization Two-pool determination from curve ﬁtting Approximate analyses to determine initial pool size Labile, holocellulose, and resistant initial litter fractions; includes lag time due to microbial colonization C and N and C only compounds distinguished

nab na na

C C, N, P, S C

No Steady state No

Continuous spectrum models Carpenter (1981) Bossatta and Agren (1991, 1994) Boudreau (1992) a

From Paustain et al. (1997). Not applicable.

b

Soluble and insoluble fractions used for initial pool sizes; formation of secondary products Quality parameter derived from curve ﬁttting Quality parameters have been related to chemical fractionations Gamma distributions for reactivities

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that the metabolic potential of the soil microbial biomass exceeds the substrate supply; more explicitly, changes in microbial biomass are not associated with changes in the rate of decomposition (Gregorich and Janzen, 1998). This model treated the litter as a uniform, homogeneous substrate and explained only shortterm decomposition rates (1 year) (Paustian et al., 1997) as it is well known that different plant components are not uniform in quality and different litter constituents decompose at different rates (Berg and Agren, 1984; Moorhead and Reynolds, 1991). This was also shown in radio-carbon dating studies showing that the age of SOM ranged from hundreds to thousands of years (Campbell, 1978; Stout et al., 1981; Goh et al., 1984). To explain different decomposition rates, multiple litter pool models have been developed based on the inherent decomposability and the factors that inﬂuence decomposition rates of different constituents of litter (Table III). Such multiple models generally assumed that each litter constituent had its own potential decomposition rate and each component decomposed independently according to ﬁrst-order relation (Table III; Berg and Agren, 1984; Moorhead and Reynolds, 1991). In these models the most common litter quality attributes were deﬁned chemically (e.g., lignin, N content, C/N ratio, cellulose, polyphenols). Juma and McGill (1986) deﬁned the litter components as structural and metabolic components, having slow versus rapid decomposition rates, respectively. In an alternative approach, litter was considered as a continuous distribution of organic materials rather than a set of discrete pools (continuous spectrum models; Table III). This approach incorporated both the inﬂuence of initial litter composition and the transformation of primary litter compounds into secondary materials and subsequent effects on the overall decomposition rate (Carpenter, 1981; Bossatta and Agren, 1991, 1994). In another novel variation of the multiple ﬁrst-order pool models, Sinsabaugh and Moorhead (1994) proposed that decomposition rates were directly related to enzyme activity and they measured litter quality by assays of potential enzyme activity. An important conclusion from these models was the realization that in order to predict total mass loss and the decomposition of primary litter fractions, the formation and turnover of secondary decay products need to be included. This led to the development of OM turnover models, where OM pools representing secondary decomposition products (e.g., microbial biomass, SOM) are included (e.g., CENTURY, Parton et al., 1987; Rothamstead model, Jenkinson et al., 1987; CERES model, Godwin and Jones, 1991). The SOM and N submodels were used where litter OM and N were partitioned into different compartments according to decomposabiltity based on chemical composition. These submodels, which predict decomposition and the flow of inorganic and organic nutrients, are combined with soil water and plant growth submodels to simulate cropping system dynamics.

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IV. CROP RESIDUES AND MANAGEMENT PRACTICES Several major options available to farmers in the management of crop residues include (i) burning, (ii) incorporation, (iii) direct drilling in surface residues, (iv) undersowing crops, and (v) baling and removing crop residues. i. Residue Burning. One of the main advantages of burning crop residues is that it clears the land quickly of residues before the next crop is established, especially in high-intensity cropping areas, thus facilitating seed germination and establishment. The extent to which residues are burnt inﬂuences its effectiveness, but this varies depending on the method, timing of burning, and moisture content of residues. Total cover burning is preferred for controlling residue-borne diseases (Staniforth, 1982; Butterworth, 1985) but this is seldom achieved. When cool, wet conditions prevail after harvest, a good burn will not be achieved if the residues are too wet. Felton et al. (1987) advised that when burning of straw was inevitable, it should be burned out after seasonal rain periods to minimize leaching losses of nutrients released after burning. ii. Incorporation of Residues. Crop residues may be incorporated partially or completely into the soil depending on methods of cultivation used (Dormaar and Carefoot, 1996). This results in various degrees of ground cover by the residues, which has important implications for controlling water and wind erosion. In certain parts of the United States where erosion is prevalent, farmers are required to carry out “conservation tillage” to maintain a minimum ground cover by residues of 30% (Grifﬁth and Wollenhaupt, 1994). Ploughing is the most efﬁcient residue incorporation method (Ball and Robertson, 1990; Christian and Bacon, 1991). Methods of incorporation can affect yield and decomposition. Deep incorporation may reduce possible yield depressions, but slows down cultivation and requires more time, labor, and energy costs (Chaman and Cope, 1994). iii. Direct Drilling in Surface Mulched and Unmulched Residues. Direct drilling is a practice that leaves straw residues from a previous crop on the soil surface without any form of incorporation; the residues may or may not be mulched. The following crop is then drilled directly into the soil with residues left on the soil surface. The large volume of residues remaining on the surface often leads to machinery failure, thus affecting the proper sowing of the seeds of the following crops (Staniforth, 1982; McGuigan, 1989). iv. Undersowing Crops. This involves either undersowing a cereal crop with clover or drilling grass, forage legumes, or green manures into cereal stubble (Scott et al., 1973; Badaruddin and Meyer, 1989; Deo et al., 1993). Grazing animals are also important in this system as they usually graze off and or trample the cereal stubble after its harvest (Fraser and Francis, 1996) and contribute to the

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nutrients cycling through their dung and urine returns (Haynes and Williams, 1993). v. Baling and Removing Straw. Concurrent with the movement toward reduced tillage systems in production agriculture is the increasing need for valueadded processing from outside agriculture as a means of diversifying production and stabilizing income levels (Stumborg et al., 1996; Latham, 1997; Powell and Unger, 1997). Surplus straw from agriculture (especially poor quality) may be used for a number of useful purposes such as stock feed, fuel, building material, livestock bedding, composting for mushroom cultivation, bedding for strawberries, cucumbers, melons, and other crops, mulching for orchards, and sources of chemicals. For example, the Cochrane group (1994) estimated that the net return to the producer in the United States was $20.84 less the cost of nutrients lost in the straw ($4.81 ton1) based on values of crop residues and their acquisition costs. These estimates did not include the value of crop residues in increasing or maintaining SOM and soil structure.

A. EFFECTS OF RESIDUES AND MANAGEMENT ON SOIL QUALITY 1. Soil Quality Indicators The increasing awareness of the progressive degradation of soils has led to the search for a reliable measure of soil quality. Traditionally, soil productivity has been used as a measure for soil quality (Hornick, 1992). More recently, the concept of soil quality has been suggested by several authors as a tool for assessing the longterm sustainability of agricultural practices at local, regional, national, and international levels (Lal, 1991; Sanders, 1992; Papendick and Parr, 1992; Parr et al., 1992; Karlen et al., 1992; Acton and Padbury, 1993; Doran and Parkin, 1994; Gregorich et al., 1994). Soil quality has been deﬁned as “the capacity of a soil to take up, store, and recycle water, minerals, and energy so that crop production is maximized and environmental degradation is minimized” (Trasar-Cepeda et al., 1998). Brookes (1989) stated that a high-quality soil should (i) assist in the reduction of contaminant levels in surface and subsurface waters, (ii) allow the production of healthy and nutritious crops, and (iii) display key characteristics of ecosystem maturity. Soil quality depends on a large number of physical, chemical, biological, and biochemical soil properties, and its characterization requires the selection of the properties most sensitive to the management practices as soil quality indicators (Elliott, 1994; Doran et al., 1994; Cameron et al., 1996). Crop residue management is known to either directly or indirectly affect most of these indicators. It is perceived that soil quality is improved by the adoption

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of crop residue management practices. Karlen et al. (1994) evaluated several proposed soil quality indicators and developed a “soil quality index” based on several parameters. Their results gave ratings of 0.45, 0.68, or 0.86 for removal, normal, or double residue treatments, respectively. Effects of crop residue management on soil properties are reviewed herein. 2. Soil Physical Properties Crop residues play an important role in maintaining good soil physical conditions. In most climates, the removal of all crop residues from the ﬁeld leads to a deterioration of soil physical properties (Kladivko, 1994). a. Soil Erosion The presence of crop residues on the soil surface is known to reduce both wind and water erosion of soil either directly by affecting the physical force involved in erosion or indirectly by modifying the soil structure through the addition of organic matter (Brown et al., 1989; Franzluebbers et al., 1996; Lafond et al., 1996). Crop residue amendments have been shown to restore yields on desurfaced or artiﬁcially eroded soils (Dormaar et al., 1988; Larney and Janzen, 1996), probably due to an increase in soil aggregate stability (Cresswell et al., 1991; Sun et al., 1995). Flat residues as a mulch on the soil surface act as a barrier restricting soil particle emission from the soil surface and also increasing the threshold wind speeds for detaching these particles. It has been reported that standing residues are more effective than ﬂat residues in reducing erosion by reducing the soil surface friction velocity of wind and intercepting the saltating soil particles (Hagen, 1996). The greater the amount of residues left on the surface, the greater the reduction in wind erosion (Michels et al., 1995). Many studies reported that incorporating residues to various degrees can reduce runoff and hence water erosion losses of soil by 27–90% (Freebairn and Boughton, 1985; McGregor et al., 1990; Cassel et al., 1995; Fawcett, 1995). However, these reductions mainly occur where considerable amounts of residues remain on the soil surface after incomplete incorporation (Freebairn and Boughton, 1985; McGregor et al., 1990; Dormaar and Carefoot, 1996). b. Soil Aggregation and Soil Structure Information on the effect of crop residue management on soil aggregation is limited, although it is well established that OM helps maintain aggregate stability (Tisdall and Oades, 1982; Oades, 1984; Hamblin, 1987; Boyle et al., 1989; Haynes et al., 1991; Haynes and Francis, 1993). The addition of crop residues is expected to have a positive effect on soil structure and aggregation (Freebairn and Gupta, 1990).

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Nuttall et al. (1986) reported over a period of 6 years in the Canadian prairies that aggregates >0.84 and 12.7 mm in size were most abundant with crop residues chopped and spread and were least abundant with autumn (fall) ploughing. Values for spring burning of crop residues were intermediate. However, other residue management treatments (removal, incorporation, twice the amount incorporated and burning) did not signiﬁcantly affect aggregation in a 14-year study at Kansas (Skidmore et al., 1986). However, Karlen et al. (1994) found that the normal rate of crop residues increased soil aggregation following no-till corn compared to removal and that the doubling of crop residue amounts increased soil aggregation and stability signiﬁcantly. Singh et al. (1994) reported greater amounts of large water-stable aggregates in no-till straw treatments. The proportion of wind-erodible (1 mm) and waterslakable microaggregates (0.25 mm) was also lower, and mean weight diameter (MWD) and geometric mean diameter (GMD) were greater compared to tillage treatments. An increase in GMD of aggregates has been observed within a week when small amounts of residue were added due to a ﬂush of fungal growth, but when a large amount of residue was added, the increase in GMD was observed after only 6 weeks (Hadas et al., 1994). It is suggested that size and strength of aggregates apparently caused by fungi increased during the ﬁrst week due to external reinforcement by hyphae, whereas changes appearing after only 6 weeks were attributed to bacteria and due to internal reinforcement by bacterial secretions (Hadas et al., 1994). Signiﬁcantly higher levels of ergosterol, a sterol related to fungal biomass (Eash et al., 1994), were found in plots receiving crop residues. This suggests that long-term crop residue treatments were affecting fungal populations at this site and that the quality of residues also affected the formation and stability of aggregates (Hadas et al., 1994). Thus the proper management of residues can provide farmers with some measure to mitigate changes due to implements and trafﬁc loads imposed during the cropping cycle (Tate, 1987) and also improves soil structure and aggregation. c. Compaction, Bulk Density, and Penetration Resistance In the mixed pasture-arable cropping system, there are more chances of soil compaction due to farm equipment trafﬁc (Wagger and Denton, 1989; Unger, 1986) and annual trafﬁc (Abel-Magid et al., 1987; Dao et al., 1994). Under the weight of soil mass, the impact of raindrops, or the compactive pressure of trafﬁc, soil particles reorient themselves and pack together more tightly, while excluding air and water contained within, resulting in higher bulk density and penetration resistance. However, surface and subsurface soil density and penetration resistance may increase naturally when using a no-tillage system (Ehlers et al., 1983; Mielke et al., 1986) resulting from the raindrops effect and the structural failure (collapse) of soils having low-stability aggregates (Bautista et al., 1996). Soils with a high

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sand content are especially prone to develop a dense zone with high penetration resistance (Awadhwal and Smith, 1990). The effect of residue management and tillage has been found to be variable. Some workers reported no effect (Blevins et al., 1977; Hill, 1990; Ismail et al., 1995) whereas others found lower soil bulk densities in a conservation tillageresidue management system (Edwards et al., 1992), residue incorporation (Sidhu and Sur, 1993), and no-tillage surface residue (Dao, 1996). Bulk density has been observed to be higher in conservation tillage and no-tilling residue ﬁelds, as soils continually consolidated in the absence of tillage or with shallow tillage (Voorhees and Lindstrom, 1983; Pikul and Allmaras, 1986; Larney and Kladivko, 1989; Rasmussen and Smiley, 1989; Unger, 1986). In most studies, increases in soil bulk density and compaction were reported in crops that were seeded in comparatively wide rows (0.7 to 1.0 m), such as corn (Johnson et al., 1989; Vyn and Raimbault, 1993), soybean (Fahad et al., 1987), or sorghum (Bruce et al., 1990) crops. d. Soil Hydraulic Conductivity and Inﬁltration Crop residues increase soil hydraulic conductivity and inﬁltration by modifying mainly soil structure, proportion of macropores, and aggregate stability. These increases have been reported in treatments where crop residues were retained on the soil surface or incorporated by conservation tillage (Murphy et al., 1993). Up to eightfold increases in hydraulic conductivity in zero-tillage stubble retained have been reported over treatments where stubble was removed by burning (Bissett and O’Leary, 1996; Valzano et al., 1997). Hydraulic conductivity under straw-retained direct drilled treatments was 4.1 times greater than that of straw-burnt conventional tillage treatments (Chan and Heenan, 1993). Observations (Hanks and Anderson, 1957; McMurphy and Anderson, 1965) of frequently burned rangelands in Kansas Flint Hills in the United States indicated that annual burning reduced the inﬁltration rate. These differences could be related to the effect of ﬁne noncapillary porosity of the topsoil, decreases in porosity of crust (Veckert et al., 1978, Pikul and Zuzel, 1994), or the development of water-repellent soil surface from hydrophobic compounds of plant residue (Scifres and Hamilton, 1993). Another reason may be leaching of ﬁne particle of ash or dispersed clay to lower layers and clogging the pores, breaking the continuity of pores (Araujo et al., 1994). This decrease in inﬁltration rate results in greater runoff and erosion (Robichand and Waldrop, 1994). Baumhardt and Lascano (1996) observed increases in cumulative infiltration from 29 mm for bare soil to as high as 49 mm under different residue and management practices. Cassel et al. (1995) reported that tillage practices that leave crop residues on the soil surface can reduce or eliminate surface crusting, increase infiltration, and reduce surface runoff and soil loss while increasing crop yields.

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Thus, although increases in soil hydraulic conductivity and inﬁltration can result through proper residue management practices, care should be taken because increased inﬁltration may lead to leaching, causing nutrient losses and groundwater pollution. e. Soil Temperature Crop residue management inﬂuences soil temperature signiﬁcantly. Major mechanisms involved are (a) a change in radiant energy balance and (b) insulation (Unger and McCalla, 1980). The radiation balance is inﬂuenced by the heating of air and soil, the evaporation of soil water, and the effect of incoming radiation by surface residues (Van Doren and Allmaras, 1978). Residue characteristics involved in the reﬂectance of incoming radiations include residue age, color, orientation, distribution, and amount (Unger and McCalla, 1980). The insulation effect of crop residues is controlled by the amount and associated thickness of residue cover. In arid and semiarid regions or in summers, crop residues left on the soil surface as a mulch as compared to incorporation, removal, or burning are known to be beneﬁcial for crop production (Dao, 1993; Tian et al., 1993b). This reduces the soil temperatures, thus inﬂuencing the biological processes (Hatﬁeld and Prueger, 1996), and enhances soil N mineralization (Tian et al., 1993a). In temperate areas, residue burning, removal, or incorporation provided greater yields because residues left on the soil surface reduced the seed zone soil temperatures, resulting in poor or delayed germination and poor crop growth and grain yields (Schneider and Gupta, 1985; Kaspar et al., 1990; Burgess et al., 1996; Swanson and Wilhelm, 1996). The removal of residue from a wider strip of row area may beneﬁt plant growth in some areas by increasing soil temperature in the row (Kaspar et al., 1990; Swan et al., 1996). f. Soil Moisture Content It has been well established that increasing amounts of crop residues on the soil surface reduce the evaporation rate (Bussiere and Cellier, 1994; Gill and Jalota, 1996; Prihar et al., 1996). Thus, residue-covered soils tend to have a greater moisture content than bare soils except after extended drought (Tanaka, 1985; Thomas et al., 1990; Felton et al., 1995; Cantero-Martinez et al., 1995; Bissett and O’Leary, 1996; Moitra et al., 1996; Peterson et al., 1996). Residue mulch or partial incorporation in soil by conservation tillage has also been shown to increase the inﬁltration by reducing surface sealing and decreasing runoff velocity (Box et al., 1996). Studies have shown that soils retained more moisture when residues were retained on the soil surface by conservation tillage as compared to residue incorporation, removal, or burning (Osuji, 1984; Freebairn et al., 1986; Unger, 1986; Dormaar and Carefoot, 1996). Boyer and Miller (1994) reported a 27 and 18% decrease in surface and subsurface soil water-holding capacities, respective-

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ly, in burned treatments as compared to nonburn. The amount of residue cover is also important in determining the moisture retention in soil (Power et al., 1986a; Wilhelm et al., 1986). In some studies, however, workers failed to obtain a signiﬁcant difference in soil moisture content between no-tillage residue retained and conventional tillage with residue incorporated, removal, or burning (Hill et al., 1985; Nuttall et al., 1986). These results may occur because of extended dry periods or because amounts of residue present were too low to be signiﬁcant. 3. Soil Chemical Properties a. Soil pH One of the most important factors determining soil fertility is pH, which may, however, be inﬂuenced strongly by cultivation and crop residue management. It has been noted in Australia, New Zealand, and many other parts of the world that soil pH decreases as a result of continuous cultivation of clovers and other leguminous crops (Mengel and Steffens, 1982; Juo et al., 1996) attributed mainly to BNF (Bolan et al., 1991) and to proton release by legume roots (Schubert et al., 1990), resulting in the accumulation of organic anions such as malate, citrate, and oxalate in plants (Bolan et al., 1991). Research has shown that if these organic anions are returned to the soil and on decomposition by microorganisms, soil pH can be increased due to the decarboxylation of organic anions (Yan et al., 1996), ligand exchange (Hue and Amien, 1989), and addition of basic cations (Bessho and Bell, 1992). Thus, one possible way of protecting soil from acidiﬁcation is by returning the crop residues to the soil (Miyazawa et al., 1993; Yan et al., 1996). Kretzschmar et al. (1991) showed that the ﬁeld application of crop residues for 6 years increased soil pH signiﬁcantly from 4.54 to 5.69. This was also reported by other workers (e.g., Hue and Amien, 1989; Bessho and Bell, 1992; Hafner et al., 1993; Karlen et al., 1994). Increases in pH by pearl millet straw (Pennisetum americanum) have been reported to decrease aluminium toxicity in acidic soils (Kretzschmar et al., 1991). Increases in soil pH occurred irrespective of whether crop residues were burnt, incorporated, or mulched (Kretzschmar et al., 1991; Ball-Coelho et al., 1993; Kitou and Yoshida, 1994). In some areas, tillage-induced reductions in pH have been reported, but the direct drilling through surface residue showed only a minor decrease in soil pH (Smettem et al., 1992). Differences in the magnitude of pH change may be because different plants species differ in their capacities in accumulating organic anions. Legumes accumulate higher amounts of organic anions than grasses (Mengel and Steffens, 1982). Legume residues often induced greater increases in soil pH than grasses or other crop residues (Hue and Amien, 1989; Bessho and Bell, 1992; Miyazawa et al., 1993). However, plants contain a large amount of organic N, such as proteins

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and amino acids, which can be mineralized to nitrate in soils producing protons during nitriﬁcation and hence acidifying the soils (Yan et al., 1996). Increases in pH after burning were generally attributed to ash accretion (Raison, 1979; Goh and Phillips, 1991; Bauhus et al., 1993; Araujo et al., 1994; Van Reuler and Janssen, 1996) as ash residues are generally dominated by carbonates of alkali and alkaline earth metals but also contain variable amounts of silica, heavy metals, sesquioxides, phosphates, and small amounts of organic and inorganic N (Raison, 1979). b. Soil Organic Matter and Nitrogen Accumulation of SOM is a reversible process and most current agricultural practices are responsible for its reduction in agroecosystems ( Jenkinson, 1981; Rasmussen and Collins, 1991; Heenan et al., 1995) with the consequent decrease of soil biological fertility and soil resilience (Lal, 1994) through an impoverishment of physical, chemical, and biological properties of soils (Kirchner et al., 1993; Wood and Edwards, 1992; Perucci et al., 1997). This can only be practically compensated by burying the crop residues in the soil. Thus, the practice of residue incorporation may be better able to sustain arable soils and represents an interesting method of managing soil fertility (MacRae and Mehuys, 1985; Prasad and Power, 1991; Geiger et al., 1992; Aggarwal et al., 1997). This has implications on amounts of soil microbial biomass as well as labile soil C and N content (McGill et al., 1986; Ross, 1987; Gupta et al., 1994; Heenan et al., 1995; Campbell et al., 1996a,b), thereby inﬂuencing the turnover of soil organic matter and consequently the availability of nutrients for crops (Aggarwal et al., 1997; Perucci et al., 1997). The addition of crop residues on OM and N increases may depend on the amount of residue added, quality of the residue environment, and the duration of addition. Short-term addition in hot climates, promoting rapid decomposition, may lead to only a slight or no increase in soil OM (Aggarwal et al., 1997), but long-term addition has been shown to increase both C and N contents (Karlen et al., 1994). The effect of crop residues on SOM content is related strongly to the amount of residues added and only weakly related to the type of residue applied (Rasmussen and Collins, 1991). Earlier studies in the United States (Larson et al., 1972), Canada (Sowden, 1968), and Germany (Sauerbeck,1982) concluded that different types of crop residues had similar effects on SOM and that it is more a function of microbial product recalcitrance than initial residue composition (Voroney et al., 1989). Several studies showed that organic C and N in soil responded linearly to an increased rate of residue addition (Larson et al., 1972; Black, 1973; Rasmussen et al., 1980; Karlen et al., 1994). Soil C and N were found to decrease with time for all residue additions except manure (Fig. 1), and the rate of decrease was related to the level but not the type of residue returned to the soil (Fig. 2) (Rasmussen and Collins, 1991). Losses of OM will continue in many of the present

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Figure 1 Effect of management practices on long-term changes in organic C in the top 30 cm of a haploxeroll soil in Oregon. From Rasmussen and Collins (1991).

cropping systems without adequate amounts of residue return to soil. While residue input may increase OM content, continued input must be sustained (Rasmussen and Collins, 1991) as some long-term studies have shown that OM and N increased compared to removal or burning (Karlen et al., 1994; Dalal et al., 1995; Perucci et al., 1997). Many reviews comparing different management strategies such as no-tillage, conservation tillage, and conventional tillage on conserving soil C and N have been reported (Prasad and Power, 1991; Rasmussen and Collins, 1991). In general, conservation tillage increased the amount of soil C and N as reported in many parts of the world (Table IV). More recently, greater C and N have been reported under no-tillage or conservation tillage compared to mould board plough or conventional tillage (Campbell et al., 1995, 1996a,b). These increases were greater in clay soils compared to sandy soils (Campbell et al., 1996a,b) due to the physical protection of organic matter by clay (Van Veen and Paul, 1981; Hassink and Whitmore, 1995). In other studies, increases in C and N were reported but were conﬁned to the soil surface (25 mm) (Dalal et al., 1991, 1995; Angers et al., 1997). However, Angers et al. (1997) found that C and N contents in 0- to 10-cm soil layer were higher under no-tillage compared to ploughing, whereas in deeper layers (20–40 cm depth), the trend was reversed. When all soil depths were combined, no differences between treatments were found in humid soils of eastern Canada.

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Likewise, no effects between tillage treatments were reported by Franzluebbers and Arshad (1996) from Canada and by Fettell and Gill (1995) from red brown earth soils in Australia. The small or even no effects reported in some studies may be due to the high initial C and N content in soils, making it difﬁcult to detect signiﬁcant increases in C and N as a result of treatments against a large background (Campbell et al., 1991a,b,c; Rasmussen and Collins, 1991). For example, using a site under subterranean clover pasture for some years with high initial C and N levels, Heenan et al. (1995) showed that under subterranean clover–wheat rotation, direct drill residue-retained treatment increased soil C and N content continuously, whereas under either grazing or cultivation, C and N were reduced. In all other treatments, C and N decreased. These decreases were more when residues were burnt or when soil was cultivated. The effects were additive in burnt and tilled soils. Whether in lupin–wheat or continuous wheat, residue retention and direct drilling showed a smaller decrease in the C and N ratio compared to cultivation and burning (Heenan et al., 1995). Decreases in C and N contents have been observed under residue burnt treatments (Pikul and Allmaras, 1986; Biederbeck et al., 1980; Wood, 1985; Collins et al., 1992; Heenan et al., 1995). Shultz (1992) showed that burning caused a decline of C and N by 6% overall for wheat/legume rotation and continuous cereal and wheat fallow rotation, whereas a 1% increase was obtained under stubble retention. However, in other studies, no effect of burning compared to residue incorporation on soil C content was reported (Nuttall et al., 1986; Rasmussen et al., 1980). However, soil N levels were reduced by residue burning (Rasmussen et al., 1980). This may be due to incomplete burning as up to 33% of the C in charred

Figure 2 Effect of the rate of carbon input on organic C change in a haploxeroll soil in Oregon. From Rasmussen and Collins (1991).

240

K. KUMAR AND K. M. GOH Table IV

Effect of Conservation Tillage on Organic C and N in Soil in Different Parts of the Worlda

Location and soil South Africa Haploxeralf Haploxeralf Germany “Podsol” “Podsol” “Podsol” Australia Western Psamment Alﬁsol Alﬁsol Queensland Pellustert Canada Saskatchewan Chernozem United States North Dakota Haploboroll Haploboroll Argiboroll Kansas Haplustoll Nebraska Haplustoll Oregon Haploxeroll Washington Haplxeroll Mean Minimum Maximum

Increase (% year1)

Annual precipitation (mm)

Soil depth (cm)

Duration of study (year)

Tillage systemb

C

N

412 412

10 10

10 10

TT NT

5.6 7.3

3.4 5.1

— — —

30 30 30

5 5 6

NT NT NT

3.2 2.4 1.3

1.4 1.6 1.3

345 307 389

15 15 15

9 9 9

NT NT NT

1.6 0.7 1.4

— — —

698

10

6

NT

1.2

1.3

—

15

6

NT

6.7

2.8

375 375

45 45

25 25

SM SM

1.8 0.1

1.3 0.1

375 —

45 15

25 11

SM NT

0.5 0.7

0.4 0.6

446 446 416

9 10 15

15 15 44

NT NT SM

2.8 1.2 0.3

2.4 1.0 0.4

560

5

10

NT

1.9

2.0

— — —

— — —

— — —

— — —

2.2 0.1 7.3

1.7 0.1 5.1

a

From Rasmussen and Collins (1991). TT, tine till; NT, no till; SM, stubble mulch.

b

residues remained on the soil surface after burning and the mulch of the charred residues left was not biologically active (Rasmussen and Collins, 1991). It is likely that burning changes the quality rather than the quantity of OM in soil.

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c. Phosphorus Most workers reported increased phosphorus (P) accumulation near the soil surface in no-tillage or minimum tillage systems (Langdale et al., 1984; Follett and Peterson, 1988; Weil et al., 1988). Stratiﬁcation of P resulting from minimum tillage is believed to result in improved P availability because there is less soil contact of organic P in crop residues and hence less P ﬁxation (Blevins et al., 1983). Burning cereal residues also resulted in a higher extractable P content in the surface 0- to 2.5-cm soil layer (Nuttall et al., 1986) and increased plant uptake (Van Reuler and Janssen, 1996). Compared to residue removal, the incorporation of residues of cluster bean, mung bean, and pearl millet has been found to increase available P, probably due to an increase in phosphatase (both acid and alkaline) enzyme activity (Aggarwal et al., 1997). d. Other Nutrients Most studies on plant decomposition have focused on N dynamics, with little or no emphasis on other nutrients that are important in describing the crop response to decomposing organic residues on soils with marginal fertility. Even if residue decomposes quickly, nutrients contained in it are not subjected to the same rapid loss as that which occurred under burning (Luna-Orea et al., 1996). Instead, nutrients are released over time by chemical, physical, and biological processes. Considerable quantities are released within a short period of time depending on the residue quality (Luna-Orea et al., 1996) and the kind of nutrients (Lefroy et al., 1995). Increases in exchangeable cations (K, Ca, Mg) and base saturation have been reported (Kretzschmar et al., 1991; Geiger et al., 1992). The greater availability of both macro- and micronutrients has been reported more under conservation tillage than other conventional tillage (Hargrove et al., 1982; Langdale et al., 1984; Hargrove, 1985; Follett and Peterson, 1988; Edwards et al., 1992). The burning of crop residue has also resulted in an increased K content in the surface soil compared with no burning, crop residue removal, or incorporation (Moss and Cotterill, 1985). Although burning induces short-term increases in nutrients, losses of nutrients due to burning can occur. e. Loss of Nutrients Due to Burning of Crop Residues The burning of crop residues can result in nutrient loss as a result of the direct convective transfer of ash (Harwood and Jackson, 1975), and subsequent losses may be increased by the action of wind and water. Simultaneous losses of C, S, and N have been reported on residue burning (Marschner et al., 1995). However, very little is known about the effects of burning intensity and frequency on nutrient losses and dynamics. O’Connor (1974) indicated that in a fairly dense tall tussock grassland stand containing 175 kg N ha1 above ground, about 40 kg N ha1 was lost when such grasslands were burnt every 3 years. In some European studies, an 8 kg N ha1 year1 loss was reported on burning

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postharvest cereal stubble and a loss of 10 kg N ha1 year1 occurred when grass or legume pastures in west Africa were burnt (Winteringham, 1984). A loss of 2600, 4800 kg C ha1 and 742 kg N ha1 has been reported during pre- and postharvest burning of sugarcane trash, respectively (Ball-Coehlo et al., 1993). Animal grazing of pastures has been shown to conserve N loss due to burning as N losses from burning grazed pastures (9 kg ha1) were half of those (18 kg N ha1) from the burning of ungrazed pastures (Hobbs et al., 1991). Likewise, S losses from high S- and low S-containing rice crop residues were 60 and 40%, respectively (Lefroy et al., 1994). High N losses in the form of ammonia volatilization can also occur in soils after stubble burning because of alkaline ash left on the soil surface and an increase in urease activity (Bacon and Freney, 1989). Emissions of ammonia from straw burning were calculated to be equivalent to 20 ktons N year1 in the United Kingdom in 1981, which declined to 3.3 ktons year1 in 1991 as a result of changes in agricultural practices because of an imposed ban on burning crop residues (Lee and Atkins, 1994). The fraction of total plant N released as ammonia was estimated to be between 40 and 80%. These data suggest that emissions of ammonia from straw and stubble burning constituted a signiﬁcant but declining source of ammonia that is currently not accounted for in European and North American emission inventories. If combustion of plant residues is nearly complete, most of the C, H, O, N, and organic S and P are transferred to air, whereas many of the cationic elements are rendered water soluble and hence are readily available to plants (Raison, 1979). Where combustion is inhibited for any reason, the ash will be black and contain residual OM (Jordan, 1965). In general, losses of nutrients due to burning decrease in the order of N Ca S K Mg P Na. These losses depend on the temperatures reached during burning. Volatile losses of P and K occur at temperatures exceeding 500C, whereas the vaporization temperature of Na is reported to be 880C (Raison, 1979). However, these temperatures may not be achieved during the burning of cereal crop residues and grass/legume pastures, and most of these elements are left in the ash. In forestry systems, where large amounts of wood and residues are burnt, higher burning temperatures were reported and greater losses of nutrients occurred (Feller, 1988; Gillon and Rap, 1989; Goh and Phillips, 1991). Also, nutrients that were left in the ash were highly soluble in water and may be prone to leaching and runoff losses (Mangas et al., 1992; Malmer, 1996). 4. Soil Biological Properties Leaving crop residues on the soil surface or incorporation provides a favorable environment for soil and surface residue dwelling organisms because of reduced water loss, amelioration of temperature extremes, ﬂuctuations, and the presence of

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a relatively continuous substrate for decomposers (House and All, 1981; Crossley et al., 1984; Wardle and Lavelle, 1997). Several aspects of crop residue management practices on soil biological properties have been studied. a. Soil Biota i. Microbial Biomass/Microorganisms. The soil microbial biomass (SMB) is the living component of the soil that comprises mainly fungi and bacteria, including soil microfauna and algae. Although it accounts for only 1–3% of organic C and 2–6% of organic N in soil (Jenkinson, 1987), it plays a key role in SOM and nutrient dynamics by acting as both a sink (during immobilization) and a source (mineralization) of plant nutrients. Because SMB and microbial activity are closely related to SOM content, they are inﬂuenced positively by organic amendments such as crop residues and animal manures (Ocio et al., 1991; Collins et al., 1992; Nannipieri et al., 1994). Perucci et al. (1997) reported that SMB-C content was signiﬁcantly greater when crop residues were incorporated in the soil instead of removed. The changes in SMB-C were also related to the type of residues incorporated (Perucci et al., 1984). Biederbeck et al. (1980) reported a decline in microbial biomass when residues were removed by burning. In an similar study, burning of residues reduced SMB to 57% of that in the manured treatment, probably because of the smaller amount of OM returned to the soil (Collins et al., 1992). In addition to crop residues, the no-till system also affects SMB by promoting C accumulation at the soil surface due to a lack of incorporation of crop residues. Results from many studies found that SMB was signiﬁcantly higher in the surface of no-till than in conventional tilled soils (Doran, 1980; Linn and Doran, 1984; Buchanan and King, 1992; Angers et al., 1993a,b). However, taking into account the whole plow layer, smaller differences existed as shown by Franzluebbers et al. (1995), where a 75 to 146% greater SMB-C concentration under no-till than conventional tillage at a 0- to 50-mm depth existed but only 12 to 43% greater SMBC concentration at the 0- to 200-mm depth was present in several wheat experiments. Higher populations of bacteria, Actinomycetes, fungi, earthworms, and nematodes have been reported in residue mulch than in incorporated residues (McCalla, 1958). Residue incorporation produced more microbial activity than residue removal or burning (Beare et al., 1996). Ten to 80% greater aerobic microorganisms and 60 to 300% greater anaerobic bacteria (including denitriﬁers) were reported in surface soils under reduced than conventional cultivated soils (Doran, 1980). This is expected because soil beneath residue has a better microenvironment as compared to conventional tillage. Gupta and Germida (1988) obtained similar results and found fungi to be particularly sensitive to tillage effects. Direct counts of viable bacteria and total fungal hyphae were consistently higher on residue from incorporated treatments compared with removed or burned treatments (Beare et al.,1996).

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Burning of residue has been shown to favor bacteria rather than fungi (Miller et al., 1955; Jones and Richards, 1978). Biederbeck et al. (1980) found that longterm burning decreased the microbial population of the soil permanently. At one location, a decline of 85% in the fungal and 70% in the bacterial population was reported. Raison (1979) also reported a severe decline in the bacteria population involved in nitriﬁcation due to burning. The extent of reduction in microbial populations would depend on the soil temperature rise during burning. Except in ﬁres, where large amounts of accumulated fuel were burnt, heat production is intense but for a short duration. In addition, because only 5% of heat energy released may be partitioned to the soil (Packham, 1969), maximum temperature may not occur near the soil surface but at a variable height in the vegetation (Kenworthy, 1963; Goh and Phillips, 1991). Temperatures below the soil surface cannot exceed 100C until all moisture evaporates (Aston and Gill, 1976). Goh and Phillips (1991) reported a temperature of 60C at the soil–humus interface and, in a few cases, it was as high as 110C. In the soil, increases in temperature are usually less than 50 – 80C and are restricted to the top 3 –4 cm of soil, which persisted for only a few minutes (Lawrence, 1966), and does not always reduce the microbial populations except in forestry systems where large quantities of fuel are burnt. Effects of ash on soil biological properties have not been studied extensively. Goh and Phillips (1991) attributed the increase in ammoniﬁcation and nitriﬁcation on burning to increase, in humus pH and increases in the population of ammonifying organisms, possibly due to the elimination of other competing organisms for the same substrate. In another study, ashed soils have been found to provide conditions favorable to nitrifying bacteria (Bauhus et al., 1993). Decreases in microbial biomass and soil respiration have been reported from treatments where residues were burned or removed in comparison to straw-returned treatments (Gupta et al., 1994; Karlen et al., 1994; Ladd et al., 1994; Beare et al., 1996). In addition to the residue management practice, residue quality has been found to affect the microbial population as less bacterial and fungal populations were reported on grass/cereal residues as compared to legumes. The difference shown is substrate-induced respiration (Parmelee et al., 1989; Beare et al., 1996). Robinson et al. (1994) observed that leaves and internodes of wheat (differing in C/N ratio and physical structure) support vastly different fungal communities. This is true especially for fungi, which depend on resource quality (Wardle et al., 1993), but bacteria seem to be less affected compared to fungi (Cornejo et al., 1994; Wardle, 1995). Biological assays have shown that bacterial communities were also affected as total substrate activity and functional diversity were higher for bacteria in incorporation treatments compared with burning treatments (Beare et al.,1996). ii. Soil Fauna. Many studies have shown that earthworms and microarthropods may assume a more dominant role in OM decomposition and nutrient ﬂux

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245

patterns in residue management systems (Prasad and Power, 1991; Tian et al., 1993b; Vreeken-Buijs and Brussaard, 1996). Predatory and saprophagus soil arthropods, as well as crop-damaging herbivores such as ground beetles (Coleoptera carabidae), spiders, and decomposer fauna such as earthworms, are reported to be in large numbers in no-tillage residue management treatments compared to conventional tillage treatments (Edwards, 1975; Barnes and Ellis, 1979; House and All, 1981; Blumberg and Crossley, 1983; House and Parmelee, 1985; Tian et al., 1993b; Chan and Heenan, 1995; Buckerﬁeld and Webster, 1996). This increase in macroorganism population and activity under residue retention is due to the fact that the mulch is known to attenuate the increase of soil temperature, decreased diurnal variations, and to retain higher soil moisture in addition to providing food for soil animals (Tian et al., 1993b; Hartley et al., 1994; Buckerﬁeld and Webster, 1996). The beneﬁts of an organic mulch on soil surface were demonstrated by signiﬁcantly higher moisture (34% increase), whereas earthworm density increased by 155% over bare soil (Buckerﬁeld and Webster, 1996). Results from many studies suggest that straw retention increased the population of earthworms and other macroorganisms compared to straw removal or burning (Nuutinen, 1992; Karlen et al., 1994). The burning of crop residues has been shown to reduce both the activity and the population of macroorganisms and it takes several months to 5 years to recover that activity (Neumann and Tolhurst, 1991; Tisdall, 1992; DeCaens et al., 1994; Doube et al., 1994). It has been shown that direct drilled plots had greater population density and biomass of earthworms and cocoons than tilled plots. On direct drilled plots, burning stubble resulted in smaller adult earthworms and a lower density of cocoons (Doube et al., 1994), whereas plots with standing stubble had fewer and smaller adults than where stubble was in close contact with soil. Although crop residue management and their effect on microclimate play a role in determining the population of macroorganisms, the nutritional quality of plant material appears to be more important in inﬂuencing earthworm population (Tian et al., 1993b). As plant residues with different chemical composition vary in their palatability for soil fauna, they are expected to have different effects on soil faunal populations (Tian et al., 1992b). In examining the food preference of leaf litter by earthworm, Hendriksen (1990) found that the earthworm population was signiﬁcantly and negatively correlated with the C/N ratio and polyphenol concentration of plant materials. Similar results have been reported by Tian et al. (1992b), where earthworm populations were negatively correlated to the lignin/N ratio of plant residues, and the population of ants was also signiﬁcantly correlated to the N concentration of plant residues (Tian et al., 1993b). iii. Mycorrhiza. Mycorrhiza play an important role in plant growth and crop production (Bethlenfalvay, 1992). However, effects of soil management prac-

246

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tices on the abundance and activity of mycorrhiza are complex. It is reported that tillage disrupts soil networks of mycorrhizal hyphae and may impair nutrient uptake and reduce crop yields (Evans and Miller, 1990). Cropping with hosts that become colonized extensively with mycorrhiza may affect the quality of crops produced (in terms of relatively greater nutrient uptake) and improve the quality of the crop residues produced. This may improve the soil quality indirectly by improving the aggregate stabiltiy, microbial biomass, and other parameters. However, there is little published evidence on the effects of tillage and residue management on mycorrhizal associations with plants and their effects on soil and plant quality need to be explored. b. Microbial Processes i. Enzymes. Crop residues and tillage have been reported to signiﬁcantly and rapidly (within few weeks) alter the composition, distribution, and activity of the soil microbial community and enzymes (Doran, 1980; Dick, 1984; Magan and Lynch, 1986; Martens et al., 1992; Scagnozzi et al., 1995; Deng and Tabatabai, 1997). Although straw amendments also contain enzymes, the increase in activity in soils with organic residues most likely results from the stimulation of microbial activity rather than the direct addition of enzymes from the organic sources. Soil enzyme activities also respond to tillage practices. Gupta and Germida (1988) compared soils cultivated for 69 years with adjacent grasslands and found that cultivation depressed phosphatase activity by 49% and arylsulfatase activity by 65%. Signiﬁcantly greater activities of acid phosphatase, alkaline phosphatase, arylsulfatase, invertase, amidase, and urease in surface soils under no-till plots have been observed (Dick, 1984). Also, activities of amidohydrolases, glycosidases, and arylsulfatase were generally greater in soil under no-till or mulch treatments compared to chisel plough and mould board plough with or without crop residues (Deng and Tabatabai, 1996a,b, 1997). A higher activity of both acid and alkaline phosphatase and dehydrogenase enzymes due to residue incorporation compared to residue removal or burning treatments has also been reported (Aggarwal et al., 1997). Differences in enzyme activities may be due to differences in the origin, states, and/or persistence of different groups of enzymes in soils (Deng and Tabatabai, 1997). Alkaline phosphatase in soils is believed to be derived entirely from microorganisms because it has not been found in plants ( Juma and Tabatabai, 1977; Tabatabai, 1994). Soil pH also controls either the rate of enzyme activity or the stability of some enzymes (Tabatabai, 1994). Signiﬁcant correlations found between the activity of different enzymes (Ladd and Butler, 1972; Ross, 1975; Frankenberger and Tabatabai, 1981; Deng and Tabatabai, 1997) suggest that tillage and residue management practices have similar effects on the activity of different enzymes involved in C, P, N, and S cycling in soils. Beare et al. (1993) found that

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microbial growth is favored in the minimum tillage soil environment, resulting in greater enzyme activities. ii. Carbohydrates. Carbohydrates inﬂuence soil quality mainly through their role in the formation and stabilization of aggregates (Cheshire, 1979). Lynch and Bragg (1985) reported that the incorporation of organic residues rather than their removal or burning provides a substrate for soils, which in turn will provide polysaccharide-binding agents. It has also been reported that no-tillage usually increases the carbohydrate content of soils (Arshad et al., 1990; Hu et al., 1995) and these changes can occur within a short period of time. However, ploughing to incorporate organic residues tends to encourage the decomposition of native SOM and thus a reduction in available substrate. The balance between these two opposing occurrences will, to a large extent, determine whether the microbial biomass and aggregation stability would increase or decrease (Haynes and Francis, 1993).

B. RESPONSES OF CROP GROWTH AND YIELD 1. Germination, Seedling Establishment, and Growth In general, the early removal of straw by burning has been used by farmers to get rid of the straw from the ﬁeld for better seedbed preparation for the seeding of small seed crops and especially for crops grown for seed production and some cereals (Hamblin et al., 1982; Mason and Fischer, 1986; Chan et al., 1987; Cornish and Lymberg, 1987; Darby and Yeoman, 1994). Since the enforcement or ban on burning in some countries, crop residues are utilized on farm, either incorporated or left on the surface. As direct drilling or minimum tillage leaves the straw from a previous crop on the soil surface without or minimum incorporation, these residues are either mulched or left on the surface as such, depending on the availability of machinery and time with the farmers. In drier and tropical climates, surface straw has been shown to provide better soil environment by reducing the soil temperature and conserving soil water, resulting in better seedling establishment (Osuji, 1990; Weaich et al., 1996). In cooler and humid climates, however, soil warming and seedling emergence can be delayed in systems that leave high levels of surface residues (Schneider and Gupta, 1985; Hayhoe et al., 1993; Darby and Yeoman, 1994; Burgess et al., 1996; Swanson and Wilhelm, 1996), which potentially affect plant growth and yields. These effects were attributed mainly to a considerable reduction in seed zone soil temperatures (Bidlake et al., 1992; Bussière and Cellier, 1994). Poor stand establishment in no-tillage surface residues has been related to difﬁculties in seeding through thick residue mulch (Staniforth, 1982; Felton et al., 1987; dos Santos et al., 1993; Burgess et al., 1996; Swan et al., 1996; Kumar,

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1998), and wet soil makes it more difﬁcult for coulters to cut through the residues. Seed was sometimes pushed into the ground, together with the residue, with little or no soil–seed contact (e.g., Weill et al., 1989; Hayhoe et al., 1993). This low soil–seed contact problem becomes more severe under dry soil conditions (Chastain et al., 1995). Shallowly buried straw may impede drilling and causes puffy seed beds with little moisture retention, which can result in poor rooting (Kirkegaard et al., 1994) and patchy establishment. In a high residue situation, trash wheels or trash whippers may be added to the planting unit, taking care to adjust the height so that residue, but not soil, is removed from the seed row [Ontario Ministry of Agriculture and Food (OMAF), 1993]. The removal of residue from a wider strip of row area may beneﬁt plant growth further in some areas by increasing soil temperature in the row (Kaspar et al., 1990; Swan et al., 1996) and minimizes developmental delays (Fortin, 1993; Swan et al., 1996), but the problem remains for cereals that are seeded in narrow rows. Incorporation of crop residues may result in a temporary immobilization of soil mineral N, with a consequent reduction in early seedling growth and development (Rooney et al., 1966; Burgess et al., 1996). Crop residues retained at the surface or incorporated may produce some phytotoxic allelochemicals, which are known to reduce the germination and seedling growth of crops (White et al., 1989; Chung and Miller, 1995; Weston, 1996). Failures in germination and a reduction in seedling growth have been related to colonization with fungi (Lynch et al., 1981) and other diseases such as take-all (Gaeumannomyces graminis), Rhizoctonia solanii, Phythium spp. (Cook and Haglund, 1991; Kirkegaard et al., 1994; Smiley et al., 1996), and other unidentiﬁed biological effects (Chen et al., 1989). These factors, especially when coupled with other stresses of the environment, including insects and diseases, temperature extremes, nutrient and moisture variables, radiation, and herbicides, often enhance allelochemical production and reduce their degradation, thus increasing the potential for allelopathic interference (Einhellig, 1996). Reduced root growth in high-strength soils is also responsible for patchy growth and losses in yield under direct drill with surface straw retained in some highstrength soils (Cornish and Lymberg, 1987; Kirkegaard et al., 1994). A clear understanding of the mechanism causing germination and growth reductions in this system is required to design management strategies to overcome them. Increasing yield under direct drilling may require more disturbance around the seed, such as with sowing points modiﬁed with a blade ﬁtted to rip or fracture a slot in the soil 5–10 cm below the seed (Chan and Mead, 1990). These points provide a narrow zone of low soil strength and disturb fungal hyphae without excessive disturbance and have improved growth and yield of direct drilled crops (Chan and Mead, 1990). Management practices should concentrate on reduction in the amount of undecomposed stubble present at sowing and appropriate rotation to control the amount and quality of residues left.

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2. Crop Yields Ideally, crop residue management practices should be selected to enhance crop yields with minimum adverse effect on the environment. In the last two or three decades, several workers have examined the effect of residue management practices on the harvested yield of the following crop. Results from these experiments are conﬂicting because of a number of factors involved, including residue quality, management, and edaphic factors and also the health of the previous crop and their complex interactions with various management factors in determining the ultimate crop yield. This indicates that no one residue management system is superior under all conditions. The constraints identiﬁed in the crop production system would guide toward the best option. Thus, effort has been made to relate the effects on yield resulting from various crop residue management practices to the constraints in crop production so that the controversy in the literature can be resolved. Under conditions of optimum fertility, adequate soil water supply, and absence of pests and diseases, grain yields achieved are largely unaffected by the different management practices as shown by several studies (e.g., Biederbeck et al., 1980; Kitur et al., 1984; Undersander and Reiger, 1985; Maurya, 1986; Rasmussen and Rohde, 1988; Wilhelm et al., 1989; Thomas et al., 1990; Njøs and Børrensen, 1991; Prasad and Power, 1991; Rule et al., 1991). Thus, in this situation, pollution and sustainability of the systems are the major considerations. However, under different environmental and edaphic constraints, different residue management practices have been found to show different grain yield trends. In cropping systems, where the previous crop was infested with pests and disease, treatments with burning of residues generally yielded more grains (e.g., Doran et al., 1984; Jenkyn et al., 1995; Sumner et al., 1995; Prew et al., 1995). Under low soil moisture and high temperatures during the growing season, no-tillage surface mulch provided higher crop yields (e.g., Dao, 1993; Tian et al., 1993b; Lafond et al., 1996) because of more water conservation, reduced evaporation (Hatﬁeld and Prueger, 1996; Prihar et al., 1996), and favorable soil temperatures for favorable root growth (Chaudhary and Prihar, 1974; Maurya and Lal, 1981) with an improvement in biological processes (Hatﬁeld and Prueger, 1996) and enhanced soil N mineralization (Tian et al., 1993b) as compared to residue removal, burning, or incorporation. Under low winter temperatures, residue burning, removal, or incorporation provides higher crop yields compared with no-till mulch due to surface residues in no-till mulch reducing the seed zone soil temperatures, resulting in poor or delayed germination and reduction in yields (e.g., Schneider and Gupta, 1985; Kaspar et al., 1990; Bidlake et al., 1992; Hayhoe et al., 1993; Bussière and Cellier, 1994; Burgess et al., 1996; Swanson and Wilhelm, 1996). Where topsoil is susceptible to both water and wind erosion (Geiger et al., 1992; Lafond et al., 1996) and loss of nutrients from the fertile upper soil (Rasmussen

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and Collins, 1991; Geiger et al., 1992; Larney and Janzen, 1996), no-tillage surface residues and reduced tillage provide better grain yields (Geiger et al., 1992) compared to other methods. In situations where allelochemicals are likely to be released from residues, residue burning and removal may provide higher grain yields (White et al., 1989; Chung and Miller, 1995; Stirzaker and Bunn, 1996; Weston, 1996). If high volumes of surface residues are maintained, failure of the seeding machinery results in poor plant populations and a reduction in yields (Burgess et al., 1996; Swan et al., 1996; Kumar, 1998) compared to residue removal, burning, and incorporation. Thus, if proper machinery for incorporating residues is not available, burning or removal of crop residues is the best option (Felton et al., 1987; Burgess et al., 1996). Poor tiller initiation has also been reported to decrease the yields of crops under no-tillage, straw-retained treatments (Rasmussen and Barrow, 1993), whereas the burning of surface residues in no-tillage treatment increased yields. Poor-quality residues (high C/N ratio, high lignin, and polyphenol content) are known to cause the temporary immobilization of inorganic N (Bhogal et al., 1997; Nicholson et al., 1997), resulting in less N uptake and lower yields (Bahl et al., 1986; Carefoot et al., 1994; Beri et al., 1995). These kinds of residues have a good mulching effect (Tian et al., 1993b) and, in some cases, burning or removal of these residues provides greater yields compared to incorporation. However, burning is only a short-term option and may show adverse effects on crop yields and soil C and N if continued (Rasmussen and Parton, 1994). Under conditions of high-fertility soils without any limiting constraint or where long-term additions of crop residues have increased the amount of available N, yield increases by the incorporation of crop residues were usually achieved (Wilhelm et al., 1986; Tian et al., 1993a; Dick and Christ, 1995), especially where the crop demand and availability of nutrients from decomposing residues are synchronized (Becker and Ladha, 1997). Otherwise, nutrients may be leached beyond the rooting zone (Francis et al., 1995; Becker and Ladha, 1997; Myers et al., 1997) and cause groundwater pollution. The effects of various constraints on crop production under variable environments are conceptualized and presented in Fig. 3. Obviously, the choice of the management practices would depend on the identiﬁed constraints in the cropping system. Integrated efforts by soil scientists, agronomists, ecologists, plant scientists, environmentalists, and economists are needed to design a system approach for the best choice of crop residue management practices. 3. Biological Nitrogen Fixation No-tillage and conservation tillage soils have often decreased amounts of soil nitrate compared to cultivated soils (Dowdell and Cannell, 1975; Doran, 1980),

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Figure 3 Conceptual diagram for the selection of residue management practice depending on environment and visualized constraints.

which may affect N2 ﬁxation, as nodulation and N2 ﬁxation in leguminous crops are increased in soils with a low concentration of nitrates (Bergersen et al., 1989; Cowie et al., 1990). Increased N2 ﬁxation under direct drilling as compared to cultivated soils has been reported in many leguminous crops (Harper et al., 1989; Hughes and Herridge, 1989; Wheatley et al., 1995). Crop residues with a wide C/ N ratio or C-rich sources have also been added with the aim of immobilizing the soil inorganic N and to increase N2 ﬁxation (Patterson and LaRue, 1983). Wagner and Zapata (1982) found that dry matter yield and N uptake of reference plants were reduced drastically by sucrose addition, but the N in soybean derived from the atmosphere was increased from 30 to 80%. It is, therefore, apparent that this technique can stimulate N2 ﬁxation by reducing the nitrate concentration in the soil. Similarly, Doughton et al. (1993) reported a close inverse relationship be-

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tween soil nitrate measured at the establishment of chickpea and percentage of N derived from ﬁxation under the wide treatments of prior crop fallowing, N fertilization, or tillage and residue management. However, in case of ﬁeld pea, there was no evidence that direct drilling increased N2 ﬁxation by decreasing the crop uptake of soil N. Because legume species differ in their symbiotic sensitivity to nitrate (Harper and Gibson, 1984), it may not be possible to generalize that direct drilling or incorporation with straw will increase the amount of N2 ﬁxed by legumes (Evans et al., 1997). Although the role of crop residues (with wide C/N ratio) in reducing the inorganic N concentration in soil and thus increasing N2 ﬁxation is well established, in some situations, for example, in acid sandy soils of Niger, west Africa, crop residues addition increased the availability of Mo and P and decreased the concentration of exchangeable Al and Mn, thus enhancing N2 ﬁxation in Arachis hypogea L. (Rebafka et al., 1993). Another factor to be considered is the release of phytotoxins from decomposing straw inhibiting nodulation and N2 ﬁxation (Heckman and Kluchinski, 1995) as residues from certain crops and weeds have allelopathic effects and may affect BNF adversely (Guenzi and McCalla, 1966; Freire, 1984). Several earlier studies on allelopathy have demonstrated that the nodulation of legumes may be inhibited by the decomposing residues of other plant species (Rice, 1971; Weston and Putnam, 1985) and this aspect needs to be studied. 4. Phytotoxicity of Crop Residues The decomposition of some crop residues has been found to have adverse allopathic effects on seed germination, seedling, and crop growth (Elliott et al., 1978; Putnam, 1994) due to substances produced during residue decomposition (Patrick et al., 1963). Microorganisms can produce a large range of substances, potentially toxic to plant roots (Lynch, 1976, as cited by Cannell and Lynch, 1984), but these toxins seldom accumulate in aerobic soils because they are metabolized rapidly by microorganisms. However, adverse effects of decomposing residues on crops under aerobic conditions have been widely reported. For example, Bhowmik and Doll (1982) reported allelopathic effects from weed residues such as giant foxtail and barnyard grass that affected plant growth and reduced corn and soybean yields. Jessop and Stewart (1983) reported that wheat growth was depressed severely by crop residues (50–70% compared with control), with the ranking of rape residue pea residue sorghum wheat straw. Kimber (1973a,b) also reported a marked depression in wheat germination under wheat straw residue treatments. By eliminating factors such as nutrients, soil structure, and soil pathogens, Stirzaker and Bunn (1996) demonstrated that phytotoxic leachates from the cover crop residues of subterranean clover and ryegrass were responsible for the reduced growth of

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vegetable seedlings. These workers found that the phytotoxic period of clover ended after 8 weeks in the ﬁeld and 6 weeks in the laboratory, where conditions favored the decomposition of phytotoxic compounds. The phytotoxic effects of ryegrass lasted longer and were more severe than clover (Stirzaker and Bunn, 1996). Allelopathic effects of rye residue on decomposition on other plant species have also been reported (Rice, 1995; Kessavalou and Walters, 1997). Vaughan et al. (1983) and Nelson (1996) found that phenolic acids, such as ferulic acid, p-coumaric acid, and p-hydroxybenzaldehyde, released from the living and dead tissue of a variety of plant species caused adverse effects on the growth of crops. Both mechanical (cultivating the soil to increase aeration) and chemical strategies exist for reducing the phytotoxic effect of crop residues on seedlings. Chemical strategies include the use of amendments such as calcium peroxide, lime formulations (Davies and Davies, 1981; Lynch et al., 1981), and calcium nitrate (Farquharson et al., 1990) to alter the environment near the seeds. 5. Weed Control and Herbicide Efﬁciency Crop residues may selectively provide weed suppression through their physical presence on the soil surface as a mulch (Facelli and Pickett, 1991; Teasdale et al., 1991; Moore et al., 1994; Martin, 1996) and by restricting solar radiations reaching below the mulch layer (Facelli and Pickett, 1991), by direct suppression (Dyck and Liebman, 1994), due to allelopathy (White et al., 1989; Weston, 1996), and by controlling N availability (Karssen and Hilhorst, 1992; Seibert and Pearce, 1993). Residues of rye and other small grains have been shown to inhibit weed emergence and growth (Shilling et al., 1986), probably due to phytotoxic effects. Crutchﬁeld (1985) reported increased weed suppression at increasing mulch rates up to 6.8 tons ha1. It is apparent that a higher quantity of mulch is required for weed suppression. Even cover crop residues in no-tillage systems have been shown to inﬂuence the species composition and population of weeds (Teasdale et al., 1991). Surface crop residues have also been shown to increase the efﬁciency of some granular herbicides on weed control (Teasdale et al., 1991; Enders and Ahrens, 1995). It is not only the residue placement but also the residue type that inﬂuence weed growth in a cropped ﬁeld (Dastgheib et al., 1999). In cropping systems where crop yield reductions have been severe due to the competition of weeds for light, water, and nutrients (Kang et al., 1980), the burning of residues is beneﬁcial in removing weeds and weed seeds (Askins, 1991; Rule, 1991; Bowerman, 1995). In addition to inﬂuencing the weed seed reserves in the soil, directly or indirectly controlling weeds, crop residue management and tillage practices also inﬂuence the efﬁciency of soil-applied herbicides (Buhler and Oplinger, 1990). Preemergence herbicides applied must enter the plants from the soil and so their

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activity might be affected by the presence of straw and ash; consequently, higher rates of preemergence herbicide may be required to control weeds (Beck and Jones, 1996). In no-till planting systems, the presence of ash from the burning of crop residues reduces herbicide efﬁciency (Butterworth, 1985; Rule, 1991; Bowerman, 1995), whereas the C in burnt crop residues is highly adsorptive of herbicides and can reduce herbicide performance substantially if such residues are retained close to the soil surface (Bowerman, 1995; Printz et al., 1995). The mulch or stubble from a previous crop present at the time of application of preemergence herbicides not only intercepts herbicides and acts as a physical barrier to prevent the herbicide from reaching the soil (Banks and Robinson, 1982), but also adsorbs the active ingredient of herbicide (Sanford, 1982; Rule, 1991), thus reducing its efﬁciency. In other occasions, however, the intercepted herbicide can be washed into the soil by rainfall and their efﬁciency remains high (Johnson et al., 1989). Although ploughing in of residues has been shown to increase herbicide efﬁciency (Rule, 1991), the long-term buildup of crop residues can reduce herbicide efﬁciency (Bowerman, 1995). However, as discussed earlier, crop residues left on the surface can suppress weed seed germination and or seedling growth and complement the effects of herbicide (Crutchﬁeld et al., 1986). Thus, the overall effect of residues on herbicide performance probably depends on many interacting factors, such as environmental and management factors and amounts of residues remaining in the soil (Mills and Witt, 1989). 6. Pests and Diseases Wheat yields, like other crops, are generally depressed when planted into a seedbed with residues of previous crop still lying on surface, a problem recognized since the introduction of stubble mulch farming into the North American Great Plains in the 1940s (McCalla and Army, 1961; Smiley et al., 1993). Similar problems have been reported for wheat planted into wheat residues in Australia (de Boer et al., 1993; Burgess et al., 1993), India (Singh et al., 1993), England (Prew et al., 1995), the U.S. Paciﬁc Northwest (Papendick and Miller, 1977), south Africa (Wiltshire and du Preez, 1993), and New Zealand (Cromey, 1996; Fraser and Francis, 1996). Fungal diseases are the most common and some of these are soilborne or seedborne whereas others are spread by airborne spores and can cause wide spread damage (Gair et al., 1987). The persistence of most of the fungi that affect cereals have a limited host range. Barley, for instance, will not contact mildew or rust from volunteer plants of wheat nor from adjacent wheat crops and vice versa. The unspeciﬁed pathogen causing take-all, however, can affect both crops and carryover from one to the other (Gair et al., 1987).

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As pests and diseases can affect crops separately or simultaneously, the system becomes very complex with few deﬁnitive principles for management. Where a particular disease or pest is a problem, some residue management options may be more suitable than others for further reduction in the incidence of disease or pests and these are reviewed. The burning of crop residues is thought to be a good option for disease control (Butterworth, 1985; Burgess et al., 1993). Although burning can destroy pests, such as aphids present in residues, the increase in temperature can reduce both harmful and beneﬁcial soil microbes at or near the soil surface (Biederbeck et al., 1980), and even cause shifts in microbial communities (Beare et al., 1996). The burning of stubble has been shown to reduce the incidence of F. graminearum infection of wheat in 2 out of 5 years at one site and in 3 out of 4 years at another site in Australia (Burgess et al., 1993). The failure to control infection in other years was attributed to susceptible weed hosts and poor burning. Effects of burning depend clearly on the burning intensity, degree, and temperatures reached during burning (Singh et al., 1993; Johnston et al., 1996) because even small amounts of inoculum left in the soil may be able to carryover the disease to the next crop (Staniforth, 1982; de Boer et al., 1993). This may be the reason for disease incidence and severity not being reduced in some years by burning (e.g., Rasmussen and Rohde, 1988; Sumner et al., 1995). In some cases, the disease index was not reduced by burning treatments and seedlings grew better compared to residue incorporation and generally yielded more grains (Jenkyn et al., 1995; Sumner et al., 1995). The effect of incorporating residues on pests and disease incidence are controversial, although Rule et al. (1991) found that straw incorporation did not usually exacerbate pests and diseases compared with burning. Take-all lesions in wheat were up to two times higher in conventionally cultivated treatments than in direct drilled treatments (de Boer et al., 1993). Tillage has been shown to have a shortterm stimulatory effect, causing increased populations of both applied and indigenous bacteria and indigenous fungi (Donegan et al., 1992). Ploughing was found to cause more severe diseases (eyespot and sharp eyespot) compared to other shallow noninversion cultivation (Jenkyn et al., 1995; Prew et al., 1995). Jenkyn et al. (1995) found that the severity of take-all was decreased by ploughing, which was also reported in a long-term study by Prew et al. (1995), but only for 2–3 years and after that the severity was increased in the ploughing treatment. A high severity of take-all has also been reported in treatments where wheat straw was incorporated into the soil compared to mulch or burnt (de Boer et al., 1993). Other evidence suggests that the disease was occasionally more severe where residues remained at the soil surface (Gair et al., 1987). Colbach and Meynard (1995) explained these variations on the basis of crop succession and soil tillage. Where the previous crop was a host crop preceded by a nonhost crop, soil inversion buried host residues, thus decreasing the primary in-

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fection risk. Where the previous crop was a nonhost crop preceded by a host crop, soil inversion carried the host residue back to the soil surface, thus increasing the primary infection risk. This phenomenon becomes more obvious in long-term studies (Burgess et al., 1993; de Boer et al., 1993; Prew et al., 1995). The presence of crop residues resulted in wetter and cooler conditions in the surface soil, thus favoring diseases, and also provides pathogens with an additional source of energy to multiply in number (Cook and Haglund, 1991). There might also be an increased incidence of slugs where residues remain on the soil surface (Butterworth, 1985; Gair et al., 1987). It has been observed that leaving straw residues on the soil surface increased the number of argentine straw weevil, pysius, springtails, and aphids carried over between crops. The number of actinomycetes, fungi, and algae also might increase in a nonburnt, direct drill situation, which leads to an increase in debris-borne disease such as eyespot, rhynchosporium, brown stem rot, yellow leaf spot, net blotch, and barley scald (Meese et al., 1991; Hermann, 1992). The incidence of yellow cereal ﬂy (Opomyza ﬂorum) has been reported to be less under mulched treatments compared to burnt and incorporated treatments (Prew et al., 1995). It is obvious that each crop residue management practice has its drawbacks for different diseases and pests. Thus, decisions on residue management should be made regarding the health of the previous crop and the potential susceptibility of the next crop, cultivar selection, crop rotation, planting date, and plant nutrition (Gair et al., 1987; Smiley et al., 1993).

C. EFFECT OF RESIDUE MANAGEMENT ON THE ENVIRONMENT Crop residue management and tillage both affect the environment through its inﬂuence on losses of soil, plant nutrients, and chemicals to the environment, causing pollution, which is of great public concern. The burning of crop residues produces smoke, which creates hazards on visibility and ash, which pollute the surrounding air if transported by high wind velocities, especially under dry climates and surface waters (Iwamoto et al., 1992; Cihacek et al., 1993; Singer and Warkentin, 1996). For this reason, a burning code was developed in the United Kingdom in 1984 (at that time the burning of residues was allowed) that required the incorporation of ashes within 36 hr of burning (Butterworth, 1985). A burning ban was imposed in Europe (Prew et al., 1995) and the United States (UNEP/FAO, 1977). It is currently being considered in New Zealand (Fraser and Francis, 1996). In addition, gases released during burning contribute to ozone depletion, acid rain, and the greenhouse effect (Bouwman, 1990). The N-based gases formed during burning include greenhouse gases such as nitrous oxide (N2O), nitric oxide (NO), and probably also sulfur dioxide (SO2). Amounts of these gases and their effects have not been determined in many parts of the world, although in the Unit-

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ed States it is estimated that agricultural burning accounts for 6% of the total annual emissions of polynuclear aromatic hydrocarbons (Wilbert et al., 1992). Soluble nutrients present in the ash after burning residues, such as K, NH+4 , SO2 4 , and, to some extent, Mg, are susceptible to loss from the farming system by leaching below the root zone and can cause groundwater pollution. Management of crop residues other than by burning may also produce greenhouse gases (Rice et al., 1988; Christensen et al., 1990; Eichner, 1990; Aulakh et al., 1991a,b; Walters et al., 1992; McKenney et al., 1993; Boeckx and Van Cleemput, 1996; Nugroho et al., 1996). The presence of crop residues on soil surface may inﬂuence ammonia volatilization by acting as a source of urease (Freney et al., 1992). Ammonia volatilization losses may also occur when crop residues decompose on soil surface (Janzen and McGinn, 1991) and denitriﬁcation can be stimulated on their incorporation (Aulakh et al., 1992). If, however, residues are burned on the soil surface, the ash, which is alkaline, can enhance ammonia emissions (Bacon and Freney, 1989). Crop residues and tillage management also affect the erosion of soil and the associated nutrients and chemicals and also leaching, which may pollute the groundwater or surface waters (Martin et al., 1978; Baker and Laﬂen, 1982; Addiscot et al., 1991; Ocio et al., 1992; Addiscot and Dexter, 1994; Fermanich et al., 1996). Another important factor to consider is the fuel energy consumed and pollution created by machinery in incorporating the residues. For example, in a desk study in the United Kingdom, Chaman and Cope (1994) found that mould board ploughing to a depth of 20 cm rather than tine and disc cultivation to a depth of 10 cm increased diesel consumption by 15 Mliters per year in England and Wales. This represented 734 1012 J per year of fossil fuel energy consumption and emissions of 807 103 kg NO2-N, 139 103 kg particulates, and 40 106 kg CO2 per year.

V. SOIL NITROGEN DYNAMICS AND CROP NITROGEN RECOVERY A. NITROGEN MINERALIZATION /IMMOBILIZATION TURNOVER Decomposition and mineralization are the means by which nutrients either held in the SOM or added through organic materials (manures, crop residues) are released into the soil as inorganic forms. The inorganic N released becomes available either for subsequent recycling and utilization by plant or microorganisms or is lost from the system. Soil N mineralization is the transformation process where ammoniacal or ammonium N (NH+4 ) or ammonia (NH3) is released by soil microorganisms as they utilize organic N compounds as an energy source (Jansson

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and Persson, 1982; Jensen, 1997). The process is complex and depends on the activity of nonspeciﬁc heterotrophic soil microorganisms under both aerobic and anaerobic conditions (Jarvis et al., 1996). Both the magnitude and the rate of mineralization are different for newly added residue and existing, already degraded organic materials of varying ages and degrees of recalcitrance. Soil N mineralization is always accompanied by N immobilization (Fig. 4); the processes are intimately connected and dependent. Much of the NH+4 or NO 3 or simple organic N compounds that are released are assimilated by the soil microbial population and transformed into the organic N constituents of their cells during the oxidation of suitable C substrates through the process of immobilization. However, immobilized N is likely to be available subsequently for mineralization as the microbial population turnover. The degradation of microbial tissue is of great importance in terms of the ﬁnal release of N originally bound in organic residues, and biomass N contributes, over the short term, substantial amounts of N to parts of mobile N. Concurrent with the release of N from the soil microbial biomass there will also be direct release from fresh residues and “native” soil organic materials of various ages. Nitrogen continually cycles between inorganic and organic phases via the mineralization/immobilization activities of soil microbes,

Figure 4 Mineralization of organic matter incorporated into soils. Nitrogen-rich matter (e.g., young leaves) produces inorganic nitrogen, whereas matter poor in nitrogen (e.g., straw) consumes nitrogen (immobilization). Both processes feed the microbial pool. From Mengel (1996).

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hence termed the biological turnover or mineralization–immobilization turnover (MIT) (Jansson, 1971). Because these processes are concurrent but in opposite directions, net immobilization or net mineralization is often used to indicate the dominant process. In general, net immobilization occurs when the soil microbial population expands due to a substrate addition and N demand exceeds the supply from the soil or fertilizer. Alternatively, net mineralization results when the residues of inorganic N exceeds the demands of soil microorganisms as during the decline of a microbial population. A basic assumption of MIT is that all immobilization occurs from the inorganic pool. Ammoniacal N has been shown to be preferred (Jansson, 1958; Recous et al., 1988). However, where NH+4 is not available, NO 3 is assimilated by the soil microbial biomass in the presence of readily available C (Azam et al., 1986; Recous et al., 1988). There has been no evidence of any difference between the subsequent rates of release of immobilized NH+4 or NO 3 (Bjarnason, 1987). It has also been proposed that at a microsite scale there may be direct immobilization of small organic compounds such as amino acids (Hadas et al., 1987; Drury et al., 1991), known as “direct hypothesis.” Although it has been demonstrated that the soil microbial biomass can utilize amino acids in this way (Barak et al., 1990), MIT generally describes overall mineralization more accurately (Barak et al., 1990; Hadas et al., 1992). The way in which the mineralization/immobilization process operates is important in the turnover, recycling, and fate of released N from the added fertilizer, crop residues, and native SOM. 1. Involving Fertilizer Nitrogen Field experiments that include plots with and without fertilizer N enable the “apparent” recovery of the fertilizer N to be calculated from the amounts of N in harvested herbage. The retention of N in stubble and roots is generally ignored in the calculation of recovery, with some justiﬁcations, as the effect of fertilizer N on the amounts of N in roots and stubble is often relatively small. The “apparent” recovery is deﬁned as difference between fertilized and unfertilized plots in the amounts of N harvested in aboveground parts, usually at the harvest of arable crops, expressed as a percentage of the amount of N applied in the fertilizer. On this basis, the “apparent” recovery of fertilizer N by arable crops is generally in the range of 40–60%. For example, Grove (1979) showed that the recovery of fertilizer N by maize was similar in temperate and tropical regions. At N application rates between 35 and 120 kg N ha1, a fairly constant 55% was recovered in harvested plant tops. Nitrogen recovery decreased quickly when application rates exceeded the assimilative capacity of the crop so that less than 40% was recovered at the 200-kg N ha1 rate (Grove, 1979). The advent of 15N tracer methodology made it possible to account for all added forms of N to soil and to distinguish between soil and fertilizer sources. Some of

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the ﬁrst ﬁndings with 15N revealed higher unlabeled soil N recovery by fertilized plants than unfertilized plants (Broadbent, 1965; Westerman and Kurtz, 1973; Barraclough et al., 1985; Porter et al., 1996). For example, Powlson et al. (1992) showed in their experiments on winter wheat at different sites in eastern England on three soils that the application of labeled fertilizer N tended to increase the uptake of unlabeled soil N by 10 to 20 kg ha1 compared to control receiving no fertilizer. This was probably due to pool substitution (i.e., labeled inorganic N standing proxy for unlabeled inorganic N that would otherwise have been immobilized or denitriﬁed). These data implied that N fertilization stimulated the mineralization of native soil organic N (SON). This phenomenon, which has been referred to in the literature as the “priming effect” or “added N interaction” (ANI) (Hauck and Bremner, 1976; Azam et al., 1985; Jenkinson et al., 1985), had been discussed earlier, when research with green manures showed enhanced mineralization of soil N (Lohnis, 1926; Bingeman et al., 1953). However, conﬁrming data were not available until 15N was used. As a result, the basic assumption of the difference method that N immobilization and mineralization processes were similar between fertilized and unfertilized soils (Pomares-Garcia and Pratt, 1978) appeared to be invalid. Other mechanisms have also been proposed to account for ANIs such as osmotic and salt effects (Broadbent and Nakashima, 1971; Westerman and Tucker, 1974), stimulation of rhizosphere microorganisms (Legg and Allison, 1967), stimulation of microorganisms (Westerman and Kurtz, 1973), increased metabolism, and greater soil exploration by expanding root systems accounted for the higher levels of soil N in fertilized plants (Sapozhnikov et al., 1968) and protonation of organic nitrogenous bases (Laura, 1975). Another ﬁnding of 15N fertilizer efﬁciency experiments was that plant recoveries of applied N were generally lower than those obtained by the difference method (Allison, 1955; Westerman and Kurtz, 1973). Vlek and Fillery (1984) found this disparity especially true in high N paddy soils, although a greater agreement between methods was noted in low N soils. In a study at seven sites in the United Kingdom using wheat (applied with 32.4- to 233.9-kg labeled N ha1), Powlson et al. (1992) reported that crop recoveries of fertilizer N by 15N methods ranged from 46 to 87% (mean 68%), whereas recoveries ranged from 30 to 96% (mean 74.3%) by the difference method. Thus, these workers concluded that from 5 to 63% (mean 16.5% of N uptake by control crop) more unlabeled soil N was taken up by the crop when 15N-labeled fertilizer was added. While several researchers consider these differences due to priming effects, another explanation postulates that they are caused by isotope exchange, resulting from interactions with soil microorganisms. An alternative explanation for the disparity between the difference method and the 15N method, as well as the apparent mineralization of soil organic N, was initially proposed by Jansson in 1958. He postulated that the biological interchange between added N and soil microorganisms was a signiﬁcant factor controlling 15N recovery. Further research is necessary to quantify the extent of microbial N

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processes under the various management conditions that are imposed in agricultural systems, which ultimately control the recoveries of applied fertilizer N by crops. 2. Involving Crop and Organic Residues The application of crop and organic residues to soil involves a substantial input of carbonaceous material and thus may result in the immobilization, at least temporarily, of some inorganic N already present in the soil. However, the balance between immobilization and mineralization changes with time and, in the long run, increasingly favors mineralization. Thus, the C/N ratio has long been known to be of great relevance to the rate with which N is released from crop residues ( Jensen, 1929; Ford et al., 1989; Quemada and Cabrera, 1995; Janssen, 1996; Whitmore and Handayanto, 1997; Fig. 5). As the inﬂuence of the crop residue type and the related quality (C, N, or lignin content, C/N and lignin/N ratios) on residue decomposition have already been discussed, their inﬂuence on mineralization of N is reviewed brieﬂy here. The addition of plant residues most often results in a net N immobilization phase followed by a net remineralization phase (as evaluated by the difference between

Figure 5 Relationship between nitrogen mineralized (or immobilized) and the carbon/nitrogen ratio of added organic matter. *, Jensen (1929); 䉱, Chae and Tabatabai (1985); , Nieder and Richter (1989); , Franzluebbers et al. (1994); 䉭, Zagal and Persson (1994); , Thorup-Kristensen (1994); , A. P. Whitmore, unpublished. The solid line represents theoretical relationship [Eq. (6)]. From Whitmore and Handayanto (1997).

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an amended soil and a control soil). The dynamics as well as the net amounts of N immobilized varied greatly in these experiments according to the nature of plant residues. Harmsen and van Schrevan (1955) reported that crop residues with a C/N ratio below 30 are expected to result in net mineralization, whereas a C/N ratio wider than 30 favors immobilization. Das et al. (1993) reported that sorghum straw with a C/N ratio of 72 resulted in the immobilization of N up to 90 days. According to Stevenson (1986), net immobilization lasts until the C/N ratio of the decomposed material has been lowered to about 20. However, as in earlier studies (Azam et al., 1993), net immobilization of N has been reported to take place during early decomposition with a residue C/N ratio as low as 15 (Jensen, 1994a, 1997), probably due to C/N ratios of readily decomposable material being different from the overall C/N ratio of the material. In the study of Quemada and Cabrera (1995), the C/N ratio of the organic materials remaining on the soil surface at the end of experiment for three cereal stems was 28, which indicates that the N mineralization can commence before the C/N ratio of the residue is lowered to 20. Results of Aulakh et al. (1991a) supported the observation of Smith and Sharpley (1990) of less drastic effects on soil N immobilization when high C/N ratio crop residues were left on the soil surface than when they were incorporated. The reason was that more N was immobilized by incorporated residues. Greater amounts of fertilizer 15N were found by Cogle et al. (1987) in incorporated straw than in surface straw in ﬁeld studies, further implicating incorporated straw as immobilizing more of applied or soil N. Tracing soil mineral N and/or residue N pools with 15N has been used to measure variations in N and 15N mineral pools or in N and 15N organic pools (soil biomass) by using calculations based on the isotopic dilution technique (Barraclough, 1991). This has been used to describe the dynamics of gross N mineralization and immobilization after residue incorporation (Sorensen, 1981; Ocio et al., 1991; Jensen, 1994a; Recous et al., 1995; Watkins and Barraclough, 1996). A similar approach has been adopted using combined treatments identical in the total amount of added N and C but differing only in the N pool being labeled (15N residues 14N mineral and 14N residues 15N mineral) (Mary and Recous, 1994). Once the residue N is mineralized, it is taken up by plants (Jordan et al., 1993), recycled in the microbial biomass for their growth (Mary et al., 1996; Jensen, 1997), stabilized in complex soil organic matter (Jansson and Persson, 1988), or lost from the soil plant system (Harper et al., 1987; Aulakh et al., 1991a,b; McKenney et al., 1993; Haynes, 1997). Finally, this mineralized N would beneﬁt the subsequent crops.

B. CROP NITROGEN RECOVERY The recovery of mineralized N by a subsequent crop from either plant residues or fertilizer is the product of net mineralization and the efﬁciency with which in-

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organic N is assimilated by a subsequent crop. The efﬁciency of uptake is similar for high N leguminous residues and that of fertilizers, but for high C/N ratio crop residues it is slightly lower (Janzen, 1990; Bremner and van Kessel, 1992). This efﬁciency depends largely on the temporal patterns of net mineralization, plant N uptake, and N losses. 1. Fertilizer Nitrogen by Crops Recoveries as high as 73–80% of applied fertilizer N by sorghum and wheat (tops roots) and 61–66% (tops only), have been reported (Haynes, 1994; Jordan et al., 1996). The range of reported values of N recovered by subsequent crops from other studies is presented in Table V. In general, between 20 and 87% of applied N is recovered by the ﬁrst crop and between 10 and 35% is retained in the soil and 1–35% is unaccounted for (Table V). The N that is retained in soil is probably immobilized and enters the soil organic N pool. The reported variation in crop recoveries may be due to different fertilizer sources used in different studies, different rates of fertilizers used, climate, management practices, and the test crop grown. The ﬁxation by OM and clay lattices can effectively lower the availability of N to plants (Nommik, 1965). Another possibility for the poor plant recovery of fertilizer N is the competition between plants and soil microorganisms. The incorporation of N into a microbial biomass through the immobilization process essentially removes N from the plant available pool. Several studies have identiﬁed biological immobilization as having a signiﬁcant role in controlling plant N availability (Ladd and Amato, 1986; Recous et al., 1988; Haynes, 1997). One might view net immobilization as a way of storing N for future crops. However, recoveries of residual fertilizer N in subsequent crops have been quite limited (Legg and Alison, 1967; Thomsen and Jensen, 1994). For example, Jannson (1963) found that only 1% of immobilized N would be remineralized per year. In other studies, between 1 and 10% recovery in second-year crops has been reported (Table V). Hart et al. (1993) found that only 16% of the labeled 15N remaining in the soil (0 –70 cm) and stubble in the year of application was taken up by subsequent crops during 4 residual years, 29% was lost from the soil/crop system, and 55% remained in the soil. Thus, added N seems to undergo stabilization and modiﬁcation to less active forms. In studying SOM dynamics and transformation, it is necessary to understand the short- and long-term effects on plant recovery of applied N. Several pools and pathways have been delineated to describe the fate of unrecovered N, which is often considered as lost from the system through various edaphic loss processes such as leaching, denitriﬁcation, and ammonia volatilization (Ladd and Amato, 1986; Aulakh et al., 1992; Porter et al., 1996; Haynes, 1997). Another source of loss is from the plant itself by ammonia volatilization from leaves and root exudation (Harper et al., 1987; Papakosta and Gagianas, 1991).

Table V Recovery, Retention, and Estimated Losses of Fertilizer N Added to Different Crops Reported by Different Workers

N recovered in crop (%) N source/ fertilizer Urea (NH4)2SO4

(NH4)2SO4 NH4NO3

(NH4)2SO4 (NH4)2SO4 NH4NO3/KNO3 KNO3

aUnaccounted bNo

N retained in soil (%)

Estimated N loss from systema (%)

N rate applied (kg ha1)

Crop

Year 1

Year 2

Year 1b

Year 1b

Reference

52 225 240 220 30 100 100 100 56 168 100 50 150–225 50 50 50

Wheat Winter Wheat Oilseed rape Potatoes Spring Wheat Barley Barley Barley Corn Corn Sugarbeet Winter rye Winter wheat Rye monoculture Rye-clover Clover

35.1 55 48 53 23–34 34.5 40.1 29.3 45 49 43–46 20–27 46–87 39 19 4

— — — — — 2.2 — — — — 1 2 — 8–10 — —

— 27 26 22 — — — — — — 26–29 — 7–14 36 40 32

— 24 23 25 — — — — — — 25–31 — 1–35 25 41 64

Palta and Fillery (1993) Powlson (1993) Powlson (1993) Powlson (1993) Bremner and van Kessel (1992) Thomsen and Jensen (1994) Thomsen and Jensen (1994) Thomsen and Jensen (1994) Hesterman et al. (1987) Hesterman et al. (1987) Zapata and van Cleemput (1986) Zapata and van Cleemput (1986) Powlson et al. (1992) Ranells and Wagger (1997) Ranells and Wagger (1997) Ranells and Wagger (1997)

for 15N was taken to be as N loss from the system. corresponding data were available for year 2.

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2. Legume and Nonlegume Nitrogen by Succeeding Cereal Crops Most estimates of the N beneﬁt of legumes are based on the use of added 15Nlabeled legume residues to the soil (Ladd and Amato, 1986) and not on in situ residues left in the ﬁeld as this has not been conducted extensively. Experiments in Australia (Ladd et al., 1981,1983; Ladd and Amato, 1986; Muller and Sundman, 1988; Armstrong et al., 1997), Canada (Janzen et al., 1990; Bremner and van Kessel, 1992), and the United States (Harris and Hesterman, 1990; Harris et al., 1994) have shown that cereals recovered between 10 and 34% of the 15N applied in legume residues (Table VI). In some cases, 50–70% of the N applied as soybean and lucerne residues has been recovered by corn (Hesterman et al., 1987). In the Philippines, Morris et al. (1986a,b) showed that in short-term green manures, 33–49% of the N applied in residues of mung bean and cowpeas was taken up by a rice crop. Only a few studies have been conducted on the recovery of pasture N by cereals. In one study, wheat was found to recover approximately 10% of N from a ryegrass/white clover (70:30, w/w) pasture (Haynes, 1997). Legume roots were found to contribute signiﬁcantly toward the nutrition of subsequent cereals (Sawastsky and Soper, 1991; Thomsen et al., 1996). Nitrogen recoveries from nonleguminous residues (Table VII) are even lower than that from leguminous residues (3–18% vs 10–34%; Table VI), except in one case, where the N recovery by a rice crop was reported to be 37% from wheat straw. The reason is because of the low C/N ratio (34) of the wheat straw and the submerged conditions (Norman et al., 1990). Although the limited plant availability of legume N is primarily due to the stabilization of N in soil organic forms, losses of legume 15N through ammonia volatilization, denitriﬁcation, or leaching may also be considerable. For example, Ladd and Amato (1986), Janzen et al. (1990), and Haynes (1997) reported that losses of legume 15N were equal or greater than N uptake by a subsequent crop. In general, 39–70% of leguminous residue N (Table VI) and 54–81% of nonleguminous residue N (Table VII) were retained in soil. The unaccounted N varied from 20 to 50% of leguminous residue N (Table VII) and only 9–16% of nonleguminous residue N (Table VII). In the case of nonleguminous residues, limited or slow decomposition and stabilization of N in organic forms may be the reason for low N availability to subsequent crops (Bremner and van Kessel, 1992). In the second and third years, only 1–5% of the leguminous or nonleguminous residue N was recovered in crops (Ladd and Amato, 1986; Ta and Faris, 1990; Thomsen and Jensen, 1994; Haynes, 1997). 3. Comparison of Nitrogen Recovery from Applied Fertilizer and Legume Nitrogen In general, several studies have shown that the recovery of 15N from labeled leguminous and nonleguminous crop residues by subsequent cereal crops was

Table VI Recovery, Retention, and Estimated Losses of Leguminous Residue Nitrogen Added to Different Crops Reported by Different Workers

N recovered in crop (%) Residues added

266

Pisum sativum (C/N 14–16) T. repens; V. faba; T. Subterraneum (% N 1.6–3.0) Glycine max (C/N 15) L. culinaris green manure (% N 4.05) L. culinaris straw M. littoralis (% N 3.2) T. pratense L. M. littoralis L. (% N 3.2) G. max Roots Trash Leaves M. sativa (% N 2.6) L. perenne/ T. repens (70:30; C/N 18) a

Crop

Year 1

Estimated loss from systema (%)

Year 1

Year 1

Year 2

Reference

Barley Barley

11 6–25

4.3 —

— —

— —

— —

— —

Jensen (1996) Muller and Sundman (1988)

Rice Wheat

11 19

— —

39 —

— —

50 —

— —

Norman et al. (1990) Bremner and van Kessel (1992)

Wheat Wheat Corn Wheat

5.5 22–28 15 16–19

— 3–4 — 4– 5

— 56–70 57 —

— — — —

— 3–20 27 —

— — — —

Bremner and van Kessel (1992) Ladd et al. (1983) Harris et al. (1994) Ladd and Amato (1986)

Oats Oats Oats Barley Winter wheat Spring wheat

0.53 9.85 18.17 11 9.9 9.0

—

—

—

—

—

4 1.5 1.5

— 60(5.2)b 60(5.8)

— 55 55

— 25 24

— 10 14

Bergersen et al. (1992) Bergersen et al. (1992) Bergersen et al. (1992) Ta and Faris (1990) Haynes (1997) Haynes (1997)

Unaccounted for 15N was taken to be as N loss from the system. Recovered in undecomposed residues.

b

N retained in soil (%)

Table VII Recovery, Retention, and Estimated Losses of Nonleguminous N Added to Different Crops Reported by Different Workers

N recovered in crop (%) Residues added

267

Wheat straw (C/N 34) Rice straw (C/N 59) Wheat straw (C/N 43) Barley straw (N% 0.5) Barley straw ryegrass cover crop Ryegrass Ryegrass barley straw Sorghum (C/N 20–44) Barley straw Wheat straw a

Crop

Year 1

Wheat Barley Barley Barley Barley Sorghum/barley Barley/mustard Sorghum

37 3 5.5 4.5 4.4 10.2 7.8 4.5–25 8 18–20.6

Rice

No corresponding data were available for year 2. Unaccounted for 15N was taken to be as N loss from the system.

b

Year 2 — — — 2.6 2.7 2.4 2.1 5 3.3 —

N retained in soil (%)

Estimated loss from systemb (%)

Year 1a

Year 1a

Reference

54 81 — —

9 16 — —

— — —

— — —

Norman et al. (1990) Norman et al. (1990) Bremner and van Kessel (1992) Thomsen and Jensen (1994) Thomsen and Jensen (1994) Thomsen and Jensen (1994) Thomsen and Jensen (1994) Vigil et al. (1991) Jensen (1996) Jordan et al. (1996)

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K. KUMAR AND K. M. GOH

about one-half and one-eighth, respectively, of that from various forms of labeled fertilizer N (Tables V–VII). For example, in Australian dryland, Ladd and Amato (1986) found that the recovery of 15N by the first wheat crop averaged 17 and 46% from legume and fertilizer sources, respectively, after 1 year (Fig. 6a). Subsequent recoveries of both fertilizer and legume N in second year were lower (5%, Fig. 6b). Total 15N recoveries in plant and soil from fertilizer and legume were 84 and 80%, respectively, indicating similar losses of N from both sources. Associated measurements revealed that more 15N was immobilized into soil organic N from legume 15N than from fertilizer N. In a greenhouse study, Jordan et al. (1996) found that the sorghum crop recovered within 8 weeks 61, 22, and 18% of applied N from fertilizer, clover, and wheat residues, respectively. Likewise, Harris et al. (1994) found that more fertilizer than legume N was recovered by crops (40% vs 17% of input), more legume than fertilizer N was retained in soil (47% vs 17% of input), and similar amounts of N from both sources were lost from the cropping system (39% of input) over a 2-year period. More fertilizer than legume N was lost during the ﬁrst year of application (38% vs 18% of input), but in the second year, more legume N was lost compared to fertilizer N. In contrast to dryland experiments, legume N was found to be as good as fertilizer N based on recoveries by rice crop (Sisworo et al., 1990). It is clear that the synchronization between N mineralization and N uptake is an important factor controlling the recovery of applied N and extent to which mineralized N is lost from the system (McGill and Myers, 1987; Myers et al., 1997; Becker and Ladha, 1997; Haynes, 1997). 4. A Conceptual Approach to Nitrogen Mineralization from Crop Residues Key processes involved in the soil–plant N cycle are the decomposition of native SOM and plant residues and litter and the accompanying mineralization and immobilization of inorganic N. Even though these processes are complex, considerable advances have been made. It has been shown that crop residues decompose in two distinct phases: an initial more rapid phase, in which about 70% of the C initially present in the residues is lost as CO2, followed by a slower phase (Jenkinson, 1977; Parton et al., 1987; Xu and Juma, 1995). These two phases represent the labile C (or decomposable) and recalcitrant C (or resistant) fractions of the crop residues (Jenkinson and Rayner, 1977; Van Veen and Paul, 1981; Hansen et al., 1991). However, with regard to N mineralization, the results are often contradictory. A clear two-phase mineralization suggesting a labile and resistant fraction similar to that observed for C has been reported in some studies (Broadbent and Nakashima, 1971); in other studies, this two-phase pattern is not observed (Amato et al., 1984).

Figure 6 Relationship between the amount of nitrogen added to soil as legume and fertlizer sources and that (a) taken up by a ﬁrst wheat crop and (b) recovered in cropped soil and that taken up by a second wheat crop. From Ladd and Amato (1986).

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K. KUMAR AND K. M. GOH

In many studies, the C/N ratio, the N concentration of residues, or both have been used as predictors of net N mineralization (Vigil et al., 1991; Quemada et al., 1997 ). Whitmore and Handayanto (1997) reported that N mineralized can be predicted for a uniformly decomposable residue using Eq. (6) (Fig. 5) as N Co{1/Z E/Y},

(6)

where N is the amount of N mineralized once decomposition is almost complete, Z is the C/N ratio of the substrate (assumed constant), E is a microbiological efﬁciency factor that can vary for several reasons, but can be taken as about 0.4, and Y is the C/N ratio of the end product of the decomposition process; humus with a C/N of 10 and Co is the release of N at the start of mineralization (i.e, at zero time, to). Replacing Co with (Co Ct) gives the release of N after time “t”. Microorganisms require N for their growth in order to decompose crop residue, and they and their products must maintain a certain C/N ratio. This value is mostly narrower than the residue themselves, but because most microbes are not 100% efﬁcient (0 E 1), the critical C/N ratio of 20, where mineralization switches to immobilization, is often wider than the residues and, with the values assumed for E and Y earlier, is about 25, so it is more of a process controlled by supply and demand than decomposition (Whitmore and Handayanto, 1997). This equation may not hold good for residues that are heterogeneous in terms of quality, and results may deviate from what is predicted by Eq. (6) (Fog, 1988). Nevertheless, Whitmore and Handayanto (1997), using data taken from a number of ﬁeld and laboratory experiments (Jensen, 1929; Chae and Tabatabai, 1985; Nieder and Richter, 1989; Franzluebbers et al., 1994; Thorup-Kristensen, 1994; Zagal and Persson, 1994; Whitmore and Handayanto, 1997), have shown that this Eq. (6) is quite general and holds well. This also forms the foundation of the mineralization sections of many other models (e.g., CENTURY, Paustian et al., 1997; DAISY, Magid et al., 1997b); as it expresses the release of N after decomposition is complete. Estimates of N mineralization can be made experimentally with a large number of diverse incubation and chemical extractants (Goh and Haynes, 1986). A few of the common predictors are listed in Table VIII. In many other studies, predictors such as the lignin-to-N ratio and N-to-polyphenol ratios appeared more reliable indicators of N mineralization (Table VIII). Berg and Staff (1981) reviewed a number of results and concluded that net N mineralization started at a wide range of C/N ratios. In actual ﬁeld conditions, this confusion arises because only net N mineralization was measured. Thus the relationships being studied are only between C/N ratios of the organic substrate and a group of soil processes, not the N mineralization alone. Changes in processes such as N immobilization and N losses (e.g., due to denitriﬁcation) may blur or completely obscure any postulated relationships between N mineralization and a factor such as the C/N ratio of the residues.

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Table VIII Chemical Predictors Used by Different Workers for Estimating N Mineralization from Crop Residues Predictor Hot water extractable N (60 °C, 30 min) 6 N HCl hydrolysable N 6 N H2SO4 hydrolysable N N and C/N ratio Polyphenols (Folin–Denin method) Lignin concentration Polyphenol:N ratio (Lignin+polyphenol): N ratio N and lignin: N ratio

Residue type

Reference

Medic

Amato et al. (1984)

Corn and wheat straw Wheat straw and roots Agroforestry species Legumes

Martin et al.. (1980) Jawson and Elliott (1986) Vanlauwe et al. (1996) Anderson and Ingram (1989)

Field crops Tropical legumes Tropical legumes Sorghum

Muller et al. (1988) Palm and Sanchez (1991) Fox et al. (1990) Vigil et al. (1991)

VI. NITROGEN BENEFITS TO SUBSEQUENT CROPS Beneﬁcial effects of legumes on the yield of subsequent crops have been demonstrated in many studies (Senaratne and Hardarson, 1988; Jensen, 1994a; Hossain et al., 1996a,b; Armstrong et al., 1997; Holford and Crocker, 1997). This residual effect was noted when the legumes were incorporated as crop residues (Ladd and Amato, 1986; Senaratne and Hardarson, 1988; Haynes, 1997), as green manures (Yadwinder-Singh et al., 1991) grazed by animals (He et al., 1994; Whitehead, 1995), and harvested for hay (Papastylianou, 1987) or for grains (Blumenthal et al., 1988; Hossain et al., 1996a; Armstrong et al., 1997; Holford and Crocker, 1997). The N beneﬁts in legume-cereal rotations have been attributed entirely to the transfer of BNF (Munyinda et al., 1988). These beneﬁts have also been explained by a greater immobilization of nitrate during the decomposition of cereal compared with leguminous residues (Green and Blackmer, 1995). Others have expressed the view that N beneﬁts may be due to a combination of legume N sparing and the transfer of ﬁxed N2 (Keatinge et al., 1988; Herridge et al., 1995). Thus, the beneﬁcial residual effect is not necessarily due to the direct contribution of nutrients from aboveground materials being returned to soil, although the magnitude of the yield increase of the subsequent crop is related to the amount of material returned to the soil. For these residual effects to occur after legumes compared to nonlegumes, it is expected that the amount of ﬁxed N returned by legumes to the soil should be greater than the amount of soil N removed in the harvested grain (Eaglesham et al., 1982; Haynes et al., 1993). There is no agreement as to whether the beneﬁcial effect of legume to subsequent crops is due to a direct N contribution from N2 ﬁxation or is a net “rotation contri-

272

K. KUMAR AND K. M. GOH

bution,” which is a complex effect combining N availability, disease control, and soil structure improvement (Papastylianou and Puckridge, 1983; Dalal et al., 1991; Janzen and Shaalje, 1992; Holford and Crocker, 1997). These effects may include reduced diseases and insects problems, residual treatment effects, and unexplained effects. The use of 15N labeling in recent experiments has permitted some degree of separation of residual and legume N effects (Prasad and Power, 1991; Chalk, 1998).

A. GRAIN YIELD AND NITROGEN RESPONSES Many cropping experiments have shown increasing yields and N uptake by cereal grown after legumes than when grown after cereals (Strong et al., 1986b). For example, a summary of the effects of the inclusion of lupin, ﬁeld pea, faba bean, and chickpea in cereal cropping systems in over 30 Australian experiments showed responses from subsequent crops of between 12 and 164% N or 0.18 and 2.28 tons ha1 increase in yield responses (Evans and Herridge, 1987). Likewise, beneﬁts of legumes in the range of 0.2–3.68 tons ha1 increases in yield have also been reported (Peoples and Herridge, 1990). The measured beneﬁts are dependent on the antecedent crop used as the reference criterion (Chalk, 1998). Relative increases in N yield were in general higher than the relative increases in grain yield, suggesting the factors other than N were limiting grain yield. Many studies have veriﬁed that N is a major factor beneﬁting cereals following legumes compared with cereals following nonlegumes (e.g., Rowland et al., 1988; Evans et al., 1991; Chalk et al., 1993). However, the reported response in grain yield may not be due entirely to N (Chalk, 1998). Improvements in soil structure, the breaking of pest and disease cycles in monoculture, and phytotoxic and allellopathic effects of different crop residues have all been responsible for the yield responses reported.

B. FERTILIZER NITROGEN RESPONSES 1. Fertilizer Nitrogen Equivalence Various experiments have examined the N beneﬁts of legumes to a subsequent nonlegume crop by comparing the effects of a number of rates of N fertilizer on the same crop grown after a nonlegume calculated as N fertilizer equivalence (NFE) (Clegg, 1982). Estimates of the NFE of lucerne (Medicago sativa L.) to the following corn crop have been as high as 180 kg N ha1 (Baldock and Musgrave, 1980; Voss and Shrader, 1984). The range of NFE values, as summarized in Table IX, is generally between 15 and 148 kg N ha1 year1. Holford and Crocker (1997) reported that values of NFE for clover, lucerne, and medic for a ﬁrst wheat crop varied from

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Table IX Nitrogen Fertilizer Equivalence (NFE) of Legumes as Reported by Different Workers

Legume/nonlegume T. subterraneum/wheat Lupinus albus L./wheat Vigna sinensis/wheat Phaseouls aureus/wheat Arachis spps./wheat T.subterraneum/wheat Medicago sativa/wheat M. scutellata/wheat Cicer arietinum/wheat M. sativa/maize M. sativa/maize Vicia spps./maize–sorghum Arachis spps./maize V. sinensis/maize Cajanus cajan/maize Sesbania rostrata/rice S.aculeata/potato Vigna radiata/potatoes Green manures/riceb aValue bFrom

Yield response due to legume (kg ha1)

NFE (kg N ha1)

Reference

84–283 29–83 30 (11)a 49 (15) 23 (10) — — — — — — — 39 95 57 — 100 600 —

66 22–182 38 (13) 68 (16) 28 (12) 35–120 25–125 20–70 15–65 180 62 65–135 60 30 38–49 50 48 44 34–148

Brandt et al. (1989) Evans and Herridge (1987) Bandopadhyay and De (1986) Bandopadhyay and De (1986) Bandopadhyay and De (1986) Holford and Crocker (1997) Holford and Crocker (1997) Holford and Crocker (1997) Holford and Crocker (1997) Voss and Shrader (1984) Fox and Piekielek (1988) Blevins et al. (1990) Dakora et al. (1987) Dakora et al. (1987) Kumar Rao et al. (1983) Planiappan and Srinivasulu (1990) Sharma and Sharma (1988) Sharma and Sharma (1988) Singh et al. (1991)

in parentheses refers to companion crop. more than 10 legume green manures in 24 experiments throughout the world.

70 to 120 kg ha1 in the ﬁrst year, 25–125 kg ha1 in the second year, and 20– 75 kg ha1 in the third year, whereas corresponding values for chickpea were 35 kg in the ﬁrst year, none in the second year, and 20 kg in the third year. Thus, the determination of NFE is resource intensive and provides an economic assessment of the value of including rotation in terms of fertilizer N saved (Chalk, 1998).

C. RESPONSES OF CEREALS TO THE ANTECEDENT LEGUME 1. Legume Nitrogen-Sparing Effects The difference between the uptake of soil N or soil fertilizer N by nonlegume and legume crops sown in adjacent plots is an estimate of N sparing, provided that net N mineralization under the different crops is equal (Chalk, 1998). Estimates of N-sparing reported ranged between 27 and 136 kg N ha1, depending on the stage

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of crop growth and reference crop (Chalk et al., 1993). Evans et al. (1991) found that less soil N was taken up by lupin than adjacent cereal crops, whereas Herridge et al. (1995) measured a higher N-sparing effect in chickpea than wheat. Differences in amounts of mineral N in the soil proﬁle also provide direct estimates of legume N sparing (Peoples et al., 1995), and values ranging from 5 to 60 kg N ha1, depending on the crops selected, have been reported (Strong et al., 1986a). However, additional mineral N under legume crops compared with cereals may result from the rapid mineralization of organic N derived from rhizodeposition rather than from a lower N demand by leguminous crops (Unkovich et al., 1997). Thus, the concept of spared N needs to be reconsidered. 2. Amounts of Biological Nitrogen Fixation It is important to have information on the actual N2 ﬁxation in the wide variety of environments (both spatial and temporal) and potential N2 ﬁxation in ideal ﬁeld situations, as only then can efforts be made to manipulate the management practices to enhance N2 ﬁxation near to potential N2 ﬁxation (PNF). The range of experimentally obtained values of N2 ﬁxation by temperate and tropical grain legumes of 0–97% N derived from the atmosphere (Ndfa) [0–450 kg N ha1 (Evans et al., 1989; Hardarson et al., 1993; Herridge et al., 1993; Peoples et al., 1994a,b; Ladha et al., 1996)] and from pasture legumes of 30–92% Ndfa [15–300 kg N ha1 (Giller and Wilson, 1991; Ledgard and Steele, 1992; Peoples and Craswell, 1992; Thomas, 1992; Gault et al., 1995)] reﬂects the inherent capacities of legumes to ﬁx and accumulate N, the environmental constraints on those capacities, and the effects of cultural practices, experimental treatments, or both. Where soil fertility is high, legumes in the ﬁeld thrive without ﬁxing atmospheric N2; under such conditions they may derive all their N requirements from soil N (Goh et al., 1996) due to the altered source-sink relationship where an soil N apparently substituted for BNF (McNeill et al., 1996). However, in the majority of soils, levels of plant available N are usually insufﬁcient to fully satisfy the requirement of legume for N and the demand will be met by BNF. For white clover in clover-based pastures, the amount of N2 ﬁxed may range from nil to more than 500 kg N ha1 year1 in New Zealand. Where conditions are particularly favorable for white clover, a maximum of 670 kg N ha1 year1 was reported during the establishment of a grass–white clover pasture on a subsoil supplied with additional P and K (Sears et al., 1965). However, the usual range for grass–clover swards in New Zealand is between 100 and 350 kg N ha1 (Caradus, 1990; Hoglund et al., 1979; Ledgard et al., 1990), with less than 100 kg N ha1 in unimproved pastures and in dry areas (Ledgard et al., 1990). High rates of ﬁxation by white clover have also been reported from The Netherlands, with up to 565 kg N ha1 year1 on a previously arable ﬁeld on a clay soil (Mannetje, 1994). In Britain, the amount of N2 ﬁxed in a mixed grass–clover pasture can vary from nil to about 400 kg N ha1 year1 and productive swards ﬁx between 100 and 200 kg

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N ha1 year1 (Reid, 1970; Munro and Davies, 1974; Laidlaw, 1988; Evans et al., 1990). In Switzerland, between 270 and 370 kg N ha1 were reported as being ﬁxed by ryegrass–white clover swards (Böller and Nösberger, 1988). Major factors contributing to high BNF are high potential growth and a high percentage of N derived from the atmosphere (%Ndfa). This last parameter generally appears to be least affected by environmental conditions than total N2 ﬁxed (Danso et al., 1992). There has to be an upper limit on BNF (Dommergues and Steppler, 1987). Herridge and Bergersen (1988) postulated a theoretical upper limit of 635 kg N ha1 for soybean (Glycine max) and more than 300 kg N ha1 for pigeon pea (Cajanus cajan) and groundnut (Arachis hypogea). Although values approaching the theoretical limits may be achieved under optimal conditions (i.e., high legume yield and low soil nitrate) in practice, levels of N2 ﬁxation in ﬁelds of farmers may often be only a fraction of the potential ﬁxation (Peoples et al., 1995) as several ecological constraints may limit N2 ﬁxation by legumes. The identiﬁcation of these constraints and the processes involved that limit N2 ﬁxation are important for the improvement of both agronomic practices and crop-breeding strategies. 3. Transfer of Biologically Fixed Nitrogen The amount of legume N transferred to a subsequent cereal crop clearly depends on the amount of ﬁxed N incorporated in a farming system. In crop rotations, the net N beneﬁt from grain legumes may be small. For example, in a range of experiments with grain legumes varying in productivity and N2 ﬁxation, the net contribution from ﬁxed N averaged only 15 to 20 kg N ha1 and was frequently negative (Table X). This negative contribution occurred because the ﬁxed N remaining in residue was less than soil-derived N removed in the harvested grains. In contrast to grain legume cropping systems, where most of the legume N is removed in harvested grains, the contribution of ﬁxed N can be substantial in legumebased pastures (Table XI). This is especially important in mixed cropping rotation (pasture/arable) systems of farming where N2 ﬁxation by the pasture phase is utilized to grow subsequent arable crops (Haynes and Francis, 1990; Francis et al., 1995).

D. RELATIVE CONTRIBUTION OF FIXED AND NONFIXED NITROGEN TO CROP NITROGEN RESPONSES An almost equal contribution of ﬁxed N and nonﬁxed N toward N beneﬁt has been reported in several studies (Kumar Rao et al., 1987; Danso and Papastlianou, 1992; Chalk et al., 1993). However, Senaratne and Hardarson (1988) found that the N beneﬁt was predominantly due to nonﬁxed N when only roots were returned and the beneﬁt declined further when both stover roots were returned. Nevertheless, in general, a greater beneﬁt is expected from the return of more leguminous residues (Chalk, 1998).

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K. KUMAR AND K. M. GOH Table X

Estimates of Amounts (kg N haⴚ1) of Nitrogen-Fixed, N Removal in Harvested Grains and Net Contribution of Fixed N to Subsequent Crop by a Range of Grain Legumes

Legume Chickpea

Pea

Lupin

Lentil

Faba bean

Cowpea Greengram Soybean

Field bean

Total plant N

Total ﬁxed N

Total N removal in grain

Net contribution from ﬁxed N

50–135 60 83–120 139 61–91 69–104 116–140 124–160 126–326 212–238 220 220–227 48–131 53–297 292–347 303–420 133–139 135–158 178–237 83 107–161 154–277 66–101 130–200 111–177 78–108 100–378 329–402 363–417 173

24–84 6 44–88 104 28–177 54–85 53–86 23–28 90–206 160–196 133 133–183 18–80 30–288 111–126 249–317 32 57–111 129–192 53 64–115 113–252 12–22 66–117 71–112 26–33 13–287 290–312 143–244 48

42–74 46 23–55 94 27–91 6–22 49–91 92–116 82– 237 154–170 148 135–162 5–78 8–153 220–278 154–266 90–106 30–57 107–333 48 18–84 117–220 50–71 82–85 52–89 47–64 63–257 262–296 187–205 98

18 to 10 40 7 to 65 10 32 to 96 52 to 73 28 to 27 64 to 93 34 to 17 3 to 30 15 2 to 21 19 to 14 41 to 135 109 to 152 51 to 95 58 to 74 0 to 70 143 to 26 5 20 to 57 8 to 40 57 to 18 16 to 32 19 to 23 30 to 21 116 to 67 6 to 50 44 to 39 50

Reference Rennie and Dubetz (1986) Smith et al. (1987) Evans et al. (1989) Armstrong et al. (1997) Evans et al. (1989) Evans et al. (1997) Smith et al. (1987) Haynes et al. (1993) Armstrong et al. (1994) Rennie and Dubetz (1986) Armstrong et al. (1997) Peoples et al. (1995) Smith et al. (1987) Evans et al. (1989) Haynes et al. (1993) Armstrong et al. (1997) Haynes et al. (1993) Smith et al. (1987) Rennie and Dubetz (1986) Evans et al. (1989) Smith et al. (1987) Rennie and Dubetz (1986) Sisworo et al. (1990) Awonaike et al. (1990) Chapman and Myers (1987) Sisworo et al. (1990) Herridge and Bergersen (1988) Awonaike et al. (1990) Bergersen et al. (1985) Haynes et al. (1993)

E. ROLE OF LEGUME IN THE GAIN OR DRAIN OF SOIL NITROGEN There is a great deal of variation in the literature concerning the potential of grain legumes in making a positive contribution to the N balance of cropping systems, given that a considerable amount of N is harvested and removed in grain. Eaglesham et al. (1982) presented a simple expression [Eq. (7)] to predict the net effect of grain legumes on soil N balance as

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N balance Nﬁx Nharv,

(7)

where Nﬁx is N2 ﬁxed by the legume and Nharv is N removed in grain or other harvested portions. McDonald (1989) expressed the concept in the form of Eq. (8) as N balance (Nleg Ndfa) (Nleg NHI) Nleg (Ndfa NHI),

(8)

where Nﬁx Nleg Ndfa and Nharv Nleg x NHI, Nleg legume N, Ndfa proportion of N from BNF, NHI proportion of crop N harvested in grain. It is obvious from Eq. (8) that the N balance will be positive as long as NHI Ndfa and it will be negative if NHI Ndfa (Haynes et al., 1993). In general, positive N balances are associated with the return of a greater amount of ﬁxed N in crop residues compared with the removal of soil N in grain (Bergersen et al., 1985; McDonagh et al., 1993). Thus, grain legumes with high biomass N, low NHI, and high BNF have the greatest potential to contribute positively to the soil N pool (Chalk, 1998). Some examples of both positive and negative estimates of N balances for grain legumes are shown in Table X. These N balances should be viewed with caution because of the overestimation of NHI as discussed previously. For example, Russell and Fillery (1996) found the NHI for lupin decreased from 0.78 to 0.56 when belowground biomass N (determined using foliar labeling) was included with aboveground N to give total plant N. In comparison to grain legumes, pastures are credited with supplying large amounts of ﬁxed N to the system (Table XI). Chalk Table XI Amounts of Nitrogen (N2) Fixation by The Legume Component of Different Legume-Based Mixed Pastures a

Legume component Trifolium repens L.

T. pratense L. Medicago littorallis L.

Lotus corniculatus T. vesiculosum aFrom

N2 ﬁxation (kg N ha1 year1)

Country

42–200 83–283 155 224–291 49–373 49–277 93–258 51–172 114–282 70–223 20–60

Uruguay Switzerland United Kingdom New Zealand Switzerland Uruguay Canada Canada Austria Uruguay United States

Ledgard and Giller (1995).

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(1998) cautioned using single-season data to predict the long-term trends because both NHI and Ndfa show considerable spatial and temporal variations.

VII. CONCLUSIONS This review reveals that crop residues of common cultivated crops are an important resource, not only as a source of signiﬁcant amounts of nutrients for crops and hence agricultural productivity, but also affecting soil physical, chemical, and biological functions and properties and water and air quality. In general, major factors controlling residue decomposition are (i) factors related to the medium (soil) to which the residues are added, (ii) properties and quality of crop residues, (iii) residue management practices, (iv) climate, and (v) other factors. The development of an effective crop residue management system depends on a thorough understanding of these factors. It is difﬁcult to predict decomposition from an individual property of organic residues, but when combined, these properties could accurately predict relative rates of decomposition from a broad range of important residues. Various indices, such as residue decomposability indices, resistance indices, and plant residue quality indices, have been developed, and although general trends have been observed, no unique relationship has been developed. This is partly due to different methodologies and approaches used by different workers to quantify the relationships. There is thus a need for standardizing methods for determining residue characteristics (e.g., lignin, polyphenol) and decomposition rates before a universal plant residue quality index could be developed and used to predict the nutrient release from decomposing residues. Theoretical and predictive models have been developed to describe the decomposition process mathematically. An important conclusion from these models is the realization that in order to predict total mass loss, as well as the decomposition of primary litter fractions, the formation and turnover of secondary decay products need to be included. Although OM turnover models incorporating submodels or pools representing secondary decomposition products (e.g., microbial biomass) have been developed and combined with soil water and plant growth submodels to simulate cropping system dynamics, these models have not been tested extensively in terms of decomposition and nutrient release from different crop residues and management practices. Crop residue management and tillage both affect the environment through its inﬂuence on losses of plant nutrients and chemicals to the environment causing pollution which is of great public concern. Awareness regarding the progressive degradation of soils has led to the search for a reliable measure of soil quality. Crop residue management is known to affect most of these soil quality indicators either directly or indirectly. It is perceived that soil quality is improved by the adoption

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of proper crop residue management practices. In addition, crop residues and their management also inﬂuence biological N2 ﬁxation by altering the inorganic N concentration in soil and hence their phytotoxicity. Considerable evidence has accumulated that the use of legumes can increase yields signiﬁcantly. Indirect estimates indicate that leguminous crop residues can supply 15 –148 kg chemical fertilizer equivalent N per hectare. Pasture legumes in mixed cropping systems can supply even higher amounts. A proper choice of legumes and their better management show a considerable potential in meeting part of the N demands for succeeding crops. Comparisons of N recoveries from crop residue N and inorganic N fertilizers have shown that, in general, N recoveries from leguminous and nonleguminous residues are about one-half and one-eighth, respectively, of that from various forms of N fertilizers. Also, more legume N than fertilizer N is retained in soil and enters the organic N pool, whereas losses of legume N and fertilizer N are generally similar. Thus, there is a need to minimize losses of N from both systems by devising proper management practices for all cropping systems so that N mineralization synchronizes with crop N demand. Several options are available to farmers in the management of crop residues. Ideally, crop residue management practices should be selected to enhance crop yields with minimum adverse effects on the environment. In the last two to three decades, several workers have examined the effect of residue management practices on the harvested yield of the following crop. Results from these experiments are conﬂicting because of a number of factors involved associated with residue quality, management and edaphic factors, health of the previous crop, and their complex interactions with various management factors in determining the ultimate crop yield. This indicates that no one residue management system is superior under all conditions. To overcome this problem, it is suggested that the effects of various constraints on crop production under different environments in each cropping system be identiﬁed and conceptualized to guide toward the best option. Multidisciplinary and integrated efforts by soil scientists, agronomists, ecologists, environmentalists, and economists are needed to design a system approach for the best choice of crop residue management practices for enhancing agricultural productivity and sustainability.

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Index A Abscisic acid, drought effects in grain crops, 63, 81–86 Actinidia deliciosa, chloride response, 118 –120, 130 Africa, tertiary surfaces, oxisol soils, 168 –169 Alfalfa, chloride response, 112, 138 Alﬁsols, global distribution, 161–162 Allium ssp., chloride response, 106, 116 –117 Alluvium, oxisol soil relations, 169 –170 Almond, chloride response, 138 Aluminum, humic acid adsorption, 43 Amazon basin, oxisol soils, 167–168 Amino acids, soil component, 32 Amino sugars, soil component, 32 Ammonium, see also Nitrogen chloride uptake interactions, 121 mineralization, 257–262 nitriﬁcation inhibitors, chloride effects, 102– 103 Andisols, global distribution, 161–162 Anthesis, drought effects in grain crops, 69 –70 Apple, chloride response, 106 Apricot, chloride response, 138 Arachis hypogaea, chloride response, 118 –119, 121, 131 Aridisols, global distribution, 161–162 Avocado, chloride response, 118 –119, 123, 130–131

B Barley chloride response, 106, 118 –119, 127, 138 drought effects, see Drought nitrogen response, 263 –267 residue analysis, 200 Barton, D. H. R., 49–50 Bean, see also speciﬁc types chloride response, 106, 112 nitrogen ﬁxation beneﬁts, 276 Beta vulgaris chloride response, 105 –106, 133, 138

nitrogen response, 263 –264 Brassica ssp., chloride response, 105–106, 118 –119 Brazil, planation surfaces, oxisol soil relations, 167 Bremner, J. M., 53 Broad bean chloride response, 106 nitrogen dynamics beneﬁts to subsequent crops, 273 –278 crop nitrogen recovery, 265 –268 Broccoli, chloride response, 106, 118 –119 Burning, crop residue management, 230, 241– 242, 247

C Cabbage, chloride response, 105 –106 Cadmium, humic acid adsorption, 43 Calcium, chloride uptake interactions, 123 –125 Carbohydrates crop residue management, 247 drought effects in grain crops, 73 –75, 84 – 85 Carbon dioxide, crop residues decomposition, 219 –223 Carbon/nitrogen ratio, crop residue decomposition role, 203 –209, 220 –221, 261, 270 Carrot, chloride response, 106 Chickpea, nitrogen dynamics beneﬁts to subsequent crops, 273 –278 crop nitrogen recovery, 265 –268 Chinese cabbage, chloride response, 105 –106 Chloride behavior in soil, 99 –103 accumulation, 99 –101, 139 clay surface reactions, 101 nitriﬁcation inhibitor, 102–103 sources, 99 –101 fertilization, 100 –101 irrigation, 100 –101, 140 rainwater, 99 –100 reserves, 99 uptake, 101–102

321

322

INDEX

Chloride (continued) management, 134–139 accumulation in soil, 134 fertilization under saline conditions, 134 – 136 irrigation foliar damage, 137–139 root zone leaching, 136 –137 sprinklers, 137–139 leaching, 136–137 monitoring methods, 135 overview, 98–99, 139 –140 plant response, 103 –133 biochemical functions, 113 –115 crops alfalfa, 112, 138 almond, 138 apple, 106 apricot, 138 avocado, 118–119, 123, 130 –131 barley, 106, 118 –119, 127, 138 bean, 106, 112 broadbean, 106 broccoli, 106, 118 –119 cabbage, 106 carrot, 106 Chinese cabbage, 105 –106 citrus fruit, 108, 112, 118 –119, 124, 129–130 coconut palm, 117, 123, 132–133 corn, 106, 112, 118, 133, 138 cotton, 106, 110, 133, 138 cowpea, 106 cucumber, 106, 138 ﬂax, 106 grapevine, 106, 111, 131, 138 kiwi fruit, 118 –120, 130 lettuce, 106, 111, 118 melon, 118–119, 123 onion, 106, 116 –117 peanut, 118–119, 121, 131 pepper, 106, 138 plum, 138 potato, 105–106, 112, 118, 121, 127– 128 radish, 106 rice, 105–106, 127 sorghum, 106 soybean, 105–106, 113, 132 spinach, 106

strawberry, 106, 110, 118 –119, 123 sugar beet, 105 –106, 133, 138 sugar cane, 105 –106 sweet potato, 105 –106 tobacco, 105 –106, 118 tomato, 106, 108, 112, 118 –119, 121, 128 –129 turnip, 106 wheat, 105 –106, 118 –119, 122, 127 disease suppression, 125 –126, 140 distribution, 112–113 ion uptake interactions ammonium, 121 calcium, 123 –125 nitrate, 117–121 phosphorus, 121–122 potassium, 123 physiological functions, 115 –117 anionic balance, 115 –116 osmotic balance, 115 –116 stomatal regulation, 116 –117 tolerant crop selection, 113 translocation, 112–113, 139 uptake mechanisms, 111–112 yield response, 103 –111 content in harvested plant parts, 108 – 111 crop sensitivity, 104 –108 positive yield response, 103 –104 tolerance, 104 –108 yield quality, 108 –111 Chromium, humic acid adsorption, 43 Citrus ssp., chloride response, 108, 112, 118 – 119, 124, 129 –130 Clay chloride surface reactions, 101 crop residue decomposition role, 213 –214 mineral adsorption, 44 Clover, nitrogen dynamics beneﬁts to subsequent crops, 273 –278 crop nitrogen recovery, 265 –268 nitrogen response, 263 –264 13C nuclear magnetic resonance spectrometry crop residue analysis, 225 –227 soil organic matter analysis analytical characteristics, 13 –16 direct analysis, 6 – 9 Cobalt, humic acid adsorption, 43 Coconut palm, chloride response, 117, 123, 132–133

INDEX Compost, see Decomposition Copper humic acid adsorption, 43 water-soluble complex formation, 42– 43 Corn chloride response, 106, 112, 118, 133, 138 drought effects, see Drought nitrogen response, 263 –267 residue analysis, 200 Cotton, chloride response, 106, 110, 133, 138 Cover crops, residue decomposition management, 218–219 Cowpea, chloride response, 106 Crop residue decomposition, 200–230 affecting factors, 201–220 accessibility, 216 –217 aerobic conditions, 211 age, 202, 207 anaerobic conditions, 211 carbon dioxide, 219 –220 carbon/nitrogen ratio, 203–209, 220 – 221, 261, 270 chemical composition, 205 –208 clay content, 213 –214 climate factors, 219 crop cover, 218–219 decomposition index, 208 –209 desiccation, 203, 210 –211, 217 edaphic factors, 209 –215 freezing, 210 indigenous organisms, 214 –215 irrigation, 213–214, 217 leaf toughness, 202–203 lignin role, 204–209, 221 loading rate, 215 –216 management factors, 215 –219 methodology problems, 206 –208, 216 – 217 nitrogen content, 203 –204, 212–213 nutrient availability, 212 ozone role, 219–220 particle size, 201–202, 207–208 polyphenols role, 205, 207–209 quality index, 208 –209 quality of residue, 203 –208 soil moisture, 210 –211, 219 soil pH, 209 soil salinity, 211–212 soil structure, 213 –214

soil temperature, 209 –210, 219 thawing, 210 deﬁnition, 199 management practices affecting factors, 215 –219 accessibility, 216 –217 crop cover, 218 –219 irrigation, 213 –214, 217 loading rate, 215 –216 crop yield responses, 247–256, 278 diseases, 254 –256 germination, 247–248 growth, 247–248 herbicide efﬁciency, 253 –254 nitrogen ﬁxation, 250–252, 272 pests, 254 –256 phytotoxicity, 252–253 seedling establishment, 247–248 weed control, 253 –254 direct drilling, 230 environmental effects, 256–257 residue burning, 230, 241–242, 247 residue incorporation, 230 soil quality affects, 231–247 aggregation, 232–233 biological properties, 242–247 biomass, 243 –244 bulk density, 233 –234 carbohydrates, 247 chemical properties, 236 –242 compaction, 233 –234 enzymes, 246 –247 erosion, 232 hydrology, 234 –235 indicators, 231–232 micronutrients, 241 microorganisms, 243 –247 moisture content, 235 –236 mycorrhiza, 245 –246 nitrogen content, 237–241 penetration resistance, 233 –234 pH, 236 –237 phosphorus content, 241 physical soil properties, 232–236 soil fauna, 244 –245 soil organic matter, 237–240 soil structure, 232–233 soil temperature, 235 straw removal, 231, 247 undersowing, 230 –231

323

324

INDEX

Crop residue (continued) modeling, 227–229 nitrogen dynamics, 257–278 beneﬁts to subsequent crops, 271–278 biological nitrogen ﬁxation, 274–275 fertilizer nitrogen equivalences, 272– 273 ﬁxed nitrogen responses, 275–276 grain yield responses, 250 –252, 272 legume nitrogen, 273 –278 nonﬁxed nitrogen responses, 275–276 crop nitrogen recovery, 262–271 cereal crops, 265 fertilizer, 263–268 legume nitrogen, 265 –268 mineralization process, 268 –271 immobilization turnover, 257–262 fertilizer, 259–261 residues, 261–262 mineralization, 257–262 fertilizer, 259–261 process, 268–271 residues, 261–262 overview, 197–199, 278 –279 study methods, 220 –227 carbon dioxide evolution measurement, 220–223 isotopic techniques, 225 –227 lignin analysis, 225 profusion methods, in vitro, 224 –225 size density fractionation, in situ, 225 weight loss measurement, 223 –224 utilization, 199–200 world production, 199 –200 Crops, see speciﬁc aspects; speciﬁc crops Crop yields, see also speciﬁc crops chloride effects, 103 –111 content in harvested plant parts, 108 –111 crop sensitivity, 104 –108 positive yield response, 103 –104 tolerance, 104–108 yield quality, 108 –111 crop residue management, 247–256, 278 crop yield, 249–250 diseases, 254–256 germination, 247–248 growth, 247–248 herbicide efﬁciency, 253 –254 nitrogen dynamics, 250 –252, 272 nitrogen ﬁxation, 250–252, 272

pests, 254 –256 phytotoxicity, 252–253 seedling establishment, 247–248 weed control, 253 –254 Cucumber, chloride response, 106, 138 Cultivation crop residue management, 230 long-term effects, soil organic matter chemistry, 9 –11 Curie-point pyrolysis, soil organic matter analysis, 26 –27

D Daucus carota, chloride response, 106 Decomposition, crop residues, 200 –230 affecting factors, 201–220 accessibility, 216 –217 aerobic conditions, 211 age, 202, 207 anaerobic conditions, 211 carbon dioxide, 219 –220 carbon/nitrogen ratio, 203–209, 220 –221, 261, 270 chemical composition, 205 –208 clay content, 213 –214 climate factors, 219 crop cover, 218 –219 decomposition index, 208 –209 desiccation, 203, 210 –211, 217 edaphic factors, 209 –215 freezing, 210 indigenous organisms, 214 –215 irrigation, 213 –214, 217 leaf toughness, 202–203 lignin role, 204 –209, 221 loading rate, 215 –216 management factors, 215 –219 methodology problems, 206 –208, 216 – 217 moisture, 210 –211, 219 nitrogen content, 203 –204, 212–213 nutrient availability, 212 ozone, 219 –220 particle size, 201–202, 207–208 pH, 209 polyphenols role, 205, 207–209 quality index, 208 –209 quality of residue, 203 –208 salinity, 211–212

325

INDEX soil structure, 213–214 temperature, 209–210, 219 thawing, 210 Disease chloride effect in plants, 125 –126, 140 control, crop residue role, 254 –256 Drought crop residue decomposition, 203 grain crop reproductive development effects, 59–86 carbohydrate metabolism, 84 – 85 hormonal sterility induction, 81– 84 injury nature, 62–68 cell division inhibition, 67 fertilization initiation, 65 – 66 ﬂower initiation and development, 62–63 gametophyte development, 63 – 65 grain initiation, 65 – 66 kernel growth and maturation, 66 – 68 pollination initiation, 65 – 66, 71–73, 85 overview, 59–61, 85 – 86 reproductive failure physiology, 71– 85 carbohydrate availability, 73 –75 grain maturation regulation, 75 – 81 kernel abortion, 73 –75 pollen development, 71–73 sensitivity, 61–62 water–tissue relationship, 68 –71 anthesis stress, 69 –70 ﬂower initiation and development, 68 grain ﬁlling and maturation, 70 –71 meiotic-stage stress, 68 – 69, 82, 85

E Ecosystem management crop residue effects, 256 –257 oxisol soils, 180–186 agriculture, 185–186 forests, 180–183 pasture, 183–185 Elaeis guineenisis, chloride response, 117, 123, 132–133 Electron microscopy, soil organic matter analysis, 19–21 Electron spin resonance spectroscopy, soil organic matter analysis, 16 –19 Entisols, global distribution, 161–162 Environment, see Ecosystem management

F Faba bean chloride response, 106 nitrogen dynamics beneﬁts to subsequent crops, 273 –278 crop nitrogen recovery, 265 –268 Ferrasols, see Oxisols Fertilizer, see also speciﬁc types drought effects in grain crops, 65 – 66 Field burning, crop residue management, 230, 241–242, 247 Field pea, nitrogen dynamics beneﬁts to subsequent crops, 273 –278 crop nitrogen recovery, 265 –268 Flaig, W., 47– 48 Flax, chloride response, 106 Flower development, drought effects in grain crops injury nature, 62– 63 water–tissue relationship, 68 Forests, management, oxisol soils, 180 –183 Fourier transformation infrared spectroscopy, soil organic matter analysis, 12–13 Fragaria ssp., chloride response, 106, 110, 118 – 119, 123 Fruit trees, see speciﬁc types Fulvic acid oxidation products, see Soil organic matter

G Gametophyte development, drought effects in grain crops, 63 – 65 Gelisols, global distribution, 161–162 Glycine max chloride response, 105 –106, 113, 132 nitrogen ﬁxation beneﬁts, 275 –276 Gossypium hirsutum, chloride response, 106, 110, 133, 138 Grain crops, see also speciﬁc crops chloride response, 105 –106, 118 –119, 122, 127, 133 drought effects on reproductive development, 59 – 86 carbohydrate metabolism, 84 – 85 hormonal sterility induction, 81– 84 injury nature, 62– 68 cell division inhibition, 67 fertilization initiation, 65 – 66

326

INDEX

Gain crops (continued) ﬂower initiation and development, 62–63 gametophyte development, 63 – 65 grain initiation, 65 – 66 kernel growth and maturation, 66 – 68 pollination initiation, 65 – 66, 71–73, 85 overview, 59–61, 85 – 86 reproductive failure physiology, 71– 85 carbohydrate availability, 73 –75 grain maturation regulation, 75 – 81 kernel abortion, 73 –75 pollen development, 71–73 sensitivity, 61–62 water–tissue relationship, 68 –71 anthesis stress, 69 –70 ﬂower initiation and development, 68 grain ﬁlling and maturation, 70 –71 meiotic-stage stress, 68 – 69, 82, 85 nitrogen response, 250 –252, 272 straw residue management, 231, 247 Grape, chloride response, 106, 111, 131, 138

H Herbicides efﬁciency, crop residue role, 253 –254 humus substance interactions, 44 – 45 Histosols, global distribution, 161–162 Hordeum vulgare chloride response, 106, 118 –119, 127, 138 drought effects, see Drought nitrogen response, 263 –267 residue analysis, 200 Hormones, see speciﬁc hormones Humus substances, 1– 53, see also Crop residue amino acids, 32 amino sugars, 32 analytical characteristics, 11–21 chemical methods, 11–12 13C nuclear magnetic resonance spectrometry, 13–16 electron microscopy, 19 –21 electron spin resonance spectroscopy, 16–19 fourier transformation infrared spectroscopy, 12–13 infrared spectroscopy, 12–13 15N nuclear magnetic resonance analysis, 33

chemical structure analysis, 21– 30 amino acids, 32 amino sugars, 32 Curie-point pyrolysis, 26 –27 gas chromatography/mass spectrometry, 26 –27 nucleic acid bases, 32 oxidative degradation, 21–24 pyrolysis-ﬁeld ionization spectrometry, 24 –26 reductive degradation, 24 three-dimensional structure, 28 – 30 two-dimensional structure, 27–28 colloid chemical characteristics, 36 – 38 deﬁnitions, 4 – 5 direct analysis 13 C nuclear magnetic resonance spectrometry, 6 – 9 pyrolysis-ﬁeld ionization spectrometry, 6–9 extraction problems, 5 – 6 functions, 45 future research directions, 46 – 47 long-term cultivation effects, 9 –11 metal reactions, 41– 44 adsorption characteristics, 43 desorption characteristics, 43 mixed ligand complexes, 42– 43 water-soluble complexes, 41– 42 mineral reactions, 41– 44 adsorption, 44 dissolution, 43 – 44 nitrogen-containing components, 30 – 31, 34 – 35 15N nuclear magnetic resonance analysis, 33 nucleic acid bases, 32 overview, 1– 4 pesticide interactions, 44 – 45 phosphorus-containing components, 35 pyrolysis gas chromatography/mass spectrometry analysis, 26 –27, 34 – 35 sulfur-containing components, 35 – 36 surface pressure, 36 – 38 surface tension, 36 – 38 uses, 46 viscosity, 36 – 38 water retention, 38 – 41 Hydrogen ion pump, chloride role, 116 –117 Hydrology, see Drought; Water

327

INDEX I Inceptisols, global distribution, 161–162 Infrared spectroscopy, soil organic matter analysis, 12–13 Iron, humic acid adsorption, 43 Irrigation chloride nutrition management foliar damage, 137–139 root zone leaching, 136 –137 sprinklers, 137–139 sources, 100–101, 140 crop residue decomposition role, 213 –214, 217

K Kaolisols, see Oxisols Kiwi fruit, chloride response, 118 –120, 130 Kononova, M. M., 50–51

L Lateritic soils, see Oxisols Lead, humic acid adsorption, 43 Legumes, see also speciﬁc types nitrogen dynamics beneﬁts to subsequent crops, 273 –278 crop nitrogen recovery, 265 –268 Lettuce, chloride response, 106, 111, 118 Ligands, metal–humic substance reactions, mixed ligand complexes, 42– 43 Lignin, crop residue component decomposition role, 204 –209, 221 study methods, 225 Lupinus, nitrogen dynamics beneﬁts to subsequent crops, 273 –278 crop nitrogen recovery, 265 –268 Lycopersicum esculentum, chloride response, 106, 108, 112, 118–119, 121, 128 –129

M Malus ssp., chloride response, 106 Manganese humic acid adsorption, 43 water-soluble complex formation, 41– 42 Mass spectrometry, soil organic matter analysis, 26–27, 34–35

Medicago sativa, chloride response, 112, 138 Meiosis, drought effects in grain crops, 68 – 69, 82, 85 Melon, chloride response, 118 –119, 123 Mercury, humic acid adsorption, 43 Metals humic substance reactions adsorption, 43 desorption, 43 mixed ligand complexes, 42– 43 water-soluble complexes, 41– 42 oxisol soils, 180 Minerals humic substance reactions clay interlayer adsorption, 44 external adsorption, 44 minerals dissolution, 43 – 44 oxisol soils, 170 –172, 187 Mollisols, global distribution, 161–162

N Nickel, humic acid adsorption, 43 Nitrate, chloride uptake interactions, 117– 121 Nitrogen chloride uptake interactions, 117–121 crop residues, 257–278 beneﬁts to subsequent crops, 271–278 biological nitrogen ﬁxation, 274–275 fertilizer nitrogen equivalences, 272– 273 ﬁxed nitrogen responses, 275–276 grain yield responses, 250 –252, 272 legume nitrogen, 273 –278 nonﬁxed nitrogen responses, 275–276 decomposition affecting factors carbon/nitrogen ratio, 203–209, 220 – 221, 261, 270 nitrogen content, 203 –204, 212–213 immobilization turnover, 257–262 fertilizer, 259 –261 residues, 261–262 management practices crop yield responses, 250 –252 soil nitrogen, 237–241 mineralization, 257–262 fertilizer, 259 –261 process, 268 –271 residues, 261–262

328

INDEX

Nitrogen (continued) recovery, 262–271 cereal crops, 265 fertilizer, 263–268 legume nitrogen, 265 –268 mineralization process, 268 –271 soil component detection, 34–35 distribution, 31 function, 30–31 management practices, 237–241 15N nuclear magnetic resonance spectrometry crop residue analysis, 225 –227 soil organic matter analysis, 33 Nuclear magnetic resonance spectrometry crop residue analysis, 225 –227 soil organic matter analysis 13C nuclear magnetic resonance spectrometry analytical characteristics, 13 –16 direct analysis, 6 – 9 15N nuclear magnetic resonance spectrometry, 33 Nucleic acid bases, soil component, 32

O Oats drought effects, see Drought nitrogen response, 265 –267 residue analysis, 200 Onion, chloride response, 106, 116 –117 Organic matter, see Crop residue; Soil organic matter Orlov, D. S., 51–53 Oryza sativa chloride response, 105 –106, 127 drought effects, see Drought nitrogen response, 265 –267 Osmoregulation, chloride role, 115 –116 Oxidative degradation, soil organic matter analysis, chemical structure, 21–24 Oxisols, 151–187 deﬁnition, 163–164 ecosystem management, 180 –186 agriculture, 185–186 forests, 180–183 pasture, 183–185 formation, 164–167 geography, 158–163 global distribution, 161–162

historical perspectives, 153 –158 early European contributions, 153 –154 modern pedology, 154 –158 landscape relations, 167–170 African tertiary surfaces, 168 –169 alluvium occurrence, 169 –170 Brazilian planation surfaces, 167 central Zaire basin, 168 localized rock formations, 169 lower amazon basin, 167–168 sur Americana, 167 overview, 151–153, 187 properties, 170 –180 chemistry, 173 –175 color, 175 –178 consistence, 173 fertility characteristics, 180 heavy metals, 180 hydrologic properties, 178 –179 micromorphology, 170 –172 micronutrients, 180 mineralogy, 170 –172, 187 nutrient retention characteristics, 179 physics, 173 –175 structure, 173 types, 163 –164 Ozone, crop residue decomposition role, 219 – 220

P Pasture management, oxisol soils, 183 –185 nitrogen response, 263 –264 residue analysis, 200 Pea, nitrogen dynamics beneﬁts to subsequent crops, 273 –278 crop nitrogen recovery, 265 –268 Peanut, chloride response, 118 –119, 121, 131 Pepper, chloride response, 106, 138 Persea americana, chloride response, 118 –119, 123, 130 –131 Pest control crop residue role, 254 –256 pesticide–humus substance interactions, 44 – 45 Phaseolus vulgaris chloride response, 106, 112 nitrogen ﬁxation beneﬁts, 276 Phosphorus chloride uptake interactions, 121–122

329

INDEX crop residue analysis, 227, 241 soil component, 35, 241 Phytotoxicity crop residues, 252–253 pesticide–humus substance interactions, 44–45 Pisum sativum, nitrogen dynamics beneﬁts to subsequent crops, 273 –278 crop nitrogen recovery, 265 –268 Planation surfaces, oxisol soil relations, 167 Plant disease chloride effects, 125–126, 140 control, crop residue role, 254 –256 Plants chloride nutrition, see Chloride crops, see speciﬁc crops drought effects, see Drought yields, see Crop yields Ploughing, see Cultivation Plum, chloride response, 138 Pollination, drought effects in grain crops, 65 – 66, 71–73, 85 Polyphenols, crop residue decomposition role, 205, 207–209 Potassium, chloride uptake interactions, 123 Potato chloride response, 105 –106, 112, 118, 121, 127–128 nitrogen response, 263 –264 Prunus ssp., chloride response, 138 Pyrolysis-ﬁeld ionization spectrometry, humus substance analysis direct analysis, 6–9 soil organic matter structure analysis, 24– 26 Pyrolysis gas chromatography/mass spectrometry, soil organic matter analysis, 26 –27, 34–35

R Radish, chloride response, 106 Rainwater, see Water Rape, nitrogen response, 263 –264 Reductive degradation, soil organic matter analysis, chemical structure, 24 Rice chloride response, 105 –106, 127 drought effects, see Drought nitrogen response, 265 –267 Rye, nitrogen response, 263–264

S Salinization chloride behavior in soil, 99 –101 crop residue decomposition role, 211–212 management, 134 –136 Salts, see speciﬁc types Secale cereale, nitrogen response, 263 –264 Soil chloride behavior, 99 –103 accumulation, 99 –101, 139 clay surface reactions, 101 nitriﬁcation inhibitor, 102–103 sources, 99 –101 fertilization, 100 –101 irrigation, 100 –101, 140 rainwater, 99 –100 reserves, 99 uptake, 101–102 organic matter, see Crop residue; Soil organic matter oxisol soils, see Oxisols quality management, 231–247 biological properties, 242–247 biomass, 243 –244 carbohydrates, 247 enzymes, 246 –247 microorganisms, 243 –247 mycorrhiza, 245 –246 soil fauna, 244 –245 chemical properties, 236 –242 micronutrients, 241 nitrogen, 237–241 pH, 236 –237 phosphorus content, 241 soil organic matter, 237–240 indicators, 231–232 physical properties, 232–236 aggregation, 232–233 bulk density, 233 –234 compaction, 233 –234 erosion, 232 hydrology, 234 –235 moisture content, 235 –236 penetration resistance, 233 –234 structure, 232–233 temperature, 235 salinization chloride behavior in soil, 99 –101 management, 134 –136 three-dimensional structure analysis, 28 – 30

330

INDEX

Soil organic matter, see also Crop residue chemistry, 1–53 colloid chemical characteristics, 36 – 38 deﬁnitions, 4–5 direct analysis 13C nuclear magnetic resonance spectrometry, 6 – 9 pyrolysis-ﬁeld ionization spectrometry, 6–9 extraction problems, 5 – 6 future research directions, 46 – 47 humus substances amino acids, 32 amino sugars, 32 analytical characteristics, 11–21 chemical structure, 21– 30 colloid chemical characteristics, 36 – 38 deﬁnition, 5 functions, 45 metal reactions, 41– 44 mineral reactions, 41– 44 15N nuclear magnetic resonance analysis, 33 nucleic acid bases, 32 pesticide interactions, 44 – 45 pyrolysis gas chromatography/mass spectrometry analysis, 26 –27, 34–35 sulfur-containing components, 35 – 36 uses, 46 water retention, 38 – 41 long-term cultivation effects, 9 –11 nitrogen-containing components, 30 – 31, 34–35 overview, 1–4 phosphorus-containing components, 35 sulfur-containing components, 35 – 36 surface pressure, 36 – 38 surface tension, 36 – 38 viscosity, 36–38 Solanum tuberosum chloride response, 105 –106, 112, 118, 121, 127–128 nitrogen response, 263 –264 Sorghum chloride response, 106 drought effects, see Drought Soybean chloride response, 105 –106, 113, 132 nitrogen ﬁxation beneﬁts, 275 –276

Spinach, chloride response, 106 Spodosols, global distribution, 161–162 Stevenson, F. J., 53 Straw, residue management, 231, 247 Strawberry, chloride response, 106, 110, 118 – 119, 123 Sugar beet chloride response, 105 –106, 133, 138 nitrogen response, 263 –264 Sugar cane, chloride response, 105 –106 Sugars, see Carbohydrates Sulfur crop residue analysis, 227 soil component, 35 – 36 Sur Americana, oxisol soils, 167 Surface pressure, soil organic matter characteristics, 36 – 38 Surface tension, soil organic matter characteristics, 36 – 38 Sweet potato, chloride response, 105 –106

T Tobacco, chloride response, 105 –106, 118 Tomato, chloride response, 106, 108, 112, 118 – 119, 121, 128 –129 Translocation, chloride distribution, 112–113, 139 Trifolium ssp., nitrogen response, 263 –264 Triticum aestivum chloride response, 105 –106, 118 –119, 122, 127 drought effects, see Drought nitrogen response, 263 –267 residue analysis, 200 Turnip, chloride response, 106

U Ultisols, global distribution, 161–162 Ultraviolet radiation, crop residue decomposition role, 220 Undersowing, crop residue management, 230 – 231

V Vertisols, global distribution, 161–162 Vicia faba chloride response, 106

331

INDEX nitrogen dynamics beneﬁts to subsequent crops, 273 –278 crop nitrogen recovery, 265 –268 Vigna unguiculata, chloride response, 106 Viscosity, soil organic matter characteristics, 36–38 Vitus vinifera, chloride response, 106, 111, 131, 138

Weed control, crop residue role, 253 –254 Wheat chloride response, 105 –106, 118 –119, 122, 127 drought effects, see Drought nitrogen response, 263 –267 residue analysis, 200

Y W Yields, see Crop yields Water chloride sources irrigation, 100–101, 140 rainwater, 99–100 crop residue decomposition role, 203, 210 – 211, 217 drought effects, see Drought humic substance retention characteristics, 38–41 metal–humic substance reactions, water-soluble complexes, 41– 42 osmoregulation, chloride role, 115 –116 oxisol hydrologic properties, 178 –179

Z Zaire basin, oxisol soils, 168 Zea mays chloride response, 106, 112, 118, 133, 138 drought effects, see Drought nitrogen response, 263 –267 residue analysis, 200 Zinc, humic acid adsorption, 43

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