Advances in Agronomy, Volume 60

V O L U M6 0E

8. .

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Advisory Board Martin Alexander

Eugene J. Kamprath

Cornell University

North Carolina State University

Kenneth J. Frey

Larry P.Wilding

Iowa State University

Texas A&M University

Prepared in cooperation with the

American Society of Agronomy Monographs Committee William T. Frankenberger, Jr., Chaimnan P. S. Baenziger David H. Kral Dennis E. Rolston Jon Bartels Sarah E. Lingle Diane E. Storr Jerry M. Bigham Kenneth J. Moore Joseph W. Stucki M. B. Kirkham Gary A. Peterson

DVANCES IN

Edited by

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

ACADEMIC PRESS San Diego London Boston New York Sydney Tokyo Toronto

This book is printed on acid-free paper. @ Copyright 0 1997 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-1997 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. 0065-21 13/97 $25.00

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Contents CONTRIBUTORS ........................................... PREFACE .................................................

vii u

NUTRIENT C Y C L ~ TRANSFORMATIONS. G. AND FLOWS: IMPLICATIONS FOR A MORESUSTATNABLE AGRICULTURE I. I1. 111.

Fred Magdoff. Les Lanyon. and Bill Liebhardt Introduction .............................................. Framework for Evaluating Nutrient Dynamics . . . . . . . . . . . . . . . . . . Soil-Plant System ......................................... Cycling and Flows a t the Field Level .......................... Farm-Scale Cycling and Flows ............................... Watershed, Regional, and Global Issues ........................

Iv. v; VI. VII. Promoting a More Sustainable Agriculture through Changes

Influencing Nutrient Cycles and Flows ........................ VIII . Conclusions .............................................. References ...............................................

2

5 13 23 38 47

56 65 66

ADAPTATIONOF PLANTSTO SALINITY Michael C. Shannon

I . Invoduction .............................................. I1. Rationale for Breeding for Salt Tolerance ......................

III. Selection for Salt Tolerance .................................. IV Salt Tolerance Mechanisms ..................................

v. Genetic Variability ......................................... VI . Breeding Methods ......................................... VII . Novel Concepts. .......................................... VIII . Summary and Conclusions .................................. References ...............................................

V

76 77

78 84 88 101 105 107 108

vi

CONTENTS

INFLUENCEOF NO-TILL CROPPINGSYSTEMS ON MICROBIAL RELATIONSHIPS

L . F. Elliot and D .E. Stott I . Introduction ..............................................

I1. Decomposition of Surf-ace-Managed Crop Residues . . . . . . . . . . . . . . I11. Modeling Crop Residue Decomposition ....................... n? Root-Microbial Relationships................................ v. Deleterious Rhizobacteria for Weed Control .................... VI . Low-Input. On-Farm Composting ............................ References ...............................................

121 122 125 129 137 141 144

PRACTICAL ETHICSIN AGRONOMICRESEARCH Don Holt I . Introduction .............................................. I1. Basic Concepts ............................................ I11. Ethics of Choosing Research Subject Matter .................... rv. Difficulties with the Utilitarian Approach ...................... v. Agricultural Ethics and the World Food Situation . . . . . . . . . . . . . . . . VI. Ethics in the Conduct of Research ............................ VII . Ethics in Research Administration ............................ References ...............................................

150 151 154 158 162 165 184 190

AREAGROECOSYSTEMS SUSTAINABLE IN SEMIARID REGIONS?

B . A. Stewart and C. A. Robinson I . Introduction .............................................. I1. Agroecosystems ........................................... ILL Semiarid Regions ..........................................

n? v.

VI.

VII .

The Issue of Sustainability .................................. Technologies for Increasing Plant-Available Water . . . . . . . . . . . . . . . Soil Organic Matter Maintenance ............................. Summary ................................................ References ...............................................

INDEX...................................................

191 193 194 198 205 223 224 225

229

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

L. F. ELLIOT (12 l), National Forage Seed Production Research Center, Corvallis, Oregon 97331 DON HOLT (149), College of Agricultural, Consumer, and Environmental Sciences, Illinois Agricultural Experiment Station, University of Illinois, Urbana, Illinois 61801 LES LANYON (l), Department of Plant and Soil Science, University of Vermont, Burlington, Vermont 05405-0082 BILL LIEBHARDT (l), Department of Plant and Soil Science, Universityof Vermont, Burlington, Vermont OS405-0082 FRED MAGDOFF (l), Department of Plant and Soil Science, University of Vermont, Burlington, Vermont 05405-0082 C. A. ROBINSON (191), Dryland Agriculture Institute, West Texas A&M University, Canyon, Texas 79016 MICHAEL C. SHANNON ( 7 9 , United States Department of Agriculture, Agriculture Research Service, U S . Salinity Laboratory, Riverside, California 92507 B. A. STEWART (191), Dryland Agriculture Institute, West Texas A&M University, Canyon, Texas 79016 D. E. STOTT (12 l), National Soil Erosion Research Laboratory, Purdue University, West Lajayette, Indiana 47906

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Preface Volume 60 contains five outstanding chapters that address cutting-edge research and timely issues in the plant and soil sciences. Chapter 1 discusses nutrient cycling transformations and flows and the implications for a sustainable agriculture. Topics that are included are the soil-plant system; cycling and flows at the field level; farm scale cycling and flows; watershed; state, regional, and global issues; and promoting a more sustainable agriculture. Chapter 2 is a state-of-the-art review on adapting plants to salinity. The most contemporary research on selection for salt tolerance, salt tolerance mechanisms, genetic variability, breeding methods, and novel biotechnological tools for improving plant adaptation to salinity, including tissue culture and molecular biology, is included. Chapter 3 discusses the effects of no-tillage cropping systems on soil microbiological relationships, including decomposition of surface-managed crop residues, modeling crop residue decomposition, root-microbial relationships, deleterious rhizobacteria for weed control, and low-input, on-farm composting. Chapter 4 discusses the very timely topic of ethics in agronomic research. This treatise should be of great interest to students in the plant and soil sciences and to practicing professionals. The author defines personal ethics and scientific conduct and then discusses the ethics of choosing research subject matter, agricultural ethics and the world food situation, and ethics in research and administration. Chapter 5 discusses the question of the sustainability of agroecosystems in semiarid regions. Semiarid regions, the issue of sustainability,and technologies for increasing plant available water are covered. The editor expresses sincere gratitude to the authors for their fine contributions.

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NUTRIENT CYCLING, TRANSFORZMATIONS, AND FLOWS: IMPLICATIONS FOR A MORE S U S T ~ A B LAGRICULTURE E Fred Magdoff, Les Lanyon, and Bill Liebhardt Department of Plant and Soil Science University of Vermont Burlington, Vermont 05405-0082

1. Introduction 11. Framework for Evaluating Nutrient Dynamics A. Historical Overview B. Definitions C. Implications of Spatial Scale and Ecosystem Relations, Seasonal Patterns, and Landscape Position 111. Soil-Plant System A. Plant Nutrition and Soil Nutrient Stocks B. Ecology of Nutrient Flows, Transformations, and Cycles C . Soil Chemical Properties D. Soil Physical Properties E. Biological, Chemical, and Physical Interactions W. Cycling and Flows at the Field Level A. Nutrient Losses B. Nutrient Additions C. Management Practices and Nutrient Flows D. Changes in Field Nutrient Flows E. Changing to Biologically Based Nutrient Sources V. Farm-Scale Cycling and Flows A. Within-Farm Nument Flows B. Nutrient Flows to and from Farms C. Nutrient Flows between Farms D. Patterns of Farm Nutrient Flows VI. Watershed, Regional, and Global Issues A. Watersheds B. Other Spatial Scales C . Energy Use and Nutrient Flows D. Possible Changes in Large-Scale Flows E. Influences on Nutrient Flow Patterns

1 Adwmcrr in A p n a r y , Voliimr 60 Copyright 0 1997 by Academic Press. All rights of rcproducoon in any fnnn reserved

0065.2 1 11/97 $25.00

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FRED MAGDOFF ETAL. VII. Promoting a More Sustainable Agriculture through Changes Influencing Nutrient Cycles and Flows A. Field-Level Changes (Short Term) B. Farm-Level Changes (Medium Term) C. Societal-Level Changes (Long Term) VIII. Conclusions References

I. INTRODUCTION The many economic, environmental, and social problems associated with conventional agriculture have elicited calls for new approaches to agricultural science as well as practices at the farm level. It is suggested that by relying on ecologically sound principles it will be possible to develop practices that enhance the economic viability of agriculture while at the same time helping to improve environmental quality (MacRae et al., 1990). Among the environmental problems associated with conventional agricultural practices are a number related to nutrient management. The most pressing of these include pollution of groundwater with nitrates and surface water with both nitrates and phosphates. Nutrients from agricultural activities have decreased drinking water quality as well as the usefulness of fresh water and estuaries for recreation and commercial fisheries. This decline of water quality is caused by leakages from farms that, although not desired, appear to be an integral part of conventional agricultural practices. Part of the explanation for the large quantity of nutrients lost to leaching and runoff waters is the use of more fertilizers and manures than are actually needed by crops. For example, it has been estimated that farmers in the Midwest have used about one-third more N fertilizer than actually needed (Swoboda, 1990). One of the reasons for the overuse of nutrients may be insufficiently precise soil test and fertilizerhanure recommendation systems. Other explanations for nutrient overuse include insufficient available cropland area to properly utilize nutrients from animal production facilities and the use of “rule of thumb” guidelines by many farmers instead of regularly testing soils or plant tissue to determine nutrient needs. In addition, the heavy reliance on the readily available (soluble) nutrients in commercial fertilizers as well as in many manures may enhance nutrient loss from soils by leaching and runoff compared to amounts lost from less soluble sources. Finally, the decreased soil tilth associated with various crop and soil management practices can result in loss of large amounts of runoff, carrying with it dissolved nutrients and eroded sediments. The loss of nutrients from soils can also have significant economic consequence.

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Any use of fertilizers above the economic optimum, where the value of increased yields just balances the extra cost of applying an increment of fertilizer, is a direct economic loss to farmers while at the same time it greatly increases the risk of pollution. This is especially important for low-value per hectare agronomic crops, where the cost of fertilizer is a significant portion of input expenditures and the margin between costs of production and crop value is very narrow. For example, for high yielding corn and wheat the estimated expenditures of fertilizers and lime in Michigan are approximately 18% of the crops’ value (including deficiency payments) and 33 and 44% of the costs of growing the crops, respectively (excluding depreciation, insurance, rent, taxes, interest, and family labor) (Nott et al., 1995). In contrast, similar data for bearing semi-dwarf apples for fertilizer and lime are approximately 1 % of the crop’s value and 2% of the costs. Therefore, although a little extra fertilizer above the economic optimum applied to an apple orchard will have minimal effects on economic returns, the situation is very different for agronomic crops. For low-value per hectare crops, it is especially critical to ensure that as little fertilizer as possible is used over that needed for maximum economic return. There are also other nutrient management issues that potentially influence the long-term sustainability of agriculture. Reliance on large amounts of energy to produce fertilizers, especially N, and to transport them significant distances to farms as well as crops to animals and food to people depends on ready availability and relatively low-cost fossil fuels. Also, runoff from agricultural land tends to carry surface sediments that are enriched in organic matter in addition to readily available nutrients. This loss of organic matter, which may contribute to pollution of surface waters, also decreases soil quality and long-term productivity. Erosion of organic matter-enriched surface soil decreases the tilth as well as the fertility of soil, decreasing water infiltration and storage for plant use and leading to more runoff. The development of the synthetic fertilizer industry, which began in the 19th century and vastly expanded during the post-WW I1 era, allowed agriculture to avoid many of the obvious consequences of depleting the natural fertility of soils. The introduction of low-cost N fertilizers also permitted the elimination of forage legumes from rotations on many farms and lead to increased farm specialization such as continual cultivation to grain crops. However, as soil organic matter (SOM) was depleted, other problems developed such as decreased soil tilth, increased soil erosion, lower soil water holding capacity, decreased buffering with respect to pH and nutrient availability, increasing plant pest problems, etc. (Magdoff, 1993). In response to these many problems as well as other powerful forces and trends, practices and grower outlook developed during the last half of the 20th century so that agriculture is now treated in a manner that mimics industry. Plant and animal outputs of agriculture are thought of in almost the same way as nonbiological industrial products that require “assembling” by using various external in-

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FRED MAGDOFF ETAL.

puts such as synthetic fertilizers, pesticides, irrigation, fuel, equipment, feeds, and labor. As cities have grown more numerous and larger and an agriculture has developed that relies on specialized production of crops and animals and high application rates of readily available nutrients from synthetic fertilizers as well as manures, there has been a dramatic increase in the magnitude of problems resulting from flows of nutrients that end up in surface and subsurface waters and in the air. It is now clear that the economic and environmental impact of these nutrient management issues is so large that a reevaluation of nutrient flows and cycles is critical to the successful development of sustainable agricultural systems. Agriculture is practiced along a broad continuum of possibilities with farmers following many different practices and philosophies. Sustainability refers to agriculture that is viable for a long period. It implies economic, environmental, and social components that interact to a high degree and are not mutually exclusive. Because humans have such a large impact on the globe, the social or human component of agriculture is very important to the subject of nutrient cycling. Some current agricultural practices and ways in which agriculture and the rest of society interact appear to be sustainable; others do not. “Sustainability” is not a formula or a recipe; rather, it may be more of a direction toward a “moving target” because society and the earth are constantly changing. What may be considered sustainable at one time may or may not be considered sustainable at another as new information is evaluated. Conventional agriculture is dependent on large quantities of synthetic chemical, capital, energy, and machinery inputs. It largely follows the theme of manipulation of nature-changing nature to suit humankind. Sustainable agriculture practitioners attempt to work with natural systems as much as possible. They endeavor to develop economically and environmentally sound practices and reduce depletion of nonrenewable resources. At the same time they strive to enhance their quality of life, as well as that for rural communities and society as a whole. This review will discuss characteristics of current nutrient flows, some of the concerns about the condition of nutrient cycles in contemporary agriculture, and opportunities for nutrient cycling in sustainable agriculture. We will view these issues at different geographic scales, including the soil-plant, field, farm, watershed, regional, and global levels. We will also discuss features of nutrient cycles that influence the relationships of agriculture and society. As the character of nutrient flows is evaluated and modified in the future, changes are likely to have implications for the nonfarm segment of society as well as on-farm practices. Thus, it is important for nonfarm citizens to become familiar with features of nutrient cycles that influence the relationship of agriculture to society. It may well be possible to significantly “tighten-up’’ nutrient cycles and make them function more efficiently in individual soils or on the farm as a whole. This is a challenge for agriculture and society. Although we will focus most of our attention on the conditions in the United States, much of the discussion will be relevant to other developed coun-

NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS

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tries in temperate regions as well as developing nations in both the temperate and tropical regions.

II. FRAMEWORK FOR EVALUATING NUTRIENT DYNAMICS A. HISTORICAL OVERVIEW The flow of energy in an ecosystem can be represented by a pyramid with those species higher on the pyramid consuming organisms or residues below. A simple trophic pyramid involving plants at the base, providing all the primary products, and humans at the top can be used to demonstrate connections within a system of food production and consumption. The energy of sunlight captured and the nutrients taken up by plants flow upward in the pyramid as the products of plants are consumed and utilized. Trophic pyramid diagrams can be used to highlight differences over time in the spatial connections between plants, animals, and humans and indicate the potential for nutrient cycling and maintenance of soil nutrient levels or stocks. What follows are generalized abstractions of complex processes and relationships that do not apply equally to all current or historical situations but help to highlight major trends over time. It is thought that for most of human history people lived in small bands that wandered over extensive territories as they spread out and eventually populated much of the earth’s land area. As populations increased and became more sedentary, preagricultural hunters and gatherers brought plants and animals back to villages and dwellings and there was some spatial separation between humans and their food sources. There was little possibility for return of nutrients to soils from where they came except that animals would cycle nutrients in urine and manure as they fed themselves prior to capture. However, because there were small numbers of people relative to the territories being exploited for food and they constantly changed the areas being used, effects on nutrient flows were probably small. During the early stages of agriculture when crops were produced near dwellings and animals were raised by seminomadic herding there was more potential for nutrient cycling. Animal manures were deposited as the animals grazed as before, but crop and animal remains were now in or near fields. It was during this stage of development when a wave of episodes of erosion occurred, such as the one in Greece and the Middle East, as a result of hillside deforestation and subsequent grazing and cropping (Runnels, 1995; Hillel, 1991).This resulted in a massive transfer of nutrients and soil from hills and mountains to valley floors. It has been argued that the agricultural changes that occurred in medieval Europe were an essential precursor to the industrial revolution. The diversification of

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crops through the raising of forages, especially N,-fixing clovers, allowed continuous cropping to take the place of the alternate year or every third year fallow systems (Bairoch, 1973). It also permitted the integration of livestock into cropping systems and ended nomadic husbandry. The enhanced productivity of the land allowed a significant increase in the annual agricultural production over the needs for farm family consumption (Bairoch, 1973). Although the industrial revolution began in England during the last half of the 18th century, it reached other countries in Europe and the United States only during the 19th century. Through much of the 19th century, and well into the 20th century in pockets, most agricultural products were consumed on the farm where produced. This was a common feature of temperate region agriculture in what eventually became the advanced economically developed countries. In the less developed temperate and tropical regions, with the important exception of plantation crops such as sugar and bananas, subsistence farming has been common through much of the 20th century, with only small amounts of products exported off the farm. In the diversified subsistence farming systems that developed in Europe and the United States before the industrial revolution, most of the plant products were either consumed directly by people on the land or were consumed by animals that were then consumed by humans (Fig. la). In this example the three parts of the pyramid are physically connected and residues and waste products can easily return to the land. The development of large cities and transportation systems to move food long distances in the United States and the industrializing countries of northern Europe created the first modern widespread physical break in the production-consumption chain. Crops and animal products were sent from the countryside to urban areas and even to other countries, decreasing the potential for on-farm nutrient cycling (Fig. 1b). In the last half of the 20th century, rapid urbanization has also been occurring in most developing countries (usually without commensurate economic development), and this, together with the development of an “advanced” commercial agricultural sector oriented toward exports, has also had a significant negative impact on nutrient flows in those countries. Concern about the consequences of interrupting the cycling of nutrients was expressed in the last century: Capitalist production, by collecting the population in great centers, and causing an ever increasing preponderance of town population . . . disturbs the circulation of matter between man and the soil, i.e., prevents the return to the soil of its elements consumed by man in the form of food and clothing; it therefore violates the conditions necessary to lasting fertility of the soil.’’ (Marx, 1887; originally published in German in 1867) Another physical break in the trophic pyramid resulted from the transformation of animal agriculture based on small diversified farms to large specialized production units separated by long distances from the farms that produce feeds (Fig.

NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS

7

b

a

nutrients

II

t

Acnnsumers

I

primary producers (plants)

Figure 1 Changes in the spatial relationships of the trophic pyramid relating plants and animals to humans. (a) Early agriculture (eighteenth to mid-nineteenth century); (b) urbanizing agriculture (mid-nineteenth to mid-twentieth century); (c) industrial agriculture (mid- to late-twentieth century).

lc). The availability of low-cost N fertilizers after WW I1 rendered forage legumes superfluous on farms producing grain crops. There was no longer the need to raise animals to utilize the forages. In the United States, the conversion to enormous production units is essentially complete for poultry, far advanced for beef cattle, and well under way for hogs. This phenomenon has further exacerbated environmental problems associated with agriculture. The heart of the issues resulting from the geographic separation of crops and animals can be summarized as two sides of the same coin: (i) the decline of SOM and nutrients on crop farms (requiring the application of large quantities of synthetic fertilizers as well as other inputs to compensate for organic matter depletion, and (ii) the simultaneous overabundance of nutrients and organic matter at animal production facilities (with the resulting pollution of surface and groundwaters).

B. DEFINITIONS Clarification of the definitions of some of the key terms that we will use will be helpful for the discussion of issues and problems of crop nutrient management. Stocks-Stocks refer to the quantity of nutrients within a defined part of a system. The total stock of nutrients may be of interest for many assessments. However, from the point of view of plant nutrition the maintenance of a sufficient stock (pool) of nutrients that are either available or easily transformed into an available state is essential for crop productivity. At the same time, available nutrient substocks must be low enough to moderate potential environmental effects of agriculture. Flows will both contribute to and be subject to the magnitude of the various stocks.

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There are numerous biological and chemical reactions that change the state of nutrients to more or less available forms. These transformations convert nutrients from one stock of the element to another but do not change the quantity of the total stock of a nutrient. Although the total stock of a particular nutrient may be important for long-term sustainability, it will not usually be of interest for the shortrun concerns of soil fertility unless the net rate of transformation to an available form is also known. The size of a stock may exert an influence on susceptibility for nutrient flow. For example, large stock of inorganic Nor of soluble P will permit significant flows of these nutrients with leaching or runoff waters. Flows-The flow of nutrients in an ecosystem is the most basic concept of nutrient movement. Nutrient flows represent linkages among various pools (or stocks). Measurements of various types of nutrient flows can suggest control mechanisms and indicators of system performance. Some nutrient flows are managed pathways, where the purpose of the operation entails the intentional addition or removal of nutrients. Managed flows occur when fertilizer is applied to meet an estimated crop need, when manure is applied to certain fields, when a crop is harvested and sold, when animals graze on pastures, etc. Although other flows, such as leaching of nitrate or nutrient losses in runoff waters, are not purposely managed, their magnitude is strongly influenced by management practices such as tillage systems, rotations, fertilizer application rates, manure application rates and application methods, and animal stocking density. Cycles-A nutrient cycle is an example of a closed loop pattern of flow in which a particular atom ends up back in the same location from where it started. Where a boundary is drawn surrounding the extent of the system has a significant impact on deciding whether a true cycle or rather another pattern of flow is occurring. Transformations-There are numerous processes that determine the “state” or form in which nutrients occur in soils. These include mineralization from organic matter, immobilization of inorganic ions by microbial uptake, precipitation of lowsolubility compounds, various oxidation reactions such as nitrification, various reduction reactions such as denitrification, dissolution from solid forms, etc. The particular form that a nutrient is in influences its availability for plant uptake as well as susceptibility to leaching or gaseous losses. When a nutrient undergoes a transformation to another form, it is not usually considered a flow because the transformation normally occurs in place. However, one transformation, biological N, fixation, is also a flow. Because soil N, is in equilibrium with the atmosphere, N, moves into the soil as N, fixation occurs, and the stock of total soil N is increased. For purposes of discussion in this chapter we will refer to N, fixation as a flow of N rather than a transformation. Boundaries-When discussing nutrient flows and cycles it is essential to define a boundary around the system of interest. The boundary becomes a reference point for evaluating relative movement of nutrients. Different objectives may

NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS

9

cause one to define a boundary to be around a certain portion of the soil or a field, farm, state, region, watershed, or country. If global-scale cycles are of interest, the boundary then includes the entire earth.

C. IMPLICATIONS OF SPATIAL SCALEAND ECOSYSTEM RELATIONS, SEASONAL PATTERNS,AND LANDSCAPE POSITION When discussing nutrient transformations, flows, and cycles it is important to take into account implications of spatial scale, ecosystem relations, seasonal patterns, and landscape position. These various considerations can either influence the nutrient flows and transformations themselves or our perception of them.

1. Spatial Scale and Ecosystem Relations The extent of the system under consideration has a huge impact on how we view and understand flows and cycles. The emphasis in the literature on nutrients has been placed on the field scale because most tactical and operational management decisions are field based. When viewing processes and flows at this scale, the issue of applying fertilizers or manures is relatively simple. When a specific nutrient application is believed necessary some is applied and this is a flow into the field from somewhere outside. Likewise, when the crop is harvested, it seems to be a simple flow of nutrients out of the field. However, the crop may be consumed on the farm or leave the farm. Also, the nutrients in manure may come from inside the farm (if animals are fed farm-grown feedstuffs without imported fertility sources) or from off the farm (if animals are fed only imported feeds) or some mix of the two (if farm-produced feeds are grown with imported fertilizers). A greatly simplified diagram of a natural soil-plant-animal ecosystem (Fig. 2) can aid the discussion of scale of consideration and nutrient cycling and flows in agriculture.In this figure, the only input flows into the soil come from atmospheric deposition while the only output flows result from erosion, leaching, and gaseous losses. There are three stocks of nutrients (boxed in Fig. 2): in the soil (including all living organisms), in living plants above ground; and in aboveground animals. Nutrients are taken up from the soil by the plant as it grows and plant residues are returned to the soil to complete a soil+plant+soil or a soil+plant+animal+ soil cycle. In general,cycling of nutrients is very efficient under natural ecosystems (Crossley et af., 1984). In most undisturbed natural systems such as forests and grasslands, there is a high degree of synchronization of the supply of available nutrients with the uptake needs of plants. This results in a low level of nutrients in the soil solution at any one time, promoting an efficient soil+plant+soil cycling of nutrients. Continuous soil cover with little disturbance helps promote water infil-

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residue

8

~

atmospKem deposmn

erosto; kaching. g~~~ LOSS

I

Figure 2 Simplified natural system nutrient cycle and flows in the soil-plant system.

tration and maintain low rates of soil erosion. There may be some spatial discontinuity between where nutrients are taken up by plants and where they are deposited in residues, such as when leaves fall on the forest soil surface while roots may take up nutrients at 10 or 20 cm or greater depth. However, soil organisms, such as earthworms, beetles, and termites, and leaching help to reintroduce the nutrients into the root zone. Plants in natural systems sometimes appear to use different nutrient cycling “strategies” to their own advantage. It is hypothesized that through an evolutionary selection process some species of plants developed characteristics that enhance the fitness of their environment for themselves at the expense of other plant species (van Breeman, 1993, 1995). For example, fast-growing species tend to have residues that decompose and turnover nutrients rapidly. On the other hand, slowgrowing species often have residues that are high in lignin and secondary metabolites that slow microbial decomposition and, thus, reduce competition from fastgrowing species that require high levels of available nutrients. Compared to a natural ecosystem, a managed agricultural ecosystem has greater amounts of nutrients flowing in and out, less capacity for nutrient storage, and less nutrient cycling (Hendrix et al., 1992). There are now inputs of nutrients from a variety of animal feeds, synthetic fertilizers, inorganic amendments, manures, and composts (Fig. 3). In this example, the boundary has been drawn around a plant and the soil below to the bottom of the root system. A major nutrient output from the field is harvested plant material, which is fed to an animal or used in another

NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS 4

1I

crop removed

PLANT

* erosion,I leaching,

'

fertlltzers. manures, lime, organlc residues, ahospherlc

deposition

+

gaseous loss

Figure 3 Simplified managed system nutrient cycle and flows in the soil-plant system.

manner. In general, nutrient losses by runoff, erosion, volatilization, and leaching are far greater in an agroecosystems than in a natural system. Compared to natural systems, there is normally a greater quantity of soluble nutrients present in agroecosystems and more soil disturbance and longer times during the year when the soil is not covered with living vegetation. These agroecosystem characteristics stimulate SOM breakdown and lead to more compact soils with less porous infiltrative surfaces and more runoff and erosion than in natural ecosystems. When looking at the soil-plant system level, it is difficult to tell whether or not an input is completing a true cycle where the nutrients removed from that particular area of soil are being returned to the same location. For example, is the origin of the nutrients in manure the location under consideration or is it another field or farm? Thus, it is necessary to look at both field- and farm-level flows and cycles to determine whether or not true cycles are occumng. When looked at regionally (or globally), the location where the nutrients are produced or mined and refined or incorporated into plants or animals and where the agricultural products are shipped to, processed, and consumed all become important considerations in understanding intraregional and interregional flow patterns. These may be as important to a sustainable agriculture as field- and farmlevel flows. Nutrients commonly travel significant distances, as when fertilizer is shipped from the manufacturer to the farm or when feed grains are transported from the Midwest to the dairy farms in the Northeast, vegetables are shipped from California to New York, or wheat is transported from the Northern Plains and the

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FRED MAGDOFF ETAL.

Northwest of the United States to China. In these situations the flow is all one way and there is no realistic means for the nutrients to cycle back to the farms and fields from where they came.

2. Seasonal Patterns Nutrient transformations and flows do not happen at a uniform rate during the year. Mineralization of nutrients from organic matter is usually very slow during the winter and at a standstill when soil is frozen. Peak rates of mineralization in temperate region soils coincide with the warming in the spring and are probably significantly enhanced by freezing and thawing over the winter (Magdoff, 1991a; DeLuca et al., 1992). When soils dry down during the field season and are then rewetted, there is also a burst of mineralization caused by the conversion of a certain portion of SOM to forms that are more susceptible to microbial attack. Significant leaching and runoff losses of nutrients in most temperate annual cropping systems are confined to the late fall, winter, and early spring when precipitation exceeds evapotranspiration and recharge requirements (Fig. 4).During the summer season evapotranspiration is usually greater than precipitation and leaching and runoff are usually minimal because of the drier soil conditions. Managed flows also occur during distinct times of the year (Fig. 4).Large quantities of lime, fertilizer, and manure are normally applied when the crop is not in the field-in the spring before the crop is planted or in the fall after the last crop is harvested (some application during crop growth as side-dress and top-dress is also common). The flow of nutrients leaving the field with the harvested crop usually occurs at a distinct time of the year-determined by climate, species and cultivar, and other management practices. Thus, nutrients may be applied in the fall, taken up by plants during the following growing season, and removed from the field as the crop is harvested 10 or 11 months after application. Also, some portion of the applied nutrients may be held by the soil so that they are taken up by plants and removed from the field only years after application. There are also changes in nutrient stocks that operate over decades and even longer. Soil stocks of N in many midwestern soils were drawn down over decades as organic matter was depleted (Hass et al., 1957). Also, the buildup of nutrient levels by a few decades of heavy fertilizer and/or manure application by many farmers has made it difficult to even find low P and low K soils in certain areas (Engelstad and Parks, 1976; Sims, 1993).

3. Landscape Position By increasing the scale of attention from the soil-plant system to the field and then to the farm and watershed or subregion, issues relating to position in the land-

NUTRIENT CYCLING, TRANSFORIMATIONS, AND FLOWS

13

managed flows fertilizer

fertilizm

harvest

J

F

M I A M denitrification leaching

J

J

A

S

0

N I D denitrification leaching

rUnOff

emion

I

unmanaged flows

runoff

erosion

1

Figure 4 Seasonal aspects of nutrient flows into and out of fields for a northern hemisphere temperate region annual crop.

scape become apparent. For example, soil eroded from the slope of a field may or may not leave the field or farm. The sediments may be deposited in a low-lying depression in the field or in an adjacent field. Sediments might also flow from a field to a stream and from there into a lake. In the first situation, there is only a redistribution within a field or a flow from one field to another. It is not the same net loss to the field or farm that usually occurs after sediments enter a stream.

111. SOILPLANT SYSTEM

A. PLANTNUTRITION AND Son, NUTRIENT STOCKS Within the soil, for each plant nutrient of interest there are three main types of stocks that can potentially supply nutrients in forms that are available to plants: (i) nutrients in the soil solution in forms that can be taken up by plants, usually as simple ions; (ii) nutrients associated with organic matter by being adsorbed on negative exchange sites or present as part of organic molecules; and (iii) nutrients as-

14

FRED MAGDOFF ETAL.

sociated with soil minerals, either adsorbed on exchange sites or as part of the structure of the inorganic mineral. Mineralization of organic compounds as well as cation exchange, solubilization, desorption, and dissolution of minerals convert the soil nutrient stocks listed in (ii) and (iii) into forms that can be immediately used by plants. Nutrients are also added to the soil in a number of forms, such as fertilizers, manures, and crop residues from other fields, in precipitation and dry deposition, and in the special case of N by biological N, fixation.

1. Satisfying Short-Term Fertility Needs of Crops at the Soil-Plant Level To satisfy short-term needs of crops during the growing season the amount of available nutrients must be greater than or equal to the uptake needs of the crop (see soil-plant flow labeled 7 in Fig. 5). Using the numbering system in Fig. 5, Solution stock + (1-2)

+ ( 3 4 ) + (5-6) + (10-13) 2 7,

(1)

where solution stock is the quantity of nutrient in soil solution at start, 1-2 are the net mineralization, 3 4 are the net desorption from SOM, 5-6 are the net desorp-

[+output (flow)V-v I

fertilizers,

atmospheric deposltlon

Figure 5 Simplified nutrient cycle, flows, and transformations in the soil-plant system with inputs and outputs indicated.

NUTRIENT CYCLING, TRANSFORMATIONS, AND FLOWS

15

tionholubilization from minerals, 10-13 are the net addition to the solution from the outside, and 7 is the flow of nutrient to plant. For most nutrients, leaching, runoff, or other such losses are normally small enough during the growing season to be omitted from the equation. However, NO, -N losses by leaching during heavy rains may be an important issue, especially on sandy soils. There are two contrasting examples relating nutrient availability to plant uptake, with most real-world situations somewhere in between. For one example, external inputs create a very large stock of available nutrients at the beginning of the growing period in comparison to the initial solution stock and potential resupply from nonavailable stocks. The external input, for all practical purposes, satisfies the entire crop need. This occurs when N fertilizer is used to supply crop N needs on a light texture soil with little organic matter. The addition to the solution (10) is %[initial solution stock (1-2) ( 3 4 ) + ( 5 - 6 ) - 131 and Eq. (1) then becomes

+

+

10 2 7.

(2)

A different situation arises when there is a very small quantity of a nutrient in the soil solution, but sufficient replenishment from nonavailable stocks occurs during the growing season so that external fertility sources are not required to satisfy crop needs. This is common in the case of P and also can occur for N if leaching and denitrification over the fall, winter, and early spring reduce solution N to very low levels but there is enough mineralization from active SOM to supply plants. Solution stock is then 4 [( 1-2) + (3-4) + (5-6) - 13)] and the equation becomes

(1-2)

+ ( 3 4 ) + ( 5 - 6 ) - 13 2 7.

(3)

Enhancing mineralization and desorption from SOM and minerals and/or decreasing immobilization, adsorption and precipitation, and leaching, erosion, and gaseous losses promote a larger quantity of nutrients available for uptake by the plant. Adding nutrients in available forms (or that are easily transformed to soluble forms) also enhances short-term nutrient availability. However, this may not be necessary for many years in naturally fertile soils such as the tallgrass prairie or in soils in which large quantities of external inputs have built up high total nutrient stocks. In these situations mineralization and/or desorption and dissolution may be able to supply nutrient needs for many years. However, the decrease of the total stock of individual nutrients cannot go on indefinitely because the supply of potentially available nutrients is finite.

2. Maintaining Long-Term Soil Fertility at the Soil-Plant Level Maintaining soil fertility and nutrient availability over the long term presents a different perspective and challenge than when considering the short-term nutrient needs of crops. Building up and maintaining high levels of SOM is essential to the long-term fertility and productivity of soil (Magdoff, 1993) although this may not

16

FRED MAGDOFF ETAL.

be the solution to low levels of every nutrient. Loss of nutrients by erosion of organic matter-enriched topsoil is an important consideration in the long term although erosion that occurs in any one year is usually of little concern for nutrient availability in that year. The equation that describes buildup and maintenance of SOM is Additions of organic materials 2 losses of organic matter 8+1121+12,

(4)

where 8 is the crop residue return, 11 is the addition of other organic residues, 1 is the mineralization, and 12 is the loss of organic matter due to erosion. Use of cover crops and additions of large amounts of crop residues and/or manures adds organic matter to the soil. Decreasing mineralizationby reduced tillage and decreasing erosion slow the depletion of SOM. When considering long-term changes in the total stocks of a particular nutrient instead of SOM, the equation for maintenance and buildup becomes Additions of nutrient 2 losses of nutrient 10 11 2 12 13 14,

+

+ +

(5)

where 10 is the additions of available forms of nutrient, 11 is the addition of nutrient in organic residues from off field, 12 is the loss of nutrient in organic matter and minerals due to erosion, 13 is the loss of available nutrient by leaching, erosion, and gaseous loss, and 14 is the nutrient removal by harvest. For the long-term consideration of N stocks, N, fixation (9) is also important. Unavailable nutrients, such as much of the Pin rock phosphate and K in rock dusts, are sometimes added to soils and will also contribute to the buildup of the total stocks. If the export of nutrients off the field in harvested crops (14) and erosion, leaching, and gaseous losses (12 and 13) are low, it may be possible to maintain high levels of nutrient stocks for years without using supplemental sources of nutrients from off the field. However, in the face of high annual losses from the field, approaches that work in the short term, such as enhancing net mineralization by plowing and relying exclusively on mineralization and desorption and mineral dissolution for particular nutrients, will inevitably lead to the long-term decline of nutrient stocks (Hass et al., 1957; Bray and Watkins, 1964). If removal of nutrients in crops and/or by erosion or other losses is moderate to large, the implication of Eq. (5) is that the only way to maintain or build up nutrient stocks over the long term is to add supplementalnutrients originating from outside the field. The only questions are in what forms will the supplemental nutrients be added to soils and where will they come from? This addition may occur as N, fixation (there is no analogous reaction for the other nutrients), importing of animal feeds from off the farm, adding of synthetic fertilizers and soil amendments, transferring nutrients from other fields on the farm in the form of crop

NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS

17

residues or manures (this only helps the particular field, but not the farm), using off-farm organic materials such as manures and composts, etc. All these types of nutrient imports and transfers have become important features of contemporary U.S. agriculture.

B. ECOLOGY OF NUTRIENT FLOWS,TRANSFORMATIONS, AND CYCLES Soil organic matter dynamics is central to the ecology of nutrient flows and transformations.It is the fixed-C input into the soil from plant and animal residues that drives nutrient cycles. Food webs in soils consist of enormous quantities of organisms at a variety of trophic levels. It is estimated that there are approximately lOI4 bacteria, lo9 fungi, lo7 nematodes, and lo2 earthworms per m2 (Smil, 1991). Carbon fixed by plants becomes the primary source of energy and structural C for soil organisms. Energy flows through a food web from primary consumers of dead plant and animal debris, such as bacteria and fungi, to secondary consumers, such as protozoa, nematodes, and some mites, to tertiary consumers such as predaceous mites, centipedes, and ground beetles. As soil organisms decompose organic residues and liberate nutrients in forms that are available to plants, a significant amount of energy is utilized. During the decomposition of residues it is estimated that approximately 70% of the C is lost as respiration (Jenkinson, 1988). Nutrients, on the other hand, are conserved during mineralization. Thus, a very high percentage of the energy contained in the residues, derived from primary productivity, is lost during mineralization of nutrients as organisms respire, reproduce, and are consumed by other organisms. The quality of the residues (C/N ratio and lignin content) and their location in the soil (e.g., surface vs incorporated) affect the speed of decomposition, the kinds of organisms principally responsible, and the types and amounts of by-products produced. During the process of decomposing residues or feeding on other organisms to obtain energy and nutrients a number of important reactions occur that have important implications for soil chemical and physical conditions. These include nutrient mineralization, acidification (which may result in enhanced P availability), production of sticky polysaccharides that help in soil aggregation (and hence enhanced porosity and air and water infiltration and decreased losses by erosion), etc. Soil organisms have a major effect on almost all aspects of nutrient flows and transformations (Fig. 5 , Table I). They cause numerous transformations from one form of a nutrient to another as well as influence the rate of a number of types of flows. In management systems in which synthetic chemical use is reduced or eliminated, the action of soil microorganisms and fauna becomes a major determinant of nutrient cycling and plant growth (Doran et al., 1987) along with the nutrient stock.

FRED MAGDOFF ETAL.

18

Table I Soil OrganismsAssociated with Nutrient Flows and Transformations" Process Transformations 1. Mineralization 2. Immobilization 5. Solubilization, desorption, oxidationheduction 6. Precipitation, adsorption, oxidatiodreduct ion Soil-plant flows 7. Plant uptake 8. Planthima1 residue incorporation into soil Input flows 9. N, fixation output flows 12. Erosion 13. Leaching/gaseous loss

Organisms

Many bacteria, fungi, protozoa, nematodes, earthworms Many bacteria and fungi Bacteria Bacteria

Mycorrhizal fungi, bacteria Beetles, earthworms

Bacteria Earthworms, fungi, ants, termites Earthworms, fungi, ants, termites, bacteria

"Numbers refer to those used in Fig. 5.

Mineralization of SOM is an important source of plant-available N, P, and S in most soils (Smith er al., 1992). It is carried out by a complex of organisms feeding on residues or preying on decomposers of residues (Hendrix er al., 1986). The initial stages are carried out primarily by bacteria and fungi, with bacteria playing a more dominant role for residues incorporated into the soil and fungi playing a more dominant role for surface residues (Hendrix et al., 1986).A number of types of grazing organisms that live in water films, such as protozoa, amoeba, and a number of nematode species, feed on the bacteria and fungi. Colembola (springtails) feed on microorganisms and a number of mite species also graze on fungi. Completing the trophic pyramid in soils are tertiary consumers such as centipedes, ground beetles, and predatory mites. A number of larger organisms such as the surface-feeding earthworms (night crawlers) feed directly on residues and help fragment and mix residues with gut enzymes and microorganisms during passage through the digestive system. This promotes further decomposition of residues following deposition as fecal material. Plant and animal residue incorporation into soil by larger organisms such as earthworms also stimulates residue decomposition. This promotes faster mineralization of nutrients because conditions within the soil are more conducive to activity of bacteria and nematodes than when residues are on the soil surface. Immobilization of inorganic nutrients such as N and P occurs as bacteria and fungi decompose residues containing low concentrations of these elements. Dur-

NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS

19

ing immobilization available forms of nutrients are taken up by microorganisms and are converted to organic forms. Although immobilization may help reduce losses when N is present in amounts in excess of plant needs, when high C/N residues are present microbes may be in direct competition with plants for available N and temporary plant N deficiencies occur. Solubility of P in calcium, iron, aluminum, and manganese minerals is greatly enhanced by the effects of numerous bacteria. The mechanism of microbially mediated P solubilization is thought to be production of acids for all P minerals and reduction processes for iron and manganese (Alexander, 1977). Many oxidation and reduction reactions of N, Fe, Mn, and S are carried out by microorganisms. As the oxygen concentration in the solution declines due to water saturation of large aggregates or the whole soil itself, electrons generated by bacterial metabolism reduce N, Fe, Mn, and S and in the process consume H+ and cause the soil pH to increase. The predominance of reduction reactions causes the pH of noncalcareous rice soils to increase after flooding (Ponnamperuma, 1972). The changes in oxidation state and in pH caused by oxidationheduction reactions can strongly influence nutrient solubility and/or mobility as well as the solubility of other elements. Iron levels in solution increase under reduced conditions because Fe2+ compounds are more soluble than Fe3+ compounds and P concentrations in solution also increase because ferric phosphate compounds are rendered more soluble. Oxidation reactions such as nitrification of NH; to NO; and conversion of S or S2- to SO; are carried out by bacteria and tend to acidify soils (Alexander, 1977). Plant uptake of nutrients is stimulated by the activity of a number of fungi and bacteria. Vesicular arbuscular mycohrrizal (VAM) fungi form a symbiotic relationship with the roots of most crop plants by infecting the root and sending out hyphae that in effect extend the root systems' area of exploitation, especially at low nutrient levels. This aids the plant in uptake of P and water as well as Cu and Zn (Barea, 1991). Uptake of some micronutrients, especially Fe and Mn, is stimulated by the presence of low-molecular-weight sidophores (chelates) that are excreted by microorganisms (Schenker et al., 1995). N, fixation is carried out by free-living bacteria such as azotobacter and clostridia as well as by the symbiotic bacterial-plant associations such as rhizobia in combination with legumes. Over long periods of time, even the low N2-fixation rates normally found with free-living bacteria may be a significant addition to the soil. Some symbiotic associations may add such large quantities of N to soils (more than 100 kg ha- 'y-I), that the N demand of the following crop is largely satisfied by the residual N from the previous legume. Erosion and leaching losses of nutrients are governed to a large extent by the soil structure. Bulk density tends to decrease (and thus, porosity is enhanced) as SOM increases (Black, 1973). In addition, microbially produced gums and fungal hyphae tend to promote both aggregation and stability of aggregates (Smith et al.,

20

FRED MAGDOFF ET AL.

1992), which results in more water infiltration and less runoff and erosion. Macropores produced by the activities of earthworms, ants, and termites allow enhanced water infiltration and less runoff and erosion. Soil structure is also influenced greatly by soil and crop management practices. Gaseous losses of ammonia from the surface placement of urea are stimulated by extracellular urease enzyme produced by microorganisms. Also, deniuification of NO, to N,O and N, is performed by a number of anaerobic bacteria.

1. Soil and Crop Management Practices and Soil Ecology Soil and crop management have profound effects on soil biology with implications for flows and cycles. Management practices have numerous influences by affecting the following: 1. The amount of organic matter returned to the soil (crop species, amount harvested, and use of harvested material all influence the amount returned to soil and, therefore, the amount of food available to soil organisms); 2. The quality of the organic matter (species and maturity at time of killing and incorporation influence C/N, amount of lignin, and secondary C metabolites, which influence the rate of decomposition and soil organism species involved and attractiveness to pests); 3. The location of the residues within the soil (degree of incorporation influences species of organisms responsible for decomposition); 4. The amount of time during the year that the soil is occupied by living plants (greater duration of occupation by living plants appears to enhance mycohrrizal fungi populations as well as resistance to soil erosion); 5 . Introduction of particular chemicals that are harmful or stimulate specific organisms (the use of pesticides may drastically reduce both target and nontarget organisms and thereby change species composition); 6. Amounts of available nutrients and soil pH (influenced by type and quantity of fertilizer and soil amendment used); 7. Soil physical properties (compaction, aeration, water status, and water infiltration and holding capacity affected by amounts of residues, tillage, and irrigation practices); 8. Extent of soil disturbance (tillage influences extent of wetting and drying, worm channel continuity, and accessibility of microbial food sources).

There are many examples of effects of soil and crop management practices on soil biology. What follows is only a small sample from the literature. Use of high levels of the pesticide carbofuran was found to reduce macroarthropod populations and fungal propagules in a shortgrass prairie (Stanton et al., 1981). Kirchner et al. (1993) found that the surface soil following crimson clover grown during the off season between corn corps contained significantly larger bacterial populations and

NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS

21

activity of a number of enzymes than soil from the well-fertilized control treatment. Plowing can be very disruptive and have a substantial impact on soil life, especially on earthworm populations (Werner and Dindal, 1990). No-till practices leave more residues on the surface and can result in a large increase in earthworm populations (Edwards and Lofty, 1982).

C. SOILCHE~C~ICAL PROPERTIES There are a number of important nutrient transformations that are only peripherally related to soil biology. Although organic matter provides significant cation exchange capacity (CEC) to hold cations such as K+, NH,', Ca2+, and Mg2+,clay also has CEC. The conversion of K from unavailable mineral forms to solution or exchangeable K + is a nonbiological chemical phenomenon. Also, decreased plant availability of K + and NH: is caused by fixation within the 2:l layers of vermiculite and weathered micas (Borchardt, 1977). Although these are nonbiological phenomena, SOM may have a critical role in decreasing the extent of K fixation by 2: I clay minerals (Cassman et al., 1989). The change in CEC of organic matter by acidification or by liming practices may influence K availability (Magdoff and Bartlett, 1985). Another example of a nonbiological nutrient transformation is the fixation of P by adsorption onto already formed A1 and Fe precipitates or onto CaCO, (Sanyal and DeDatta, 1991) or direct precipitation as Al-, Fe-, and Ca-containing compounds. Although precipitation is usually considered to be a strictly inorganic process, soluble SOM may lessen the amount of P fixation as Ca-phosphate (Inskeep and Grossl, 1992) as well as the amount of adsorption onto A1 and Fe oxides (Lopez-Hernandez er al., 1986). The size of a particular nutrient stock frequently influences the amount that is transformed from one form to another. For example, as the stock of total K or P increases as a result of long-term additions of nutrients, the level in solution tends to increase as does the amount that will easily desorb into solution in response to depletion by plant uptake, leaching, and runoff.

D. SOILPMSICALPROPERTIES Soil structure controls the partitioning of incoming precipitation between infiltration into the soil and runoff from the field. Soils that are porous, contain many macropores, have considerable depth to a restricting layer, and are resistant to surface crusting tend to have high rates of water infiltration. This tends to reduce runoff and, therefore, erosion losses of nutrients and organic matter as eroded sediments. Although more water infiltrates into and through such soils, two important

22

FRED MAGDOFF ETAL.

phenomena help explain why this does not necessarily greatly increase nutrient leaching losses. With the important exception of N, which is commonly found in large quantities as the mobile NO;-N anion in the soil solution, nutrients tend to be present in the soil solution in very low concentrations. Thus, the magnitude of potential leaching losses is not anywhere near as large for other nutrients as it is for N. In addition, well-structured soils demonstrate a significant amount of preferential water flow through macropores (Bouma, 1991). As water moves through large pores it bypasses the smaller pores, in which the water that contains most of the nutrients in solution is located. For example, after applying 89 mm of irrigation water to a well-structured silt loam soil, Shuford et al. (1977) found a relatively small change in the distribution of N0-T-N in the soil profile. On the other hand, given sufficient precipitation on soils with little preferential flow, much NO;-N can be rapidly leached deeper in the profile. In work in Connecticut on sandy soils, approximately 200 mm of rainfall occurring in the month between corn planting and side-dress time resulted in significant movement of NO,-N from preplant fertilizer to the 30- to 60-cm soil layer at one site, whereas approximately 400 mm of rainfall moved most of the preplant fertilizer out of the root zone (Magdoff et al., 1993). Although nutrients can leach below the root zone, most do so gradually either because they are cations retained on the CEC or because they react with soil solids and form slightly soluble compounds. Over a period of 5-10 years, leaching losses of even those elements held on the CEC, such as Ca and Mg for noncalcareous soils and K for soils lacking large quantities of K-supplying minerals, must be remedied by addition of amendments and/or fertilizers. In general, the susceptibility of nutrients to loss by leaching and runoff waters and denitrification (for N) increase greatly as excess nutrients remain at the end of the growing season. Because a large quantity of N is present as mobile and reactive NO;-N during the growing season, it is the most common element to leach below root zone in considerable quantities. The potential for NO;-N pollution of groundwater increases greatly when N is applied in excess of crop needs (Nelson and MacGregor, 1973; Jolley and Pierre, 1977; Legg and Meisinger, 1984; Roth and Fox, 1990).

E. BIOLOGICAL,CHEMICAL, AND PHYSICAL INTERACTIONS Sometimes there are important interactions between soil biological, chemical, and physical conditions. Internal drainage (and aeration) and the chemical environment influence the populations of organisms present. A different interaction is at the heart of the concern that excess levels of N0-q-N produced from either fertilizer ammonium or urea or from some organic sources such as legumes may stimulate acidification and loss of other nutrients (Patriquin et al., 1993). The production of NOT-N by nitrification is an acidifying reaction, but as NOT-N is taken up by plants some acid is neutralized. This occurs because when plants utilize

NUTRIENT CYCLING, TRANSFORMATIONS, AND FLOWS

23

NO; as their main N source, they excrete HCO, to help balance the excess uptake of anions over cations. However, if the amounts of NO;-N produced are much larger than uptake needs, although some of the excess may be immobilized, there is a great potential for leaching out of the root zone. The greater the amount of NO-;-N that is leached, the greater is the net decrease in pH because there is no opportunity for plant roots to excrete HCO, in response to the NO-;-N leached out of the root zone. As soils become more acid, pH-dependent CEC decreases as H+ replaces other cations and significant amounts of nutrients, such as Caf and K + , enter the soil solution and may be leached along with NO;-N.

JS? CYCLING AND FLOWS AT THE FIELD LEVEL The agricultural field is usually the smallest unit of uniform management. Decisions about tillage methods, crops and rotations, and fertilizer sources, rates, and application timing/methods are made at the field level. In this section we will discuss field-scale management practices as they relate to nutrient flow and cycling issues. By looking at the field level (Fig. 6), we see that some nutrients return to

other forages and grains

N2 / /

' f

/

Nrtlxation

/ /

4

field boundary

)L

harvest

ANIMAL

4

incorporation

7f /

bedding

fertilizers, manures, lime, organic residues, atmospheric deposition

0 0

*d

N.+atlon

erosion, leaching L)

I

/

/

N2

7

Figure 6 Managed \ystem nutrient cycle and flows with a boundary around the field.

24

FRED MAGDOFF ET AL.

the same field on which the plant was grown and complete a true s o i l j p l a n t j soil or soil+plant+animal+soil cycle at the field scale. Nutrients may also be lost from the cycle as they flow away from the field in both managed and unmanaged flows and do not return. On the other hand, nutrients that did not originate in the field can flow onto it in the form of manure from animals fed partially or completely with feeds from other fields or farms. Also, crop residues applied to a field can originate in other fields or farms. These flows, although not nutrient cycles at the field level, may represent cycling at the farm or regional scales. There are numerous factors that control the efficiency with which nutrients are utilized by crops and/or lost from soils. These include the characteristics of the nutrient source, the quantity of nutrients added, the method and timing of the application, the uptake needs and abilities of the crop, soil structure, and the climate.

A. NUTRIENT LOSSES 1. Harvest Removal Crop harvest normally represents the only intentional loss of nutrients from fields. The amounts of nutrients removed in the harvested portion of crops are influenced by the characteristics of the plant species and the portion of the plant removed. Although there are differences in biomass production and elemental composition among crop species, some of the most striking differences in nutrient removal are caused by differences in the amounts of plant material removed. With a crop such as broccoli, many nutrients are taken up by the crop, but only approximately 20% of the nutrients in the aboveground biomass is removed with the heads, with the remaining 80% in the unharvested stalk and leaves. Therefore, relatively small amounts of nutrients leave the fields with the harvested broccoli heads (Table 11). Potatoes offer a contrast to broccoli because a large portion of the total plant biomass and nutrients are in the harvested tubers. Nutrient removals of N, P, and K in potato harvests are approximately four to nine times greater than for broccoli (Table 11). Occasionally, the same crop can be harvested for different purposes, with resulting effects on the quantity of nutrients removed from the field. For example, the removal of nutrients from fields is much greater with corn silage, where close to the entire aboveground corn plant is removed, than occurs with corn harvested only for grain (Table 11). In a 9-year study, Vitosh er al. ( 1 973) found that N, P, and K removal in silage was 60, 28, and 509% more from the soil, respectively, than when grain was harvested. The relative amounts of the various nutrients removed in crop harvest depends on the species and the plant part(s) removed. For example, there is much less K relative to N harvested in grain than when whole plants are harvested (Table 11).

NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS

25

Table I1 Approximate Amount of Nutrients Removed in Harvested Portion of Selected Crops (representativecommercial yields)

Crop Agronomic" Corn grain Corn silage Alfalfa GrdS hdy Soybeans Vegetable" Broccoli Brussel sprouts carrots Lettuce Onions Potatoes Tomatoes Fruits Honeydew mellons' Apples" Peaches"

N

P

K

I12 157 224 I79 I68

8 39 29 22 20

28 130 186 I40 52

22 I56 90 I06 123 I68 112

2 22 22 13 22 21 II

50 I40 224 123 I23 224 20 1

78 39 76

9 10 11

73 71 96

"Derived from data from Pennsylvania State University State Agronomy Guide," assuming yields of 6, 15, 9, 6.7, and 2.5 tons ha-' for corn grain, corn silage, alfalfa, grass hay, and soybeans, respectively. "From Lorenz and Maynard (I 980). 'From "New England Vegetable Guide," (Ferro, 19961997). '/From Westwood (1978).

( 1994),"Penn

Some cropping systems, such as on a dairy farm or vegetable farm, that rely on mixes of crops in the rotation can export significantly different amounts of nutrients and the net effect may be moderate average yearly field nutrient exports. By contrast, when high yielding forages, such as corn silage and alfalfa, are grown in the same field over a long time period, high levels of nutrients will be consistently exported from fields because most of the aboveground biomass leaves the field. If grains are produced in the same field for some years there will be fewer nutrients exported out of the field than if forages are produced. A field producing vegetables, such as potatoes and tomatoes, for long periods will also be continually exporting large amounts of nutrients. However, when high-nutrient export pota-

26

FRED MAGDOFF ETAL.

toes and/or tomatoes are rotated with crops such as broccoli and lettuce, the average annual nutrients exported from the field will be decreased.

2. Inadvertent Losses Although removal of nutrients in crops is a necessary part of agriculture, conventional agriculture systems have had particularly large inadvertent nutrient losses from fields. The pathways of these losses, sometimes referred to as unmanaged flows, include soil erosion (by water andor wind), soluble nutrients dissolved in runoff waters, leaching of nutrients to below rooting depth, and gaseous losses (mainly for one nutrient, N, as NH, volatilization and loss of N, and N,O resulting from denitrification). These losses are an economic loss to the farm and at the same time may lead to decreased water or air quality. In the United States, soil erosion alone is estimated to cause losses of $0.5 billion worth of available nutrients and $18 billion of total plant nutrients (Crosson, 1985; Troeh et al., 1980). In addition, off-farm economic damage from runoff and erosion is estimated at between $2 and $8 billion (USDA, 1987). Soil erosion leads to movement of soil particles that enrich natural waters with N, P, and other nutrients as well as pesticides. Nitrate leaching from fields contributes to poor groundwater quality and is a special health threat to infants. In many cases, systems leak because of the use of excess nutrients. In the case of N, this is partially a result of fertilizer recommendations not taking sufficient account of the nitrate in the soil, soil organic matter mineralization, additions of manure, legumes in the rotation, or nitrate in irrigation water. In addition, some farmers do not follow the recommendations and apply excess or extra N because they want insurance that they will have sufficient N or they assume they are starting with little or no sources already in the soil that might provide N for their crops. However, concentrations of NO;-N in the percolating water underneath a corn field will most likely be above 10 mg kg-I, even when fertilizer applications do not exceed the economic optimum rate (Magdoff, 1992). Holding unintended losses to an absolute minimum is an important goal for sustainable agriculture. Methods for doing so include many different techniques and practices. Of particular importance are those that reduce runoff and erosion such as contour tillage and planting, strip cropping, use of terraces, grassed waterways, minimum tillage, use of sod crops as part of rotations, enhancing soil structure through building up organic matter, etc. In addition to conserving soil and water by improving moisture retention and reducing surface runoff and erosion, reduced tillage systems also allow farmers to reduce costs through saving on fuel, labor, and equipment (Magleby et al., 1985; Phillips and Phillips, 1984). In order to minimize unintended losses it is also important to maintain nutrient applications close to levels needed to obtain maximum economic yield (see Quantity of Added Nutrients) as well as follow other nutrient management practices

NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS

27

that reduce losses (see Sources of Added Nutrients and Nutrient Application Timing and Methods). For one element (N), there are other significant sources of loss aside from runoff and erosion and leaching: volatilization of NH, and denitrification to gaseous N,O and N,. Much research has been done on N use efficiencies and loss. In general, approximately 5-10% of fertilizer N may be lost to the atmosphere in gaseous forms (Kundler, 1970: Westerman et al., 1972).

B. NUTRIENT ADDITIONS 1. Sources of Added Nutrients Some sources are more susceptible to unintended loss than other sources. This may be related to method of application as when ammonia volatilization occurs with surface-applied urea or manures. The loss of ammonia from anhydrous ammonia can also be a problem when the soil does not seal quickly after injection. Some biologically based sources of fertility, such as raw manure, may have high quantities of available nutrients (Klausner, 1995). However, most of the nutrients in other biological sources, such as plant residues and composted materials, are usually less available in the short run and must first undergo mineralization. There is some evidence that farms relying on slowly available organic sources of fertility might have more efficient soil-plant nutrient flows, thereby lessening the potential for loss in leaching waters or as eroded materials. For example, in a comparison of organic and conventional farms producing tomatoes in California, Drinkwater et al. ( 1995) found that soils from the organic farms had lower nitrate levels and a greater portion of the inorganic N in the ammonium form. In an extensive evaluation of the fate of lSN from fertilizer and legumes, Harris et al. (1994) found that more fertilizer than legume N was recovered by crops (40 vs 17% of input), but less fertilizer than legume N was retained in the soil (17 vs 47% of input), and similar amounts of N from both sources were lost from the cropping systems over the 2-year period. Less legume than fertilizer N was lost during the year of application (18 vs 38% of input), but more legume than fertilizer N was lost the year after application (17 vs 4% of input). Residual fertilizer and legume IsN were distributed similarly among soil fractions. Soil microbial biomass C and N was substantially larger in the legume-based system. A larger, but not necessarily more active, soil microbial biomass was probably responsible for the greater soil N-supplying capacity in the legume-based system compared with the fertilizer-based system. These data suggest that legume- and fertilizerbased systems do have some similarities and some very significant differences. The main difference is the amount of microbial biomass C and N. The yields of the resulting crops are often very similar, and the N recovered from 1 year’s input

28

FRED MAGDOFF ETAL.

is usually higher in fertilizer systems. This suggests that in the legume system, numerous years of N inputs may contribute to the N-supplying ability of these systems, and that their N supply is related to a much larger microbial fraction. It may also explain why yield losses result when the system is being changed from one that relies exclusively on commercial synthetic fertilizers to legume- or organicbased practices. It is also important to recognize that organic sources of fertility have many potential benefits aside from direct effects on plant nutrient supply. For example, when a variety of sources of organic residues are added to soils they promote greater biodiversity of soil organisms and may help reduce outbreaks of plant pests (Dick, 1992). Organic residues also help microorganisms to produce phytohormones (Frankenberger and Arshad, 1995) and help promote better soil physical properties (Smith and Elliott, 1990), etc.

2. Nutrient Application Timing and Methods The efficiency of nutrient use by plants and, hence, the amount of unintended losses, is often influenced by the timing of the application and/or the application method. [Timing of application is frequently implied by method (for example, side-dress and top-dress are done during the growing season, whereas in-ground band placement is usually done near the seed at planting, and broadcast is either done in fall or spring when the ground is bare or as a top-dress for certain crops during season).] When using amendments with low solubilities, such as rock phosphate, limestone, and gypsum, it is important to mix them with the soil as thoroughly as possible and have some time for them to react before corps are planted. Complete incorporation of fertilizer P and K is frequently done to raise the general fertility level of soils that test low in these elements. Incorporation of manures is also often desirable in order to reduce ammonia losses, reduce risk of runoff losses, and introduce organic matter deeper into the topsoil. However, either in-season or atplanting localized placement of soluble synthetic sources of N and P (which are susceptible to loss in runoff, erosion, and leaching or are commonly converted into forms that are not available to plants) helps to enhance uptake and thus may reduce losses (Mahler et al., 1994; Lathwell ef al., 1970; Welch ef al., 1966, 1971). There are certainly pros and cons to preplant, at-planting, and in-season N fertilizer application. In-season fertilizer N application relies on precipitation to bring the nutrient into intimate contact with roots and thus is not a reliable system for relatively dry but unirrigated conditions. However, under humid conditions, the longer the time between N application and plant uptake, the greater the possibility for nutrient loss. Randall and Hoeft (1988), in a review of the literature, related fertilizer efficiency to soil test level and placement with corn, soybeans, and small grain. They

NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS

29

concluded that corn and soybeans usually do not respond positively to P and K on medium or high soil test levels. At low soil test level corn responds more positively to banded fertilizer than broadcast fertilizer, whereas broadcast is sufficient with soybeans on low testing soils. Small grains generally respond better to banded rather than broadcast fertilizer where a response is achieved. There has been considerable interest in application systems that involve more intensive soil sampling and changing the rate of fertilizer application within fields based on soil test levels. Referred to as variable rate or precision farming technology, these systems involve the use of specialized equipment to map fields, monitor location within the field, and to change the fertilizer application rate rapidly. Usually used for applying preplant broadcast fertilizers, some systems involve onthe-go sensors, eliminating the need for previously testing and mapping the field and providing the possibility of use for N side-dress of corn. These systems can theoretically enhance the efficiency of nutrient use by taking soil variability into account and applying only the amounts of fertilizer needed to different parts of the field. However, the degree to which they influence total nutrient application rates and flows has not been established.

3. Quantity of Added Nutrients The goal of most soil fertility programs is to have the amount of nutrients present that is needed to produce maximum economic crop yield. If a nutrient is not supplied in sufficient quantities to achieve this goal, then yields suffer. If a nutrient is added above the level needed to achieve maximum economic yields, then the extra flow onto the field is both an unneeded expense and a potential environmental hazard. Many technical and human factors combine to determine the actual quantity of nutrients added to a particular field. Although the use of soil testing and plant analysis as means of assessing the nutrient status of fields is common, the majority of farmers do not utilize these tools. They use experiential knowledge based on their observations of soil conditions and crop yields, intuition, observations of their neighbors’ fields, and discussions with fertilizer salesmen or other agricultural suppliers and professionals. The land base available for manure application frequently affects the nutrient loading rate on farms with animals. The soil testing process and the complexities and pitfalls of recommendation systems are poorly understood by farmers as well as by many agricultural professionals. It is commonly believed that soil testing provides a precise and scientific assessment of the soil’s nutrient status. In actual practice soil testing and recommendation systems are a mix of science, art, probabilities, compromises, and other human factors. There are two separate but critically important parts of the process: (i) A good soil test must reflect plant availability (low, medium, and high soil test levels must

30

FRED MAGDOFF ETAL.

reflect high, medium, and low probability, respectively, of increasing plant yields by adding a particular nutrient); and (ii) the system for recommending nutrient additions based on the soil test should suggest amounts of added nutrients that will not sacrifice yield nor recommend greater application rates than are economically and environmentallyjustified. Rates of nutrient additions recommended by soil testing laboratories vary substantially for N, P, and K (Liebhardt et al., 1982a,b,c). These variations are partially due to all the technical and human factors involved in soil testing and recommendations, but primarily result from differing soil test philosophies and motivations of people interpreting the soil test. There are three main recommendation philosophies currently being used. One approach is the “sufficiency level,” which was derived from research that reveals no yield response to an applied nutrient above a certain “critical” soil test level. Soils testing low and very low have a high probability of a significant yield response to added nutrients. Crops grown on soils testing medium have a lower probability of responding to nutrient additions and soils testing high and very high are unlikely to respond to nutrient additions. A second philosophy, the “cation saturation ratio” concept, suggests a certain balance or ratio of Ca, Mg, and K and calls for additions to bring the ratio of these cations into certain ranges. A third approach is the “build-up and maintenance” concept. This system calls for adding nutrients (mainly P and K) to attain high soil test levels and then adding annually what is removed in the harvest. Field research has conclusively shown that the sufficiency level approach, which generally recommends lower rates of nutrients, is superior to the cation saturation and the maintenance approaches with respect to agronomic, economic, and environmental factors (Olson et al., 1982; McLean et al., 1983; Liebhardt, 1981). Another factor that affects the accuracy of recommendation systems is that good soil tests only separate sites with high probability of response to added nutrients from those with low probability of response. It is frequently found that within the population of sites that will likely benefit from nutrient additions (i.e., low soil test values), the extent of the response is not sensitive to soil test level (Magdoff et al., 1993). Thus, although more fertilizer is generally recommended as soil test levels decrease further below a critical value, there are little crop response data to justify that approach. Other factors in addition to the soil test level of the particular nutrient, such as rooting depth, organic matter mineralization, bulk density, plant pests, and weather, are apparently influencing the extent of the response at these sites. In addition, even when using the sufficiency level system for making recommendations, different models for describing the same data can result in varying fertilizer recommendations. For example, Cerrato and Blackmer (1990) found that economic returns were greater when using a linear-plateau model to describe plant response to fertilizer N than for other models, such as a quadratic curve. Because of the complex nature of the biological and chemical reactions N un-

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31

dergoes, the development of soil testing and recommendation procedures for this element has been a greater challenge than for P or K. Nitrogen recommendations for corn, the crop on which the largest amount of fertilizer N is used in the United States, were historically based mainly on the farmer’s yield goal. For example, if the yield goal was 9408 kg ha-’ (150 bu acre- I ) and it was estimated that crop needs 0.022 kg N kg-’ (1.25 Ibs N bu- I ) , then N required was approximately 210 kg kg-l (187.5 Ibs acre-’). In this system it was assumed that there was no legume or manure, no soil N carryover or mineralization, and no N in irrigation water or in any other source. This “put back what you take out” philosophy is simple but does not take into account many important factors and results in economic and environmental costs. An improved “tax form” recommendation system continues where the previous system leaves off and attempts to credit various sources of available N. In the case in which the estimated total requirement is 2 10 kg ha- I , estimates of contributions from sources such as SOM, fertilizer N carryover from the previous year, legume plowdown, manure additions, or N in irrigation water can be subtracted from the total estimated need. If these sources are not sufficient, then N fertilizer should be added. It is an improvement but it still makes many guesses and assumptions about N behavior, crop growth, and environmental conditions. The first soil test for N that gained widespread acceptance was in the semiarid western corn belt where there is little leaching and denitrification and SOM levels are low and not very active. In this system, soil nitrate in the soil profile to at least 3 ft, and sometimes to 6 ft, is determined in the fall or spring and is used to estimate fertilizer N needed. The pre-side-dress nitrate test (PSNT) (Magdoff et al., 1984 and Magdoff 1991a), is a new approach to N testing now used by many states in the humid regions of the Northeast, mid-Atlantic, and Midwest. The premise of this concept is to evaluate soil nitrate in the top 30 cm of soil when corn height is 15-30 cm. The test is late enough in the season to account for a significant quantity of mineralization of SOM, legume residues, and manure and early enough to still be able to side-dress N fertilizer when soil nitrate is below the critical value. During the past few decades, there have been improvements in many fertilizer recommendation systems, but few major advances in soil testing and recommendation systems have been adopted for routine use by testing laboratories. These have been limited to adoption of a new extract, the Mehlich 3 (Mehlich, 1984), by a number of laboratories, and the PSNT (Magdoff et al., 1984; Magdoff, 199I a). Soil testing will become more important in the future as farmers, testing laboratories, and various governmental agencies try to “fine-tune” recommendations to take into account concerns of both yields and the environment. More research is needed to enhance the accuracy of the quantity of nutrients recommended for sites that will probably show a positive response.

32

FRED MAGDOFF ETAL.

C. MANAGEMENT PRACTICES AND NUTRIENT FLOWS Soil and crop management practices that do not involve direct additions of nutrients from off the field or harvest losses also have profound impacts on nutrient flows. These include the use of crop residues, system(s) of tillage practiced, use of cover crops, and the characteristics of the growing crop and the other crops grown in rotation. The influences of these factors are primarily on the magnitude of nutrient loss, but there are a number of influences on nutrient additions as well. 1. Crop Residues Cycling within the field setting is mainly confined to the soil+crop+soil pathways through residues remaining in the field after harvest and by use of cover crops. When residues are returned to the soil they help retain plant nutrients, maintain soil porosity and tilth, enhance water infiltration, and act as an effective control against water erosion (Lindstrom and Holt, 1983). Crop residues may contain substantial amounts of biomass as well as nutrients, as in the case of broccoli or corn grown for grain. On the other hand, certain crops such as corn silage leave few residues except for the root systems. Cycling of nutrients in residues is relatively rapid under conventional moldboard plow-harrow tillage systems in which residues are annually incorporated into the soil. Under reduced tillage systems, especially no-till, residues accumulate near the surface where they generally decompose more slowly. When residues are incorporated into soil by tillage or biological activity or accumulate on the surface, organisms begin the process of decomposition, during which nutrients are mineralized. The amount of nutrients contained in crop residues in the United States is sizable, with estimates that they contain 24% of the N, 13% of the P, and 34% of the K applied to cropland (Follett et al., 1987). Nutrients in Midwest crop residues as a percentage of those in commercial fertilizer are also large, averaging approximately 75% for N, 35% for P, and 90% for K (Holt, 1979). In addition to the direct effects on cycling nutrients, residues left on the soil surface reduce wind and water erosion, thus reducing loss of nutrients from the field. Although crop residues may be plentiful on a national basis, not all available residues contribute to a field-scale nutrient cycle. Residues are sometimes removed to become another commodity for sale as when straw from small grain is sold to another farm for use as animal bedding. Other potential uses of crop residues that are under exploration are to produce “biofuels” and feedstocks for industrial biochemical products. In some regions of the United States crop residues, such as rice and wheat straw, are routinely burned, with loss of most of the N and S as volatilized gases. This practice is being discouraged because of the resulting air pollution. Competition for crop residues is particularly critical in developing countries, where they commonly serve as important sources of fuel or building ma-

NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS

33

terials. Although there may be good economic reasons to remove crop residues, this results in the loss of valuable nutrients as well as a source of material to help maintain SOM. 2. Tillage Systems Tillage practices have both short-term and long-term consequences, particularly in relation to soil erosion and soil structure, and thus are key components of soil management strategies for sustainable land management, crop production, and agriculture as a whole (Cannell and Hawes, 1994). The early criticisms of the widespread reliance on the moldboard plow, such as by Faulkner (1943), were considered controversial. However, it is now widely accepted that there are potentially severe problems associated with reliance on it as the main form of tillage. Hillel(1991) summarized the situation as follows: Although the development of the plow represented a huge advance in terms of convenience and efficiency of operation, it had an important side effect. As with many other innovations, the benefits were immediate, but the full range of consequences took several generations to play out, long after the new practice became entrenched. The major environmental consequence was that plowing made the soil surface-now loosened, pulverized, and bared of weeds-much more vulnerable to accelerated erosion. The soil disturbance caused by plowing and subsequent harrowing also accelerates SOM decomposition, and more organic materials must be returnedadded to the soil to maintain or build up SOM levels. Thus, conventional tillage systems tend to promote loss of nutrient stocks from soils by wind and water erosion as well as by accelerated SOM mineralization. Loss of cation-exchange capacity with decreasing SOM also encourages the leaching of cations such as Ca, K, and Mg. Plowing is different from a natural system in that it disrupts soil and buries organic matter deep in the soil rather than allowing it to be worked in at the top layers by soil organisms. One of the main ways to decrease runoff and erosion losses of soluble nutrients and soil solids with their associated nutrient stock is to follow practices that enhance the development of macropores for conducting water deep into the profile. In addition, a stable and porous structure should be maintained at the soil surface so that water can infiltrate rapidly even under the potentially damaging impact of rainfall and agricultural machinery traffic. Reduced systems of tillage allow macropores to develop (such as by earthworms under notill practices) and maintains more of a vegetative cover than with moldboard plow systems. Organic residues on the surface provide food for earthworms and also lessen the force of rainfall on the soil surface. Higher organic matter content of notill surface soils also helps to build and stabilize soil structure. Tillage methods are changing rapidly. Conservation tillage in the United States

34

FRED MAGDOFF ETAL.

is increasing and is currently used on approximately 30% of cropland, whereas notill methods are employed on approximately 10%of cropland (Cannell and Hawes, 1994). Experiments in Mississippi demonstrate the dramatic positive effects of the conservation tillage systems, where no-till planting with the residues left on the surface reduced soil losses due to water erosion by 85% (from 17.5 to 2.5 Mg hayear-’) compared to plowing (McGregor etal., 1975). Another study on soils that were susceptible to severe soil erosion compared conventional tillage with flutedcoulter and in-row chisel techniques and found that the latter two methods essentially eliminated sediment losses (Langdale and Leonard, 1983). Fluted-coulter tillage reduced runoff 50% compared to conventional tillage, whereas in-row chisel tillage reduced runoff 90%. Although concentrations of soluble N and P increased in runoff in the reduced tillage treatments, except for PO,-P associated with the fluted-coultertillage, nutrient loss in runoff was drastically decreased with both reduced tillage systems. Stinner etal. (1988) found that tilled agroecosystems are much more susceptible to nutrient loss than are natural systems such as forests. No-tillage conditions tend to favor nutrient conservation, as measurements in both sorghum and soybean systems demonstrate. In addition to positive effects on runoff and erosion, conservation tillage reduces costs of fuel, labor, and equipment and conserves soil and water by improving moisture retention and reducing surface runoff and erosion (Magleby et al., 1985; Phillips and Phillips, 1984). In addition, no-till creates conditions resembling undisturbed ecosystems (Blumberg and Crossley, 1983; Fleige and Baeumer, 1974). Despite similarities between natural and no-till soils, it is not clear how much conservation practices modify decomposition process, thereby altering nutrient cycles in agroecosystems (Hendrix et al., 1986). Hendrix et al. (1986) concluded that tillage increases nutrient mobility, and no-till soils are usually physically stratified with more nutrients localized near the surface.

’

3. Type of Crops a. Rotations Rotations have long been the cornerstone of well-managed agricultural systems. There are a number of sound reasons for rotating crops, including an increase in yield that is frequently obtained over that obtained under monoculture-sometimes called the “rotation effect.” Rotations help manage insect, weed, and disease cycles and confer a degree of protection against weather and pest problems. In addition, crop sequences, which include forage legumes or fibrous-rooted grass sod crops, often enhance soil microbial populations and activity (Bolton e f al., 1985) as well as soil structure. Although NOT-N leaching to groundwater from soils under corn is especially problematic (Magdoff, 1991b), little leaching of NO;-N occurs under sod crops (Olson et al., 1970) except where very high N rates are used

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35

for intensively managed grass stands. Thus, rotations in which corn appears fewer years and sod crops are more common may help reduce the average amount of NO;-N leaching from fields over the rotation. b. Pastures If animals are fed crops grown on the farm, manure is another potential source of nutrients for cycling back to the field. On many specialized farms producing only animals or animal products, a substantial portion of the animal feed is imported from off the farm, making manure only a temporary repository in a flow of nutrients from off-farm to the field. When pastures are utilized there is more potential for a direct soil+plant+animal+soil cycle. A recent review of soil fertility issues relating to pastures (Haynes and Williams, 1993) highlights a number of unique aspects that are relevant to our discussion of flows and cycles. With cows on pasture, there are concentrated deposits of nutrients in urine spots and manure deposits. These provide a mosaic of low or medium and very high nutrient status zones within the pasture. In addition, P is contained mainly in feces and K mainly in urine, and the percentage of N in urine increases as dietary N increases while remaining fairly constant in feces. Thus, there is to some extent a physical separation of elements in pastures. In addition, at low stocking densities, there is frequently a significant transfer of nutrients within the pasture from steeper to flatter areas or to around the water trough or under trees where cows congregate. As stocking density increases and the field is used as pasture for a number of years, nutrient distribution becomes more uniform. Weather conditions soon after deposition influence the rate of manure decomposition and loss of nutrients. If it is dry for a few days, the surface of manure pies starts to dry and a crust is formed, slowing down subsequent decomposition. The formation of a crust also slows down volatilization losses of ammonium from manure. The high concentration of mineral N found under urine spots, with more N than plants within the spot can utilize, may be conducive to higher leaching losses than would otherwise occur. c. Cover Crops Farmers use cover crops for many reasons such as retention of nutrients during the season when soil would otherwise be bare, decreasing erosion, building up SOM, weed suppression, N addition to soil through using N,-fixing legumes, and providing habitat for beneficial insects (Brusko er af., 1992). By taking up nutrients that might otherwise be lost by leaching and denitrification (for NO;-N) or along with eroded sediments (most nutrients), cover crops contribute to maintaining the stock of soil nutrients and to cycling within the field. Legume cover crops in particular have received considerable attention in recent years because of their potential to increase the N stock and contribute significant amounts of biologically fixed N to the subsequent summer crop (Ebelhar et al., 1984; Hargrove, 1986).

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FRED MAGDOFF E T A .

Average estimates of the equivalent amount of fertilizer N replaced by hairy vetch (Vicio vilfosa Roth) and crimson clover (Trifolium incarnatum L.) in these studies were 90-100 kg ha-' for corn (&a mays L.) and 7 2 kg ha-' for grain sorghum [Sorghumbicolor (L.) Moench], respectively. Legume cover crops also provide N in a timely manner for the following corn crop (Wagger, 1989). In addition, due to higher rates of nutrient accumulation, larger quantities of P,K, Ca, and Mg may be available following legume cover crops than grasses (Groffman et al., 1987). Legumes may also contribute to VAM diversity and abundance because they are highly responsive to VAM (Rabatin and Stinner, 1989). Grasses that have greater root surface and greater rooting depth are more efficient then legumes at recovery of residual N, which reduces leaching losses when they are used as winter cover crops in the humid East or the South (Shipley et al., 1992; Moms er al., 1986). Growing cover crops when the soil would otherwise be bare also serves to reduce runoff and losses of nutrients that occur with eroded sediments. An indication of the potential effects of cover crops on soil biology is the finding that surface soil from crimson clover grown during the off season had significantly larger populations of bacteria and a number of microbial enzymes than did soil from well-fertilized control treatment (Kirchner et al., 1993).

D. CHANGES IN FIELDNUTRIENT FLOWS As a result of the multitude of management and natural factors influencing gains and losses of nutrients in a particular field, the stocks of individual nutrients may follow a number of different patterns over the course of a rotation.

1. Drawdown (losses > additions)-The amount of time that this can continue without a severe yield decline depends on the initial fertility status of the particular soil as well as the extent of the drawdown. For rich Midwest prairie soils, drawdown went on for decades after virgin soils were first cultivated before nutrients limited yields. In the situation of a tropical Oxisol, where slash and burn practices are used, the low initial soil fertility and the rapid oxidation of SOM and extensive leaching combine to cause severe nutrient limitation in 2 or 3 years. 2. Buildup (losses < additions)-If the soils were nutrient deficient to start with, a buildup of nutrient stocks is important. However, this pattern may continue even after reaching nutrient levels that were sufficient for crop production. Sometimes, the excessive nutrients are stored in a form in which plants may be able to use them years later, in which case the farmer has put nutrients (and money to purchase them) in the equivalent of a noninterest bearing bank account. However, frequently leaching, runoff, erosion, and other reactions will cause the loss of some portion of the added nutrients. In addition, the high nutrient levels attained in some soils may pose an extra environmental hazard, for example, when P-enriched sediments eroded from fields reach a bay or a lake.

NUTRIENT CYCLING, TRANSFORMATIONS, AND FLOWS

37

3. Remain the same (losses = additions)-If the initial fertility of the soil was medium to high, this can proceed indefinitely. However, if the soil was initially low in fertility, this may pose a perpetual barrier to attaining high crop yields. If the nutrient levels were already very high, this may only perpetuate a potential environmental hazard (in the cases of N and P). 4. Alternate between buildup and drawdown (losses < additions followed by practices that cause losses > additions)-An example of this pattern can be found on many dairy farms. Dairy farmers commonly apply manure to corn at rates to satisfy estimated N needs. This sometimes causes a buildup of P and/or K that can be begun to be drawn back down a year or two later when alfalfa is grown. There is no problem with this pattern unless (i) the extent of the drawdown phase is too great and nutrient deficiency develops or (ii) the extent of the buildup phase is too great and potential environmental hazards develop (for N and P).

E. CHANGINGTO BIOLOGICALLY BASEDNUTRTENT SOURCES Soil organic matter buildup and maintenance plays a central role in field-level sustainable agricultural practices because of the overwhelming positive influence of organic matter on soil biological, chemical, and physical properties (Magdoff, 1993). Although the effects of organic matter do not all directly influence nutrients, most either directly or indirectly impact nutrient flows and cycles. Good SOM management practices reduce nutrient losses in runoff and erosion, provide high CEC levels to protect exchangeable cations against leaching, provide available N to nonlegumes by rotations or cover crops that include forage legumes, increase soil pH buffering, improve root development and health (thereby enhancing uptake of available nutrients), help make more P available, and enhance micronutrient availability. Alternative systems to conventional agricultural practices frequently include crop diversification and sometimes also involve adding an animal enterprise (MacRae et al., 1990). There is usually great emphasis on using the biological N,fixing capabilities of forage legumes (as either cover crops or sod crops for on-farm use or sale) and various other organic sources to supply N to nonleguminous crops. However, N deficiency is frequently found to be the major fertility problem during the early years of a transition from reliance on synthetic fertilizers to systems that use biologically based fertility sources such as legumes, manure, and composts. This probably results from the previous depletion of active organic matter pools (particulate SOM) by conventional systems prior to a change to the new system. Crop sequence was found to be a very important factor during the 4 years of a transition experiment from synthetic fertilizer N to organic (legumes and manure) sources (Culik, 1983; Liebhardt et al., 1989). This research was conducted on land that was conventionally farmed for many years before the experiment was initiat-

38

FRED MAGDOFF ETAL.

ed, with corn and wheat as the preceding crops. Nitrogen sufficiency and weeds were the main problems during the early years of the experiment. It was found that oats, soybeans, and red clover hay all performed adequately biologically because they are either legumes or have low N requirements. Red clover and oats had neither N deficiency nor weed problems. Soybean yields in the organic and conventional system were the same, but weed pressure was greater in the organic system. However, corn yield was reduced up to 40% in the organic system because of N deficiency and weed competition. From the fifth year of the experiment on (1986-1994), corn yields were not significantly different (P > 0.2) for organic (legume and legume plus manure) and conventional corn (Drinkwater and Peters, 1994). Compared with conventional management, the lower corn grain yields in the legume cash grain rotation during the early transition period were associated with lower soil nitrate levels and a greater proportion of N present in weed biomass and below-ground microbial biomass pools. The mineralization-immobilization turnover phenomenon is important to the cycling of soil organic N and subsequent availability. Until sufficient active organic matter has been accumulated in the soil, the N-supplying ability may not meet the need of high N demand crops, especially under weedy conditions. The productivity of alternative management systems employing legumes as sources of N may depend largely on the growers’ ability to synchronize supplies of available soil N with periods of maximum uptake by nonleguminous crops. Corn is commonly grown with little or no N fertilizer during the first year following alfalfa on conventional farms, but the effect of the high N availability from alfalfa residues dissipates rapidly (Fox and Piekielek, 1988). Thus, after the first year following alfalfa in a system in which synthetic N fertilizer is not used, the use of leguminous cover crops or other means of adding biologically based N to soils will be necessary to maintain yields of nonlegumes. Totally removing N fertilizer from the soil results in a system that must then function in a different manner. A new biologically based N system requires time to build up SOM, organic N content, and the populations of organisms that participate in N cycling so that the crops can obtain sufficient N. In order to avoid this problem in the early years, some N fertilizer, heavier rates of compost and manure, or higher producing green manure legume crops need to be incorporated into the system. The lower yields due to lower N availability during such a transition can be prevented by aggressive management (Fauci and Dick, 1994).

V. FARM-SCALE CYCLING AND FLOWS To decipher whether flows are farm-level nutrient cycles it is necessary to use the scale of the entire farm (Fig. 7). Manure produced by livestock may leave the farm, be used on a field that supplied the animal with feed, or be used on another

NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS

39

concentrates & other on-farmfeeds N2

I

f

I

/

farm boundarv’

to other fields

/ /

I bedding

offifann manures and other organic residues, fertilizers, lime, atmospheric deposirion

NZ

Figure 7 Managed system nutrient cycle and flows with a boundary around the farm.

field. An expansion of Fig. 7 to include all the farm’s fields would be needed to examine the details of flows within the farm. For example, nutrients may be “cycling” on the farm, but in a very uneven transfer pattern that depletes nutrients in some fields and builds up nutrients to excessive levels in others. This type of nutrient redistribution can occur when, for the convenience of the farmer, manure is applied only to fields near the barn.

A. WITHIN-FARM NUTRIENT FLOWS Within-farm nutrient flows represent the allocation of the nutrient stocks available to the farmer. Although there can be within-farm flows on farms producing only crops, with residues from one field removed and applied to another field, there

40

FRED MAGDOFF ET AL.

is a much greater ability to transfer nutrients on farms based on animal enterprises. On an integrated crop and animal farm these flows reflect the movement of nutrients from crop fields to animal facilities and application of manure to the crop fields. These within-farm flows have not been routinely characterized. Bacon et al. (1990) measured the flow of materials and calculated the nutrient balances for all the fields on a Pennsylvania dairy farm. The balances were influenced by crop type and management activities. There was little similarity in balances in P for adjacent fields. There was also little relationship between distance from the dairy manure storage and the nutrient balances. This farm may have been small enough and the goal to use manure across the farm important enough that such a pattern did not develop. Others (Bouldin et al., 1984;White and Safley, 1984) have suggested that manure applications are usually greater close to the animal facility.

B. NUTRIENT FLOWSTO AND

FROM

FARMS

Nutrient inputs to contemporary farms come in a variety of sources such as commercial fertilizers, lime and other inorganic soil amendments, organic amendments, biologically fixed N, and purchased feed. Manure produced on the farm is not a source of nutrients for the farm, although it can be a significant source for individual fields. It represents a material to redistribute nutrients within the space of the farm and to link the elements of the trophic pyramid. Nutrients contained in the manure either came from farm-grown feeds or bedding or from purchased feeds, bedding, and minerals. The particular farm inputs and outputs and the resulting nutrient loading depend on the strategic direction the farmer has taken. The socioeconomic and political influences of society play an important role in the farmer’s decisions regarding nutrient flows. Fertilizer inputs are generally related to the goal of enhanced crop production. However, biological N, fixation is most closely associated with farms that also produce ruminant livestock. This occurs because the most productive legumes are typically forages. Purchased feeds are common for most farms that include animal production, but they are particularly significant for nonruminant animal production. Adams and McAllister (1975) measured the nutrient balance of a group of farms in Northern Ireland. They observed that the P and K nutrient balances for the farms with ruminant livestock tended to be lower than the balances for farms with nonruminant animals, especially hogs. This reflects the differential emphasis on on-farm feeds for the contrasting animal types. For a farm producing ruminant animals, taking advantage of biologically fixed N may no longer be an important consideration in the management organization of the farm. Westphal ef al. (1989) calculated that when there was a limited crop area used to grow corn relative to alfalfa, there could be a significant limit to dairy herd size if biologically fixed N was fully accounted for and manure was used only

NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS

41

to supply the N need of corn. With this limit in herd size came an estimated decrease in the potential net farm income. A cash crop farm in a southeastern Minnesota study had the lowest excess N balance per cropland hectare, whereas a dairy farm had the highest (Legg et al., 1989). On the other hand, it is possible to use animal manures on forage legumes without adverse environmental consequences. In this situation, however, the N,-fixing capability of the legume is not fully utilized (Daliparthy et al., 1994). Kaffka and Koepf (1989) measured nutrient balances on a mixed crop and livestock farm in southern Germany over a 30-year period. They calculated greater losses of P and K in farm outputs than additions in farm purchases, but a high degree of self-sufficiency for N because of the production of legume crops (54% of the fields in a legume plus undersown clover on an additional 23%). Nutrients contained in purchased feedstuffs and bedding for livestock were essential to the maintenance of soil fertility. Nolte and Werner (1994) measured N in the products of a cropanimal organic farm in Germany as 183% of the inputs, without accounting for biological N, fixation. Most of the nutrients left the farm in crop sales, with less than 40% in animal products. The negative N balance of the farm contrasted with the observations of Kaffka and Koepf (1 989). However, this farm had only 29% of the area in legumes as contrasted with the almost 75% on the livestockbased farm of Kaftka and Koepf (1989). Granstedt (1992) suggested that a farm needs a minimum of one-third of the area in legumes to do without supplemental N fertilizer. A minimum animal enterprise may also be necessary to utilize the forages produced and to contribute to maintenance of the soil nutrient stocks. Feed inputs from off the farm generally increase with the animal density. Granstedt (1995) observed such an increase on three Swedish farms. Nutrient export from these farms did not increase in the same proportion as the nutrient inputs with feeds so that farm balances of each nutrient increased with animal density. There was little relationship between N fertilizer purchased and N balance and there was a negative relationship between P fertilizer inputs and P balance on these farms. Granstedt ( 1995) concluded that potential nutrient losses from enlarged nutrient stocks are strongly related to the intensity of animal production and the extent of use of purchased feed, whereas fertilizer use is greatest in areas without livestock. Westphal et al. (1989) found that purchasing feed could make dairy herd increases feasible compared to the herd size supported by on-farm crop production while still balancing crop nutrient needs with the available farm stocks. However, if the farm performance goal of balancing soil P was eliminated, more feed could be purchased, the herd size increased, and the net returns to the farm increase again. Purchased inputs can also affect soil organic matter stocks. Kaffka and Koepf (1989) observed increases soil organic matter (and N) in the last period of a long-term study after the purchased feed increased on a farm in Germany. As animal production intensifies, the efficiency of nutrient use as measured by the fraction of the inputs exported in the animal products often decreases. Van der

42

FRED MAGDOFF ETAL.

Werff et al. (1995) calculated nutrients in the produce from three organic mixed crop-dairy farms in The Netherlands as 3 1, 83, and 27% for N, P, and K, respectively, of the inputs, whereas a conventional farm was projected to yield only 12, 29, and 15%, respectively. The major difference between the farms was the greater stocking density on the conventional farm (2.4 vs 1.25 cows ha-') that was supported by feed and fertilizer purchases. Frink (1969) estimated that as the dairy cow numbers (density) increased on a farm, the N balance would increase to the point where nitrate losses from the field-applied manure would be significant. The long-term effects of large annual manure applications can be significant. Kingery et al. (1994) observed that total N increased to 30 cm, P and K increased to depths of 60 cm, and nitrate N levels were greater to or near bedrock after 15-28 years of broiler litter application in Alabama. Concentrations of Cu and Zn, common additives to poultry feeds, increased to 45 cm. Long-term studies with cattle and swine manure have measured increases in P and heavy metals (Chang et al., 1991; King et al., 1990). Reliance on biological-based nutrient sources does not necessarily lead to decreased nutrient loss to the environment. Nitrate leaching has been identified as a potential problem on organic farms in Germany where Nolte and Werner (1995) estimated that losses could be 25 kg ha-' year-' for a case study farm. Leaching of nitrate appears to be within the current EC limit for drinking water on three organic mixed crop-dairy farms in The Netherlands, but will not meet the future recommendation of only 50% of the existing standard (Van der Werff et al., 1995). Nguyen et al. (1995) estimated the N, P, and S budgets for three pairs of conventional or organichiodynamic farms in New Zealand. The marketed outputs from the alternative farms were only 5 I % of the N inputs (largely biologically fixed N) compared to approximately 90% for the conventional farms. Grain yields were lower on the alternative farms and N concentrations in the grain on the alternative farms tended to be lower. With more N inputs than the conventional farms, but limited crop performance, more N is probably lost through other pathways than crop export. Nutrient stock mining on farms that try to minimize the use of off-farm nutrient sources may make them unsustainable over the long run. The alternative farms in the New Zealand study of Nguyen et al. (1995) marketed considerably greater fractions of the soil P stock compared to the conventional farms, even though the outputs from the alternative farms were less. Lockeretz et al. (1980) found more P and K removed by organic corn production than under conventional approaches on farms in the United States. Net Pand K losses have been measured for an organic farm in Germany (Nolte and Werner, 1994). This contrasted with the typical conventional farm in Germany, which gained nutrients at 5-10 times as much per hectare as this organic farm lost. In some situations the export of nutrients may equal the value of the product sold so that wealth of the soil is marketed through soil mining and not the value added due to crop growth (de Wit er al., 1995).

NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS

100

43

XEX

1 . 4 more ha than needed to produce all feed 2.0.8 more ha than needed to produce all feed 3. just the right area to produce all feed

80

4. all grain imported (corn and soybeans) 6. all grain plus halfthe forage imported 6. all grain plus 3/4 of the forage imported

20

0

-20

0

1

2

3

4

5

hectares cow1 Figure 8 Implications of various crop areas available per dairy cow on P accumulation/depletion (see text).

An example of estimated net imports or exports of P on a dairy farm under various animal density scenarios is given in Fig. 8. Calculations were made with the following assumptions: (i) The diet consists of corn silage, haylage, corn grain, and soybean meal diet and is 0.4% P; (ii) total dry matter needs for a 636-kg (1400Ib) cow for an entire year and lactation (with production of 8172 kg milk) plus dry period is 7530 kg; (iii) yields of corn silage, haylage, corn grain, and soybeans are 12.I , 6.7,6, and 2.5 tons ha-', respectively; (iv) P export in milk and meat is 9.1 kg cow- I year-'; (v) all land in excess of needs for the animal will be devoted to haylage for sale; and (vi) no nutrients enter the farm except as animal feeds. When there is much more land than required to produce feed for animals (a total of 5.2 ha cow I ) , forage crops are exported in addition to animal products and there is a net loss of I8 kg P ha- I year- I from the farm if no fertilizer P is applied. If all the feed is produced on the farm, there is a slight negative balance as exports exceed imports as P minerals to supplement the diet. If all the grain and half the forage requirements are imported, there is a positive balance of approximately 35 kg P hayear I . -

-

C. NUTRIENT FLOWSBETWEEN FARMS Modern farm specialization has separated the elements of the classic natural ecosystem so that new linkages have developed among the farms. The emerging

44

FRED MAGDOFF ETAL.

pattern of nutrient flow in a specializing agricultural sector is that nutrients in fertilizer inputs to cash crop farms are transferred in the crops produced to animal operations (Lanyon, 1995). Thus, much of the nutrient input for animal enterprisebased farms is not as fertilizer. Approximately 8 5 6 5 , and 95% of the N, P, and K inputs to a Pennsylvania dairy farm were in materials going directly for the animal enterprise as feeds, bedding, and minerals (Bacon er al., 1990). Nutrient flows are increasingly among these specialized farms and the pattern of flow in many cases is not a local land-based cycle at all. This pattern of flow should be recognized because it disrupts the spatial integrity of the trophic relationship in which the waste products of one element in the relationship were the inputs for another. A very high percentage of the U.S. cropland is used to produce grain for animal consumption on other farms. Of the approximately 176 million hectares (435 million acres) of cropland harvested in the United States in 1992, corn grain, soybeans, and sorghum were grown on approximately 3 1 % (55 million hectares, or 137 million acres) (U.S. Department of Commerce, 1992). Although some of this grain is certainly used on farm, typically more than 55% of the corn produced is not used on the farms where it is grown (Watson, 1977) and approximately 25% is exported abroad (NCGA, 1996).

D. PATTERNS OF FARMNUTRIENT FLOWS After observing nutrient flow reports for a large number of natural areas Kelly and Levin (1986) noted that the “requirement” for recycling nutrients diminishes as the inputs increase. They expressed the requirement as the potential nutrient uptake divided by the inputs. Since the evolution of the fertilizer industry following WW 11, nutrient inputs for production are not as scarce as they once were. The result is that the “biological necessity” for recycling has been eliminated. Now it is possible for farms to be organized in different ways along the continuum of the requirement for recycling. Each of the patterns of farm-scale nutrient flows have very different implications. The main patterns discussed in the following sections along with potential implications at different scales are summarized in Table 111. 1. Farm Nutrient Exports > Imports This is a mining process whereby nutrients contained in soil organic matter or associated with minerals are being depleted. This pattern is more common on farms exclusively producing crops than for mixed livestock-crop farms where animal products are a significant component of sales. If soil organic matter is plentiful (as it was in the virgin tallgrass prairie soil of the corn belt) or if the minerals are geologically young and easily weathered (as is the case for K minerals in many soils in the northern Great Plains), the mining of various nutrients may go on for

Table IIl Potential Implications of Different Nutrient Flow Patterns Geographic extent Nutrient flow pattern"

Plantkoil and field

Farm

Implications if occurs on individual fields or whole farm Export > import

Decreasing fertility

Yields unsustainable

Export < import

Increasing fertility

High pollution potential; system unsustainable

Export = import

Maintaining fertility

Goal, but may not be most profitable under politicaVeconomic environment

"Assume starting with sufficient, but not excessive available stocks.

Regiodnation

Earth

Implications if occurs on majority of farms Decreased agricultural production Water pollution; depletion of nonrenewable resources (fuel, K and P deposits): enhanced market position Many changes needed in rural and urban areas developed and developing countries; animal production limited

Limitation to human poputaion Depletion of nonrenewable resources (fuel, K and P deposits) Moderated impact of agriculture

46

FRED MAGDOFF ET AL.

decades. However, Crews et al. (1991) suggest that soil fertility should not be mined in an ecologically based sustainable agriculture. To sustain soil productivity it will eventually be necessary to either export fewer nutrients or import more nutrients. By introducing, or increasing, a livestock component, fewer nutrients may be exported.

2. Farm Nutrient Exports < Imports The accumulation of nutrients with this pattern is both wasteful of nutrients and a potential environmental hazard as N and P accumulate. This is primarily a problem on livestock farms with relatively high numbers of animal units relative to cropland and a reliance on purchased feed. To bring import and export more into balance, fewer animals or more cropland may be needed. Another way to deal with the problem at the farm level is to export manure. Some farms are already disposing of manure on neighboring farms or producing composted manure for sale. Although this may solve the problem of oversupply of nutrients for the individual farm, when there are many farms with the same pattern of nutrient flow in a given region the opportunities for local export of manure may be limited and long distance export may be very costly (Young et af., 1985).Kloen and Vereijken (1995) observed that the soil P and K reserves on a group of organic farms in The Netherlands were greater than the agronomic requirement. These farms would need to pursue a management approach with greater nutrient outputs than inputs in order to reduce the soil reserves. This could complicate the supply of N to the crops because only biologically fixed N would meet crop needs without additional P or K as in the commonly used animal manures. The authors promote the concept of setting annual input/output balances in conjunction with the use of “acceptable” sources. On crop farms where excess use of fertilizers has caused the buildup of nutrient stocks, these stocks can be drawn down over a period of time by ceasing imports as fertilizer and maintaining sales of crops.

3. Farm Nutrient Exports = Imports with Regard to One or More Nutrients, but Not with Regard to Others Some farms follow a philosophy of trying to depend on nutrient cycles with little or no importation of nutrients. Biological N, fixation is relied on to provide a significant portion of the N inputs, but few other nutrients are used. In this situation, exports and imports of N may be balanced, but there may be a net export of other nutrients.

4. Farm Nutrient Exports = Imports As long as nutrient stocks are not too low, causing a yield sacrifice, or too high, causing environmental problems, farms should strive to have this pattern as a long-

NUTRIENT CYCLING, TRANSFORMATIONS, AND FLOWS

47

term goal. Crop productivity is maintained and the threat of future environmental degradation is minimized. However, animal production may need to be limited under this scenario to avoid buildup of nutrients. This pattern may not be economically sustainable for many specialized farmers who rely on animal production under the current political and economic climate.

VI. WATERSHED, REGIONAL, AND GLOBAL ISSUES Although the specialization of agriculture and various incentives have broadened the scale of nutrient flow in agriculture, there are few studies that have extrapolated from small plot or whole farm sustainable production tactics to other geographic units. There have been studies of different land uses in relation to nutrient flow, but these have been largely conventional practices on spatial scales larger than small plots.

A. WATERSHEDS The fate of inputs to a watershed is sometimes difficult to determine. A southeastern U.S. watershed appeared to retain 40% of the N, 58% of the P, and 63% of the annual inputs to the watershed (Woods et al., 1983). Fertilizer accounted for the majority of the inputs. Because of the mixed nature of the land uses (45% rowcrop agriculture, 13% pasture, and 30% forested), both soil storage and riparian forests were suggested as significant sinks. However, the researchers note that storage capacity is certainly limited. In subsequent studies of this area, Lowrance et al. ( 1986) found positive correlations between fertilizer inputs and harvested nutrients as well as the balance (unaccounted for nutrients). They concluded that the field, farm, or landscape response was dominated by the removal of material in the harvest but the watershed response was due to the interactions among the ecosystem components. Fluck et al. (1992) estimated that 73% of the P inputs to the 600,000 ha Lake Okeechobee watershed in Florida were from fertilizer and 20% from dairy and beef feeds. Outputs from the watershed were approximately 18% of the inputs and the yearly loading to the lake was only approximately 8% of the inputs. The remainder of the inputs were retained in the watershed P stock. Nutrient losses to the environment usually are greater from agriculture than from forests or other undisturbed vegetation. Correll (1983) reported 5-10 times greater losses of total N and P from a watershed dominated by conventional corn production than a forested one. Losses from a pastured watershed were intermediate. However, the fraction of the N inputs lost in the drainage was comparable at about 10% for the two agricultural land uses. The fraction of P lost was greater for the cornfield (1 3%) than for the pasture (5%). Highest nitrate discharges oc-

48

FRED MAGDOFF ETAL.

curred in the winter for both agricultural land uses. Nitrate losses from southeastem U.S. agricultural watersheds tended to be greater than losses from forested ones, whereas P loads were similar or slightly less than those coming from the agricultural watersheds (Lowrance et al., 1986). Kilmer et al. (1974) measured consistently higher N and P concentrations in discharge from a more heavily fertilized watershed than one with lower fertilizer inputs. Hallberg et al. (1983) tracked increases in nitrate-N draining from an agricultural watershed in northeastern Iowa as agricultural practices changed following the introduction of N fertilizer. However, Thomas and Crutchfield (1974) observed that the "background" nitrate levels from small Kentucky watersheds with crop, pasture, and forested land uses were highly variable. A strong relationship existed between the geology of the watersheds and the P contents of the stream flow rather than the land use. Stream P levels corresponded closely to those compiled 50 years earlier when few nutrient inputs were available to agriculture. The aerial extent of intensive agricultural operations has been related to nitrate observed in groundwater (Beck et af., 1985; Pionke and Urban, 1985) and surface water (Ritter, 1984). Patterns of elevated nitrate in a southwest Georgia study were coincident with an area of intensive cropland (Beck et af., 1985). Nitrates in groundwater in a Pennsylvania watershed were approximately four times greater in wells under cropland than under forest (Pionke and Urban, 1985). The mixing of recharge from the two areas diluted the nitrate concentration before the flow entered the surface water stream. Nitrate concentrations in drainage from watersheds in Delaware with >60% cropland were more than 30% greater than those with <60% cropland (Ritter, 1984). Klepper (1978) determined that both N fertilizer use for corn and the fraction of watersheds in row crops (primarily corn and soybeans) were significant factors in explaining the variation in nitrate concentration in surface waters from a central Illinois watershed. The watershed studies suggest that making meaningful observations of nutrient cycling in a sustainable agriculture at aggregated geographic scales is difficult. Lowrance et af. (1986) suggest that watershed-level studies are essential in relating management to external environmental impacts. However, the relationships may not be simple nor sufficiently sensitive for most decision making. Small plot and field results do not seem to aggregate well to the higher levels of spatial resolution where the land use patterns are mixed and complex. Furthermore, there must still be some connection between the activities and the responsibilities (for both the farmer and the beneficiaries) before changes can be promoted. The appropriate level for observation and for responsibility remains a question. Background concentrations and differential, as well as relatively slow nutrient transfer rates may limit the usefulness of observations at the real or perceived scale of the problem. Perhaps an indicator at another level of resolution will be useful. The German guidelines for organic agriculture have selected the farm as the unit to limit off-farm inputs of both fertilizers and feeds as a means to meet the expectations they associate with organic agriculture (Nolte and Werner, 1994).

NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS

49

B. OTHERSPATIALSCALES The introduction of clover in Europe during the 17th through 19th centuries (Kjaergaard, 1995) is a historical illustration of the impact a change in the potential nutrient stocks can have on a region beyond the scale of an individual farm or watershed. For example, the input of N to cultivated land doubled with the introduction of clover to Denmark during this period and an ecological dilemma of declining yields and degrading landscapes was averted. Grain yields doubled and cattle numbers increased by one-third in less than 35 years. Even the vast increase in potato production has been linked to the newly available quantities of N. The new clover-based system relied heavily on animals to utilize the legume forage crop and provided an opportunity to retain P and K by marketing animal rather than crop products. Individual cultivators influenced the flow by the allocation of land resources to the N “collectors,” but it was the introduction of a production factor from outside the original system that made the change feasible for the individual to consider. Nutrient balance information at other spatial scales, such as county, state, andor country level, can be used to identify where clustering of nutrients may occur when inputs exceed outputs and/or utilization potential, assess potential nutrient impact on water resources, and provide a basis for future decision making (Barker and Zublena, 1995). Almost 20% of North Carolina counties were found to have nutrients in excess of crop requirements with excess P more common (18% of the state) than excess N (3% of the state) (Barker and Zublena, 1995). When Keeney (1979) estimated the N budget for Wisconsin, he identified N fixation by legumes and N fertilizer as significant sources of N for the state (54 and 22%, respectively). However, approximately 40% of the N inputs could not be accounted for and were assumed to be lost to the environment through denitrification and leaching. Based on estimates of total N use in all kinds of farming in The Netherlands and the total removed from farms in produce, 75% of the N is wasted (Whitmore and van Noordwijk, 1995). Granstedt (1995) estimated that 74,60, and 74% of the N, P, and K, respectively, imported onto the farms in Sweden are not exported in food products. Of the nutrients actually taken up by plants only 20,25, and 5% of the N, P, and K, respectively, are actually exported from the farm to the community food system. Isermann (1990) recognized that the surpluses of N and Pat the country level in Western Europe are due to increasing levels of nutrient application not resulting in similar increases in nutrient exports in farm products.

1. Intercontinental Flows Cooke (1989) highlighted the significance of intercontinental plant nutrient transfers. Not only is there a concern for soil mining at the farm level, but these concerns should be nationwide. The value of nutrients lost may not be offset by the prices paid for the commodities. This transfer affects the countries producing

50

FRED MAGDOFF ETAL.

the commodities, which are often developing countries, and the countries importing the commodities. Furthermore, he estimated that developed countries import 12% more N and more than 60% more K than they export, whereas comparable depletions exist in developing countries. Only in Pdo the developing countries appear to be gaining while the developed countries are losing. Nutrients in a single animal feedstuff, casava, imported from Thailand to The Netherlands were equivalent to more than the K fertilizer used in The Netherlands; one-third of the P fertilizers used; and slightly less than 10% of the N fertilizers used. Of course, these data do not represent the whole range of conditions from various countries. Symptoms of the importing countries may be excessive nutrient accumulation that will have adverse environmental impacts. In 1983, 40, 72, and 59% of the total imported N, P, and K, respectively, to The Netherlands could be attributed to feedstuffs. Cooke (1989) suggested that all countries need to develop national nutrient balances as policy guideposts for the future prosperity of their countries. Issues of sustainability, especially the contribution of maintaining the integrity of the resource base, either from depletion or enrichment, to intergenerational equity should be a major consideration. Part of the difficulty in implementing policies that might be derived from such analyses is that the controllers of the nutrient flow may not correspond with the authority of national jurisdictional boundaries and that future generations and other considerations are routinely underrepresented in the evaluation of impacts of a wide array of decisions.

2. Finite Geologic Deposits Although global reserves of P and K are large, the deposits will eventually be exhausted. It is estimated that, at current rates of mining, the known global reserves of phosphate and potash will last for approximately 200 years (Louis, 1993). Even though there are larger “potential” reserves that may extend the time span for mining these nutrients, the rate of growth of demand for P and K may work to limit the increase in time until deposits are exhausted. Regardless of the time until reserves are depleted, the extent of the potential supply must be compared to the ability of society to sustain the pattern of nutrient flow. Gains in efficiency of P and K utilization may extend the life of the geological reserves, but if the connections in the patterns of nutrient flows are not sustainable devoting management and other resources to enhanced efficiency in order to prolong the reserves may contribute little to true sustainability.

C. ENERGYUSEAND NUTRIENT FLOWS Patterns of nutrient flow depend on the energy contributed to the process from a variety of sources. Energy use and flow is the central purpose of the food chain

NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS

51

as solar energy is captured in primary production and moves along to the succession of consumers. Ultimately, energy is dissipated as heat, becomes part of decomposer organisms, or remains in the undecomposed residues. Animals consume solar energy that was recently concentrated by plants. In addition to relying on capturing current solar energy, agricultural production systems also utilize historic solar energy, recovered from geologic reserves, to create inputs. The inputs have a variety of functions. These are generally recognized as substituting for labor, for reducing the deficiency of production inputs such as crop nutrients or water, or creating conditions that could not otherwise exist. However, substituting animal power or machine inputs in corn production for human labor generally reduces the amount of solar energy captured per unit of expended energy (Pimentel and Burgess, 1980). Nitrogen fertilizer can eliminate a N deficiency and actually increase the potential for crop production, but its use also tends to decrease the energy captured per unit of expended energy. Energy accounting can be used to identify the various forms and the amounts of energy used in agricultural production. The accounting may be conducted for enterprises such as corn, tomatoes, alfalfa, or animal production. Nitrogen fertilizer is typically a major energy input to cereal production such as corn. This one input may account for 20% of the total energy in the production inputs and fuel can be over 40% (Pimentel and Burgess, 1980). For legume crops such as alfalfa the total energy requirements can be approximately 60% of corn production (Heichel and Martin, 1980).There are usually no N fertilizer inputs to an established stand, and the fuel requirements for cultural practices become approximately 98% of the total energy used. Animal agriculture benefits from the controlled environment conditions provided by modern housing systems (Spedding, 1982). Energy inputs to animal production vary widely with species and geographic location, but the ratio of energy inputs to outputs is generally much greater than for common crops (e.g., <0.5 MJ MJ- for cereal crops and 2.8 and 10 MJ MJ-' for milk and broilers, respectively; Spedding, 1982). Farms are rarely single enterprises and it is essential to evaluate energy consequences of the entire system instead of focusing only on one aspect such as nutrients. For example, changing crop sequences from continuous corn to corn-soybeans in a dairy farm simulation resulted in a more dramatic decrease in energy requirements (36%decrease) than when a corn-alfalfa rotation was selected (Vinten-Johansen et ul., 1990.The greater reduction in N fertilizer requirement for the corn-alfalfa rotation was partially offset by the greater requirement for fuel to manage the forage crop. The structure of contemporary agriculture has significant implications for the energy requirements to sustain the connections among the various components. Pimentel(1980) estimated that farm input supplies travel an average of 650 km and require 257 kcal kg-' for transport. With emerging regional and global markets (Lanyon, 1995)since Pimentel's original calculation, the energy consumed for this

'

52

FRED MAGDOFF ETAL.

purpose is likely to have increased. A further link in nutrient flow that is seldom considered is the energy requirements to “return” the nutrients to some former location. Identifying the relevant controls and the responsible parties is likely to be an essential part of acting on an understanding of the role of nutrient cycling in a sustainable agriculture. Thus, the ultimate energy concern regarding the nutrient flow pattern may have less to do with farm practices that replace fertilizer inputs than with the extent of the connections between sources of nutrients, production of cash crops that are transferred to animal production locations, and even the transfer of animal products to consumers. Another important issue is that the deposits of the two main types of fossil fuels used in production have different projected lifetimes. For example, the natural gas reserves to produce N fertilizer may well outlast those for the liquid fuels that are the keystone of modern agriculture and its associated transportation networks (Lanyon, 1995). The significance of energy use in maintaining the pattern of nutrient flow can be evaluated using several criteria. For instance, there are environmental consequences that develop as energy is depleted from reserves, as energy and mineral extraction takes place, as nutrients are used with decreasing efficiency in crop production, or as nutrients accumulate in areas of animal concentration. Consideration of proposed changes will need to include these criteria and to develop relative priorities for addressing them. Also, if energy scarcity is a concern, then evaluating both the relative magnitude of competing needs and the flexibility for change may need to be considered. Production agriculture used <3% of the total U.S. energy consumption in the 1970s and was <20% of the energy in the food system from field to table (Fluck and Baird, 1980). Reforming the pattern of nutrient flow is likely to involve a reconsideration of the strategic direction of agriculture. Thus, putting energy use into perspective with the competing uses across the spectrum of energy uses in society may be a first step. From this perspective will emerge a new set of questions and associated technical challenges. Questions such as who decides the characteristics of the new pattern of nutrient flow, who benefits from the new formulation, and who pays for the new arrangement are likely to all become part of the debate.

D. POSSIBLE CHANGES IN LARGE-SCALEFLOWS Isermann ( 1 990) cites the long-term possibilities of balancing nutrient applications and use at the field and farm levels by reintegration of animal and crop production, improved animal feeding programs, modified animal housing to reduce potential ammonia volatilization, increased emphasis on N and P recycling, and even changes in human diets toward less animal protein. He projects a degree of success in reducing emissions if these fundamental changes are implemented within 20-30 years. Kaffka and Koepf (1989) suggest that not just a site-adapted pro-

NUTRIENT CYCLING, TRANSFORMATIONS, AND FLOWS

53

duction program but a mixed plant and animal husbandry are prerequisites for the success of an approach to sustained production and balanced fertility. This view is supported by Granstedt (1995) because he suggests that a good balance between animal production and crop production at local, regional, and national levels is required to reduce the losses of nutrients from agriculture. He envisions that certain regions of Sweden would emphasize animal production less and animal production would be decentralized in other areas in order to achieve ecologically based, resource-conserving agriculture. He also believes nutrient balancing can be achieved through cooperation among farmers with different enterprises and through returning nutrients from the community to the farm. Stopes (1995) supports the notion of fundamental changes in the food production and delivery system and the human diet if widespread adoption of organic farming systems (as one form of an agriculture presumably with nutrient cycles) is to become a reality. There is tension apparent between the value of animal agriculture in conserving nutrients on the farm and the perceptions that less animal production would be desirable. Land application of human waste has been viewed as one mechanism to return nutrients to farms. Witter and Lopez-Real (1 987) suggest that nutrients in sewage sludge are only 3% of the fertilizer requirement in Britain, but that with increased emphasis on reducing waste and emphasizing return flows 40% would be possible. The impact of industrial pollution of the waste stream and a variety of policies that keep fertilizer prices low and emphasize productivity will not encourage “recycling.” Although between 40 and 60% of the N, P, and K of food products in Sweden is in slaughterhouse and domestic wastes that could be recycled with little risk (Granstedt, 1995), it would require extensive change in the relations between agriculture and the rest of society. Loehr ( 1969)recognized animal waste management as a “national” problem and highlighted increasing reliance on readily digestible ration ingredients as a practice to reduce the costs of animal waste management. Increased digestion of nutrients supplied in the rations would reduce the quantity required for the same production. Cromwell et d.(1993) project considerable reductions in required P supplementation of corn- and soybean-based rations with the use of an enzyme that increases P availability from these sources. These approaches also are likely to provide only incremental, short-term improvements. The challenge is to recognize that apparent farm-based solutions may not be that at all if they do not address the appropriate scale of the problem, which is the transfer of nutrients between specialized farms with different ecosystem elements. Increasing the efficiency of feed nutrient utilization through changes in the ration components or better balancing of rations has also been suggested. The effect of livestock production on nutrient balance can be managed by exporting fewer animal products (cutting production) or reducing losses of nutrients by on-farm management practices. A biodynamic farm was found to be self-sufficient in nu-

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trients because the livestock rate was adjusted to the carrying capacity of the farm crop production (Granstedt, 1995). Organic farms in Germany cannot purchase more than 10% of the feed for cattle (Nolte and Werner, 1994), and their production is, therefore, less than that of farms with unlimited potential to purchase feeds. Sustainable livestock production is an issue of manure management, effects on soil erosion and soil organic matter, and the export of nutrients in products sold (de Wit et al., 1995). Postharvest and postplowing N losses have been suggested as a focus for reducing nitrate losses when legume crops are rotated to nonlegume crops (Kopke, 1995). Other losses from manure application and manure “heaps” need to be considered. Sustainability must be determined for various agroecological criteria in a system-specific analysis (de Wit et al., 1995).

E. INFLUENCES ON NUTRIENT FLOWPATTERNS There are controls on flow patterns that are exerted at different scales. In general, controls exerted at the regional, national, and global levels have ramifications that influence the choices and decisions made at the farm level (Table IV). The control that the farmer then exerts over the direction of the farm influences the choices and decisions made regarding practices in particular fields. One of the main factors influencing farm decisions affecting nutrient flows is the perceived economic benefits of particular management options. The growth of specific crops or raising of animals, the amount of purchased feed for animals vs farm-grown feed, and the enterprise mix affects the farm nutrient balance. The use of fertilizers at rates recommended by reputable soil testing laboratories is believed to help ensure maximum profits from crop sales. Reliance on off-farm feeds frequently enhances the economic performance of an animal farm. In some circumstances, dairy operations may have greater net returns if off-farm feeds are purchased rather than if the dairy cows are fed only from on-farm sources (Westphal et al., 1989). However, purchased feed arrangements potentially create a gap between the location of crop production and the location of animal production. Because cost is a major factor in purchased feed decisions, rather than the biology of production, the least costly feed may come from widely dispersed sources. “Return’’ of nutrients to these locations is not considered as part of the cost when they are purchased. Nor are the full costs of growing and transporting the crops necessarily paid for when feeds are purchased. A variety of government programs, actions, or subsidies commonly mask the true costs of water (as in California), of transportation (such as the construction of the interstate highway system), or of maintaining a low-cost energy supply (such as military expenditures to help maintain a flow of oil from the Middle Ease under favorable terms). Consequentially, there is no economic incentive to maintain biological integrity in the transaction. Profit maximization is not the only farmer goal influencing the pattern of nutri-

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Table IV Summary of Controls and Their Outcomes on Nutrient Cycling and Flows at Different Geographic Resolutions Controls Regional. national, and glohal

Impacts

Political process, corporate decision-making process, geologic reserves of nutrients and energy, transportation and infrastructure

Government programs, international trade patterns, externalization of + certain costs, corporate decisions about where to locate facilities and how to organize their supply of agricultural products. advertising, etc.

/ Farm

Farmer orientation (outlook). Enterprise mix (types of crops and economics (potential sale animals raised), products exported price - costs), consumer + off-farm, products imported, preferences, agribusiness animal density relative infrastructure and preferences to land area, etc.

SoiVplant and field

Amounts and forms of nutrients added, method and timing of nutrient additions, tillage system and surface residue status. types of crops grown, portion of crop removed, etc.

K- Soil structure, runoff and erosion, -W

nutrient stock levels, bioligical diversity and activity, amount of active SOM, amount of total SOM, etc.

ent flow to and from a farm, especially on animal production farms. Although rates of fertilizer application are likely to be reinforced by the economics of crop production (Legg et d., 1989),sometimes N fertilizer use in addition to the available N from legume residue and manure is thought to be a risk-reducing management tactic for livestock farmers. Farm-scale nutrient flows are influenced by other factors in addition to individual farmer goals such as profit maximization in crop production or intensive animal production, risk aversion, or some alternative ideology. Government programs that create incentives for particular practices also influence farm-level nutrient flows. Actions taken by large corporations to ensure their profits have transformed the very nature and structure of agriculture. For example, the production of broilers in the United States is almost 100% controlled under production and marketing contracts, with a limited number of integrators or they are raised under integrated corporate ownership (Welsh, 1996).For the most part, the direction of the hog industry is following poultry, with the decisions as to where animals are raised and what they are fed coming increasingly under the control of a small number of companies.

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Although not shown in Table IV, there can be feedback mechanisms that move from the field or farm scale upwards and may influence farmer as well as governmental decisions. For example, when soil structure declines, a farmer might choose to change tillage, crop rotation, and /or the farm enterprise mix. Widespread problems with soil erosion and water pollution have also been recognized nationally with the implementation of government-financed soil conservation programs aimed at decreasing soil and nutrient loss to water. If changes in the pattern of nutrient flow are to be made to more closely approximate a cycle for a sustainable agriculture, consideration of the character of the existing linkages and the incentives behind them will be essential. Nutrient cycling must become a priority in farmer decision making before it can play a significant role in farm performance. For instance, a nearly closed nutrient cycle is a goal for individual farms in Germany for them to be recognized as organic (Nolte and Werner, 1994).However, widespread adoption of such a goal cannot be based solely on resources and personal perspective of individual farmers. It must be a goal that is supported in the messages communicated to the farmer from the “surroundings.” Efforts to associate nutrient cycling with sustainable agriculture or management of nutrient flow to meet any criteria other than those present in marketplace transactions will have marginal results unless they are strongly endorsed outside the farm and then communicated to the farmer.

VII. PROMOTING A MORE SUSTAINABLE AGRICULTURE THROUGH CHANGES INFLUENCING NUTRIENT CYCLES AND FLOWS A sustainable agriculture is not simply an assemblage of numerous possible onfarm techniques, but one that will have to confront the realities of functioning within dynamic social and economic constraints on agricultural activities. The decisions to structure farms in particular ways are shifting from resource-based constraints to emerging choices based on a variety of social factors. Stopes (1995) suggests that a new orthodoxy of multiple goals for agriculture that are not limited to production but rather that include environmental and social outputs and a greater degree of “sustainability” will emerge. Will mainstream society consider the diversity of approaches that could be utilized to meet production, efficiency, and consumption goals (Whitmore and van Noordwijk, 1995)? Many changes in the way farms are managed and in the way society at large interacts with the agricultural sector have been suggested in order to enhance sustainability. A number of these approaches either directly involve nutrient flows and cycles or have important implications for these flows. For the long-term sustainability of agriculture, ways need to be found to decrease the dependence on fossil

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fuels to manufacture fertilizers or other amendments and to transport fertilizers and feeds over long distances. With the fertilizer industry (especially N) so intimately tied to the price of energy, a sharp increase in the cost of fuel will make agriculture especially vulnerable. Ways also need to be found to reduce nutrient pollution of surface and groundwater. Enhanced efficiency of nutrient cycling and uptake by plants can increase individual cash crop farm profitability by decreasing the amount of purchased inputs. Finally, although there are large reserves of K- and P-bearing mineral deposits, the pace of mining these nonrenewable deposits may increase, thus decreasing the time needed to exhaust the deposits. This indicates that the nutrients should be used carefully and efficiently. Changes in agriculture and society that are related to nutrient flows and that might promote a more sustainable system include those that can happen quickly as well as those that will take longer to accomplish (Fig. 9). Some of these suggestions appear to hold more promise than others. In general, short-term changes help to “tighten up” the efficiency of the use of nutrient sources and the cycling of nutrients in the field setting. Those changes that might occur at the farm level will take longer to put into effect but will have a more long-lasting impact. However, farm-based changes may not turn out to be appropriate if the scale of the problem, such as the transfer of nutrients between specialized farms, is not addressed. Fi-

centuries 8d

7 decades

8a 8b 8c

8

b

4c 6 6

years

4a

2

current field season

Ib la

3

4b

field

farm

1c

I

plantkoil

regionhation

earth

Spatial Scale Figure 9 Relationship between spatial scale of possible changes affecting nutrient flows and estimated time needed to complete changes (see text for explanation of numbers).

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nally, long-term changes at the societal level are those that will eventually have the greatest potential impact on the development of sustainable nutrient flows. These changes are also the most difficult to ultimately carry out. A variety of possible changes that might impact nutrient flows are discussed in the following sections.

A. FIELD-LEVEL CHANGES (SHORTTERM) 1. Enhance the Uptake Efficiency of Already Available Nutrients in the Soil This promotes cycling by reducing nutrient loss and the flow of added nutrients from off the farm, might reduce costs, and may also maintain environmental quality. a. Increase the Degree of Synchronization between Nutrient Availability and Uptake Needs of Plants The idea behind this approach is to decrease the amount of time when there are large quantities of available nutrients present (mainly N) and thereby decrease the potential loss of nutrients by leaching or runoff or through gaseous loss. Poor synchronization resulting from the application of fertilizers applied long in advance of crop needs may lead to higher amounts of fertilizer use. Also, when well-drained soils rich in organic matter are intensively worked, high amounts of nitrate are produced (Magdoff, 1991a,b). If the crop being grown does not have a high N need, much of the NO;-N that accumulates cannot be used and is subject to leaching and denitrification losses. Greater synchronization between supply and demand of nutrients decreases the opportunity for loss to the environment and allows nutrients to be taken up by plants more efficiently. Nutrients have greater potential for participating in a true soil+plant+soil, soil+plant+human+soil, or soil+plant+animal-+human+ soil cycle when taken up by plants than when lost from the soil as runoff or leaching water or as gaseous forms. Along these lines, there is interest in planting of spring wheat instead of winter wheat on northern plains to take advantage of better synchronization between the supply and demand for available nutrients. Reducing the intensity of tillage operations will also reduce the burst of mineralization that follows. In addition, keeping the soil more evenly moist during the growing season may reduce minibursts of mineralization that occur after rewetting a dry soil. Mulches (organic residues or plastic) are commonly used in vegetable crop and small fruit production and the surface of no-till soils usually will have plentiful residues, thus helping reduce evaporation from the soil surface. Although higher moisture soils may have less SOM mineralization than soils that dry

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down and then rewet, NOT-N may be more prone to denitrification in the moist soil. However, more available C for microbes to use following rewetting also allows for a burst of denitrification in alternately dry and wet soils. b. Optimize the Root Environment By optimizing conditions such as soil structure or pH, enhanced root health and exploration may help plants take up nutrients more efficiently. Soil biological activities produce a multitude of active plant hormones (Frankenberger and Arshad, 1995). Humic materials have been shown to promote root growth and branching (Lee and Bartlett, 1976). Also, practices that enhance mycorrhizal associations such as using cover crops will increase the ability of plants to extract available nutrients from the soil. Using diverse residue sources to maintain high levels of SOM may reduce soil pathogens and other pests. However, if root systems are healthier and explore a greater portion of the soil, native stocks will be drawn down quicker than would have occurred otherwise. Thus, although this may help from an environmental point of view and reduce the need for imported nutrients from off the farm and enhance profitability in the short run, it only delays the need for nutrient inputs. c. Use Cover Crops A vigorous grass cover crop such as winter rye when the soil would otherwise be bare (fall, winter, and early spring for temperate region summer annuals) will minimize leaching losses of N (Meisinger et al., 1991) and runoff losses of both available and unavailable N and P.

2. Increase the Availability of Soil Nutrients (from SOM, Newly Added Residues, or Soil Minerals) By more reliance on the soil to supply nutrients, there will be less need to use commercial fertilizers or other nutrients from off the farm. Emphasizing practices that result in building up and maintaining high levels of SOM will promote the more efficient use of nutrients. Use of cover crops, reduced tillage, and rotations that include sod-type crops are practices that enhance soil structure and thereby reduce loss of nutrients by both runoff and leaching. The use of cover crops also helps to reduce loss of inorganic N and can enhance myccorhizal development in the following crop, thus helping the following crop obtain needed nutrients. In addition, active microbial populations help make P and micronutrients more available to plants. It must be kept in mind that if large quantities of nutrients are removed from soils in harvested crops and not returned (as manure or other residues), this strategy plus the ones discussed under Section VII,A, 1 must cause a decrease in total stocks of the available nutrients. Thus, although for a period of time (which might

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be quite long or short) it will be possible to replace fertilizer nutrients with those derived from soil, this strategy of more efficient exploitation of soil nutrients cannot go on forever.

3. Utilize Fertilizers and Amendments More Efficiently If fertilizers and amendments are used more efficiently, application rates can be decreased. a. Use Manures More Efficiently Available on-farm sources of nutrients such as manures should be distributed according to soil/crop needs. This best management practice calls for reliance on regular soil testing and on making sure that manures are applied where they are needed and not just where it might be most convenient to put them. This will help build up nutrients in depleted fields and not oversupply nutrients on fields with large amounts of available nutrients. Other best management practices include better timing of applications, more rapid incorporation, and recommendation systems that take better account of (credit) nutrients in manures. b. Better Use of Synthetic Fertilizers Better soil and tissue tests and/or recommendations based on tests are needed to ensure that only the amounts of fertilizers needed for crop production are actually applied (Beringer, 1985). Most routinely used soil tests do not adequately account for contributions from organic matter mineralization (Beringer, 1985). As discussed previously, better timing and methods of fertilizer applications may enhance the percentage of fertilizer nutrients that are taken up by crops.

B. FARM-LEVEL CHANGES (MJZDIUM TERM) 1. Use Nutrient Sources That Are More Efficiently Taken Up and/or Require Lower Energy Inputs Instead of Synthetic Fertilizers a. Rely Primarily on N, Fixation by Symbiotic and Nonsymbiotic Organisms This will allow the substitution of biologically fixed N for the energy-intensive chemical fixation process, as well as decreasing transportation and application energy and machinery requirements. This is also the only example of a soil biological process that results in increasing the total stock of a nutrient. However, it is the forage legumes that are the best in terms of fixing N, and contributing available N to following crops. Perennials such as alfalfa can be grown as an economic crop in a rotation, whereas annuals such as hairy vetch can be used as a cover crop.

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However, if there are insufficient animals on the farm or nearby farms, the use of perennial legumes may not be profitable. b. Use Only Sparingly Soluble Nutrient Sources One of the suggestions for enhancing nutrient cycling efficiency is to rely exclusively on sources of applied nutrients that are slowly available, such as composts and partially processed rock minerals. This will keep losses from the available stock lower than when large amounts of available nutrients are used and could potentially allow nutrients to be used more efficiently. During the period of high plant demand mineralization and/or dissolution of minerals must proceed rapidly enough to keep up with plant needs. Thus, to make sure that crop production does not suffer when first using sparingly soluble sources, it will be necessary to build up total stocks to higher levels than would be needed with more soluble sources. It must also be kept in mind that many of these low-solubility materials, such as rock phosphate and greensand, travel significant distances and require energy for application. Thus, although processing rock phosphate requires only 9 GJ tonp1 P compared to the 22 GJ ton-' P for producing concentrated super phosphate, they both consume an additional 22 GJ ton- I P for packaging, transportation, and application (Boswell et al., 1985). Using such partially processed mineral sources may decrease field losses and may require less energy for production but may not be contributing to cycling as a closed-loop pattern of flow in any meaningful sense, except that losses may be decreased somewhat. c. Utilizing Nearby Nutrient Flows for Agriculture The agriculture of a number of societies has taken advantage of nearby flow nutrients. The Aztecs, as well as other indigenous peoples of Central and South America, and people in parts of China built mounds for cropping in naturally swampy regions or in shallow lakes. The fertility for these raised bed chinampas came primarily from the sediments used for building and maintaining the mounds, from weeds harvested from the water, and from the water itself as it flowed from upstream into and through the system. Fish were commonly produced in the water and a mixture of crops was grown on the beds. Animals raised on crop residues also provided some nutrients to cycle back onto the beds. Other important early agricultural systems, such as those along the Nile and the Euphrates, relied on another type of nutrient flow-sediments from upriver. The rich alluvial soils were replenished by the annual floodwaters. From the point of view of nutrients, although these systems relied on upriver soil erosion, they may have been able to function for a very long time. However, salinization resulting from insufficient internal soil drainage is believed to have had a serious adverse influence on productivity. In modern times, the building of dams to reduce flooding and to gain better control over water distribution has greatly decreased the amounts of sediments available for adding nutrients to many alluvial soils around the world.

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2. Change Miu of Crops or the Animal-Crop Mix on a Particular Farm so That Fewer Nutrients Are Exported Because the export (sale) of some crops removes a particularly large amount of nutrients, it is possible to change the mix of crops grown on a farm and place more emphasis on those that result in smaller quantities of nutrient removal. In addition, increasing the importance of animal enterprises on the farm relative to crop sales will result in less nutrient loss in agricultural products. However, economic barriers and personal preferences of farmers make it difficult for selection of the farm enterprise mix to be made only on the basis of nutrient management issues.

3. Change Animal Feeds to Promote More Efficient Use of Nutrients If ways can be found to make the nutrients in the feeds more available to the animals, less N and P will be required in the diet, and thus less N and P will occur in the manure. The use of an enzyme that increases P availability and balancing the correct proportions of essential amino acids in feeds can reduce the total amount of P and N in feeds (Cromwell et al., 1993; Chase, 1994). Because these approaches only deal with a small part of the problem, they are likely to provide only incremental improvements.

C. SOCIETAL-LEVEL CHANGES(LONGTERM) 1. Stop Using Practices or Growing Crops That Promote Large Nutrient Leaks and Wasteful Use of Soil Resources a. Promote the Philosophy of Eating “Lower” on the Food Chain This is a euphemism for eliminating or greatly reducing consumption of animal products. The interest in this approach is because of (i) the belief that animals are treated inhumanely in conventional agriculture, (ii) the belief that consumption of large quantities of animal products are potentially harmful, and (iii) the concern for using resources to more efficiently supply food to hungry people. If a significant portion of the U.S. population started eating fewer animal products, there should be a decreased demand for feed grains. Thus, less land would be planted for crops such as corn and soybeans. The way these crops are currently grown tends to promote especially large leaks due to inefficiencies in nutrient flows and to breakdown of nutrient cycles. Because less hayland would be needed, the total area in crop production would be decreased even more. There would also be fewer large animal production facilities, which can cause considerable environmental damage resulting from the concentration of animal wastes. The mas-

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sive interregional transfer of nutrients for the production and utilization of grains for animals would be dramatically decreased. However, there are a number of potential negative side effects to the reduction of animal products in the diet as well as countertrends that may offset potential benefits. Most important, the market for feed grains is now global and trends in many developing countries such as China are toward more animal products in the diet. Thus, decreased internal use of feed grains may be countered by increased exports abroad. The remaining livestock enterprises in the United States may be of even larger size than the enormous units that currently exist. The reduction of hayland that would occur with less animal production also removes an important practice for build up of soil organic matter and N as well as improvement of soil structure. There would also need to be at least some increase in the area devoted to production of grains, vegetables, and fruits for humans that substitute plant for animal products (and, thus, not as large an area removed from production as would be estimated from the reduced use of animal products). Exporting crops from individual farms (for direct human consumption in this scenario) also entails a much larger loss of nutrients than export of animal products from cropanimal farms. In addition, as opportunities shrink in animal production and less farmland is used for animal feeds, the numbers of farmers would probably decrease. The net effect of decreasing the extent of animal agriculture may be to decrease overall nutrient flows (assuming increased feed grain exports are less than the decreased use in the United States), although nutrient loss from an individual crop farm may be larger than would have occurred on an integrated croplivestock farm. Thus, even if there is a widespread movement toward eating lower on the food chain, it may be important to enhance the possibility of nutrient cycling by promoting ways to lessen distances between production and consumption of human food crops. Another similar idea is to promote production systems in which ruminants eat lower on the food chain with a decreased amount of grain and an increased amount of perennial forage crops in their diets. This would diminish the flow of nutrients for production and utilization of feed grains, promote the utilization of perennial forages, and may also lead to decreased numbers of very large production facilities. This change could occur whether or not large numbers of people choose to follow new diets. However, it may take longer and be more costly to bring animal products to market with reduced feed grain inputs.

2. Overcoming the Physical Separation of the Plant-Animal-Human Trophic Pyramid Segments When consumption of food by farm animals or humans is in relatively close proximity to where the crops are grown there is a great potential for true cycling of nutrients.

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FRED MAGDOFF ETAL.

a. Integrate (or Reintegrate) Animals into Many Cropping Systems When crops leave a farm, normally a relatively high percentage of the nutrients in the aboveground biomass are exported. The extreme situation occurs with crops such as hay and corn silage, but for grain crops such as corn and soybeans a high percentage of the N and P,although less than half of the K, in the aboveground biomass is still exported off the farm. By raising animals on farms that produce the bulk of their feeds, true cycling of nutrients from animal agriculture becomes more feasible. In an integrated livestock and crop farm, fewer nutrients are exported than on strictly crop farms. Because a high percentage of the nutrients eaten by animals are excreted in urine and feces (Azevedo and Stout, 1974), only approximately 1 ~ 3 0 % of nutrients in crops harvested on integrated animal-crop farms end up in exported animal products, with the remainder available for cycling back onto cropland. Thus, there is less depletion of nutrient stocks in animal-based systems than in cash grain or cash forage systems. Another reason for the proposal to reintegrate animal and crop production is to give more rotation options to crop farmers. Without animals present on the farm or on nearby farms, there is little incentive to put land into legume or grass-legume hay. Growing sod crops help make nutrient cycles operate more efficiently because they help to maintain or increase organic matter (and hence CEC) and promote better soil structure and reduce loss of nutrient stocks in leaching waters or as eroded sediments. Using a forage legume as part of the rotation also helps to build stocks of soil N. Use of manures also helps to build up and maintain soil structure and CEC. Although there will always be niche local markets for animals produced on small- to medium-sized farms, reversing the national trend to concentrated animal production units and reintegrating animal and crop farms over large areas will require major national political and economic changes. b. Consume Mainly Locally Produced Food Increased reliance on locally produced food has been proposed as a means to better support regional farmers and, it is hoped, create a more viable agriculture that relies less on long-distance shipping to markets. In addition, it will be easier to “close the loop” by returning nutrients to the farms if more products are sold locally and in return local organic waste materials are applied to farmland. Regardless of the extent that locally grown food becomes a greater part of people’s diets, there will always be the need to import certain foods produced only in other regions. c. Transport Nutrients from Urban Areas (Contained in such Materials as Leaves and Sewage Sludge) and from Livestock Production Facilities “Back” onto Farmland This is suggested because it will help build up andor maintain SOM and at the same time help municipalities deal with waste materials. It is also a way of di-

NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS

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verting nutrients from the solid or liquid waste stream and promoting their flow to cycle back to agricultural land. In most cases this would entail long-distance transport of wastes. However, because of the significant costs (and energy use) associated with this practice as well as the concern for toxic material accumulation when sludges are used, long-distance waste transport is probably not a sustainable solution to these problems. d. Reduce the Number of People Living in Cities and Suburbs and Encourage Living in Smaller Communities Close to Farmland As mentioned previously, the concentration of large numbers of people into urban areas creates an overabundance of nutrients in these areas while nutrient-exporting crop farmers must rely heavily on commercial fertilizers to maintain soil fertility. However, the problems associated with nutrient cycling are but one aspect of the issues arising from concentrating large numbers of people in cities and their associated sprawling suburban developments. These other problems include excessive reliance on personal automobiles for transportation (wasting energy and construction materials and labor for highways, gas stations, auto construction, etc.), concentrating social problems in urban areas, etc. By promoting the revitalization of small towns and encouraging a reverse migration back to rural communities, nutrient cycling problems resulting from the separation of people from the land can be more easily resolved.

WII. CONCLUSIONS The purpose of agriculture is to provide food and fiber to people, and when people live removed from the land a flow of nutrients off the farm in the form of harvested products is unavoidable. Cycling from plant residue or animal or human wastes back to soils can never be 100%efficient, even under the most favorable conditions. Thus, external sources of nutrients will, over the long run, continue to be an essential part of agriculture. The efficiency of nutrient use can be controlled through on-farm decisions and changes in soil and crop management will certainly help tighten up the nutrient cycle and build stocks of nutrients to optimal levels. This can foster a decreased reliance on imported nutrients onto farms and reduce environmental damage and resource depletion caused by excess nutrients as well as reduce the use of fossil fuel. However, the control of nutrient flow patterns lies beyond the farm boundaries. Thus, promoting long-term sustainable nutrient management will ultimately require radical changes in the way agriculture and society are organized. These include reintegration of livestock and crop farms and the encouragement of closer physical association of people with farmland. Establishing and implementing per-

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formance expectations for contemporary patterns of nutrient flow is especially difficult because the scale of transactions is broad and control of most aspects is far removed from the farm level. As long as the nutrient cycle is perceived to be only within the boundaries of a farm and the responsibility for its management exclusively within the purview of the farmer, there will be little progress in substantive modification and movement toward a sustainable nutrient flow pattern.

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ADAPTATIONOF PLANTSTO SALINITY Michael C. Shannon United States Department of Agriculture Agricultural Research Service US.Salinity Laboratory Riverside. California 92507

I. Introduction 11. Rationale for Breeding for Salt Tolerance 111. Selection for Salt Tolerance A. Measurement B. Neld and Productivity C. Growth Stage D. Specific Ion Tolerance E. Environmental Interactions W. Salt Tolerance Mechanisms A. Ion Selectivity B. Ion Accumulation C. Osmotic Adjustment D. Organic Solutes E. Water Use Efficiency V. Genetic Variability A. Grains B. Field Crops C. Oil Seed Crops D. Grasses and Forages E. Vegetable Crops F. Fruits, Nuts, and Berries C;. Ornamentals VI. Breeding Methods A. Genes for Tolerance B. Heritability C. Field Screening Techniques D. Selection Methods VII. Novel Concepts A. Tissue Culture B. Molecular Biology C. Modeling VIII. Summary and Conclusions References

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I. INTRODUCTION If life evolved in the sea, and if ancient seas were saline, why then are crop plants sensitive to salt? This somewhat naive rhetorical question is worthy of consideration. Geologic processes contributed to the slow dissolution of the earth’s crust and led to the deposition of significant amounts of sodium, calcium, magnesium, chloride, sulfate, carbonate, and numerous other inorganic compounds into the oceans. Thousands of salt-tolerant plant species, ranging from unicellular algae and diatoms to the giant plankton and seaweeds, inhabit the oceans and seas of the earth. As terrestrial environments arose from the abating seas, new niches were provided for plant exploitation-environments in which wetting and drying cycles occurred. Survival and success under such conditions required root and vascular systems to harvest and transport water, mechanisms to sequester and recycle nutrients to aerial shoots, and tolerance to desiccation. Plants colonizing land areas distal to the oceans were dependent on rain. Those species that could rise above the ground to intercept light from their neighbors had a competitive advantage. This kind of competition may have led to the gradual loss of characters contributing to salt tolerance, a loss that has been exacerbated beginning a few hundred thousand years ago following the discovery of agriculture, when early plant breeders based selections on high growth rates. The art of agriculture eventually led to irrigation. Irrigation practices began about 2500 years ago and have increased dramatically during the past 30 years (Postel, 1989). They have contributed substantially to increases in world agricultural productivity (Rechcigl, 1982; Shannon, 1987). Irrigation has also contributed to increasing salination of agricultural lands and has caused the destruction of agriculture in some areas (Armillas, 1961; Rhoades, 1974; van Schilfgaarde, 1984). It is now estimated that 10% of the world’s croplands are affected by salinity. Of the irrigated lands, as much as 20-27% may be salt affected and up to 37% may be saline, sodic, or waterlogged (Ghassemi et al., 1995). In the United States, 23% of irrigated cropland is saline or sodic and, if both irrigated and nonirrigated lands are considered, saline-affected soils cover more than 19.6 Mha. Salination is inevitably associated with irrigation. Typically, irrigation water contains 0 . 1 4 kg salt m-3 and is applied at rates from 1.O to 1.5 m annually. Thus, from 1 to 60 metric tons of salt per hectare are applied to croplands annually. In order for agriculture to be sustainable, this salt, minus water used in evapotranspiration, must be removed from the crop root zone by leaching and drainage. There is evidence that farmers first dealt with salinity by replacing salt-sensitive crops with more salt-tolerant ones; barley (Hordeum vulgare) replaced wheat (Triticum aestivum) in early Ethiopia (Marr, 1967). Thus, crop substitution was probably used as a method of dealing with salinity long before technologies were developed to leach salts from soils and to avoid salinity problems using various

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management strategies. Substitution of salt-tolerant crop species for sensitive species is still practiced in all the saline growing areas of the world. Some crops, such as sugar beet, barley, cotton (Gossypium spp.), asparagus, sugarcane, and dates, are very salt tolerant; however, climatic, nutritional, and economic needs often require a greater diversity of salt-tolerant crop species than is now available. The improvement of salt tolerance in cultivated plants has only recently attracted the attention of plant breeders and scientists. Vast reclamation projects have been implemented to recover some croplands, but strategies have also been proposed to improve the salt tolerance of crop species not as an alternative to reclamation but as an augmentation to it (Shannon, 1982, 1984; Epstein, 1985). Recently, interests in maintenance of the environment, preservation of natural resources, and a consciousness toward human health and nutrition have placed a new impetus on water quality standards (Ghassemi et al., 1995).These issues, and the occurrence of drought conditions in the western United States during a period when urban water requirements are at an all-time high, have forced severe demands on high-quality water resources (McClurg, 1993). The use of recycled water, drainage water, or other poor-quality water on crops with improved salt tolerance has been proposed as part of the solution to some of these problems (van Schilfgaarde and Rhoades, 1979). Thus, within the past 15 years, interests have gradually increased to improve the salt tolerance of both crop and ornamental plant species. Four major strategies have been proposed; (i) to gradually improve the salt tolerance of crops through conventional breeding and selection; (ii) to introgress crops with their wild progenitors, which may already possess salt tolerance; (iii) to domesticate wild species that currently inhabit saline environments (halophytes) by breeding and selection for improved agronomic characteristics; and (iv) bold strategies have been suggested in which genes for salt tolerance can be identified, cloned, and manipulated across conventional genetic barriers using the advanced techniques of molecular biology.

11. RATIONALE FOR BREEDING FOR SALT TOLERANCE Crops are generally selected for marketability and how well they fit the environment and rotational system. Specific varieties are selected for resistances and tolerances to local diseases and environmental stresses. Where soil or water salinity is a potential problem, the crop salt tolerance is an important determinant. Conventional selection and breeding can be used to improve salt tolerance in both sensitive and tolerant crops. Increased salt tolerance in sensitive species has great economic potential beyond the improvement of yield in moderately saline areas. It provides the farmer greater flexibility for crop selection when markets for the

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limited number of salt-tolerant crops are saturated. Additionally, in areas where reclamation is required through leaching, relatively small increases in salt tolerance may greatly reduce the leaching requirement. Leaching efficiency decreases, as measured by percentage decrease in soil salinity per unit water applied, depending on the soil salinity that can be accepted (Hoffman, 1980). Improved salt tolerance in sensitive and tolerant crops would allow more extensive use of brackish water supplies-an especially important consideration where water costs are high or water availability is low. The development of new crops that have a high degree of salt tolerance is also a tenable strategy. Jojoba, although not too salt tolerant, is a high cash value crop in areas unsuited to conventional farming practices. Other new crops, such as Atriplex, Salicornia, and Disfichlis, have high salt tolerance but have not yet emerged as widely accepted crops (Glenn and O’Leary, 1985; Glenn e f al., 1991 ; Yensen, 1988). New crops could be produced with recycled or reclaimed water that is no longer suited for municipal purposes or irrigation on conventional crops. However, even this strategy faces competition; reclaimed municipal water is now commonly used to irrigate golf courses and parks. Consequently, there is a corresponding interest in maintaining the quality of municipal water, which has led to restrictions on the use of water softeners and general water quality guidelines for sewage. Clearly, water quality has become an important socioeconomic issue and conflicts between water use for agriculture versus municipal and industrial purposes will continue to increase. All these potential advantages draw attention to the fact that too little progress has been made in improving salt tolerance of crops. However, in a recent review, Flowers and Ye0 (1995) conclude that salinity has not yet become a significant agricultural problem to warrant an intensive breeding effort. There is currently a need to develop new direction and cohesive impetus in the area of salt-tolerant crops development. Realistic short- and long-range goals need to be established and continually pursued. The information that we now have is extensive but fragmented. The purpose of this chapter is to outline some of the foremost issues and strategies concerning selection and breeding for plant salt tolerance, to highlight crucial advances in knowledge in recent years, and to identify some of the fundamental gaps in our understanding.

In. SELECTION FOR SALT TOLERANCE Salt tolerance is a complex, quantitative, genetic character controlled by many genes. A few of these genes have been identified and provide information that can be useful in screening and selection programs (Shannon, 1996; Shannon and Noble, 1990).Information is lacking on how most genes function in concert with other genes that may have influenced the mechanisms of salt tolerance.

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A. MEASUREMENT To be able to improve salt tolerance, we must first be able to measure it in a meaningful way. Plant salt tolerance is generally thought of in terms of the inherent ability of the plant to withstand the effects of high salts in the root zone or on the plant’s surfaces without a significant adverse effect. Salt resistance is another term that is often used for this phenomenon, and although some have tried to differentiate the two terms (Levitt, 1972), the terms are used interchangeably. In an agronomic context, salt tolerance is described as a complex function of yield decline across a range of salt concentrations (Maas and Hoffman, 1977; van Genuchten and Hoffman, 1984). Using a simple convention, salt tolerance can be measured on the basis of two parameters: the threshold (EC,), the salinity that is expected to cause the initial significant reduction in the maximum expected yield (Y,,,), and the slope (s) (fig. 1). Slope is simply the percentage of yield expected to be reduced for each unit of added salinity above the threshold value. Relative yield ( Y ) at any salinity exceeding EC, can be calculated as Y = 100 - s(ECs - EC,),

(1)

where ECe > EC,. Usually, salinity is measured in units of electrical conductivity of a saturated soil paste extract (EC,) taken from the root zone of the plant as averaged over time and depth. Soil paste extracts are soil samples that are brought up to their water saturation points. Electrical conductivities are measured on the filtered water extracts from these samples in units of decisemiens per meter (dS m-’), or previ-

s

90

9 607 0 5n 5

t

/”

Typical Salt Tolerance Curve Threshold (t) = 2 dSlm Slope ( 8 ) = 7 14

60-

p

.-5

2

5040-

3020 10

-

0-

Figure I Typical salt tolerance graph depicting the threshold ( 1 ) and slope (s)parameters. Threshold is defined as the salinity at which yield decline is significantly reduced relative to nonsaline conditions. Slope is a function of the amount that yield is reduced by salinity beyond the threshold.

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ously as millimhos per centimeter. New methods use electronic probes or electromagnetic pulses to calculate ECe with less time and effort (Rhoades, 1976; 1993a). Reliable data to describe the salinity functions can only be obtained from carefully controlled and well-replicated experiments conducted across a range of salinity treatments. In order to provide information to growers concerning the potential hazards of a given saline water or soil, data of this type have been compiled for 127 crop species, which include 68 herbaceous crops, 10 woody species, and 49 ornamentals (Mass, 1986, 1990). Thus, crop substitutions can be made if the potential hazards indicate that expected yield reductions may be economically disasterous. A brief examination of the threshold and slope parameters gives an indication of the potential range in variability that is found among the major domesticated plant species. Although the information that comprises this database is considered to be reliable, it is significant that multiple varieties were examined in trials for only 28 of the species. Clearly, the variability for salt tolerance based on yield criterion has not been adequately explored.

B. YIELDAND PRODUCTMTY Unfortunately, traditional measurement of salt tolerance as just described is not directly applicable to selection methods. One component of the measurement, the threshold or the salt concentration at which yield decline begins, is highly sensitive to environmental interaction and is dependent on both the accuracy of salinity measurements and the method by which they are integrated over plot area, depth, and time. Because of this, there is a degree of error in evaluating the slope at salt concentrations near the threshold. At the highest salt concentrations, there is a tendency for the slope to “tail-off.’’ This results in added uncertainty at this part of the curve. For agronomic purposes, salt tolerance at high salinities has little economic importance but the merit of selecting for tolerance at these salinities has not been thoroughly evaluated. It has been speculated that the physiological and genetic factors that contribute to the growth of glycophytes at very high salt concentrations may be proportionally related to survival more than to high yields and probably are not of interest to the grower except in cases of subsistence agriculture at the most meager level (Shannon and Noble, 1990).Among glycophytes, genetic variance is usually lower at these high salinities than at lower salinities. The fundamental selection criteria in plant breeding are mean yield and yield stability across environments. Richards (1983, 1995) indicates that because of the heterogeneity of saline soils it is best to select for productivity rather than salt tolerance. When the genetic correlation for yields across environments is highly negative and genetic variance in the stress environment is less than that in the nonstress environment, selection for productivity will normally increase yields in both environments (Rosielle and Hamblin, 1981). Alternatively, if it can be shown that there is some capacity for selection under a particular stress environment, i.e., ge-

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netic variance is high compared to that under nonstress, tolerance might be improved without a concomitant yield decrease in a nonstress environment. These principles were demonstrated by Johnson er al. (1992), who found that selection for increased yield in alfalfa (Medicago sariva) was effective under low and moderate salinities but not under nonsaline conditions. Salt tolerance ( S ) can be described as a reduction in yield at a given salinity (Y,) with respect to a measured yield under nonsaline conditions (Y,):

s = YJY,. This index may change with the degree of the salinity stress that is imposed. Another index for stress was proposed by Fischer and Maurer (1978) that allows comparisons where the mean population relates the ratio of yield under stress and nonstress conditions to the ratio of the means of all genotypes under stress and nonstress conditions ( D is the mean of all genotypes under stress/mean of all genotypes in a nonstressed environment):

One of the difficultiesin making selections for salt tolerance is that low-yielding varieties seem to be proportionately less sensitive to the effects of salinity than are high-yielding varieties. It has been previously found that there is a negative correlation between high mean yield and phenotypic stability across environments (Finlay and Wilkinson, 1963; Frey, 1964). Selection for salt tolerance under the wrong conditions or using the wrong genetic material can result in low-yielding selections that are not competitive with higher yielding, nontolerant varieties (Richards, 1983). Thus, salt-tolerant lines selected on the basis of Eqs. (2) or (3) may be lower yielding lines at low to moderate salinities. This almost universal phenomenon may be due to the fact that high-yielding lines are nearing their capacity to divert as much of their assimilated carbon to yield potential as possible, whereas, lowyielding lines may still retain some mechanisms for stress adaptation. In addition, certain complications are inherent in the measurement of salt tolerance for purposes of screening. As noted, assessments for tolerance as measured in Eqs. (2) and (3) cannot be made on single plants in a segregating population because information must be collected on a relative basis. How well a plant grows under saline conditions depends on both salt tolerance and vigor. Comparisons between performance under control and saline conditions can be made on genetically segregating material only if progeny lines with some degree of homozygosity are established first. This is a time- and labor-consuming process. Because of the difficulties in accurately measuring salt tolerance, indices other than yield have been suggested for breeding work. These include tolerance during germination; conservation of shoot dry weight, root weight, or shoot number; resistance to leaf damage; maintenance of flowering, seed and fruit set, leaf size,

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canopy volume, or quality; and plant survival under salt stress. The selective value of these assessments depends on the agronomic situation and will be discussed more thoroughly in a later section. Other indices of tolerance have been proposed that are based on specific physiological characters; for instance, specific accumulation of an ion in shoots or leaves or the production of a metabolite. No such criteria have been unequivocally correlated with salt tolerance, but some, as will be discussed, have higher degrees of correlation than others. The value of any parameter undeniably depends on species and, in certain cases, varieties.

C. GROWTHSTAGE Another nuance associated with assessment and measurement of salt tolerance is variation with ontogeny or growth stage (Lunin et al., 1963). Rice (Oryza sutiva),for example, is sensitive during the early seedling stages and at flowering (Akbar and Yabuno, 1977), sugar beet is tolerant during later growth stages but is sensitive during germination (Beatty and Ehlig, 1993), and corn (Zea mays) is tolerant at germination but is more sensitive at seedling growth than for ear and grain yield (Maas et al., 1983). Efforts to evaluate salt tolerance in a species on the basis of tolerance during germination and emergence have not generally been successful; tolerance at one growth stage usually is not related to another. Salinity often affects the timing of development. In wheat, sorghum (Sorghum bicolor), and oats, ear emergence, anthesis, and grain maturity occur earlier under saline conditions, whereas, in barley and rye maturity is unaffected by salinity (see Shannon et al., 1994). In cotton, flowering occurs earlier under salt stress, but salinity delays flowering of tomato, Lycopersicon esculentum (Pasternak et al., 1979). Yield components and growth parameters also show differential responses to salinity stress. Ayers et u1. (1952) found that in barley and wheat seed production was decreased less than shoot dry weight (wt) by salinity. Likewise, at low salinities root growth is often less affected, or sometimes even stimulated by salinity, compared to shoot growth. In muskmelons, salt tolerance decreased in the following order: total vegetative dry wt > total vine yield > fruit yield > marketable yield (Shannon and Francois, 1978). Consequently, the degree of salt tolerance between and within species is likely to vary according to the criteria used for evaluation. In a review, Jones and Qualset (1984) assert that plant growth attributes must be measured throughout the growth period so that particularly salt-sensitive growth stages can be identified. Because of the differences in salt tolerance between growth stages, some investigators have resorted to selection for tolerance by imposing salt stress over the entire growth cycle (Epstein et al., 1980). However, if a constant salt concentration is used in this strategy, the degree of selection pressure will vary with growth stage. For some species, independent selection at more than one growth stage may be appropriate. This would permit the development of lines with optimal tolerance

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at each specific growth stage followed by a crossing program to combine these tol-

erances into a single variety. In some agricultural situations, selection for salt tolerance at only one growth stage may have a significant benefit. For instance, sugar beet is very salt tolerant during vegetative growth stages but is sensitive to salinity during germination (Bernstein and Hayward, 1958) and selection during this stage could remove a limiting step to tolerance throughout its growth. For a large number of crops, adequate information is not available concerning salt sensitivities during development. Sometimes salt tolerance at specific growth stages may be used to advantage. Moderate salinity applied during fruit development can change the partitioning of photosynthates and improve soluble solids in melon and tomato (Shannon and Francois, 1978, Mizrahi and Pastemak, 1985; Mizrahi et al., 1988). Any small yield decrease due to salinity is offset by the higher marketable quality of the fruit. Some grain crops, such as sorghum, wheat, and barley, are extremely insensitive to relatively high concentrations of saline water applied during orjust prior to anthesis (Maas et al., 1986; Maas and Poss, 1988). Such tolerance could be exploited by substituting brackish water for irrigation water during later plant growth stages. This strategy has been used successfully for both field and vegetable crops (Rhoades, 1986; Grattan et al., 1987). Genetic variation for salt tolerance at specific growth stages has not been adequately examined.

D. SPECIFIC ION TOLERANCE The relative salt responses of various crops is often dependent on soil type and other environmental factors (Levitt, 1972).Saline soils and waters include those with high concentrations of dissolved salts of many kinds, any of which may be critically limiting to plant growth. Saline soils may be sodic or acidic and cover a wide range of soil types and moisture conditions. Genotypes that show similar salt tolerance in one environment may differ in response in a different environment. Rana (1985) has cited the complexity of soils and environmental interactions as major obstacles to successful breeding for salt tolerance. He noted that crops adapted to alkali soils are usually tolerant of nonalkaline saline soils, but the converse was not true. Most salt tolerance data have been collected based on the effects of saline waters predominated by sodium chloride, sometimes with varying amounts of calcium added as needed to avoid the development of soil permeability problems associated with soil sodicity. However, specific ion sensitivities may be critically limiting to crop growth in some geographic locations. For example, iron, aluminum, boron, selenium, arsenic, manganese, or zinc may be found in toxic or growth-limiting concentrations in certain areas. Drainage waters or waters reused from agricultural processing or manufacturing operations may have high concentrations of boron, selenium, arsenic, or other ions that may pose environmental hazards (Francois and Clark, 1979a; Clark, 1982). Plant species have demonstrat-

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ed a wide degree of variation in their abilities to accumulate, exclude, or withstand the toxic effects of individual ions (Shannon et af., 1994; Flowers and Yeo, 1986). Even so, the potential for variability between species and varieties remains as one of the research areas that has not been adequately explored. The genetic variability associated with plant tolerance to these ions has been reviewed in detail (Epstein, 1963; Vose, 1963; Epstein and Jefferies, 1964; Lauchli, 1976; Wright, 1976; Jung, 1978; Christiansen and Lewis, 1982).

E. ENVIRONMENTAL INTERACTIONS Identificationof a quantitative character is difficult at best, and the interactions between salinity and other other environmental stresses complicate accurate assessments using yield or growth as an index of tolerance. Important environmental factors that show significant interaction with salinity include temperature, wind, humidity, light, and pollution. High temperatures and low humidities may decrease crop salt tolerance by decreasing the effective value oft in Eq.(1) and increasing the value of s. Thus, significant reductions in yields will be realized at lower salinities, and yields will decrease more rapidly with increasing salinity under hot, dry conditions. Two other environmental factors that can influence the measurable effects of salinity include elevated atmospheric levels of carbon dioxide and ozone. Salinity causes leaf stomata to restrict the volume of air exchanged with the environment.This usually improves plant water use efficiency somewhat but reduces the amount of carbon dioxide that can be fixed by the plant and be used for growth. High carbon dioxide concentrationsin the air due to the so-called “greenhouse effect” may, in part, offset the reduction in air exchange. However, if pollutants, such as ozone, are present, reductions in air exchange may also reduce the volume of pollutants that enter the plant, thereby decreasing any adverse effects of salinity (Mass and Hoffman, 1977). Root zone waterlogging is another environmental hazard that can be exacerbated by salinity. Root zone salinity and waterlogging greatly increase salt uptake compared with nonwaterlogged conditions (West, 1978; West and Taylor, 1984). Salt tolerance in saline, drained conditions can be quite different from that in saline, waterlogged conditions.

W. SALT TOLERANCE MECHANISMS Salinity exerts complex effects on the plant as a result of ionic, osmotic, and nutritional interactions, although the exact physiological mechanism of salt stress is unknown. Salt tolerance often depends on the anatomical and physiological complexity of the organized plant. This fact makes it difficult to find ways to increase

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salt tolerance to large degrees. However, it does give hope that salt tolerance can be increased by finding the factor that is most limited by salt stress during growth and development. Several investigators have demonstrated salt tolerance mechanisms based on factors such as ion accumulation (Rush and Epstein, 1976, 1981b; Tal and Shannon, 1983), ion exclusion (Abel, 1969; Noble et al., 1984), compatible solute production (Grumet and Hanson, 1986; Wyn Jones et d., 1977), late maturation (Bernal et al., 1974), and pollen sterility (Akbar and Yabuno, 1977; Akbar et al., 1972). Some investigators have suggested that several of these factors can be selected and combined in a reengineered individual, a process referred to as pyramiding characters (Pasternak, 1987; Ye0 and Flowers, 1983).

A. ION SELECTIVITY Salt sensitivity in some crops has been attributed to the failure of plants to keep Na+ and C1- out of the transpiration stream and, consequently, the cytoplasm of the shoot tissues (Flowers et al., 1977; Harvey, 1985). Under salt stress a plant must absorb nutrients and restrict the uptake of toxic ions at lower water potentials than usual. Munns and Termaat (1986) divided salt stress into short- and longterm effects. Short-term effects occur in a matter of days and involve decreased shoot growth, possibly as a result of the root response to water deficit. Long-term effects occur over weeks and result in maximum salt loads in fully expanded leaves and a reduction in photosynthetic activity. Flowers and Ye0 (1986) noted that salt damage in leaves of sensitive species may be the result of excess apoplastic ion concentrations or ion toxicity effects on metabolic processes in the symplast. Plants that limit uptake of toxic ions and maintain normal ranges of nutrient ions could be more salt tolerant than those that do not restrict ion accumulation and lose nutrient balance. Selective ion uptake mechanisms capable of discrimination between chemically similar ions such as Na+ and K+ could have adaptive value. The mechanisms responsible for ion discrimination probably are located in the membranes of tissues and various organelles throughout the plant (Bliss et al., 1984; Kuiper, 1968). Breeding for efficient nutrient uptake or low ion accumulation under salt stress may be among the simplest ways to improve salt tolerance in sensitive varieties of some species. This also may be accomplished by finding tolerance to the toxicity of a specific ion associated with salt stress. Munns et al. ( 1988) concluded that high salt concentrations in the phloem of the salt-sensitive Lupinus albus are not directly related to either growth reduction or leaf injury but are, more important, a symptom of disrupted regulation of ion transport properties in the root. Among plant species, mangroves undoubtedly have the most efficient system of restricting salt uptake through the development of a passive root membrane filtration system. The gray mangrove (Avicennia marina) can

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exclude 90% of the salt in the medium surrounding its roots (Burchett et al., 1984). It has maximal growth at 25% seawater. Other mangrove species can survive salt concentrations two or three times that of seawater (Clough, 1984). The system in mangrove is unique and, unfortunately, has not been reported in other species; most crop species limit salt uptake into the transpiration stream to some degree through membrane-mediated compartmentation in organelles (vacuoles) or tissues (Shannon, 1997). Some species may be able to rid themselves of ions through ionsequestering organelles (salt glands) or by storing salt in the root, old leaves, petioles, stems, or tracheids (Jacoby, 1964). Salt restriction from the cytoplasm is not complete; the plant will eventually succumb to salt unless its growth rate is high enough that its salt storage sinks do not become filled, thereby preventing salt overflow into sensitive tissues (Flowers and Yeo, 1986). Selective ion transport differences among species and varieties are the result of specific gene differences (Vose, 1963; Epstein and Jefferies, 1964). The genetic variations that may occur in each of these systems are numerous and little research has been done to evaluate the extent of that variation.

B. IONACCUMULATION Restriction of ions into roots or shoots is one of the most frequently reported differences between salt-tolerant and -sensitive varieties. It is well known that halophytes take up substantially high concentrations of ions as an adaptation to saline environments (Flowers et ul., 1977); however, some can sequester toxic ions not only in vacuoles but also in specialized organs such as salt glands and bladders (Levitt, 1972; Schirmer and Breckle, 1982). The accumulation of salt in the plant or its excretion onto leaf surfaces is believed to reduce the requirements for increased wall extensibility that might otherwise be required to maintain positive growth and turgor at low soil water potentials. The wild tomato species (Lycopersicon cheesmunii) is considered to be more salt tolerant than the cultivated species due to its capacity to accumulate ions (Rush and Epstein, 1981b), and the salt-tolerant “Edkawy” tomato also accumulates higher concentrations of Na+ in leaf tissues than does more sensitive cultivars of L. esculentum (Hashim et al., 1986).As with salt restriction, salt accumulation within tissues is believed to be well regulated and generally sequestered away from cytosolic compartments containing the salt-sensitivemetabolic machinery of the cell. In both glycophytes and halophytes, salt may accumulate preferentially in vacuoles, interstitial compartments, stems, or older leaves. The physical and genetic factors that influence ion compartmentation and distribution within plants are mostly unknown. Only a few crop species, e.g., sugar beet, are halophytes. It may not be practical to attempt to transfer halophytism into glycophytic crop species. However, several investigators have shown an interest in developing the agronomic potential of wild halophytes into new and useful salt-tolerant crops.

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C. OSMOTIC ADJUSTMENT Osmotic adjustment is a decrease in plant osmotic potential through an increase in solute content (or a decrease in water content) in response to a decrease in external water potential to the extent that turgor potential is maintained. There is some controversy whether osmoregulation even occurs in higher plants (see Munns and Termaat, 1986). Nevertheless, substantial differences in their capacity for osmoregulation have been noted among wheat genotypes (Morgan, 1977). High humidities improve the tolerance of corn, bean, onion (Allium spp.), radish, and barley but not of cotton, wheat, and red beet (Gale et al., 1967; Hoffman et al., 1971; Hoffman and Rawlins, 1971; Hoffman and Jobes, 1978; Prisco and O’Leary, 1973). The relative sensitivity of crops to osmotic stress may vary with external salt concentrations. This may indicate that certain crops may benefit from selection pressures, which improve their capacity to adjust osmotically or maintain more favorable water relations under salt stress (Tal and Gardi, 1976; Shannon et al., 1987). Generally, poor osmotic adjustment leads to turgor loss and stomata1 closure, which is soon followed by reduced gas exchange and photosynthesis. Turgor loss, in turn, can also have detrimental effects on cell division and elongation.

D. ORGANIC SOLUTES Sugars, proline, glycinebetaine, and other organic solutes are believed to improve salt tolerance by contributing to osmotic balance and preserving enzyme activity in the presence of toxic ions (Greenway and Munns, 1980; Grumet et al., 1985; Tal et al., 1979). Rathert (1984) noted that salinity causes greater leaf sucrose increases in salt-sensitive species than in tolerant species. He suggested that leaf sucrose and starch concentrations could be used as a selective index in screening for improved salt tolerance. High betaine genotypes of barley maintained lower solute potentials than near-isoline, low-betaine genotypes grown at the same salinities (Grumet and Hanson, 1986). This also suggests that betaine could be used as a selection index for improved salt tolerance, although these characters alone may prove to be inadequate criteria if other salt-tolerant characteristics are not maintained.

E. WATERUSEEFFICIENCY Other mechanisms that could prevent turgor loss and better water efficiency are increased leaf resistance (fewer stomata, increased mesophyll resistance, and increased cuticle thickness) or a higher root-shoot ratio. Plant diversity provides evidence that each of these strategies may be accomplished in various ways. Most of

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these strategies, however, are associated with some aspect of growth and metabolism that is detrimental to maximum crop production. Most measurements of water relations are not accurate or reliable enough to be useful in screening techniques for salt tolerance. Future advances in instrumentation and more thorough understanding of water relations mechanisms may some day improve the breeder’s ability to select genotypes based on the maintenance of optimum water relations during salt stress.

V GENETIC VARIABILITY One requirement for breeding for salt tolerance is that genetic variation exists for the character in the gene pool. Such variation may be between individuals, varieties, or even species that have some degree of sexual compatibility so that genes may be transferred from one individual to another. Another necessity is that salt tolerance can be identified in segregating generations or that specific information exists concerning its genetic control in terms of numbers of genes involved and their heritability. Although considerable research has been devoted to quantifying the salt tolerance of various crop species (Francois and Maas, 1978, 1985; Maas and Hoffman, 1977; Maas, 1985, 1987), data for many species are usually based on comparisons among only a few varieties. In studies that have examined a range of varieties, some species exhibit wide intraspecific variation of salt tolerance, whereas others have limited variation. Usually, only a relatively small portion of the existing germplasm base has been screened. Many wild progenitors of cultivated species have not been tested.

A. GRAINS Grain crops include both tolerant (e.g., barley) and sensitive (e.g., rice and corn) species (Fig. 2). There are many examples in which salt tolerance has been indirectly developed in varieties selected for high yield in naturally saline environments. Some wheat, barley, cotton, and rice varieties developed primarily for high yield in saline regions of Pakistan, India, Egypt, and the United States have better salt tolerance than varieties developed in nonsaline areas (Akbar et al., 1972; Bernal et al., 1974; Kingsbury and Epstein, 1986). For example, individual plants selected directly from fields in the Kharchi-Pali area of Rajasthan led to the development of the salt-tolerant Kharchi-Rata wheat line (Rana, 1986). Other wheat varieties in which salt tolerance has been demonstrated include Sakha-8 (Egypt), LU26s (Pakistan), and SARC-1 (Pakistan). Measurement of salt tolerance in wheat varies with growth stage (Srivastava and Jana, 1984; Ashraf and McNeilly, 1988).

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Figure 2 Comparative differences in salt tolerance among a number of grain crops. Values in parentheses represent the threshold and slope values for each species (t. s). Data serve only as a guideline to relative tolerances and may vary depending on climate, soil conditions, and cultural practices.

Barley is one of the most salt-tolerant grain and forage crops. Salt tolerance in varieties such as CM67 and Albacete have been correlated with their abilities to exclude Naf from the shoot (Royo and Aragues, 1993; Wyn Jones and Storey, 1978). Extensive screening for salt tolerance in wheat and barley has been conducted among thousands of accessions of the world collections (Kingsbury and Epstein, 1984). Breeding and selection efforts in wheat involve crossing the cultivated hexaploid species with diploid wheat or closely related wild relatives in an effort to improve the variability for salt tolerance (Dvoifik et al., 1985; Dvoihk and Gorham, 1992; Dubcovsky etal., 1996; King et al., 1996). Wheat collections have been extensively screened for salt tolerance. Approximately 9% of 5000 hexaploid, tetraploid, and diploid accessions of wheat and triticale survived to the seedling stage in pots irrigated with the equivalent of 50% seawater (Sayed, 1985). In solution cultures, 29 accessions from more than 5000 entries of spring wheats grew to seed set at 50% seawater concentrations (Kingsbury and Epstein, 1984).Among 400 Iranian tetraploid and hexaploid accessions, high grain yield under salinity stress (see Eq. (3)) was found to be a better criterion for salt tolerance than biomass, harvest index, or relative salt tolerance (Jafari-Shabestari et al., 1995). With respect to yield parameters, tolerance has shown a high coefficient of correlation with grain number per ear (Singh and Rana, 1985), but Maas et al. (1996) reported that the loss of spike-bearing tillers accounts for most of the yield reduction with salinity. Water use efficiency or photosynthetic capacity as measured by carbon assimilation rate are only two of the parameters in wheat that have not been

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shown to be reliable measures of discrimination for salt tolerance. However, a relationship has been shown between tolerance and high K+/Na+ in grain and straw (Ashraf and O'Leary, 1996; Chhipa and Lal, 1995), and salt-tolerant varieties such as Kharchia and Sakha-8 were found to have lower concentrations of leaf Na+ than sensitive varieties (Salama et al., 1994). Sorghum has intermediate salt tolerance and is more tolerant at germination than during later growth stages, and vegetative growth is less affected by salinity than yield (Francois et al., 1984). Seedling response in two studies, one with 48 cultivars and another with 51 accessions, indicated that significant variation in salt tolerance as measured by dry weight increase exists and that improvement through recurrent selection should be possible (Taylor el al., 1975; Azhar and McNeilly, 1987).Varieties that were most tolerant included Desert Maize, Shallu, and Hegari in one study, and Double-TX, INRA 383, PAK SS 11, and Red Jampur in the other. Previously, Double-TX was selected for salt tolerance (Francois er al., 1984). Genetic variation for osmotic adjustment was found among 10 inbred lines (Basnayake et al., 1994). Variation was attributed to more than a single gene and both GCA and SCA were found to be significant. It has been shown that higher levels of salt tolerance can be induced in sorghum by certain pretreatments. Pretreatment of seedlings with 75-150 mol mP3 NaCl for 20 days resulted in plants that could withstand 300 mol mP3 NaCl, whereas controls without pretreatment did not survive (Amzallag et d., 1990). Corn is more sensitive to salinity during the seedling stage than during germination or later growth stages, although genetic variation exists at all stages (Maas et al., 1983). No clear relationship has been found between salt tolerance at different growth stages. Maize responds primarily to osmotic, rather than ionic, effects (Kumar and Singh, 1982; Cramer et al., 1994). A natural recurrent selection program using saline imgation water (2000-4000 mg liter- I total soluble salts) over a period of 20 years was used to develop Arizona 8601 maize (Day, 1987). This maize outyielded Pioneer 3183 in both silage (+38%) and grain (+26%) production under the conditions in which it was selected. In salt-sensitive rice (0. saliva), injury is directly related to the toxic effects of NaCl in the shoot tissues that cause leaf burn. Salt-tolerant genotypes regulate both the absorption and the translocation of ions better than sensitive genotypes (Yan et al., 1992). Interactive factors that influence salt tolerance include the sensitivity of the variety to Na+-induced Ca2+ deficiency and the capability of the variety to maintain shoot K+ levels against high concentrations of Na+ in the root zone (Grieve and Fujiyama, 1987). Transpirational bypass flow has been found to be a major contributing factor in sodium transport and is a heritable trait (Yadav et ul., 1996). In plot and field trials based on yield, Giza-159 rice was selected for salt tolerance in Egypt from a cross between the salt-tolerant Agami 1 and the locally adapted Giza 14 (Gad El-Hak, 1966). In subsequent efforts to further improve tolerance, Giza I59 was crossed with the salt-tolerant variety Reiho. Recently, a good correlation among varieties was reported between high paddy yield and shoot

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weight after 15-day-old seedlings were grown for 15 days in solution cultures containing 100 m M NaCl (Aslam et al., 1993). Most studies report that salt tolerance at different growth stages in rice is not well correlated to yield under saline conditions. Despite the limited number of released varieties specifically bred for improved salt tolerance, breeding efforts to produce such varieties are rapidly expanding. There are many studies under way that may result in more salt-tolerant crops in the near future. The International Rice Research Institute (IRRI) has screened more than 10,000accessions of rice for salt tolerance in greenhouse tests and has identified many promising varieties (IRRI, 1977).

B. FIELDCROPS This category of crops covers a wide range of salt tolerance, from the salt-tolerant cotton and sugar beet to the sensitive bean and peas (Fig. 3). Cotton is very salt tolerant, although it is more sensitive during germination and emergence (Francois and Bernstein, 1964; Ghorashy et al., 1972). Screening for tolerance at germination and selection was conducted on a number of Egyptian cotton cultivars and Bahtim 110, Menoufi, Ashmouni, and Bahtim 108 were shown to be most tolerant (Ashour and Abd. El-Hamid, 1970). Gossypium barbadense, G. herbaceum, and G. neglectum are generally more tolerant than G. hirsutum; of 37 American cotton varieties (G. hirsutum), only 7 were tolerant (Novikov, 1943). Sugar beet (Beta vulgaris) is very salt tolerant. Iyengar and Pandya (1977) stud-

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ied salt tolerance among 20 varieties representing a range of germplasm. irrigation with seawater dilutions equivalent to 22,000 mg liter-' in sand reduced tuber yield in only half of the entries. Sugar content was only slightly reduced in the tolerant entries. The varieties U.S.H.9, Maribo Magnapoly, and Kawep Precopoly were the most tolerant. Potato (Solanurntuberosurn) is moderately sensitive to salinity (Ahmad and Abdullah, 1979). Variability for tolerance exists among varieties and wild relatives (Arslan et al., 1987; Bilski et al., 1987), but little effort has been made to improve tolerance in this species. A close correlation has been observed between salt stress response of cultured roots and that of whole plants (Naik and Widholm, 1993). In field screening trials ion sites in which poor leaching had failed to remove excessive KCl fertilizer, Parker et al. (1 986) evaluated chloride-induced leaf scorch in 65 varieties of soybean (Glycine rnux) in four maturity groups. As noted, susceptability to C1- damage is the result of a single gene. However, it has been shown that an interaction exists between Cl--induced injury and high inorganic phosphate concentrations above 0.12 mol m-3 in solution culture (Grattan and Maas, 1984, 1988). In addition, a mixture of NaCl:CaCl, was found to be more toxic than NaCl salts at the same osmotic potential because of the added proportion of C1-. These observations are critical in designing screening studies with soybean, especially under controlled conditions in hydroponics and sand cultures. Interactions between phosphate and C1- also have been reported in sesame (Sesurniurn indicurn) and corn (Bernstein et al., 1974; Cerda et al., 1977). Salt tolerance comparisons among a number of salt-sensitive, cool-season food legumes has shown that faba bean is more tolerant than lentil and chickpea, rated in both germination and yield tests (Saxena et al., 1994). Lentil and faba bean are more sensitive at germination than at later growth stages, but the converse is true for chickpea. Screening for tolerant genotypes had been done in both field and controlled conditions based on yield and visual symptoms.

C. OILSEEDCROPS Canola (Brussica spp.) is a derivative of rapeseed (Brassicanapus) that has low saturated fat and, hence, has increased in importance among the oil seeds (Fig. 4). This species is very salt tolerant and in salt-tolerance trials did not exhibit yield decline until soil salinity exceeded 10 dS m-' in the saturated soil extract. Yield declined at a rate of only 11.2% per unit increase in salinity above this threshold (Francois, 1994). Oil content and protein content in the oil-free seed meal were not affected by salinity. Safflower (Carfhamustinctorius) is less tolerant than sugar beet but more tolerant than the legumes. Like cotton, it is more sensitive during germination and seedling growth than during later growth stages (Abul-Naas and Omran, 1974). In

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safflower, high salinity does not affect the fatty acid composition of the oil in standard varieties but does decrease oleic acid in a high-oleate variety (Yermanos et al., 1964; Irving et al., 1988). Safflower has a high threshold salinity (approximately 7.5 dS m- I ) and a yield decline of approximately 6 dS m-' (Francois and Bernstein, 1964). Little variation was noted among 11 cultivars (Irving et al., 1988). Sunflower (Helianthus annuus) is moderately tolerant to salinity; yield is unaffected by salinities up to 4.8 dS m-' ( t ) in the saturation soil extract and declines by approximately 5% per unit increase in salinity thereafter (Francois, 1996). Yield decreases were attributed to reductions in the number of seeds per head, but no significant genetic differences were found among four varieties. In a more extensive study for varietal differences, 45 accessions were evaluated for seedling growth in salinity trials, and HO-1, Predovik, and Euroflor were found to be the most tolerant (Ashraf and Tufail, 1995). A wild relative of sunflower, H. paradoxus, has been found to be more tolerant at the seedling stage by virtue of a single dominant gene, Sa,, but a modifier gene may also be involved (Miller, 1995). This work could lead to an increase in salt tolerance in progeny derived from interspecific crosses. Another oil seed crop, linseed (Linurn usitatissimurn), by comparison, is more sensitive to salinity. Based on screening tests conducted on 36 accessions, threshold values for two sensitive genotypes (P-70 and Aver-Rec-Seic) and two tolerant genotypes [T-5(69-S9)Band LS-21 were calculated to be 0 and 2 dS m-I, respectively (Ashraf and Fatima, 1994). Above the threshold, yield declined at a rate of approximately 10 dS m- I .

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D. GRASSES AND FORAGES Some grasses are sensitive to salinity, but many, such as Bermuda grasses and wheatgrasses, have high tolerance (Fig. 5). Growth rate, leaf chlorosis, and recovery rates were found to be useful indices for screening 33 accessions of wheatgrass (Shannon, 1978). Hybridization of two salt-tolerant lines indicated that tolerance was an additive character (Weimberg and Shannon, 1988). The desert wheatgrass (Agropyron desertorurn) variety Nebraska 10 was selected as a salt-tolerant strain in artificially salinized field trials (Dewey, 1962). Variation for salt tolerance

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among Bermuda grasses (Cynodon spp.) has been demonstrated (Dudeck et al., 1983; Francois, 1988). At high salinities Na+ increased in shoots and K + decreased, despite the fact that Bermuda grasses have salt glands (Breckle, 1992). Among five Kentucky bluegrasses (Poa pratensis), Adelphi and Ram I were shown to be significantly more tolerant to NaCl than other cultivars tested (Torelle and Symington, 1984). Seedling root length in NaCl solution cultures was used as an indicator to determine heritability estimates and demonstrate that improvement in salt tolerance could be made in several grasses including Lolium perenne, Dactylis glomerata, Agrostis stolonifera, A. castellana, Puccinnellia distans, and Festuca rubra (Ashraf et al., 1986a,b). An ecotype of red fescue (Festuca rubra), selected from a tidal marsh, has been developed into the variety Saltol for use in the revegetation of roadsides that are affected by deicing salts (Cordukes, 1981). Alfalfa is intermediate in tolerance among the forages (Fig. 5). There are a number of alfalfa varieties that have been selected for salt tolerance. The alfalfa varieties AZ-Germ Salt I , AZ-Germ Salt 2, and AZ-90NDC-ST were selected for high germination rates and superior forage production from Mesa-Sirsa using recurrent selection under pressures as high as 99% in laboratory and greenhouse salt-tolerance tests (Dobrenz et al., 1983; Johnson et al., 1991).Al-Khatib et al. (1994) used seedling tests to identify salt tolerance in Cargo, Punjab Selection, and Local Syria. Indirect selection at germination and seedling growth, however, has not been found to confer significantly higher tolerance in the field (Johnson et al., 1992). Screening salt-sensitive and -tolerant lines based on leaf damage under saline conditions has shown some utility. Tolerant and sensitive selections have been made from the variety CUF- 101(Noble et al., 1984).High shoot dry weights under saline conditions wee highly correlated with low shoot C1- (Noble and Shannon, 1988). Plot trials in Australia have demonstrated significantly superior salt tolerance in Haifa and Irrigation white clover (Trifoliurnrepens) (Rogers et al., 1994). Clover selections also have been made based on the ability of plants to exclude NaCl from the shoot. It was found that C1- exclusion had high heritability and was the result of a different gene action than Na+ exclusion, but neither criterion resulted in an improvement in salt tolerance based on yield (Noble and Shannon, 1988). Among six varieties of subterranean clover (Trifolium subterraneum), high yield under saline conditions was found to be related to low shoot Na+ and the maintenance of leaf K + (Shannon and Noble, 1995). Salt-tolerance rankings among varieties differed with growth stage for tests conducted during germination, emergence, and vegetative growth.

E. VEGETABLE CROPS Vegetable crops are generally more salt sensitive than grains and forages; notable exceptions are asparagus, red beet, and zucchini squash (Fig. 6). Salt affects the growth of vegetables predominantly through osmotic effects, but

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Figure 6 Comparative differences in salt tolerance among a number of vegetable crops. Values in parentheses represent the threshold and slope values for each species (1, s). Data serve only as a guideline to relative tolerances and may vary depending on climate, soil conditions, and cultural practices.

specific ion effects and salinity-nutrient interactions have significant influences on vegetable quality (Bernstein, 1959). In addition, some lettuce cultivars often develop Ca*+-deficiency symptoms when sulfate levels in the soil are too high, and high Na+/Ca*+ ratios can cause heart rot in artichoke (Francois, 1995).Excessive Ca2+ may restrict the uptake of K+,which may be a factor in reduced yields of bean and carrot (Bernstein, 1959). Under marginal conditions of salinity many vegetables are stunted and exhibit a reduction in growth rate without showing other visible injury symptoms (Hayward, 1955).At high salinity levels, some vegetables exhibit pronounced injury symptoms at the later stages of growth. Bean leaves develop a marginal chloro-

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sis-necrosis with an upward cupping of the leaves (Bernstein and Ayers, 1951). Onions have also been shown to develop leaf necrosis (Bernstein and Ayers, 1953a); the growth parameter, other than root weight, that shows the highest correlation coefficient (0.50-0.65) to bulb yield was leaf number (Malik et al., 1981). High levels of exchangeable sodium frequently restrict vegetative growth because of the unfavorable physical conditions associated with sodic soils. Most vegetable crops appear to be at least moderately tolerant to exchangeable sodium. Bean plants, however, are sensitive to nutritional factors in sodic soils and may be severely affected, even before the physical condition of the soil is impaired. Most vegetable crops produced on saline soils are not of prime market quality. This is seen in such diverse ways as smaller fruit size of tomatoes and peppers (Bernstein, 1959), reduced petiole length of celery (Francois and West, 1982), and misshapen potatoes (Blodgett and Snyder, 1946). However, not all quality effects are detrimental. The flavor of carrots (Bernstein and Ayers, 1953b) and asparagus (Francois, 1987)is enhanced by a measurable increase in sugar content when grown under saline conditions. Likewise, several studies have shown that total soluble solids in tomatoes are significantly increased as salt stress is increased (Adams and Ho, 1989; Jobesetal., 1981; ShalhevetandYaron, 1973; RushandEpstein, 1981a). Unfortunately, this gain in quality is more than offset by lower yields. A tomato variety with very high salt tolerance has been developed by growers along the northern coast of Egypt (Hassan and Desouki, 1982). The variety Edkway is believed to have been selected from the variety Marmande by regional farmers who grow their crops on sand dunes using drainage water of 4-6 dS m-' for irrigation. Even though tomato is highly self-pollinated and therefore has limited genetic variability within a variety, enough variation evidently was present for the selection of salt tolerance under high selection pressure. The cultivated tomato species has been hybridized with several salt-tolerant wild species in an effort to improve its tolerance. A salt-tolerant line was developed through backcrossing L. cheesmanii to the cultivated parent (Fredrickson and Epstein, 1975; Rush and Epstein, 1976, 1981a). Another wild tomato species, L. pennellii, has been hybridized and backcrossed to the variety New Yorker for nine generations and selected for cold tolerance and earliness. Many breeding lines from this cross had salt tolerance higher than that of the cultivated parent (Sacher et al., 1982). In Israel, a processing tomato variety with improved salt tolerance was selected from an interspecific backcross between a cultivated processing variety and a closely related wild tomato, L. pennellii. Four cycles of recurrent selection have produced a line that has high salt tolerance and superior quality characteristics (see Shannon and Noble, 1990). Differences between varieties have been shown in a number of melon (Cucurnis melo) varieties screened for salt tolerance at germination and seedling stages and at maturity (Shannon and Francois, 1978; Mendlinger et al., 1983; Shannon et al., 1984). Tolerance at one growth stage is not correlated to tolerance at another stage

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nor is there a consistent correlation between tolerance and the accumulation or exclusion of ions in the shoot. An F, muskmelon hybrid has been produced from selected parents of salt-tolerant and locally adapted melon varieties in Israel (see Shannon and Noble, 1990). The selected hybrid, BG84-3, had improved productivity and salt tolerance in preliminary yield trials. In Egypt, a muskmelon variety, Shad El-Dokki, has been selected for superior yields under saline conditions. Screening tests conducted on 85 lettuce (Lactuca sativa) varieties and breeding lines indicated a high degree of variability among entries (Shannon et ul., 1983). Lettuce was found to respond primarily to osmotic effects at low salinities and shoot growth was affected more than root growth. Salinity decreases both growth and head-to-frame ratio in crisp head lettuce, but both characters can be improved through selection (Shannon, 1980). Screening tests showed that 115 lettuce introductions had a higher degree of salt tolerance and had a greater degree of variability for the character than the cultivated varieties and breeding lines (Shannon and McCreight, 1984). In field trials, Romaine varieties were more salt tolerant than iceberg varieties (Pasternak et al., 1986).

F. FRUITS,NUTS,AND BERRIES Most fruit trees are relatively sensitive to salinity with the exception of date palm (Phoenix dactylifera), pomegranate (Punica granaturn), and a few other species believed to be moderately tolerant (Fig. 7). Stone fruits, citrus, and avocado (Persea americana) have all shown specific sensitivity to foliar accumulations of NaCI. The accumulation of these ions to harmful levels, as well as the general osmotic growth inhibition, contributes to the reduction in tree growth and fruit yield. Different cultivars and rootstocks absorb chloride and sodium at different rates, so tolerance can vary considerably within a species. Injury by sodium can occur at concentrations as low as 5 mol m-3 in the soil solution (Maas, 1990). However, injury symptoms, which are characterized as tip, marginal, andor interveinal necrosis, may not appear for a considerable time after exposure to salinity. Some of the more sensitive fruit crops may accumulate toxic levels of sodium or chloride or both over a period of years from soils that would be classified as nonsaline and nonsodic (Ayers et al., 1951; Bernstein, 1980). Initially, the sodium is thought to be retained in the sapwood of the tree; most fruit trees are relatively sensitive to salinity. This may partly explain why stone fruits and grapes appear to be more sensitive to salinity as the plants grow older. With succeeding years, the chloride and sodium accumulate more rapidly in the leaves, causing leaf burn to develop earlier and with increasing severity (Hoffman et al., 1989). Chloride toxicity in woody species is generally more severe and observed in a wider range of species than is sodium toxicity. Differences in susceptibility to chloride among species, varieties, or rootstocks usually reflect the capability of the plant to prevent or retard chloride accumulation in the plant tops. Recent studies

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Figure 7 Comparative differences in salt tolerance among a number of fruit (A) and nut and berry (B) crops. Values in parentheses represent the threshold and slope values for each species (r, s). Data serve only as a guideline to relative tolerances and may vary depending on climate, soil conditions, and cultural practices.

have shown that sodium accumulation in plum leaves did not significantly increase until the leaves were already severely damaged by chloride accumulation (Hoffman et ul., 1989; Mead et uf.,1990). These studies indicate that, when chloride and sodium are present in the soil solution, chloride is the primary damaging ion on stone fruits. Sodium accumulation only occurs after the leaf membranes have already been damaged. The initial symptom of excess chloride accumulation in fruit crops is leaf tip necrosis developing into marginal necrosis. With citrus, a chlorosis and bronzing of the leaves occur without a well-defined necrosis. As chloride continues to accumulate, the effects become more severe with premature leaf drop, complete de-

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foliation, twig dieback, and, in extreme cases, death of the tree or vine (Hayward et af., 1946; Bernstein, 1980). Growth and yield reduction may occur with woody fruit species in the absence of specific ion toxicity. Francois and Clark (1980), working with Valencia orange, reported a 50% reduction in fruit yield from salinity with no visible leaf injury symptoms. However, it is generally believed that growth and yield of most woody fruit crops suffer from both osmotic effects and toxicities caused by chloride or sodium accumulation (Bernstein, 1980). The tolerance of many fruit tree and vine crops can be significantly improved by selecting rootstocks that restrict C1 and/or Na accumulation. Crosses between the salt-tolerant Rangpur citrus line and Rubidoux trifoliate orange have been used to produce progeny with a range of salt tolerance (Furr et al., 1963; Furr and Ream, 1968). Although citrus is not considered to be very salt tolerant, there are differences in salt tolerance among the various rootstocks (Cooper et al., 1951; Shalhevet e? al., 1974). These differences are attributed to salt exclusion and particularly to chloride exclusion (Cooper, 1961; Walker and Douglas, 1983). Citrus apparently excludes chloride from shoots not by sequestering it in the root but rather by restricting its entry into and/or movement within the roots. The chloride concentration differences found in leaves and to a lesser extent in stems emphasize pronounced rootstock differences in root to shoot transport of chloride and have led to the development of screening procedures based on this characteristic (Walker and Douglas, 1983; Sykes, 1985).The scion appears to have no major influence on chloride transport from the roots to the shoot (Behboudian et af., 1986). Differences among rootstocks are much greater for chloride accumulation than for sodium and there appears to be no correlation between chloride tolerance and sodium tolerance (Cooper, 1961). These differences are due to the existence of apparent separate mechanisms that operate to limit or regulate the transport of sodium or chloride to the leaves (Grieve and Walker, 1983). The chloride tolerance range for avocado rootstocks is much narrower than for citrus. In addition, because of the wide variation among varieties of the same rootstock, the rootstock tolerances tend to overlap (Embleton et af., 1962). However, the average ranking of chloride tolerance is generally West Indian > Guatemalan > Mexican (Cooper, 1951; Ernbleton et af., 1962; Haas, 1950a). The general pattern for sodium accumulation with avocado rootstocks tends to follow that for chloride accumulation, and, like chloride, it shows differences among varieties on the same rootstock (Haas, 1950b, 1952). Cold hardiness has been implicated in the salt tolerance of citrus and avocado rootstocks. Wutscher ( 1979) reported that citrus rootstocks with good chloride-excluding characteristics were relatively cold hardy. For some citrus species, a shortterm, moderate salt stress enhanced cold hardiness in seedlings by modifying growth, water relations, and mineral nutrition (Syvertsen and Yelenosky, 1988). In contrast, the salt-tolerant avocado rootstocks, such as West Indian and West Indian-Guatemalan hybrids, are the least cold tolerant. Likewise, the salt-sensitive Mexican is the most cold-tolerant rootstock (Cooper et af., 1957). The salt-toler-

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ant variety, Arsola I - 18, was developed from a cross between a salt-tolerant West Indian variety and a cold-hardy Mexican variety (Cooper, 195I). Chloride toxicity has been the principal limiting factor for grapevines grown on their own root. However, a significant reduction in chloride accumulation has been shown to occur in chloride-sensitive scions when grown on Dog Ridge or 1613-3 rootstocks (Bernstein et al., 1969). The salt tolerance of these two rootstocks would probably be limited by soil osmotic effects long before chloride reached toxic levels.

The salt tolerance of ornamental trees and flowers is determined by the aesthetic value of the plant species. Bernstein et al. (1972) determined salt tolerance of 25 shrub and ground cover species. Injury or loss of leaves or flowers due to salt stress is unacceptable even though growth may be unaffected. A significant growth reduction might be acceptable and possibly desirable for some species, as long as they appear healthy and attractive. The type of injury seen on woody ornamentals and trees is similar to damage recorded for fruit trees and vines. Although some woody ornamentals and trees accumulate sodium, the salt tolerance of these species is closely associated with the ability to limit chloride uptake and accumulation (Dirr, 1976; Francois, 1982b; Townsend, 1980). In northern climates, where NaCl and/or CaCl, are used as deicing salts, typical salt injury symptoms occur on roadside trees. These trees are subjected to both soil salinity from runoff and saline spray from passing automobiles. Although salt spray is thought to be the more detrimental of the two modes of deposition (Hall et al., 1972; Lumis et al., 1973), the soil salinity effects may be accumulative and, over a period of years, may result in a slow but progressive decline of the trees. A limited number of floricultural plants have been tested for salt tolerance. Although chrysanthemum, carnation, and stock (Matthioh sp) are considered to be moderately tolerant to salt stress, aster, poinsettia, gladiolus, azalea, gardenia, gerbera, amaryllis, and African violet are somewhat sensitive (Hayward and Bernstein, 1958; Sonneveld and Voogt, 1983; Devitt and Morris, 1987). The aesthetic value of floral plants is the determining factor for salt tolerance.

VI. BREEDING METHODS A. GENESFOR TOLERANCE Salt sensitivities in barley (Greenway, 1965), soybean (Abel and Mackenzie, 1964), citrus (Cooper et al., 1951, Cooper and Gorton, 1952), and grape (Bern-

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stein et al., 1969; Downton, 1977; Francois and Clark, 1979b) have been found to be related to the inability of the plant to adequately regulate C1- transport from the root to the shoot. In some cases (e.g., soybean), a single gene may be responsible, but in other cases multiple genes are involved, making the response quantitative (e.g., citrus). Sometimes it is difficult to distinguish whether toxic effects are due to Na+ or C1- because the concentrations of both increase concomitantly. Woody species seem to be sensitive to C1-, whereas beans an corn varieties that better regulate accumulation of Na+ in the shoot can tolerate low salinity better than those that cannot (Richter and Marschner, 1973; Jacoby and Ratner, 1974). It is important that each species be examined without bias for variability in ion toxicity thresholds, ion restriction ability, and the capacity for ion selectivity. Hannon and Barber (1972) found that salt-tolerant clones of Festuca rubra and Agrostis stolonifera restricted shoot uptake of both Na+ and C1-. Populations of A. srolonifera collected from maritime habitats had lower Na+ and higher K+ concentrations in the shoots than inland populations grown at the same salinity. Often, specific ecotypes have been found to be tolerant to salinity. Another closely related species, A. tenuis Sibth., was sensitive to high Mg2+ concentrations in seawater. Tolerant accessions of tall wheat grass (Elytrigia pontica) limited Na+ and C1- uptake into shoots more effectively than sensitive accessions (Shannon, 1978). Hybridization between tolerant lines yielded progeny with improved tolerance; however, improvement in salt tolerance at this level was not correlated with differences in ion uptake or osmotic regulation (Weimberg and Shannon, 1988). Dvokik and co-workers (DvoW and Ross, 1986; Dvofhk et al., 1985) have been successful in transferring salt tolerance from E. pontica and E. elongatu to wheat by hybridizing the two grasses to wheat. Some of the derivatives, which varied in chromosome composition, had improved salt tolerance compared to wheat; furthermore, amphidiploids showed tolerance to a wide range of ions (Na+, K+, Mg2+,C1-, and SO:-) as well as to seawater. Genes that control K/Na discrimination in wheat have been located on the long arm of chromosome 4D through the use of conventional genetic manipulation of chromosomes and chromosome fragments (Gorham et al., 1987). These investigators made use of disomic D-substitution lines of tetraploid wheat previously developed from crosses between tetraploid and hexaploid wheats with unbalanced A, B, and D genome sets (Sears, 1966). The distinction of a line as salt sensitive or salt tolerant should depend on its relationship to the average response of the breeding population. Certain gene differences may result in changes that increase sensitivity to a specific ion or salt effect. These genotypes should be differentiated from those that confer greater salt tolerance than is found in the mean population. An example of salt sensitivity is the soybean variety Jackson (Abel and MacKenzie, 1964; Abel, 1969). Similarly, an introduction of wheat PI 9434 I was found to be more sensitive to Na+ than PI 178704 (Kingsbury et al., 1984; Kingsbury and Epstein, 1986).

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B. HERITABILITY Breeding procedure depends on the pattern of inheritance (qualitative or quantitative), the number of genes with major effects, and the nature of gene action. Knowledge of the heritability and inheritance of salt tolerance in a species will determine the selection intensity and number of selection cycles necessary to effectively improve tolerance. Breeding for improved salt tolerance has been discussed for several decades and has been reviewed on numerous occasions (Nieman and Shannon, 1976; Gupta, 1978; Pasternak etal., 1979; Norlyn, 1980; Ramage, 1980; Duvick etal., 1981; Shannon, 1982, 1984, 1985; Noble, 1983; Blum, 1984, 1988; Downtown, 1984; Jones and Qualset, 1984; Rana, 1985; Tal, 1985; Epstein and Rains, 1987; Jones, 1987). Salt tolerance in Agropyron intermedium was found to be a heritable characteristic that could be selected based on growth in mature plants (Hunt, 1965). Norlyn (1980) found that salt tolerance in barley was heritable, but that genetic control was complex. The lack of genetic variation and a poor understanding of genetic control are most often the greatest bamers to improvement of salt tolerance. The nature of salt tolerance as a multigenic, quantitative character imposes several constraints on the type of breeding approach that might be successful. Ramage (1980) emphasized that a favorable background genotype is needed on which to improve the salt-tolerance characteristics. He suggested the use of recurrent selection for both crosspollinated species and those self-pollinated species that have male sterility systems that can be genetically manipulated. Use of male sterility systems would reduce the work involved in hybrid production during the recombination phase of the recurrent selection procedure with self-pollinators.

C. FIELDSCREENINGTECHNIQUES Selection for tolerance under field conditions is inaccurate because of the difficulty in assessing the physical and temporal variability in salinity in soil solutions proximal to the root zone. The ability of plants to extract preferentially water from the most nonsaline portions of their root zones further complicates this assessment. It may be possible to compensate for these problems to some extent by using very large populations, making a judicious selection of breeding strategy and experimental design, and precisely controlling imgation in artificially salinized plots. Nevertheless, a low selection efficiency can be expected even under ideal conditions and the efforts will be very expensive. The advantages of field screening are that space is not a major limiting factor to replication and population sizes, and that the environmental conditions are more nearly what will be encountered under real field conditions.

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D. SELECTION METHODS Several screening and selection schemes have been proposed for salt tolerance (Greenway and Andrew, 1962; Dewey, 1962; Beatty and Ehlig, 1973; Epstein et al., 1980; Shannon et al., 1983; Sykes, 1985). Because of the many interactions of salinity with environmental variables and specific ions, a target environment is important to a screening program for salt tolerance. Initial considerations should be made concerning the balance of major and minor ions that could prove toxic or have detrimental effects on nutrients. Recent advances in soil physics and chemistry allow the prediction of soil water compositions in the plant root zone based on applied water quality and soil chemistry (Suarez and Simbnek, 1995). Many examples have been cited to support the generalization that salt tolerance varies with ontogeny and development. Salt tolerance measured at one growth stage usually does not correlate well with salt tolerance at other growth stages. Intervarietal variation in salt tolerance during germination has been found, in a number of cases, to differ with tolerance at other growth stages. Germination rates as measured in saline filter paper or gels have very little relationship to emergence potential. Many seeds that germinate on salinized filter papers or in gels may be too weak to break a soil crust and establish as viable plants. Emergence rate might be a more practical screening criterion than germination rate, and seedling vigor may also be an additional useful screening factor for conditions in which saline soils form hard crusts. Germination or emergence trials under saline conditions should always be conducted using healthy and viable seed lots. Hard or damaged seed should be discarded; however, too much selection may also be a problem. Large seed size has been shown to have greater salt tolerance than small seed selected from the same lot in wheat and sorghum (Amthor, 1983; Grieve and Francois, 1992). Available information on the salt tolerance of the crop should be collected. Such information should include relative salt tolerance range, potential variability among cultivated varieties and closely related species, and sensitivities to specific ions and environmental interactions. The process of developing salt-tolerant varieties should begin with the identification of the precise growth stage that is limiting to productivity. It should be determined if economical management techniques can be used to overcome the limitation. For example, if stand establishment is a limiting factor, this limitation could be overcome by better bed preparation to move dissolved salts away from the seed, by applying a timely irrigation of high- or medium-quality water; or by more dense seeding or plant spacing. Seed priming has also been proposed as a method to enhance seed germination and emergence under conditions of salinity and low temperatures (Nerson and Covers, 1986; Shannon and Francois, 1977). Determine whether other locally adapted varieties are more tolerant at this growth stage. Develop a screening procedure for the sensitive growth stage. The procedure must be based on information concerning average salt concentration and

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composition of the soil water during sensitive growth periods and the environmental conditions during the period of salt damage in the field. A selection criterion needs to be one that is related to mean yield response in the field. Field experiments should be conducted at an early stage to prove the relationship of the criteria selected to the desired field characteristics. Evaluate a range of varieties and introductions to determine genetic variance for the desired characteristics. Proper controls must be included to separate genetic from environmental effects under both nonsaline and saline conditions. Information from the collected data can be used to determine if intravarietal selection will be effective. If genetic variance is low or if a greater degree of tolerance is required, wild relatives and lines developed from hybridizations can be evaluated. A point of interest is that a newly selected rice variety, Giza 160, was found to be sensitive to two new blast races prior to its release, and germplasm derived from the salt-tolerant Kharchi wheat line was been found to be susceptible to both rusts and powdery mildew (Rana, 1985). Research has not been conducted to determine whether there is a mechanistic link between salt tolerance and susceptibility to rusts and mildews. It has been speculated that a weaker cell wall structure could favor easier expansion growth under saline conditions. Weaker cell walls could also decrease resistance to rusts and mildews.

VII. NOVEL CONCEPTS There are a number of new methods and techniques that promise to overcome the barriers that have prevented rapid development of salt-tolerant crops. Among these, and in approximate chronological order of their application to salt tolerance, are tissue culture, molecular biology, and crop modeling. None of these technologies has yet led to breakthroughs in the realm of salt tolerance. However, if applied integratively, they will make valuable contributions in the future.

A. TISSUECULTURE The use of undifferentiated cells in tissue culture to improve salt tolerance has been explored in many crops (Dix and Street, 1975; Nabors et al., 1975, 1980; Croughan et al., 1978; Ben-Hayyim and Kochba, 1982; Rangan and Vasil, 1983, Pandey and Ganapathy, 1984; Winicov, 1991). Large populations of cells in cultures can be maintained under precisely controlled environmental conditions and easily manipulated. Mutagenesis, haploid production, somatic hybridization, and transformation are relatively easy in tissue cultures. The major limitation of tissue cultures with respect to salt tolerance is that the selected character cannot be maintained during the regeneration process. In addition, there is still insufficient under-

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standing of the relationship between cellular and whole plant response (Petolino and Collins, 1984). An additional limitation is the possibility that epigenetic change, or physiological adaptation, may occur. Tolerance mechanisms that depend on the integrated function of differentiated tissues cannot be readily identified in cell cultures, but it is hoped that some intracellular tolerance characteristics can be improved. Potential characteristics that may be selected from cell cultures include the ability to regulate or partition ions more efficiently, the production of organic solutes to improve osmoregulation under salt stress, and improved metabolic efficiency (Hasegawa et al., 1980; Ben-Hayyim and Kochba, 1983; Ben-Hayyim et al., 1985; Binzel et al., 1983; Watad et al., 1983; Stavarek and Rains, 1984). Recently, some unconfirmed successes in using tissue cultures to improve salt tolerance have been reported in alfalfa (Winicov, 1991) and bentgrass (Kuo et al., 1994). In potato, cultured stem segments and cell suspensions differed in their response to salinity to that expressed by whole plants, but the response of cultured roots was more closely related (Naik and Widholm, 1993).

B. MOLECULAR BIOLOGY Recent advances in molecular biology have broadened the possibilities for gene manipulation at the level of cell cultures and higher units of organization. The development of new or improved technologies for monoclonal antibodies, endonuclease digestion, cDNA libraries, purified protein isolation, and rapid amino acid sequencing now make gene identification, isolation, and transformation realities. The major problems that prohibit the use of these techniques to develop salt-tolerant crops are that salt tolerance is a complex, multigenic trait and is often a composite response of the integrated biological system. At its current level of technology, molecular biology is not able to engineer traits that involve whole plant responses. However, the power of molecular biology goes beyond the ability to manipulate genes: It is also useful as a method to monitor environmental effects at the level of the genome. As new and improved technologies are developed, it can be expected that the molecular approach will contribute significantly to our knowledge of both genetic and physiological events associated with salt stress. Perhaps salt-tolerant genes will someday be transferred to salt-sensitive species by direct integration of DNA or through bacterial or viral plasmid vectors. However, because the number and location of specific genes affecting salt tolerance are unknown, molecular biology techniques that transfer substantial portions of genetic material from one species to another may prove most useful, particularly for somatic hybridization of related species. Electrofusion, for example, can overcome natural interspecific barriers to hybridization by fusing cells from species in different families. Complete transfer of genetic material between species is possible. However, fused cells need to

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be cultured, plants regenerated, and the interaction of genes from different species studied. Certainly, we are on the threshold of a new and exciting era.

C. MODELING Under saline conditions plants respond to increased osmotic potential in the root zone. The initial effects of salinity are a consequence of stomate closure and the resultant decrease in assimilated carbon. In addition, the plant is forced to divert some of the energy normally expended for growth into increased maintenance. Thus, the efficiency with which the plant utilizes its energy resources may be an important aspect of salt tolerance. Salination of the rooting medium initially increases respiration rates in many species (Nieman, 1962; Livne and Levin, 1967), but total respiration then decreases concomitant with decreased photosynthesis, with the latter directly related to stomata closure induced by water stress effects (Schwartz and Gale, 1981,1983,1984). Energy costs for cell maintenance increase with salinity. Active transport processes that maintain ion compartmentation and fuel protein synthesis are two of the most costly energy sinks in the plants (Penning de Vries, 1975). Increased maintenance costs during salinity stress, no doubt, are directed toward ion regulation (compartmentation and transport of toxic ions), osmotic adjustment, and maintenance of membrane integrity (Yeo, 1983). It is almost impossible to conceptualize all these facets in a quantitative manner to determine the given effect of salinity in a given environment. The outcome of subtle genetic modifications on the biochemistry and physiology is modified by the complexities of the aerial environment as well as the physics and chemistry of the root environment. Physiological or process-based crop growth models provide powerful tools for this type of integration. Simultaneous efforts are being made to develop the necessary soil chemistry, physics, and plant water use models that will provide a more comprehensive conceptual framework to describe the processes that influence plant salt tolerance (Grant, 1995; Suarez and Simhek, 1995). The development and accessibility of powerful computers and object-oriented languages provide a tool that has not been available so far.

WII. SUMMARY AND CONCLUSIONS There is a wide range of salt tolerance in vascular plants. Several genera include species that are extreme halophytes and are ecologically competitive only under saline conditions. Crop species primarily are glycophytes, salt-sensitive, and grow well only under cultivated conditions. Domesticated species typically are provided optimum conditions in order to obtain high yields. In effect, this has constituted

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a negative selection pressure for those characteristics that influence adaptation to extreme environments (Rosielle and Hamblin, 198 1). Thus, a great many crops are moderately to highly sensitive to salinity and there is a relatively narrow range of variation for salt tolerance and other environmental stresses among crop species. Alternatively, many wild species are moderately to extremely salt tolerant, and although it may be possible to develop the agronomic potential of some halophytes into new and useful salt-tolerant crops, much additional effort is needed to improve agronomic characteristics of new crops. The subject of salt tolerance has received a tremendous amount of attention during the past two decades and many reviews have been published that describe the mechanisms of salt tolerance, the possibility of breeding for salt tolerance, and the potential of new technical advances in the development of salt tolerant varieties. The concept of developing salt-tolerantplants, even to the degree that they can be grown with seawater, is an interesting one that captures the imagination of both the scientific and the public sector (Boyko, 1966;Epstein and Norlyn, 1977). However, based on the progress that science has made within the past half a century, it would be safe to conclude that the task is not a simple one. Because of its complexity, the elusive goal of routinely developing salt tolerance has not been realized. It is more likely that progress in developing salt tolerance will evolve slowly until a number of technological breakthroughs develop. There is a need for an increased understanding of the mechanisms of salt tolerance, the development of methods to comprehensively integrate and predict the effects of environmental factors on plant response throughout all stages of growth and development, and the development of improved methods to identify and manipulate genes and gene groups between species.

ACKNOWLEDGMENTS My appreciation is extended to Dr. Prem Jauhar and Dr.Nicholas Yensen for their helpful comments during preparation and editing of the manuscript.

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Rush, D. W., and Epstein, E. (1976). Genotypic responses to salinity: Differences between salt-sensitive and salt-tolerant genotypes of the tomato. Plant Physiol. 57, 162-1 66. Rush, D. W., and Epstein, E. (1981a). Breeding and selection for salt tolerance by the incorporation of wild germplasm into a domesticated tomato. J. Am. SOC. Hot?. Sci. 106,669470. Rush, D. W., and Epstein, E. (1981b). Comparative studies on the sodium, potassium and chloride relations of a wild halophytic and a domestic salt-sensitive tomato species. Plant Physiol. 68, 1308-1 3 13. Sacher, R. F., Staples, R. C., and Robinson, R. W. (1982). Saline tolerance in hybrids of Lycopersicon esculentum x Solanum penellii and selected breeding lines. In “Biosaline Research: A Look to the Future” (A. San Pietro, Ed.), pp. 325-336. Plenum, New York. Salama, S., Trivedi, S., Busheva, M., Arafa, A. A., Garab, G.,and Erdei, L. (1994). Effects of NaCl salinity on growth, cation accumulation, chloroplast structure and function in wheat cultivars differing in salt tolerance. J. Plant Physiol. 144,241-247. Sayed, H. I. (1985). Diversity of salt tolerance in a germplasm collection of wheat (Triticum spp.). Theor: Appl. Genet. 69,651-657. Saxena, N. P., Saxena, M. C., Ruckenbauer, P., Rana, R. S., El-Fouly, M. M., and Shabana, R. (1994). Screening techniques and sources of tolerance to salinity and mineral nutrient imbalances in cool season food legumes. In “Expanding the Production and Use of Cool Season Food Legumes” (F. J. Muchlbauer and W. J. Kaiser, Eds.), pp. 457-471. Kluwer, Dordrecht. Schirmer, U.,and Breckle, S. W. (1982). The role of bladders for salt removal in some Chenopodiaceae (mainly Atriplex species). In “Contributions to the Ecology of Halophytes” (D. N. Sen and K. S. Rdjpurohit, Eds.), pp. 215-232. Junk, The Hague. Schwartz, M., and Gale, J. (1981). Maintenance respiration and carbon balance of plants at low levels of sodium chloride salinity. J. Exp. Bof. 32,933-941. Schwartz, M., and Gale, J. (1983). The effect of heat and salinity stress on the carbon balance of Xanrhium srrumarium. In “Effects of Stress in Photosynthesis” (R. Marcelle etal., Eds.), pp. 325-33 I. Nijhoff Junk, Dordrecht. Schwartz, M., and Gale, J. ( 1984). Growth response to salinity at high levels of carbon dioxide. J. Exp. Bot. 35, 193-196. Sears, E. R. ( 1966). Nullisomic-tetrasomic combinations in hexaploid wheat. In “Chromosome Manipulations and Plant Genetics” (R. Riley and K. R. Lewis, Eds.), pp. 2 9 4 5 . Oliver & Boyd, Edinburgh. Shalhevet, J., and Yaron, B. (1973). Effect of soil and water salinity on tomato growth. Hunt Soil39, 285-292. Shalhevet, J., Yaron, D., and Horowitz, U.(1974). Salinity and citurs yield-An analysis ofresults from a salinity survey. J. Hort. Sci. 49, 15-27. Shannon, M. C. ( 1978). Testing salt tolerance variability among tall wheatgrass populations. Agron. J. 10,7 19-722. Shannon, M. C. (1980). Differences in salt tolerance within ‘Empire’ lettuce. J. Am. Soc. Hort. Sci. 105(6), 944-947. Shannon, M. C. (1982). Genetics of salt tolerance: New challenges. I n “Biosaline Research: A Look into the Future” (A. San Pietro, Ed.), pp. 271-282. Plenum, New York. Shannon, M. C. (1984). Breeding, selection, and the genetics of salt tolerance. In “Salinity Tolerance in Plant Strategies for Crop Improvement” (R. Staples and C. H. Toenniessen, Eds.). Vol. 13, pp. 23 1-254. Wiley-Interscience, New York. Shannon, M. C. (1985). Principles and strategies in breeding for higher salt tolerance. Plant Soil 89, 227-24 I . Shannon, M. C. (1987). Salinity-An environmental constraint on crop productivity. In “Agronomy 1987-Responding to Change,” Proc. 4th Australian Agronomy Conference, pp. 9-18. La Trobe Univ. Melbourne. Victoria, August.

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Shannon, M.C. (1997). Genetics of salt tolerance in higher plants. In “Strategies for Improving Salt Tolerance in Higher Plants” (P. K. Jaiwal. R. P. Singh, and A. Gulati, Eds.). pp. 265-289. OxfordllBH, New Delhi. Shannon, M. C.. and Francois. L. E. ( 1977). Influence of seed pretreatments on salt tolerance of cotton during germination. Agron. J. 69,6 19-622. Shannon, M. C., and Franccris. L. E. (1978). Salt tolerance of three muskmelon cultivars. J . Am. Soc. Horf. Sci. 103, 127- 130. Shannon, M. C., and McCreight. C. (1984). Salt tolerance of lettuce introductions. HorrScience 19, 673675. Shannon. M. C.. and Noble, C. L. (1990).Genetic approaches for the developing economic salt-tolerant crops. I n “Agricultural Salinity Assessment and Management” (K. K. Tanji, Ed.), ASCE Manuals and Reports on Engineering Practice No. 71, pp. 161-185. ACSE, New York. Shannon. M. C., and Noble, C. L. ( I 995). Variation in salt tolerance and ion accumulation among subterranean clover cultivars. Crop Sci. 35,798-804. Shannon, M. C., McCreight, C.. and Draper, J. H. ( 1983). Screening tests for salt tolerance in lettuce. J. Am. Soc. Horf. Sci. 108,225-230. Shannon, M. C., Bohn, G. W.. and McCreight, J. D. (1984). Salt tolerance among muskmelon genotypes during seed emergence and seedling growth. HorfScience 19,828-830. Shannon, M. C.. Gronwald, J. W., and Tal, M. (1987). Effects of salinity on growth and accumulation of organic ions in cultivated and wild tomato species. J . Am. Soc. Horf. Sci. 112,416-423. Shannon, M. C., Grieve, C. M., and Francois, L. E. (1994). Whole-plant response to salinity. In “Plant-Environment Interactions” (R. E. Wilkinson, Ed.), pp. 199-244. Dekker, New York. Singh, K. N.. and Rana, R. S. (1985). Genetic variability and character association in wheat varieties grown on sodic soil. Indian J. Agric. Sci. 55,723-726. Sonneveld, C.. and Voogt, W. (1983). Studies on the salt tolerance of some flower crops grown under glass. P/ci/lt Soil 74, 41-52. Srivastava. J. P.. and Jana, S. (1984). Screening wheat and barley germplasm for salt tolerance. 111 “Salinity Tolerance in Plants-Strategies for Crop Improvements” (R. C. Staples and G. H. Toeniessen, Eds.). pp. 273-284. John Wiley & Sons, NY. Stavarck. S . J . . and Rains. D. W. ( 1984).The development of tolerance to mineral stress. HorfScience 19,377-382. Suarez, D. L.. and SimBnek, J. (1995).Modeling equilibrium and kinetic major ion chemistry with COz production/transport coupled to unsaturated water flow. In “In-Situ Remediation: Scientific Basis for Current and Future Technologies” (G. W. Gee and N. R. Wing, Eds.), Part 2, pp. 1215-1 246. 33rd Hanford Symposium on Health and the Environment, Pasco, WA. Sykes. S. R. ( 1985).A glasshouse screening procedure for identifying citrus hybrids which restrict chloride accumulation in shoot tissues. ANSI.J . Agric. Res. 36, 779-789. Syvertsen. J. P., and Yelenosky, G. ( 1988). Salinity can enhance freeze tolerance of citrus rootstock seedlings by modifying growth. water relations, and mineral nutrition. J . Am. Sue. Horf. Sci. 113, 889-893. Tal. M. ( 1985). Genetics of salt tolerance in higher plants: theoretical and practical considerations. Pltr17f Soil 89, 199-226. Tal. M.. and Gardi, 1. (1976). Physiology of polyploid plants: Water balance in autotetraploid and diploid tomato under low and high salinity. Physiol. Plunr 38,257-261. Tal. M.. and Shannon, M. C. (1983). Salt tolerance in the wild relatives of the cultivated tomato: Responses of Lycopersicon esciilenfum. L. cheesmanii, L. peritvianuin, Solunum pennellii and F, hybrids to high salinity. Ausl. J . PIunf Physiol. 10, 109-1 17. Tal, M.. Katz, A,. Heikin. H.. and Dehan, K. (1979). Salt tolerance in the wild relatives of the cultivated tomato: Proline accumulation in Lycopersicon esculen~um.L. peruvianum and Solunitni penne/li under NaCl salinity. New Phyrol. 82, 349-355.

MICHAEL C. SHANNON Taylor, R. M., Young, E. F., Jr., and Rivera, R. L. (1975). Salt tolerance in cultivars of grain sorghum. Crop Sci. 15,734-735. Torello, W. A,, and Symington, A. G. (1984). Screening of turfgrass species and cultivars for NaCl tolerance. Plant Soil 82, 155-161. Townsend, A. M. (1980). Identifying trees tolerant of soil salts. Proc. Metria 111 Con$, June, p. 10. Rutgers Univ., New Brunswick, NJ. van Genuchten, M. Th., and Hoffman, G. J. (1984).Analysis of crop salt tolerance data. In “Soil Salinity under Irrigation-Process and Management” (1. Shainberg and J. Shalhevet. Eds.), pp. 258-27 1. Springer-Verlag, New York. van Schilfgaarde, J. (1984). Drainage design for salinity control. In “Soil Salinity and ImgationProcesses and Management,” Vol. 6, pp. 190-197. Springer-Verlag. Berlin. van Schilfgaarde, J., and Rhoades, J. D. (1979). Benefits from reuse of drainage water for irrigation. ASAE Paper No. 79-2552. Presented at the 1979 Winter Meeting ASAE, New Orleans, LA., December 11-14, Vose, P. B. (1963). Varietal differences in plant nutrition. Herbage Abst,: 33, 1-13. Walker, R. R. (1986). Sodium exclusion and potassium-sodium selectivity in salt-treated trifoliate orange (Poncirus rrifoliata) and Cleopatra mandarin (Citrus reticulatu) plants. Aust. J . Plant Physi01. 13,293-303. Walker, R. R., and Douglas, T. J. (1983).Effect of salinity level on uptake and distribution of chloride, sodium and potassium ions in citrus plants. Aust. J. Agric. Res. 34, 145-153. Watad, A.-E. A., Reinhold, L., and Lerner, H. R. (1983). Comparison between a stable NaC1-selected Nicoriana cell line and the wild type. Plant Physiol. 73,624-629. Weimberg, R., and Shannon, M. C. (1988).Vigor and salt tolerance in 3 lines of tall wheatgrass. Physi d . Plant. 73,232-237. West, D. W. ( I 978). Water use and sodium chloride uptake by apple trees. 11. The response to soil oxygen deficiency. Plant Soil 5 0 , 5 1 4 5 . West, D. W., and Taylor, J. A. (1984). Response of six grape cultivars to the combined effects of high salinity and rootzone waterlogging. J. Am. SUC. Hort. Sci. 109,84485 1. Winicov, I. ( 1991). Characterization of salt tolerant alfalfa (Medicago sativa L.) plants regenerated from salt tolerant cell lines. Plant Cell Rep. 10,561-564. Wright, M. J. (Ed.) (1976). “Plant Adaptation to Mineral Stress in Problem Soils,” pp. 420. A spec. Publ. of Cornell Univ. Agric. Exp. Stn., Cornell Univ., Ithaca, NY. Wutscher, H. K. (1979). Citrus rootstocks. Hurt. Rev. 1,237-269. Wyn Jones, R. G., and Storey, R. (1978). Salt stress and comparative physiology in the Gramineae: II. Glycinebetaine and proline accumulation in two salt- and water-stressed barley cultivars. Aust. J . Plant Physiul. 5, 8 17-829. Wyn Jones, R. G., Storey, R., Leigh, R. A,, Ahmad, N., and Pollard, A. (1977).A hypothesis on cytoplasmic osmoregulation. In “Regulation of Cell Membrane Activities in Plants” (E. Marre and 0. Ciferri, Eds.), pp. 12 1-1 36. Elsevier, Amsterdam. Yadav, R., Flowers, T. J., and Yeo, A. R. ( I 996). The involvement of the transpirational bypass flow in sodium uptake by high- and low-sodium-transporting lines of rice developed through intravarieta1 selection. Plunt Cell Environ. 19,329-336. Yan, X., Zheng, S., and Kuang, Y. (1992).Rice genotypes differing in salt tolerance. 11. Short-term kinetics of NaCl absorption and translocation in intact plants. J. Plant Nutr: 15,2667-2678. Yensen. N. P. (1988). Plants for salty soil. Aid Lands Newslert. 27,2-10. Yeo, A. R. (1983). Salinity resistance: Physiologies and prices. Physiol. Plant. 58,214-222. Yeo, A. R., and Flowers. T. J. (1983). Varietal differences in the toxicity of sodium ions in rice leaves. Physiol. Plant. 59, 189-195. Yermanos, D. M., Francois, L. E., and Bernstein, L. (1964). Soil salinity effects on the chemical composition of the oil and the oil content of safflower seed. Agrun. J. 56,35-37.

INFLUENCE OF NO-TILL CROPPING SYSTEMSON MICROBIAL RELATIONSHIPS L. E Elliott' and D. E. Stott2 'National Forage Seed Production Research Center Cowallis, Oregon 9733 1 National Soil Erosion Research Laboratory Purdue University West Lafayette, Indiana 47907

I. Introduction 11. Decomposition of SurfaceManaged Crop Residues 111. Modeling Crop Residue Decomposition A. Residue Decomposition Models in Expert Systems and Erosion Models B. Theory Used in the RESMAN Tv. Root-Microbial Relationships V. Deleterious Rhizohacteria for Weed Control VI. Low-Input, On-Farm Composting References

I. INTRODUCTION No-till seeding is essential for soil erosion control, reducing seeding costs, and protecting or enhancing soil quality. However, no-till seeding has resulted in several microbiological related problems, which include predicting residue decomposition and resultant soil cover during critical periods, plant root health, weed control, and the interference of heavy crop residues with no-till seeding and crop growth. We have been able to solve or gain insight into many of these problems. Most of the studies reported here deal with the establishment of no-till seeded winter wheat. Development of machinery that is economical for no-till seeding and that is optimal for plant-microbial relationships has been a major problem but that will not be covered here.

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11. DECOMPOSITION OF SURFACE-MANAGED CROP RESIDUES Knowledge of the decomposition of surface-managed crop residues is important so that protection of soil against wind and water erosion can be predicted during critical erosion periods. This capability is essential for determining the efficacy of tillage and crop residue management practices. Soil protection by crop residues occurs by two processes: (i) The residues protect the soils mechanically by absorbing water and wind energy; and (ii) after a period of residue decomposition, the bodies of the microbes and metabolic by-products can bind soil particles increasing soil resistance to erosion. The ability for specific residues to protect against soil erosion depends on quantity, their chemical composition, and management. These characteristics and properties will determine longevity of the residue and the types of metabolic by-products produced. The bulk of the following discussion relates to the decomposition of winter wheat residues. A useful model must predict residue decomposition across climatic zones. This accomplishment requires knowledge of the mechanisms regulating residue decomposition. Many of the mechanisms are interdependent. For example, residues left on the soil surface will affect both soil temperature and moisture (Parr and Papendick, 1978). Residue placement and handling will result in quite different environmental effects and in residue decomposition rates. Surface residues disappear at about two-thirds the rate of buried residues (Brown and Dickey, 1970; Douglas et al., 1980). Partially or completely incorporated residues have more intimate soil contact and are not subject to the temperature and moisture extremes of surface residues. Also, the perceived rate of residue decomposition is greatly affected by the method of measurement. Caution has to be exercised with data collected by mesh bags because the fiberglass bag method underestimates residue decomposition by approximately 30% when compared to residue placed directly in or on the soil (Witkamp and Olson, 1963; Wieder and Long, 1982; Wiegert and Evans, 1964). Residue decomposition was affected little by placement if water and temperature were held constant (Stott et ul., 1986). Jawson and Elliott (1986) also showed that soil inoculum was not needed to provide organisms for straw decomposition. The residue contained sufficient indigenous flora for this purpose. The chemical composition of the crop residue is another important regulator of decomposition. We have known that this is so for some time when comparing, for example, a cereal such as wheat straw with a legume such as pea straw. However, there are large differences in the decomposability of wheat straws and among the components of wheat straw (Reinertsen et al., 1984; Collins et af., 1990a). The work by Collins et al. (1990b) with the Daws winter wheat variety showed that the ratio of component plant parts (leaf, leaf sheath, chaff, and stem) remained rela-

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tively constant between growing seasons and yield differences; however, the individual components of the residues decompose at different rates. The total C, total N, soluble C, and nonstructural carbohydrate concentrations of the wheat residues were not correlated with residue or grain yields (Collins et al., 1990b). The decomposition rates of the individual parts were closely related to the carbohydrate, lignin, C, and N contents. When the individual residue components were mixed in the ratios found in the intact residues the residue mixes decomposed approximately 25% more rapidly than what was predicted from the decomposition rates of the individual parts (Collins et al., 1990b). McClellan et al. (1987) observed that more than 30% of the wheat residue mass after harvest consisted of chaff. The potential decomposition rate of wheat straw can be based on the size of the readily available C and N pools (Knapp et al., 1983a,b; Reinertsen et al., 1984). From this work, it was postulated that microbial extracellular materials such as polysaccharides might dominate the aggregation process shown by decomposing straw if the wheat straw contained low N and if alternate sources of N were unavailable. Elliott and Lynch ( I984a) aerobically degraded three wheat straws containing 1.09,0.5,and 0.25% N in the absence of added N. The 0.25% N straw treatment produced significantly more aggregation in the soils tested than the other treatments. The 0.5% N straw treatment generally caused more aggregation than the I .095% N straw. The largest microbial biomass would be generated from the straw containing the most N; thus, these results confirm the postulate that the increased aggregation resulted from the microbial production of extracellular gums. Electron micrographs also showed more gum production from the 0.25% N containing straw than the 1.09% N straw (Figs. 1 and 2). Polysaccharides have long been implicated in having a positive effect on the soil aggregation process (Tisdall and Oades, 1982). The results of these studies show that there is potential for improving soil aggregation through residue management on the soil surface and by reducing tillage because tillage increases the rate of soil organic matter mineralization and N availability (Rovira and Greacen, 1957). Improved soil aggregation increases water infiltration, resistance to wind and water erosion, and probably soil productivity. Reinertsen et ul. (1984) and Stroo et al. (1989) considered cereal substrates to consist of three separate fractions (pools) based on availability to microorganisms. They designated the pools as readily available, intermediately available (cellulose and hemicellulose), and resistant (lignin). With their model, they were able to predict wheat residue decomposition across climatic zones. The amount of readily available C and N controls the size of the initial microbial biomass and the initial decomposition rate. Readily available C and N content of the residues increases as the C/N ratio decreases (Reinertsen et al., 1984). Residues high in total N tend to be high in soluble N (Iritani and Arnold, 1960). Most models treat decomposers entirely as microbes. When fauna was exclud-

Figure 1 Microbial growth and gum production during the decomposition of wheat straw containing 0.25% N.

Figure 2 Microbial growth with no visible gum production during the decomposition of wheat straw containing 1.09% N.

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ed from litter bags, decomposition rates in grasslands, in contrast to forests, are little affected (Curry, 1969). Stroo er al. (1989) showed fauna accounted for 5% or less of the CO, respired during wheat straw decomposition. It must be noted these studies were conducted under a specific set of conditions and the environment will vary greatly depending on location. For example, in warmer climates, termites play a significant role in residue degradation. Distinguishing each group of decomposers and their relative contributions would be almost impossible with current technology (bacteria, fungi, actinomycetes, fauna, etc.). This precision is unnecessary for current predictive needs. Therefore, models generally assume that under optimal environmental conditions there is an adequate decomposer population present to sustain the maximum decomposition rate. There must be favorable moisture and temperature conditions for biological activity. However, each group, and even subgroup, of decomposers has a range of climatic conditions in which they are active and an optimum at which they are most active. Fluctuating climatic conditions, especially those that go beyond the range of the decomposer, are more deleterious to decomposers than constant conditions (Parr and Papendick, 1978). For practical considerations in the field, it is unlikely that much biological decomposition occurs above 20°C. Effects of climatic conditions are different even with different phases of decomposition. For example, Stott et al. (1986) found that low water potentials or low temperatures had significant effects on microbial activity only during the initial phase of decomposition. Thus, both long-term and diurnal fluctuations of temperature and moisture must be inputs.

III. MODELING CROP RESIDUE DECOMPOSITION Generally, decomposition is evaluated across some time frame. Decomposition can be related to degree days (DD) and calculated from air temperature (Douglas and Rickman, 1992). Degree days are determined by measuring the daily mean air temperature in "C and summing over the desired period. If the mean daily temperature is less than O"C, it is considered as zero. Zero is used as a base value because of reports by Wiant (1967) that microbial reactions follow the Van't Hoff and Arrhenius' laws at temperatures below 40°C. Reiners (1968) confirms that this is true down to 0°C. However, these relationships must be viewed with caution because dramatic change occur in the flora makeup and activity as temperatures change. Stott et al. ( I 986) showed the response to temperature was more likely related to changers in the microflora as temperature was changed and this was the reason the system did not respond according to Van't Hoff's and Arrhenius' laws. Also, degree days may accurately predict in a defined climatic zone but will likely be inaccurate as moisture varies.

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A. RESIDUEDECOMPOSITION MODELSIN EXPERTSYSTEMS AND EROSION MODELS In 1989, Stroo et al. published a mechanistic-based model for surface-managed wheat residue decay in the field. The model simulates decay under constant environmental conditions using C and N dynamics. It then determines the impact of environmental conditions, calculating the fraction of an optimum “decomposition day” occurring in a4-h period. Information concerning initial residue C and N pool availability is required for this model. The model was developed from mechanistic relationships developed in the laboratory and a series of field decomposition studies in different climatic zones (Stott el al., 1990). Once the theoretical model for crop residue decomposition was developed, the next step was to simplify the model for use as a component in an expert system on residue management (RESMAN) (Stott and Rogers, 1990; Stott et al., 1988; Stott, 1991). The goal in developing RESMAN was to incorporate residue decomposition knowledge with site- and situation-specific tillage considerations including residue burial. Inputs for the expert system needed to be relatively simple and readily available to a wide variety of users.. Another feature was a user-specified need for quick run times; thus, the expert system was unable to deal with the complexity of a true research model. Since its release in 1990, RESMAN has been used widely by industry personnel, extension and soil conservation advisors, and private consultants throughout the United States, Canada, and several other countries to develop crop residue management strategies for soil conservation. Due to its wide acceptance, the theory and equations used in RESMAN were incorporated into several erosion models being developed by the USDA, including Revised Universal Soil Loss Equation (RULSE), Revised Wind Erosion Equation (RWEQ), Water Erosion Prediction Project (WEPP), and Wind Erosion Prediction System (WEPS), RUSLE was implemented by the USDA-NRCS in its southeastern field offices in 1995 and will be implemented in the remainder of the United States in 1996. RWEQ is to be implemented in late 1996. WEPP and WEPS, utilizing new erosion prediction technologies, are expected to be completed and implemented by the end of the decade.

B. THEORY USEDIN THE RESMAN To simulate the decomposition process, the decomposition day concept as presented by Stroo et al. (1989) for winter wheat residue decomposition was used as a basis for the residue mass loss calculation. The Stroo et al. (1989) model simulates residue decay under constant environmental conditions using C and N dynamics based on Knapp et al. ( 1983a,b)and Bristow er nl. (1986). The residue C is split into three pools based on availability for use by the soil microbial population and chemically defined. This information is not readily available for a wide variety

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of crops, thus in the RESMAN model the equations describing the C and N dynamics were replaced with the following single equation (Stott and Barrett, 1994): M, = M y * e - (Rapt . EF),

(1)

where M,is the residue mass per unit area remaining on the surface today, Myis the mass per unit area left on the ground the previous day, Ropt is a decomposition constant for a given residue type for the amount of mass lost in I day under optimum conditions for microbial activity, and EF is the environmental factor determining the fraction of a decomposition day that has occurred during day t. The value for Ro for a given crop can be calculated from the more mechanistic Stroo et al. (1984) model if the nutrient data are available. Alternatively, Ropt can be estimated from field or laboratory studies measuring rates of residue mass loss. In the field, residue decomposition rates are controlled by environmental factors (Martin and Haider, 1986). Especially important are the residue water content and temperature (Pam and Papendick, 1978).The effects of water content and temperature on the rate of residue decomposition were assumed to be independent of one another (Strooet al., 1989; Stott et al., 1986).To estimate the influence of these factors on residue decomposition in the field, the following relationship was used (Stott and Barrett, 1994): EF = Minimum (WFC, TFC),

(2)

where EF is the environmental factor used in Eq. (l), and WFC and TFC are water and temperature factors, respectively, with normalized values between 0 and 1. The changes in the water content and temperature within the soil and residue layers are calculated with a simplified version of the Bristow er af. (1986) model. Bristow et al. (1986) divided the residue into five layers and the soil into three layers. Temperature and moisture fluctuations were calculated for each layer at 4-h time intervals. In the RESMAN model, three residue and five soil layers are used, and calculations are done at 24-h time intervals. The data needed to calculate the temperature and water content for the soil and residue layers include daily maximum and minimum air temperatures and precipitation. It is assumed that most of the residue decomposition occurs in the bottom residue layer that interfaces with the soil surface. Thus, WFC and TFC are calculated using the temperature and water content for the bottom residue layer. RUSLE, RWEQ, and WEPP do not calculate the temperature or moisture within the residue layer; therefore, WFC is estimated based on the water content in the top soil layer and TFC is estimated from the air temperature. The water function (WFC) is calculated as (Linn and Doran, 1984):

0

WFC= - i f @ < @ @opt

?,pt

opt

orWFC= - i f @ > O o p t ,

(3)

0

where 0 is the actual water content (g water kgg ') of the residue or soil, and Oopt is the optimum water content. The latter value was set at 3500 g water kg- residue as determined by Stroo et al. (1989) from data published by Stott et al. ( 1 986). This residue water content is equivalent to a water potential of -33 kPa, which is

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considered optimal for microbial activity (Sommers et al., 1981). RESMAN assumed that the soil texture was silt loam. For that soil textural class, a -33 kPa water potential is equivalent to approximately 60%water-holding capacity. Assuming a bulk density of 1.2 and 50% porosity, Qopt for the soil layers was set at 833 g water kg- soil. The submodel used in WEPP allows for variations in soil texture and water absorptive capacities of the soil. This includes changes in the amount of pore space within the soil as compaction and tillage occur. The temperature function (TFC) is based on an equation for photosynthetic activity (Taylor and Sexton, 1972):

'

TFC =

2(T

+ A)2(Tm + A)* - (T + A)4 (T,

+ A)4

(4)

where T is the average temperature in "C, T, is the optimum temperature, and A is an experimentally derived constant. T is calculated as the mean of the daily minimum and maximum temperatures. If T is plotted against TFC, T, is the temperature at which TFC equals 1. TFC equals 0 at two points-the first where the average daily temperature is too low for microbial activity, and the second where it is too high for activity. The constant, A, is equal to the absolute value of the lower of the two TFC zero values. Stroo et al. (1989) used Tm = 33"C, and A = 6.1"C for calculating TFC at 4-h time intervals. RESMAN uses average daily temperature for T rather than 4-h averages; thus a T,,, of 30°C and an A of 0°C were used based on laboratory data from Stott et al. (1986) and personal unpublished field data. Because Eq. (4) is a quartic formula, TFC was set to 0 when T < 0.0 or T > 42.4. Each operation or pass through a field with a tillage implement inverts residue under the soil surface, reducing erosion protection. Modeling the effects on percentage residue cover of all possible tillage procedures is difficult because each tillage pass not only turns some of the residue under but also can, in a few cases, return some of the previously buried residue to the surface. Depth and speed of the equipment operation impacts the amount of residue buried, as does the amount of residue present at time of tillage. For a specific piece of equipment, the shallower the operating depth, the greater the amount of residue left on the surface, whereas deeper operating depths will bury more residue. Additionally, slower operating speeds tend to leave more residue on the surface. In RESMAN (Version 2.0) and the erosion models, the tillage burial coefficients used were derived from the USDA-SCS-EM1 tillage implement list (1992). The effectiveness of a residue layer in protecting the soil against water erosion is dependent on the percentage of the soil surface covered. Because residue decomposition is calculated in terms of residue mass per unit area, a conversion from residue mass to percentage cover is needed. The equation used in RESMAN and the erosion models for this conversion is based on the following one published by Gregory ( 1982):

C, = tOO * ( I

-

e--Mt.K 1,

(5)

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I29

where the percentage surface are covered (C,)is a function of the M,from Eq. (1) and a constant ( K ) that is dependent on crop type and represents the area covered by a specific mass of residue. C, is the critical value for determining that adequate erosion protection will be achieved using a given management practice. The amount of erosion and sediment loss occurring is very sensitive to changes in this value. Because accurate predictions are essential for planning erosion control measures, we are attempting to gain a better understanding of how soil biological and environmental characteristics impact the plant residue decay process as well as the mass-to-cover relationship.

IV. ROOT-MICROBIAL RELATIONSHIPS Brian et al. (195 1) showed that the metabolic products from some saprophytic rhizosphere microorganisms could adversely affect plant root development. Woltz ( 1978) said that saprophytic deleterious rhizobacteria usually live outside their hosts and inhibit plant growth through the production of a water-soluble toxin. Salt ( 1979) felt that some rhizosphere microorganisms not normally considered pathogens may decrease crop yields. Suslow and Schroth (1982) called them deleterious rhizobacteria (DRB). They postulated that these organisms are a significant pathogen group that limits plant growth and yield and influence other hostpathogen interactions at the root surface. Schippers et al. (1987) defined these organisms as deleterious rhizosphere microorganisms that were minor pathogens affecting plants by their metabolites without parasitizing plant tissues; both bacteria and fungi are included in this definition. We conducted several studies on the relationship of DRB, primarily pseudomonads, on winter wheat (Triticurn aestivum L.) growth. Pseudomonads were isolated from the surface of winter wheat roots. Large numbers of these isolates were inhibitory to winter wheat root growth (Elliott and Lynch 1984b). Incidence of the organisms appeared to correlate with poor winter wheat growth when it had been no-till seeded into heavy residues from the previous crop. The organisms inhibited wheat root growth by the production of a toxin (Fredrickson and Elliott, 1985a). The organisms grew well at 5°C and were detectable on wheat roots in early spring (Elliot and Lynch, 1985) or after prolonged cold and wet periods in late fall and early winter (Rovira et af., 1990). Inoculation of wheat seed, which was no-till seeded into residues, resulted in severe inhibition of crop growth (compare Fig. 3 with the uninoculated check in Fig. 4). When wheat straw was inoculated with a wheat DRB at 1 X I O8 organisms per square meter and wheat was no-till seeded into the residues, severe stand loss and stunting occurred (Fig. 5 ; plots on left inoculated; uninoculated check on right). When seedlings were dug from the plots and compared, the three plants on the right were severely stunted when compared with the plant on the left (Fig. 6).There were no obvious effects, such as discol-

Figure 3 Loss o f a no-till seeded winter wheat stand caused by bacterization of the winter wheat seed with a DRB at I X IOh organisms per seed (the three center pairs of rows were bacterized).

Figure 4 Growth of no-till seeded winter wheat seed with nonbacterized seed.

Figure 5 The effect o f a DRB on the growth of no-till seeded winter wheat. The straw on the plot on the left was sprayed with 1 X IDx DRB organisms per square meter before no-till planting. whereas the plot on the right was lightly sprayed with water only.

Figure 6 The plants on the right were dug from the plot that was sprayed with DRB before planting. whereas the plant on the left was from a nonsprayed plot.

I32

L. F. ELLIOT AND D. E. STOTT

oration on the roots-just reduced growth. The severity of the stunting that can be caused by DRB is shown in Fig. 7 with the inoculated roots on the right. Pseudomonads isolated from winter wheat roots and incubated at 5°C or in a medium containing NO; showed significantly greater numbers of TOX+ isolates than those incubated at 20°C in the absence of NO;. Possibly, nutrification of ammo-

Figure 7 An uninoculated check (left) compared with winter wheat plants (right) growing in soil inoculated with a DRB. Reprinted from Soil Biology and Biochemistry 20(2), H. F. Stroo, L. F. Elliott, and R. 1. Papendick, Growth, survival, and toxin production of root-inhibitory pseudomonads on crop residues, pp. 201-207, 1988, with kind permission from Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington OX5 IGB, UK.

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Table I Effect of Root Zone Temperature and Soil Q p e on Root Populations of Inhibitory Pseudomonads Rhizoplane population (cfu mg-' dry root X Soil Palouse

Temperature ("C)

Time" (days)

S 10

22 16 10 22 16 10

IS Ritzville

5 10 1s

88 rif'"

3.3 1.3 I .o 7.3 6.4

9.8

LSD 0.05

4.3

NTIS rif'"

I .7 2.0 4.8 11.0 8.5 12.0 7.1

Note. Reprinted from Soil Biology and Biochemistry 19(2), J. K. Fredrickson, L. F. Elliott, and J. C. Engibous, Crop residues as substrates for host-specific inhibitory pseudomonads, pp. 127-134, 1987. with kind permission from Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington 0x5 IGB, UK. "Length of time required at the given temperature to reach the two-leaf stage (Fredrickson and Elliott, 198Sa).

nia fertilizer and the overwinter cold induces toxin expression by the inhibitory pseudomonads (Rovira et al., 1990). The pseudomonads produced a toxin that reduced winter wheat root and Escherichia coli C l a growth. However, this antibiosis toward E. coli and wheat seedling root inhibition in agar was reversed by Lmethionine (Fredrickson and Eliott, 1985a,b). No correlation between plant root amino acid exudation and plant susceptibility to the toxin could be demonstrated (J. Fredrickson and L. Elliott, unpublished data). Studies were conducted on the wheat root colonization potential of rifampicin resistant mutants of two inhibitory pseudomonads as affected by temperature and soil type (Table I). Temperature did not affect winter wheat root populations of isolates B8 riflm or NT15 but soil type did. The organisms colonized the wheat roots more vigorously in the Ritzville than in the Palouse soil. The Ritzville soil is much lower in total C (0.73%)and microbial biomass (1 1 4 6 pg biomass C g-' soil) than the Palouse soil (2.09% C and 450490 pg biomass C g- soil). However, when the soils were pasteurized, wheat root colonization by the mutants B8 riflm and NT15 rif" was still significantly greater in the Ritzville than in the Palouse soil (Frederickson and Elliott, 1985a). The main difference between the two soils is C content, whereas physical and chemical properties are similar. These data show that the inhibitory bacteria were very aggressive root colonizers in view of the number that colonized the roots under nonpasteurized conditions. Colonization increased approximately

'

134

L. F. ELLIOT AND D. E. STOTT

I log phase when the soil was pasteurized showing that there was competition from the residual microbial biomass. However, the effect was proportional, which may indicate that size of the microbial biomass is the controlling factor. Unfortunately, we have no measure of population diversity. Evidence is accumulating that DRB are affected by crop rotation (Schippers et a/., 1987; Rovira et a/., 1990). Schippers et af. (1987) found that yields of wheat and especially potatoes (Solanum tuberosum) were sensitive to the frequency of the rotation but were unable to attribute the low yields to known soil-home pathogens and postulated that the yield reductions resulted from nonspore-forming DRB. Studies in the Pacific Northwest on DRB pseudomonads indicated that as the frequency of winter wheat in the rotation increased, the populations of DRB (pseudomonads) increased on the wheat rhizoplane (Rovira er af., 1990). Other causes of the plant growth retardation cannot be ruled out, and the direct effect of the DRB cannot be established in natural soil with current technology. Additional studies are needed to determine factors favoring DRB because the presence or ahsence of these organisms could be an important measure of sustainability of a cropping system. In related studies, the colonization potential and root inhibition by NT15 rifIo0 and B8 rif'* on winter wheat, spring wheat, winter and spring barley (Hordeum vufgare L.), oats (Avena saliva L.), lentils (Lens culinaris L.), and peas (Pisum sutivum L.) were compared (Table 11). Root growth of winter wheat was most severely inhibited by the bacteria followed by spring wheat and winter barley, where-

Table I1 Wheat Rhizoplane Populations of Inhibitory Pseudomonads and Their Influence on Crop Root Growth

NT15 rifItMknu

Crop

~8 ripin)<,

Root length (% of control)

Population (log cfu mg-' dry root)

48 56 74 100 I08 86 85 12

5.16 5.11 5.63 5.98 563 5.38 4.60 0.35

Root length (% of control)

(log cfu mg dry root)

~

I

~~~

Winter wheat Spring wheat Winter barley Spring barley Oats Lentils Peas LSD (0.05)

41 54 61 80 85 90 87 13

5.72 5.34 6.08 6.04 6.08 5.12 5.20 0.4 I

"Resistant to rifamipicin at 100 p g ml-l [(from Fredrickson and Elliott (198%) with permission].

INFLUENCE OF NO-TILL CROPPING SYSTEMS

135

as spring barley, oats, lentils, and peas were either not significantly different from the controls or only slightly inhibited. The magnitude of rhizoplane colonization did not correlate with root inhibition (Fredrickson et al., 1987). Other studies have shown that the inhibitory effect resulted from the production of a toxin that exhibited plant specificity (Fredrickson and Elliott, 1985b), which agrees with the colonization data in Table 11. The toxin appeared to be unique from toxins produced by recognized plant pathogenic bacteria (Bolton et al., 1989). It was mentioned previously that the organisms appeared to be associated with no-till seeding into heavy crop residues. Evidence is accumulating that some of these detrimental organisms may be closely associated with the previous crop residue, especially when the residue remains on the soil surface. When nonsterile winter wheat straw was inoculated with a rifampicin-resistant inhibitory pseudomonad at 1/1000th the population of the native bacterial flora and incubated at 5 and 15"C, the organism multiplied rapidly to almost the same numbers as the native bacterial flora. Fairly high proportions of the introduced rifampicin-resistant mutant were also obtained from growth on pea residue, winter barley straw and chaff, and winter wheat chaff (Fredrickson et ul., 1987). Another series of studies were conducted to determine the association of the inhibitory pseudomonads with crop residues in the laboratory and field. A nonfluorescent rifampicin-resistant Pseudomonas sp. capable of producing a toxin inhibitory to the growth of wheat roots was inoculated onto nonsterile wheat straw at lo4 or lo6 colony-forming units (cfu) g-' dry weight and incubated at several water potentials. Water potential between -0.6 and -0.9 MPa appeared optimum for the introduced microorganisms, which reached populations in excess of 1O'O cfu g-I within 6 days of incubation and accounted for up to 80% of the bacteria recovered on tryptic soy agar. These data show the highly competitive ability of these organisms to colonize straw. Bacteria inoculated onto barley residues in the field in October maintained populations greater than lo6 cfu g-' straw throughout the winter and declined in mid-March when the residues dried (Fig. 8). Populations on residues in the no-till plots were approximately 10-fold higher than those in the tilted plots. It was also found that the organism B8, sprayed on the straw, colonized the rhizoplane of the winter wheat no-till seeded into the residues and the plants showed root growth inhibition (Stroo et al., 1988). These data show crop residues have the capability of carrying and transmitting these organisms. The results also indicate that residue management will dramatically affect populations of inhibitory microorganisms. Although the organisms showed specificity toward the plants tested, they had a broad antimicrobial activity (Bolton et ul., 1989). The aggressive growth of these organisms on plant roots and residues indicates that they compete well for available substrate. In other studies, a pseudomonad inhibitory to winter wheat growth, RC 1, was mutagenized with the Tn5 transposon to obtain TOX- mutants. TOX(loss of inhibition to both E. coli C l a and wheat root growth) and partial TOXf (partial loss of inhibition of E. coli Cla and wheat root growth) Tn5 mutants were

L. E ELLIOT AND D. E. STOTT

136

Uninoculated

0

100

50

Days

150

Figure 8 Numbers of rifampicin-resistant pseudomonads on barley residue in tilled and no-till plots at varying times after inoculation of the residue (from Stroo et al., 1988).

isolated to be compared with a TOX rifampicin-resistant spontaneous mutant. When the RC1 rif (rifampicin TOX+ spontaneous mutant) was coinoculated with the TN5 mutant, colonization proportions remained roughly the same (Table 111). The wild-type mutant ratios changed somewhat but one would have to conclude that, in this case, toxin production by these organisms is not their primary mechanism of competitive advantage for root colonization (Kennedy et al., 1992). The data also indicate that the TOX- bacteria could be used to alleviate the deleterious effect of these types of inhibitory bacteria because the TOX+ rif-TOX- combination did not inhibit plant growth. It appears that the effects of these organisms will have to be considered during the development of sustainable cropping systems because they can decrease the efficiency of the system. The organisms are predominately associated with heavy crop residues and no-till seeding. It is possible that these organisms will become more of a problem as crop rotations are decreased. Changing tillage to no-till affects C sequestration in the system and also changes the system ecology. Hendrix et al. (1986) showed that the no-till system was dominated by fungi and earthworms, whereas the biota of the conventional till was dominated by bacteria, nematodes, and enchytraeids in studies of Georgia soil. Kennedy (Turco et al., 1994) showed that substrate utilization patterns of microorganisms varied depending on the history of the soil from which they were isolated. Substrate utilization patterns were different when no-till seedings were compared with conventional tilled seed+

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Table Ill Colonization Potential of Pseudomonas sp. Strain RCl Mutants on the Rhizoplane of Wheat When Inoculated into NonautoclavedSoil at lo6 CFU g- * Dry Soil" Competitive ability

I : l Ratio Colonization potential

Mutant RClr RClh RClc RCle RClf RClo Control

log CFU g-I root

Rooushoot ratio

5.48a

0. I8a 0.25a 0.28b 0.28b 0.27b 0.21a 0.28b

5.65a 5.58a

5.28a 5.67a 5.58a -

RClr

mutant

IogCFUg-'

root

-

-

4.68 4.46 4.49 4.63 4.57

4.60 4.64 4.49 4.66 4.57 -

-

Wild typePJtant ratio

1.21b 0.78a 1.mdb 0.96a 1 .OOab -

Note. From Kennedy et al. ( 1992) with permission. "Values followed by a different letter in the same column are significantly different ( p \ < 0.05 level, tz = 6). Rifampicin spontaneous mutant, TOX' b,c,e,f,o Tn5 mutant.

ings and when rotated plots were compared with nonrotated plots. The differences in C utilization likely reflected changes in microbial diversity. These findings show that tillage and residue management dramatically affect system ecology. It appears likely that DRB play a role in the efficiency of cropping systems and, therefore, are a consideration for sustainable cropping systems. The numbers of these harmful root colonists may indeed be a measure of the sustainability of cropping systems. It seems that DRB are affected by residue management, rotation, and tillage. They may even be an indicator of soil quality. Substrate utilization patterns may also be a measure of soil quality and ultimately the sustainability of a cropping system.

V. DELETERIOUS RHIZOBACTERIA FOR WEED CONTROL Studies have shown that deleterious rhizobacteria have the potential to be used as biological weed control agents (Cherrington and Elliott, 1987; Kremer, 1987). Several DRB isolates were obtained from the root surface of winter wheat and downy brome (Bromus tectorum L.). After testing in the laboratory and growth

138

L. E ELLIOT AND D. E. STOTT

chamber, three isolates were suitable for field testing. Plots were established at three sites and after the winter wheat was seeded in the fall the isolates were applied at lo8cfu m-*. At the Washtucna and Eureka sites, the isolates D7 and 2VI 9 significantly reduced the downy brome pouplation. At harvest, downy brome control by isolates D7 and 2V19 significantly increased winter wheat yield at the Washtucna and Eureka sites (Table IV).Yields were increased 3.5 and 18%, respectively, and the winter wheat population was unaffected (Kennedy ef d., 1991). Weed density, biomass, and seed production were reduced. Downy brome reduction is shown by the plot treated with D7 (Fig. 9a) and the untreated check (Fig. 9b). There was no benefit from the application of the organisms at the Dayton site. The wheat was well established at the Dayton site before downy brome growth began so the weed was not as competitive as it was at the other two sites. Also, the Dayton site was conventionally tilled and seeded, whereas the Washtucna and Eureka sites were no-till seeded. There was no surface crop residue at the Dayton site. Downy brome control in Kentucky bluegrass by D7 is shown in the foreground compared with the background (Fig. 10). Current research efforts on the use of DRB for weed control were summarized by Kremer and Kennedy (199.5). They stated that weeds cause greater economic losses on agricultural lands than all other pests combined. They summarized the use of DRB for weed biocontrol as follows: The DRB strategy is to regulate or suppress weed growth, the mode of action of DRB is primarily through the production of phytotoxins, and the DRB should have adequate specificity and efficacy so the weed is inhibited and the crop is not harmed. DRB have been obtained that are effective against both narrow- and broad-leaf weed species, and there has been some suc-

Table IV Winter Wheat Population and Yield from Fields Inoculated withRhizobacteria and Planted inwinter Wheat at Three Locations in Eastern Washington Washtucna

NI" D7 2V19 3366

30 31 32 29

Eureka

3230 4360* 4 I OO'* 3490

30 30 28 30

Dayton

3200 3780*

3620*

25 28 25

3180

23

3860 4070 3620 3500

Nore. From Kennedy el a / . (1991) with permission.

"NI, noninoculated (Kennedy et al., 1991). *Statistically different from the noninoculated control from the Same location at the 0.05 level using Dunnett's LSD.

INFLUENCE OF NO-TILL CROPPING SYSTEMS

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Figure 9 (a) No-till seeded winter wheat with downy brome in the interrow retarded and inhibited by the application of D7. (b) lnterrow growth of untreated downy brome in no-till seeded winter wheat (from Kennedy CI a / . , 1991).

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L. F. ELLIOT AND D. E. STOTT

Figure 10 Inhibition of the weed downy brome in Kentucky bluegrass seed stand by the application of 1 X lo8 deleterious rhizobacteria organisms per square meter (foreground) compared with the untreated, check background.

cessful application in the field. Currently, there are several projects under way in Canada and the United States. They discussed some studies that indicate DRB may be more effective when combined with low rates of herbicides. The potential use of DRB with other biocontrol agents was also mentioned. The successful use of DRB for biological weed control still suffers from unpredictability. However, this is the case for many biological control agents (Elliott and Lynch, 1995). The development of successful weed biocontrol approaches using DRB will require additional knowledge in several areas, including (1) establishment of guidelines and procedures for isolating and selecting organisms; (ii) determination of root colonization mechanisms; (iii) design of effective carriers including the possibility of using crop residues previous studies have shown strong competitive ability of winter wheat DRB growing on crop residues-this may be a useful approach for the use of DRB for weed control; (iv) determination of the mechanism of growth inhibition (current information strongly implicates phytotoxins but the evidence is not conclusive); (v) the mechanism regulating specificity must be determined; (vi) the role of DRB with herbicide use must be explored more thoroughly; and (vii) the effect of field management practices such as seeding method (till versus no-till), fertilizer management, etc., on DRB biocontrol strategy requires more investigation. For example, as mentioned previously, preliminary data indicated that NO;

INFLUENCE OF NO-TILL CROPPING SYSTEMS

141

and incubation at 5°C increased the number of TOX+ DRB isolated from the rhizoplane of winter wheat roots. The potential for the use of DRB for weed biocontrol appears good. The approach is environmentally friendly and, if successful, should be beneficial to the development of sustainable cropping systems.

VI. LOW-INPUT, ON-FARM COMPOSTING Crop yields often suffer when conservation tillage systems are implemented (McCalla and Army, 1961; Papendick and Miller, 1977). These yield reductions have been attributed to a variety of problems. These include short-chain fatty acids produced during residue decomposition (Cochran et al., 1977; Lynch et al., 1981), infection by plant pathogens such as Pythium sp. (Cook et al., 1980); and colonization of roots by deleterious rhizobacteria (Alstrom, 1987; Fredrickson and Elliott, 1985b; Schippers et al., 1987; Suslow and Schroth, 1982). Hairpinning of the residues around the seed can result in poor seedling growth because of poor seed zone environmental conditions (Elliott et al., 1984). No-till seeding into heavy crop residues can cause high crown set when the residues fall back onto the seed row (Fig. 11). This is very undesirable because the exposed roots are subject to drying, herbicide effects, and inhibitory substances produced during straw decomposition. Many of these problems are more severe as residue production becomes heavier. The solution has been to bum the residues. Burning of residues is causing increased public concern because of air pollution. Various options such as using the residues for heat or power production have been explored but are not economically feasible at this juncture. The management of heavy residues for conservation tillage systems has been an unyielding problem in many cases. Heavy specialized no-till drills that can seed through the heavy crop residues and manage them effectively have been developed. However, the drills are too expensive for many farmers. The development of low-input, on-farm composting of high C/N ratio residues is providing an innovative residue management option and the potential for assisting the development of sustainable farming systems. Composting of crop residues will overcome the negative aspects of crop growth using conservation tillage systems where crops are seeded back into heavy crop residues and will provide added benefits when put back onto the field. The potential benefits of crop residues to soil structure have been demonstrated (Elliott and Lynch 1984a).The possible benefits of compost for alleviating some soil-borne diseases has also been shown (Hoitink et al., 1991). Compost additions may alleviate problems associated with DRB. These possibilities have not been explored thoroughly. The benefits of compost applications also include fertilizer content, soil conditioning value, and benefits to soil quality (Bangar er al., 1989; Nelson and Craft, 1992; Thomsen, 1993; Zaccheo e? al., 1993). Compost applications have shown large benefits for land reclamation (Dick and McCoy, 1993). Optimum use of crop residues for conservation tillage systems was

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L. F. ELLIOT AND D. E. STOTT

Figure 11 High crown set in winter wheat caused by seeding into heavy crop residues and by not keeping the residues out of the seedling row.

not feasible until the development of low input, on-farm composing of high C/N ratio crop residues. Churchill etal. (1995) have developed the approach for grass seed straw. Our laboratory studies show the process will work as well with wheat and rice (Oryzasativa L.) straw. Studies by Honvath and Elliott (1995a,b) and Horwath et al. (1 995) have explained the mechanisms of the process, which appears to be rapid delignification. Previously, it was thought that successful composting required a combined substrate C/N ratio of 30/1 or less (Biddlestone et al., 1987). If the C/N ratio was higher than 30/1, it would have to be cocomposted with an organic material such as manure or sludge to reduce the C/N ratio. Cocomposting greatly complicates the process for on-farm application. The low-input, on-farm composting method consists of gathering the straw into large piles at the side of the field. When rainfall occurs the stacks are turned to allow maximum water intake. The turning with a front-end loader also compacts the straw, which helps the stack to retain heat. Over winter and early spring the straw is turned when temperatures cool. The compost is ready for field application after about three turns in as little as 16-weeks time (Churchill et al., 1995). Even with two turns with a commercial compost turner, after 16 weeks less than 20% of the original volume of straw remained (Fig. 12). Studies on the economics of the process and the value of the compost additions to succeeding crops are incomplete.

INFLUENCE OF NO-TILL CROPPING SYSTEMS

1

143

100 80 6o

YOof original volume

40 20 1 Weeks

0

-ns Figure 12 Percentage of original volume remaining in grass straw wind rows with zero to six turns (from Churchill e t a / . . 1995, with permission).

In laboratory studies to determine the mechanisms of high C/N ratio substrate composting, grass straw was incubated under mesophilic and thermophilic conditions. After 45 days of incubation, the loss of lignin C in the Klason lignin fraction was 25 and 39% under mesophilic and thermophilic conditions, respectively. Changes in elemental composition indicated that 94% of the lignin fraction had been altered under both incubation conditions. These data showed that changes in the lignin fraction were much more extensive than measured by the Klason method. C mineralization from the straw was 46 and 52% under mesophilic and thermophilic conditions, respectively. The addition of N decreased the rate of C mineralization. C mineralization per unit of microbial biomass under thermophilic conditions was twice that under mesophilic conditions. These data indicate that the microbial biomass was less efficient under thermophilic conditions, which lead to greater C mineralization per unit of microbial biomass. These data also established that the C and N pathways were largely independent. Plate counts of bacteria, fungi, and actinomycetes did not show any definite patterns between the two incubation conditions. These studies indicate that there is the possibility for regulating the quality of the compost end-product (Horwath and Elliott 1995a,b; Horwath el d., 1995). Low-input, on-farm composting could be a valuable asset to developing sustainable cropping systems. Low-input, on-farm composting will allow no-till seeding or shallow conservation tillage on fields that have contained heavy residues. These cropping techniques will be possible without the risk of severe yield reductions of the succeed-

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L. E ELLIOT AND D. E. STOTT

ing crop if the heavy residues can be removed before seeding and composted. It is also likely that weed control problems will be expedited in the absence of the heavy residues. Finally, the lack of extensive tillage and return of the residues should enhance soil quality over systems using conventional tillage practices.

REFERENCES Alstrom, S. (1987). Factors associated with detrimental effects of rhizobacteria on plant growth. Plum Soil 102,3-9. Bangar, K. C., Shanker, S., Kapoor, K. K., Kukreja, K.,and Mishra, M. M. (1989). Preparation of nitrogen and phosphorus-enriched paddy straw compost and its effect on yield and nutrient uptake by wheat (Triricum uestivurn L.). Biol. Fertil. Soils 8,339-342. Biddleston, A. J., Gray, K. R., and Day, C. A. (1987). Composting and straw decomposition. In “Environmental Biotechnology” (C. F. Forster and D. A. Wase, Eds.), pp. 135-175. Wiley, New York. Bolton, H., Jr., Elliott, L. F., Gurusiddiah, S., and Fredrickson, J. K. (1989). Characterization of a toxin produced by a rhizobacterial Pseudomonus sp. that inhibits wheat growth. Plant Soil 114, 279-287. Brian, P. W., Wright, J. M., Stubbs, J., and Way, A. W. (1951). Uptake of antibiotic metabolites of soil microorganisms by plants. Nurure 167,347-349. Bristow, K. L., Campbell, G. S., Papendick, R. I., and Elliott, L. F. (1986). Simulation of heat and moisture transfer through a surface residue-soil system. Agric. FOEMeterol. 36, 193-2 14. Brown, P. L., and Dickey, D. D. (1970). Losses of wheat straw residue under simulated field conditions. SoilSci. Soc.Am. Proc. 34, 118-121. Cherrington, C. A., and Elliott, L. F. (1987).Incidence of inhibitory pseudomonads in the Pacific Northwest. Plunr Soil 101, 159-165. Churchill, D. B., Bilsland, D. M., and Elliott, L. F. ( I 995). Method for composting grass seed straw residue. Appl. Eng. Agric. 11(2), 215-279. Cochran, V. L., Elliott, L. F., and Papendick, R. 1. (1977). The production of phytotoxins from surface crop residues. Soil. Sci. SOC.Am. J . 41,903-908. Collins, H. P., Elliott, L. F., and Papendick, R. 1. (1990a). Wheat straw decomposition and changes in decomposability during field exposure. Soil Sci. SOC.Am. J. 54, 1013-1016. Collins, H. P., Elliott, L. F., Rickman, R. W., Bezdicek, D. F., and Papendick, R. I. (1990b). Decomposition and interactions among wheat residue components. Soil Sci. SOC.Am. J. 54,780-785. Cook, R. J., Sitton, J. W., and Waldher, J. T. (1980). Evidence for Pyrhium as a pathogen of direct drilled wheat in the Pacific Northwest. Plunr Dis. 64, 102-103. Curry, J. 0. (1969). The decomposition of organic matter in soil. Part I. The role of the fauna in decaying grassland herbage. Soil Biol. Biochem. 1,235-258. Dick, W. A.. and McCoy, E. L. (1993). Enhancing soil fertility by the addition of compost. In “Science and Engineering of Composting: Design, Environmental, Microbiological and Utilization Aspects” (H. A. J. Hoitink and H. M. Keener, Eds.), pp. 622-644. Renaissance, Worthington, OH. Douglas, C. L., Jr.. Allmaras, R. R., Rasmussen, P. E., Ramig, R. E., and Roager, N. C., Jr. (1980). Wheat straw composition and placement effects on decomposition in dry land agriculture of the Pacific Northwest. Soil Sci. SOC.Am. J. 44,833-837. Douglas, C. L., Jr., and Rickman, R. W. (1992). Estimating crop residue decomposition from air temperature, initial nitrogen content, and residue placement. Soil Sci. SOC.Am. J. 56, 272-278. Elliott, L. F., and Lynch, J. M. ( I 984a). The effect of available carbon and nitrogen in straw on soil and ash aggregation and acetic acid production. Plunr Soil 78,335-343.

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PRACTICAL ETHICSIN AGRONOMIC RESEARCH Don Holt College of Agricultural, Consumer, and Environmental Sciences University of Illinois Urbana. Illinois 61801

I. Introduction 11. Basic Concepts A. Definitions B. Personal and Group Ethics C. Scientific Misconduct D. Practical Principles of Ethical Behavior 111. Ethics of Choosing Research Subject Matter A. Agricultural Ethics and Sustainable Agriculture B. Ethical Dimensions of Agricultural Themes C. Criticism of the Science Paradigm D. Resolving Ethical Disputes N. Difticulties with the Utilitarian Approach A. Difficulty in Evaluating Outcomes B. The Sea of Uncertainty C. Utilitarian Evaluation of Principles D. Ethical Codes as Rules of the Game E. Abiding by the Rules V Agricultural Ethics and the World Food Situation A. Driving Forces B. Moot Questions C. Sources of Support for Research D. %ke-Home Message for Agricultural Scientists VI. Ethics in the Conduct of Research A. Initiating Research B. Ethics and Scientific Documents C. Whistle-Blowing D. Conflicts of Interest E. Intellectual Property Rights F. Ethical Treatment of Animals in Agronomic Research

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Ethics and Competition Performing to Specifications Ethical Issues Associated with Technology Transfer Ethics of Consulting VII. Ethics in Research Administration A. Ethical Hiring and Termination B. Nurturing Scientists C. Fair Evaluation D. Honest Communication with Constituents References

I. INTRODUCTION The first part of this chapter is a discussion of broad ethical and philosophical issues. The second part focuses on the role of ethics in day-to-day decisions and actions taken by agronomic researchers and research administrators in the practice of research. I think most of the ideas presented are relevant to other areas of food and agriculture research as well. Perhaps not surprisingly, not much has been written in the agricultural literature about the ethical dimensions of our everyday tasks. Apparently, we take it for granted that everyone who is educated enough to be a researcher knows how to behave ethically as a researcher. In introspecting on this topic, I find that most dayto-day decisions I make have some ethical content, but I do not perform a systematic ethical analysis on each one. From an ethical standpoint, I just do what feels right. When I tried to articulate in this chapter what feels right ethically, I encountered a situation very similar to the one I encountered when some colleagues and I developed our first computer simulation model. I found that my mental model of ethical behavior for agronomists was at least as vague as my mental model of alfalfa growth and development had been, even though I was supposed to be an alfalfa expert. It was hard to put it on paper or describe it precisely in terms of processes. In this chapter, I share with you the results of my effort to develop a clearer conceptual framework for the important ethical decisions with which agronomic researchers are confronted every day. Because I am not trained in the disciplines of ethics or philosophy, I cannot produce a scholarly treatise on this subject. I can only make a personal statement, which is no doubt very much conditioned by my own unique genetics, cultural background, upbringing, education, and experience.

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II. BASIC CONCEPTS A. DEFINITIONS I think of ethics as a process by which we distinguish between right and wrong behavior. The concepts of bad and good are different than right and wrong. Thus, ethical behavior and unethical behavior can each lead to both good and bad consequences. Ethics is also a discipline sometimes referred to as moral philosophy. The adjective ethical connotes moral approval and implies that the actions or behavior it describes are in accord with some approved standard or code. The standard or code may be formally stated or implicit. Unethical, of course, describes behavior that does not comply with accepted standards or codes. The word ethic is sometimes used to describe a particular group of moral principles or practices, e.g., the Christian ethic.

B. PERSONAL AND GROUPETHICS Each of us has a personal code of ethics, that is, some internal standard of behavior. We might find it difficult to articulate the code, but it is evident in our behavior. Needless to say, personal codes of ethics differ a lot among individuals. It is worthwhile to examine our personal codes of behavior to see how well they conform to codes of ethics and behavior of various groups with which we identify. To some extent, ethical and unethical behaviors are learned behaviors, partly the result of our past experience, including early childhood experience. Given counseling, both adults and children are known to change patterns of behavior, usually but not always from unethical to ethical behavior. This lends further support to the idea that ethical behavior is learned. One should not underestimate the power of early indoctrination in moral and ethical concepts. Minimally, such training enables humans to function and share responsibilities within family and larger groups, thus providing a basis for civilized life. At the extreme, such indoctrination apparently can cause persistent and paralyzing feelings of shame and guilt. Some people must anesthetize those feelings to function and thereafter their behavior is not inhibited by conscience. An important function of education and socialization should be to help people interpret the inner voice and subject its admonitions to rational analysis. Heichel (1 99 1 ) challenged agronomists to develop or adopt a code of ethical conduct to reassure various constituencies. The American Society of Agronomy (ASA) officially adopted a Statement of Ethics in late 1992. The statement is

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reprinted frequently in the various journals published by ASA. Other scientific societies developed or adopted codes of ethics fairly recently. The flurry of interest in codes of ethics was triggered by different factors in different areas of science.

c. SCIENTIFIC MISCONDUCT Among other outcomes of some well-publicized cases of alleged scientific misconduct in the 1980s, the National Academy of Sciences, National Academy of Engineering, and Institute of Medicine convened a Panel on Scientific Responsibility and the Conduct of Research (1992). The panel’s report identifies falsification and fabrication of data and plagiarism as scientific misconduct. These clearly unethical practices are distinguishedfrom another list of practices that the panel labels “questionable scientific practices.” The latter include such practices as failing to retain important data for a reasonable period of time, maintaining inadequate research records, denying peers reasonable access to materials or data supporting published papers, inadequately supervising research subordinates and exploiting them, and several others. If these questionable practices are part of an intent to deceive or mislead, they are clearly unethical. In many cases, however, they are the result of neglect or sloppiness, which are in a gray area from the standpoint of ethics. The panel identified a third category they labeled “other misconduct.” This includes behavior that is clearly unethical but not unique to science. Behaviors such as sexual harassment, misuse of funds, and vandalism of research facilities fall into this category.The three categories were perceived by the panel to be distinctly different and require different remedial approaches and disincentives.

D. PRACTICAL PRINCIPLES OF ETHICAL BEHAVIOR From the extensive discussion of unethical behavior, one can draw some conclusions about what constitutes ethical behavior. In my opinion, there are four key, practical principles of ethical behavior. They are honesty, integrity, fairness, and service. Although we may not agree precisely on the meaning of these words, I think that everyone likely to be reading this chapter has concepts of honesty, integrity, fairness, and service. As each of us attempts to give them meaning in our everyday lives and to apply them in individual research situations, we do not have to start from scratch. My concepts of honesty, integrity, fairness, and service, and the way they have been influenced by others, are as follows. Honesty is the quality of truthfulness. Senge (1990, p. 159) identifies telling the truth as a simple but “profound” strategy for achieving goals. He says commitment

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to the truth is “a relentless willingness to root out the ways we limit or deceive ourselves from seeing what is, and to continually challenge our theories of why things are the way they are.” I will illustrate my concept of integrity with a story. Many years ago, a visitor whose name and affiliation I have long since forgotten spoke at Purdue University. He lamented the loss of a “national secular ethic” in the United States. The principle or code he wanted restored could be stated simply as follows: unethical behavior is wrong even if no one finds out about it. His evidence for the loss of a national secular ethic included studies of public response to reports of cheating on quiz shows, shoplifting in grocery stores, and refusing to cooperate with police. In each case, a majority of persons surveyed judged the behavior to be unethical only if the perpetrators were caught. They did not seem to identify with either the perpetrators of the unethical behavior or the persons who were injured by it. They apparently recognized the difference between honesty and dishonesty but did not have a concept of integrity. Integrity broadens the concept of honesty to include the context in which honest or dishonest behavior occurs. You have integrity if you are honest when you will likely suffer bad consequences from being honest andor you are unlikely to suffer bad consequences from being dishonest, perhaps because you will not be found out. Fairness involves treating other people fairly and not seeking to gain unfair advantage. Obviously, this is not a definition because I used derivations of the word fair in the explanatory statement. Fairness has dimensions of equal treatment, equal opportunity, and the “level playing field.” The reader already has a concept of fairness, although we might arrive at different answers if asked to identify fairness or lack of it in any specific situation. Perhaps we could resolve that difference through a good analysis of costs and benefits in that situation; perhaps not. Agronomists generally value science in the service of mankind (Holt, 1989). The concept of service as ethical behavior seems consistent with the utilitarian approach to ethical decision making and with seeking to optimize benefits. The fact that an individual or group stands to benefit from service to others, i.e., enlightened self-interest, does not diminish the value of a particular act of service. Of course, totally unselfish service is held up as an ideal by many groups, for example, “greater love hath no man . . . Covey (1989) lists fairness, honesty, integrity, and service among other principles on which one should center hisher life and base hisher decision making. He believes these are self-evident values and thus may be to some extent innate. For example, little children on the playground may be heard to exclaim, “That isn’t fair!” They seem to have a sense of fairness even before they have been taught the full meaning of fairness. Although 1 know of no data on this question, I perceive that many if not most ”

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U.S. agronomic researchers are associated in some way with organized religion. Religions usually prescribe codes of ethical behavior, establishing these by appeal to the authority of a deity or prophets. A basic philosophical issue is whether (i) we should behave ethically because it is the will of a higher power or authority or otherwise woven into the fabric of nature or whether (ii) we should behave ethically because it is the practical thing to do. The latter stance is congruent with a utilitarian approach to ethical decision making and with the concept of enlightened self-interest. For purposes of this discussion, I do not think it makes any difference why one accepts these as ethical principles. Even if one accepts them as divinely revealed, decisions on how to apply them will inevitably be utilitarian because each such decision depends on the situation. If we accept these principles, we have a basis for deciding what constitutes ethical behavior in specific practical situations. We can ask, “Is this behavior honest and fair? Is this action characterized by integrity and service?’ Of course, accepting these principles is just the first hurdle. Answering the questions for specific situations is often extremely difficult. Nevertheless, in my opinion if we can, with integrity, answer yes to the questions and we have not deluded ourselves, the behavior is ethical, even if the consequences are ultimately bad. In many situations, only the person making the decisions will know if the resulting behavior is ethical. A commitment to the principles previously discussed establishes the foundation for ethical behavior, but adherence to the principles does not guarantee good outcomes or optimal benefits. Unfortunately, the old maxim that “the road to hell is paved with good intentions” applies in this situation.

III. ETHICS OF CHOOSING RESEARCH SUBJECT MATTER A. AGRICULTURAL ETHICSAND SUSTAINABLE AGRICULTURE The focus of ethical concern in the biomedical field (Monsen er al., 1991) is on behavior that is generally agreed to be right or wrong, such as the misconduct identified by the panel described previously. In a sense, ethical concern in the biomedical field was very inwardly focused. The published literature on agricultural ethics, however, is much more outwardly focused, addressing much more controversial issues, especially the selection of subject matter for research (Thompson, 1988). I think the sustainable agriculture movement played a key role in sensitizing agronomists to ethical issues in research and development (R&D).In the 1980s,

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sustainable agriculture advocates maintained that some kinds of research, even if conducted properly, lead to technologies that damage the environment, consume inordinate amounts of natural resources, andor render small, family farms less able to compete in highly competitive markets. Many agricultural scientists were taken aback when accused of being unethical because their research addressed a certain subject matter. They had neither fabricated nor falsified data nor committed plagiarism, the sins usually associated with scientific misconduct. They thought they had performed a valuable service by creating new or improved technology that was adopted in practice. One of my colleagues, a soil fertility specialist, said, “I feel like they are trying to nullify my whole career.” I do not remember a time when agronomic scientists were not occasionally criticized by people who thought they were unduly influenced by private sponsors of research, especially when such sponsors were suppliers of inputs for production, including fertilizers, pesticides, seed, etc. Some of these critics suggested that agronomic researchers actually falsified or contrived data to please their sponsors and assure continued support. Such behavior, of course, would be unethical.

B. ETHICAL DIMENSIONS OF AGRICULTURAL THEMES People trained in philosophy and ethics became interested in agriculture and have written extensively about agricultural ethics, especially in the past decade. No attempt was made to review that extensive literature here. I find the aforementioned article by Thompson (1988) to be a useful summary of broad ethical issues in agriculture. I drew heavily on it for the perspectives provided in this section. Thompson (1988) identifies productivity as the ethical theme of traditionakonventional agriculture and agricultural research and lists three other themes favored by critics. Critics argue that new technology is developed and implemented in conventional agriculture without sufficient concern for its externalities, that is, its potential effects on environment, international justice, and agrarianism. I generally use a slightly different classification of the cross-cutting issues or themes, namely, environment, food and worker safety, natural resources, and social impacts. The externalities are, by definition, effects that are not measured or accounted for in R&D leading to new technology. Some critics of traditional agricultural R&D are skeptical of attempts to internalize externalities by adding additional dimensions to R&D efforts. Others would go so far as to require an economic and social impact study before a research project could be launched. Most researchers would agree that in many research situations it is impossible or impractical to evaluate all the possible externalities.

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C. CRITICISM OF THE SCIENCE PARADIGM Some critics oppose the traditional agricultural research paradigm, rooted as it is in systematic analysis, experimentation, and statistical analysis. They argue that this approach inevitably ignores important nontechnical issues, does not place enough value on indigenous and intuitive knowledge and practical experience, and fails to anticipate or provide quantitative and qualitative estimates of externalities. The tendency toward reductionism, that is, reducing a system to its component processes and focusing research on individual components, is frequently criticized (Busch, 1989). Critics argue that when the systems are reconstituted by practitioners, changes in component processes cause unforeseen and deleterious changes in the system as a whole. To some extent, reductionism is the result of increasing specialization among scientists. A component of a system can be researched by an individual specialist. Studying the whole system, which is the sum of its component processes plus the interactions among them, almost always requires a team. Critics maintain that current university culture and protocols favor individualresearch and discourage team research.

D. RESOLVINGETHICAL DISPUTES I know few if any people who do not want agricultural technology to be productive, environmentally sound, safe, resource conserving, and socially benign or positive in its impact. The disagreements lie in the relative degree of importance to be attached to these themes, especially in situations in which there are potential trade-offs. People also disagree as to how these differences might be resolved. Some favor absolute legal or regulatory constraints. For example, some would prefer that no research be conducted on chemicals or biotechnology. They view these things as being potentially dangerous because they are not “natural” or because they have potential to disadvantage some categories of producers. Others attribute religious (or at least deep philosophical significance) to one or more of the themes and thus would wish those themes to be overriding considerations in any decision-making process, including decisions on the subject matter of research. Some people try to influence the research agenda andor research outcomes by labeling the attitudes, philosophies, or theologies of researchers or advocating certain philosophies. Elmore (1996) identifies a theocentric philosophy of nature as one more likely to lead to a truly sustainable agriculture than a geocentric, acentric, or anthropocentric view.

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Several years ago there was a symposium on ethics at the annual meetings of the American Society of Agronomy. The moderator got my attention immediately when he stated that people who adopted the philosophy that “farming is a business’’ were likely to conduct research in the context of competition. This would probably lead, in his view, to the development and implementation of new, productivity-enhancing technology that benefitted those who adopted it but disadvantaged those who could not or would not adopt it. The moderator went on to postulate that research predicated on the concept of farming as a business would be especially harmful to farmers in undeveloped nations. He used this as an example of scientists failing to face the fact that decisions on the subject matter of research were value laden. He strongly implied that espousing and even entertaining the concept of farming as a business was unethical. I believe that farming is a business. In fact, I think that agricultural researchers who do not at least consider the business implications of their research may very well obtain results that are at best irrelevant and at worst misleading to their constituents. I agree, however, that research sometimes leads to dislocations and stress, especially among those who are unable or unwilling to apply research results in practice. This is an example of how two people looking at the ethics of the same research situation may arrive at much different conclusions. Researchers create new possibilities and potentials. Typically these possibilities have the potential to have both good and bad consequences. Research itself does not assure that the new possibilities will be realized in practice. The “acid” test of a new technology is whether or not it is successful in the practical environment. In that environment, a new technology not only has to “work” technically but also must work economically and politically. It has to work initially for the producers, processors, etc. and ultimately for the consumers. The ethical problem arises when a new technology works to improve conditions within one so-called ethical theme but causes harm in the context of one or more other themes. To prevent this by precluding certain areas of research, however, prevents both good and bad outcomes. Often, the extent of good and bad cannot be estimated accurately until much of the research is accomplished. The appropriate response to this dilemma, in my opinion, is to internalize as many as possible and practical of the externalities. Thompson ( 1988) describes a “pragmatically modified” utilitarian approach to ethical decision making in the food and agriculture arena. In general, this involves seeking, through R&D, to optimize benefits. The word optimize is used here instead of maximize, recognizing that not all objectives in a multiobjective situation can be maximized simultaneously. There are inevitable trade-offs. Thompson would entertain the imposition of some absolute constraints, if a sound philosophical basis for such constraints were established.

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Iv. DIFFICULTIES WITH THE UTILITARIAN APPROACH

A. DIFFICULTY IN EVALUATING OUTCOMES Although I believe strongly in the utilitarian approach to ethical decision making, I realize that there are great difficulties associated with it. It is often very difficult and costly to assess accurately the goodness and badness of outcomes. Whether outcomes are perceived as good or bad often depends on who is perceiving them. The benefits to individual consumers of any particular new agricultural technology may be small, but because so many individuals are affected the aggregate benefit may be enormous. Of course, if a new technology harms consumers, even marginally, the aggregate harm can be great. Typically, successful new agricultural technology marginally benefits a large number of people, most of whom are consumers, and severely disadvantages a relatively small number of people, most of whom did not or could not adopt the technology. This situation presents challenges to those who weigh good and bad outcomes of research. DDT prevents malaria and harms raptors. Bovine somatotropin conserves resources, makes milk cheaper, and puts smaller, less efficient operators out of business. The mechanical tomato harvester kept the tomato processing industry in California, greatly expanded the processing industry there, but eliminated thousands of stoop-labor jobs that sustained migrant workers. Good short-term outcomes may be associated with bad long-term outcomes and vice versa. Only recently has there been strong disciplinary focus on assessing impacts of research. At best, it is very difficult to predict outcomes. Thus, there is inevitably a great deal of uncertainty associated with the utilitarian approach to ethical analyses. This uncertainty is indeed frustrating and drives many toward a more authoritarian approach.

B. THESEAOF UNCERTAINTY In Atherton’s (1961) book, “The Cattle Kings” (third Bison Book printing), he quotes a “noted” theologian who was supposedly “addicted” to reading western novels. He says, If just once I could stand in the dust of frontier main street facing an indubitably bad man who really deserved extermination, and with smoking six-gun actually exterminate him-shoot him once and see him drop dead. Just once to face real and unqualified evil, plug it and see it drop. (p. 250)

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Alexander Solzhenitsyn (1973) expresses similar frustration in “The Gulag Archipelago.” He says, If only it were all so simple. If only there were evil people somewhere, insidiously committing evil deeds, and it were necessary only to separate them from the rest of us and destroy them. But the line dividing good and evil cuts through the heart of every human being. And who is willing to destroy apiece of his own heart. (p. 168) Many years ago, I participated in a discussion with several scientists, campus ministers, and philosophers and a representative of the Creationist Society. When asked why he always appealed to the authority of the Bible when discussing scientific issues associated with evolution, the young Creationist said, “If you don’t accept the Bible as absolute truth on all issues, you are launched on the sea ofuncertainty.” What the young man said is obviously true. In fact, I gave serious thought to titling this chapter “Navigating on the Sea of Uncertainty.” It was particularly interesting to me that in this conversation it was the scientists and philosophers who became emotional during the debate. They were very frustrated with the young man, who steadfastly refused to engage in rational discourse on the issue of creation versus evolution. With red faces, loud voices, and scathing sarcasm, the scientists and philosophers berated their opponent for what they regarded as his anti-intellectual stance. The young man, obviously serenely confident in his beliefs, admonished the group to be patient and held forth a biblical promise that the scales would fall from our eyes and we would fully understand, as was experienced by St. Paul (Acts 9: 17-19). Needless to say, this merely provided more fuel for the fire. The sea of uncertainty was stormy that day. In many ways, science is a method of navigating on the sea of uncertainty. Scientists generally accept the fact that they will not, in their lifetimes, know or understand anything with complete certainty. We try to estimate the level of uncertainty or probability associated with our hypotheses but recognize that even that estimate is fraught with uncertainty. The best we can do is be committed to the truth as Senge described that commitment. However, that will not carry us through all the ethical dilemmas we may encounter.

C. UTILITAR~AN EVALUATION OF PRINCIPLES One might hope that at least one principle of ethics, e.g., honesty, would be universal in application, thus simplifying ethical evaluation. Recognizing that it is extremely difficult to be objective in interpreting data, we try to be scrupulously honest in reporting our observations so that others may draw their own conclusions if

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they do not agree with ours. If we do not report data honestly, we obviously do not help ourselves or others get a better understanding of reality, which is what we are about. It is not surprising that fabrication and falsification of data and plagiarism are almost universally accepted as scientific misconduct and unethical behavior. In fact, in my experience, these have been treated as unforgivable sins, even if committed by fledgling scientists. Punishment is permanent banishment from the scientific community. There is no practical mechanism of appeal and no effective program of rehabilitation. Once the faith has been broken, it is virtually impossible to reestablish trust. To illustrate the utilitarian nature of this analysis, however, most of us would not think it unethical to falsify or fabricate data to be communicated to an enemy who would use the real data to harm our nation, families, etc. In wartime, scientific subterfuge may represent the difference between victory and defeat (Brown, 1975). People who are able to refrain from telling the truth under extreme duress or even torture are often regarded as heroes. The military situation is described with chilling candor by Sir Garnett Wolseley (as quoted by Brown, 1975) in the “Soldier’s Handbook” published in 1869. Wolseley writes, We are bred up to feel it a disgrace ever to succeed by falsehood . . . we will keep hammering along with the conviction that honesty is the best policy, and that truth always wins in the long run. These pretty little sentiments do well for a child’s copy book, but a man who acts on them had better sheathe his sword forever. (p. 9) Perhaps it is most accurate to say that the principle of honesty or other principles perceived as universal do not apply unless one is willing to take an uncompromising position and suffer the consequences. However, even those with the courage and fortitude to hold to principles regardless of the consequences are usually reluctant to expose to those consequences their families, loved ones, and others for whom they may be responsible. If there are no revealed truths, immutable principles, or even practical, utilitarian generalities, such as the importance of honesty and integrity, that apply to science, how can we determine what constitutes ethical behavior? How can we make any ethical judgments? If there are no points of reference visible from the sea of uncertainty, how do we find our way?

D. ETHICAL CODESAS RULESOF THE GAME It seems to me the analogy of rules of ethics to rules of a game is appropriate and useful in this situation. Ideally, a set of rules makes a game possible, enjoy-

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able, and interesting. It serves to bring out the best in the players while protecting both players and observers from the consequences of unrestrained behavior. Over time, the community of scientists adopted rules in the form of codes of behavior by which we conduct ourselves. Most of those rules are closely tied to the concept of honesty, but the honesty-related rules are different for different realms in which science is practiced. The differences are distinct in the realms of pure science, commerce, and the military.

1. Pure Science As a community of scientists seeking to understand the structure and function of the universe and all its parts, we have, in effect, agreed to “sheathe our swords forever.” We have agreed to pursue an improved understanding of reality, pursue the “truth,” as best we can ascertain it, through sound analysis, experimentation, and rational discourse and not through force of arms or other means. We have agreed that we will be honest in our pursuit of that truth, even and perhaps especially when dishonesty cannot be detected. In science, there are many situations in which false or fabricated data will not be detected until substantial damage has been done or there has been much wasted effort. The scientific community embraces honesty because being dishonest would prevent us from accomplishing our goals; this is a good utilitarian reason.

2. Commercial Realm The principle of honesty holds when science is practiced in support of commerce. To fabricate or falsify data or plagiarize the work of others is definitely unethical in this realm. However, there are situations in the commercial arena when it is both legal and ethical for a scientist to withhold information from other scientists or practitioners. This situation is manifested in such things as trade secrets and, to a lesser extent, patents, which will be discussed later. Most scientists in my acquaintance, especially those who understand and appreciate the concepts of markets, competition, and capitalism, accept the occasional need to withhold information in the commercial realm. They tend to see this as a necessary evil. They would really rather share their findings with other scientists.

3. Military Realm I believe it is ethical for scientists working for or within the military to withhold, and in certain circumstances, fabricate and/or falsify data. Certainly, plagiarizing the enemy’s data would not be considered unethical in the military arena. In that situation, the scientific community within which honesty is mandated is de-

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fined more narrowly. It is “our” scientific community. It may only include the scientists working on a certain project. There may not be full information exchange even among them. If we accept dishonesty as ethical behavior for our military, we must also accept similar dishonesty on the part of our enemies as ethical behavior. Like us, they see it as a necessary evil that will somehow diminish bad consequences. Of course, that recognition does not prevent either us or our enemies from punishing that behavior severely so as to deter it.

E. ABIDINGBY THE RULES Members of the scientific community abide by certain rules if they wish to remain within the community. The rules are not defined as clearly and specifically nor are they documented as thoroughly as, e.g., the rules of basketball and baseball. Much is left to judgment and, at the margin, there is considerable disagreement.The rules are always subject to scrutiny and, by common agreement, can be changed. Nevertheless, the rules have been adequate to allow the “game” of science to be played successfully for several centuries-to the great benefit of mankind. I do not want to take the analogy of ethical principles to rules of the game too far. I would not favor a force of officials looking over our shoulders to assure that we behaved ethically. Ethical behavior needs to become a habit for researchersa commitment to truth, integrity, fairness, and service as described previously. It needs to be internalized. It is the mark of a professional and an emotionally welladjusted human being. It needs to be manifested in everything he/she does. We need to cultivate it in the young people we train to be scientists.

V AGRICULTURAL ETHICS AND THE WORLD FOOD SITUATION I believe the world situation now renders most of the broadly focused ethical debates of the past decade moot. That is one reason I devoted little effort to trying to detail those debates here.

A. DRMNGFORCES Associated with the debates about appropriate themes for agricultural research is an apparent understanding that legislative and regulatory activity, manifested in government programs and influenced by public opinion, drives changes in the food

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and agriculture sector (Flora, 1986). Those who lament the increasing industrialization of agriculture, especially advocates of environment, natural resource, and social impact themes, attribute that change to government influence, manifested through various programs including research. The word industrial as I use it in this context connotes scale and scope economies, vertical coordination, customer focus, and differentiated products. It does not connote mass production, top-down management, or supply-driven strategy. Many groups seem to believe that if they can “educate” the public and the public’s representatives, their preferred themes will emerge as the dominant driving forces and appropriate changes will be implemented by legislation and regulation. They work hard and expend resources to persuade the body politic that theirs is the most ethical approach, compelling need, forceful argument, and sound philosophy. The various themes are constantly reinforced by public statements of government and organizational leaders. It seems to me, however, that agriculture moves ever more rapidly toward the industrial model despite all the debate. The trend strongly suggests that changes in agriculture are being driven by other forces. I think the driving force operates as follows. Urbanization changes the logistics of feeding large, relatively affluent populations. To meet those demands, production and marketing enterprises grow in scale, scope, customer focus, productivity, and efficiency. Information becomes of paramount importance. Relatively affluent consumers go into large, modern grocery stores and select from a very large number of diverse products. The products are carefully arranged on the shelves and well labeled so that consumers can make price, quality, and other comparisons. As they check out, the automated inventory system collects and analyzes information on their choices. Based on this information, powerful messages go back down the value chain. The messages state, in amazing detail, We want a very diverse selection of high quality, safe, convenient, and affordable food and agriculture products and services. We want them in very large quantities and we want them available close to our homes. If you can’t provide what we want, we will find somebody who can. PS: We prefer that you would not destroy the environment or consume all the nonrenewable resources to produce these products, but we have a hard time evaluating that from here. Urbanization, affluence, and demand in the populous parts of the world continue to grow rapidly. The populous nations are entering global food markets, especially markets for animal products and feed grains, making these markets larger and even more global. As more consumer’s around the world gain access to modern grocery stores and their automated inventory systems, even more powerful and detailed messages will be sent. The food and agriculture sector will respond with even more rapid industrialization.

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This is a juggernaut. We cannot stop it or even substantially change its direction, even if we wanted to. As scientists, we can only create the possibility that the effective demand for food and agriculture products and services can be met within the constraints imposed by environment, natural resources, and social stability. We cannot even do that unless the will and resources are available to do the necessary research. In addition to being essential to meeting world food needs, increased productivity and quality will be essential to sparing the sensitive agricultural ecosystems and natural resources of the world. Driven by these needs, agronomists and others will continue to conduct research aimed at increasing productivity and efficiency of food and agricultural systems. In fact, the level of this activity must increase. The food and agriculture sector of the world will have to become much more productive to meet food and agriculture needs in the future (Ruttan, 1994). There are optimistic (Avery, 1995) and pessimistic (Brown and Kane, 1994) views on whether this can be accomplished.

B. MOOTQUESTIONS In the global market environment, the question of whether it is ethical to conduct research on certain subjects, including products or services that might do harm to the environment or make food unsafe, e.g., chemicals, is moot. The need for effective pest control will be so great that no options can be precluded before research reveals the potentials. The question of whether or not it is ethical to do research on technologies that might cause dislocations among farmers or other food and agriculture practitioners is moot. The trend toward market-oriented economies and less government intervention in domestic and global food and agricultural markets will assure that less efficient and productive practitioners will be dislocated. It is hoped that attention will be given to safety nets and other programs that mitigate these effects and ease transitions for those dislocated. This is not to suggest that themes other than quality, productivity, and efficiency are not important. The pressure to meet world food needs will override other themes as central and separate issues and cause them instead to be constraints that must be dealt with in the context of increasing quality, productivity, and efficiency. They will be constraints in an ongoing utilitarian analysis. The degree to which other themes are allowed to constrain quality, productivity, and efficiency will be determined by the consumers as they walk down those aisles and make their selections. To keep these powerful forces from destroying the environment or consuming nonrenewable resources, scientists will have to create possibilities for food and agriculture practitioners to be productive and efficient without harmful consequences. Some will continue to hope that public opinion will eventually shift the agri-

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cultural emphasis from a quality, productivity, and efficiency ethic to an environmental, social justice, or other ethic. Given the great political interest and activity in environment, food safety, natural resource conservation, and social issues, I can see why this hope is sustained. Increasingly, the public opinion that will count is the opinion individuals express as they walk down the aisles of the grocery store. This is not necessarily the one that is expressed in the political arena. It is influenced more by price and quality and less by concern about other agricultural themes and externalities.

C. SOURCES OF SUPPORT FOR RESEARCH The driving forces described previously will strongly influence the subject matter of food and agriculture research. There will be funds to address problems and opportunities perceived as important by consumers and other participants in the rapidly evolving world food and agriculture system. Those who want to pursue some other ethical theme to the exclusion of those that dominate in that system may have trouble finding support. Efforts to brand some themes as unethical or less ethical than other themes will not be constructive in this situation. There will be a great need and opportunity for researchers who can integrate other themes with quality, productivity, and efficiency. They will create possibilities for good outcomes with relatively few harmful consequences.

D. TAKE-HOME MESSAGE FOR AGRICULTURAL SCIENTISTS I think there is an important message for agronomists and other agricultural researchers and research administrators in this analysis of driving forces. It says that as you plan and implement research on environment, natural resources, safety, social issues, or other themes, keep it within the context of quality, productivity, and efficiency. In order for any technology or information generated in that research to find fruition in practice, it will have to contribute to quality, productivity, or efficiency or at least not detract from them.

VI. ETHICS IN THE CONDUCT OF RESEARCH As is obvious in previous sections of this chapter, I believe debates over the relative merits of various research themes are more appropriately technical discussions than discussions of ethics. People with impeccable records of scientific conduct and ethics can be found on both sides of such debates. From an ethical

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standpoint, it is more important to think about how we as scientists engage in those debates and resolve those and other issues. How do we develop and test hypotheses, marshal1 data, draw and report inferences, engage in rational discourse, and work with colleagues and students in an ethical manner? In this section, I describe my perceptions of ethical and unethical behavior in several specific situations encountered in agronomic research and in agricultural research in general. In each case, I move from situations in which I believe there is considerable agreement on ethical choices to ones in which ethical behavior is harder to specify clearly. This is not an exhaustive treatment by any means but will serve more as a partial list of research situations with ethical dimensions. In this section, I used the terms scientist and researcher interchangeably.

A. INITIATINGRESEARCH 1. Selecting Topics for Research Scientists embark on specific research efforts because they are interested in the subject matter, have expertise in the subject matter, wish to learn something, think the subject matter is important, wish to perform a service, wish to gain personal reward, or some combination of these. The decisions are conditioned by employment opportunities and the availability of resources to support research. I am not convinced that these decisions, except those involving public service, have much practical ethical content. Of course, each person must let hisher own ethical perceptions guide these decisions.

2. Designing Experiments Creative scientists generate hypotheses. The more creative and knowledgeable they are, the more likely they are to generate hypotheses that depart from the current paradigm. Although there is a definite resistance to new paradigms in the scientific community, researchers who bring about paradigm shifts are often rewarded. The greatest reward is the feeling of having gone where no one went before, having been the first human in the history of the world to understand a phenomenon, no matter how minute and unimportant. Other more tangible rewards include recognition, accolades, fame, and money. Scientists may become emotionally involved with their hypotheses. They want these products of their intuition and insight to be true. Unless scientists are vigilant, they will unconsciously do things in their research and analysis that will “stack the deck” in favor of their hypotheses. Lay people placed in the role of experimenters do this in the extreme (Folwell, 1969). It is important for scientists to design experiments that will disprove their hy-

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potheses if they are not true. Thus, scientists are put in the position of having to scrutinize their own creations with cold objectivity and with a critical eye. They must subject their creations to rigorous tests. Intellectual honesty and integrity are of paramount importance in this situation. Only if a hypothesis is not proven false by rigorous experimentation is it ethical for the scientist to promote it as a new paradigm.

3. Collecting and Reporting Data Both practical and ethical considerations dictate that scientists should carefully and accurately record data. The levels of precision and accuracy required are functions of the size of the differences investigators need and wish to detect. It is generally accepted that fabrication and falsification of data constitute unethical behavior, except in special situations such as espionage. In reports, data should be accurately represented so that readers are not mislead. In many situations, it is hard to determine whether data have been fabricated or falsified. The more a scientist knows about the subject matter of an experiment, the easier to fabricate or falsify data without being detected. A knowledgeable person can shade the data so that they support a certain hypothesis without changing them so much that the changes are apparent. Moving a few observations toward or away from a treatment mean can alter the statistical analysis such that it supports one hypothesis over another. Such alterations of data are clearly dishonest and unethical. It is unethical for an investigator to provide only data that support a certain hypothesis if he/she collected other data. There are cases, however, in which extraneous forces ruin treatments and even entire experiments, rendering part or all of the data useless. Whether or not experiments have been ruined or data are worthless or misleading is often a judgment call, requiring objectivity and integrity. There are statistical techniques that accommodate missing data without changing variance estimates. These should not be used to deal with outliers, however, unless the outliers are clearly artifacts. Intellectual honesty and integrity are very important in this situation. It is best in these situations to err on the side of caution.

4. Analyzing Data Most agronomic data are subject to statistical analysis. Statistical analyses are conducted and reported so that those interested in an experiment can get a more accurate perspective on the degree of variation, both controlled and extraneous, encountered in the experimental material and conditions. With statistical information, others can decide whether they think an investigator’s conclusions and inferences are appropriate. Typically, agronomists test hypotheses at P = probability of type one error =

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0.05. Frustrated with the high degree of variation encountered, especially in field environments, they may elect to conduct and report statistical tests at higher levels of R They may elect to report the actual level of P computed for each test. Researchers may perform various transformations so that data conform more closely to the assumption of normality associated with many standard analytic procedures. Such practices are not unethical, so long as they are accurately portrayed to readers and other recipients of reports. It is not a case, however, of “let the buyer beware.” It is ethically incumbent on researchers to perform and present statistical analyses with as much objectivity, accuracy, and intellectual rigor as they can reasonably muster. Again, the role of experiments and statistical analyses is to disprove hypotheses that are not true, not to support hypotheses. Statistics can be used to mislead, either intentionally or unintentionally. For example, high correlation or multiple correlation coefficients result when there are linear trends in dependent and independent variables, regardless of whether any causal connections exist. As the number of parameters in a multiple regression model approaches the number of observations in an analysis, the multiple correlation approaches 1 .O. These spurious relationships may mislead both the researcher and hidher audience. It is obviously unethical to use statistical anomalies to mislead people deliberately. Whether it is unethical to use them unintentionally is in that gray area mentioned previously.

5. Drawing and Reporting Inferences Technically, inferences drawn from an agronomic experiment only apply to the materials and conditions of that experiment. At the same time, it is usually impractical to test hypotheses with all possible materials and under all possible conditions. Compounding the problem is the necessity to provide research information to practitioners working within a wide range of materials and conditions. Thus, there is great pressure on agronomists to extend their inference space beyond that included within their experiments. It is not unethical to interpolate and/or extrapolate the results of experiments. It is risky, however. It is important for the researcher to evaluate that risk and communicate it to audiences to the extent possible. It is unethical to interpolate or extrapolate with the intent to mislead. As in most research situations, there is a gray area that requires good judgment, intellectual honesty, and integrity.

6. Establishing and Maintaining Credibility When a researcher is competent, professional, ethical, and intellectually honest in measuring, recording, analyzing, and presenting the results of experiments and

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inferences drawn from them, he/she establishes credibility and contributes to the credibility of hisher organization. Without credibility, neither scientists nor their organizations can function effectively. Both scientists and practitioners have different expectations of public-sector and private-sector scientists. Public-sector scientists are expected to be honest and thorough in their portrayal of research results and inferences, even when they are comparing competing products. A private-sector scientist might reasonably be expected to portray hisher company’s product in the best light and to remain silent with respect to performance of competing products. Private-sector scientists lose credibility for both themselves and their companies if they misrepresent their products and services or portray competing products dishonestly. Markets are good mechanisms for determining value and customers have long memories, so the punishment for lack of credibility can be severe for private-sector scientists and their organizations. There is a very large gray area associated with credibility. My experience suggests that, contrary to some expectations, public institutions and public-sector scientists do not have a corner on credibility compared to private-sector scientists. There is considerable variation in each category. In both arenas, rigorous peer review and peer pressure provide strong incentives for intellectual honesty and ethical behavior.

B. ETHICSAND SCIENTIFIC DOCUMENTS 1. Preparing Proposals In the quest for program and project support, scientists and science administrators spend much time preparing proposals. These may take the form of technical proposals submitted to competitive grants programs in response to solicitations. They may be solicited or unsolicited proposals to private firms seeking support in the form of gifts, grants, or contracts. They may be political proposals, often prepared for lobbyists or legislative champions to use in supporting research-related legislation. These categories of proposals are similar in requesting an investment in return for something. Timeframes and deliverables may or may not be specified precisely, but some new information or technology, either prototype orproven, is expected in return for the investment. In research situations, outcomes are uncertain, almost by definition. There are some things the investor can expect, however. Ethical behavior of proposal authors in this situation includes promising only what can be delivered, clearly informing the investor of the risks involved, and providing an objective estimate of the probability of achieving a goal.

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If the agreed-on goal of a proposed research effort is to achieve some desired practical outcome, both investor and applicant should be clearly informed of all activities that will be required to achieve the goal, including those outside the proposed research effort. It is important that both researchers and investors start projects with reasonable expectations. I find terminology associated with the recent Government Performance and Results Act useful in thinking about research organized around desired practical outcomes. The key terms are goals, activities, outputs, and outcomes. Outputs of research efforts often include publications. Sometimes prototype products and/or processes are produced. Rarely is the desired outcome a direct result, that is, output, of a single research activity. Usually, other activities, including other kinds of research, technology transfer, and commercialization efforts, are required. It is unethical for a researcher to accept money from sponsors if the researcher does not intend to help achieve the agreed-on goals. To do so is to accept money under false pretenses. If a researcher does not believe the goal is appropriate or ethical, he/she should not enter into an agreement specifying that goal. Often, problems can be avoided by clearly specifying and agreeing on goals during contract negotiations. Conversely, sponsors are obligated not to exploit researchers by using research results for other than intended purposes. In my own experience, a manufacturer of hay preservatives supported university research that supposedly showed a considerable benefit from using the product. The preliminary research results, which were never subjected to peer review or published in scientific journals, were used extensively in the firm’s advertising. I pointed out to the firm that the design of the preliminary experiments was flawed. My colleagues and I proved in other experiments that the product was ineffective. Our results were peer reviewed and published in a reliable journal. The manufacturer continued to manufacture and market the product. Unless the manufacturer really believed the product was cost-effective, it was unethical to continue marketing it. I participated as an expert witness in litigation initiated by dissatisfied users of the product. I did not enjoy or wish to be involved in this litigation, but I felt morally obligated to make sure that research results were used properly. It is appropriate to ask in this situation if it was ethical for researchers of three universities to perform flawed experiments and allow the firm to use the results in advertising. I believe there was no intent on the part of the researchers to mislead the company or its customers. It was simply incompetence. The researchers involved were not Ph.D. scientists and might not have been expected to understand all the nuances of experiment design. They were, however, authorized to conduct independent research. Whether it is unethical to be incompetent or for a university not to have some safeguards against incompetence of its researchers is one of the issues in the gray area.

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2. Proposal Budgets Research is an open-ended process. There are almost always great uncertainties about the future of any research effort. Researchers want to make sure there are plenty of funds to support a research effort, be able to do the research thoroughly and accurately, and deal with contingencies along the way. They want funds to provide continuity and stability so that they are not constantly expanding and contracting the project, which is wasteful, inconvenient, frustrating, and tends to fragment the effort. A researcher’s organization needs to recover indirect costs, one way or the other. Sponsors, on the other hand, need to keep costs to a minimum. They want to be assured that, within reasonable limits, they are only paying for the research contracted for and not for other research that may be of interest to the researcher or hisher organization. They definitely do not wish to pay for other activities or costs of the researcher’s organization, such as publicity and fund-raising, unless they have given their approval for such expenditures. Budget numbers should be the result of careful, objective analyses of the costs of achieving mutually agreed-on objectives within the agreed-on timeframe. As much as is possible given the relative unpredictability of the future of a research project, contingencies should be clearly identified and the methods to deal with them should be explicit. It is unethical and almost always counterproductive for either a researcher or a sponsor to try to mislead the other in budget negotiations. If a budget represents a researcher’s best effort to estimate the costs of proposed research and the sponsor is unable to invest enough to cover the costs, the project may be scaled back, its scope diminished, or its proposed duration lengthened. Either the researcher or the sponsor may decide that the project is not financially feasible. As in most such situations with considerable gray area, intellectual honesty and integrity are of paramount importance.

3. Indirect Cost Recovery As a community of agricultural research scientists and administrators, we have failed to come to grips with the ethical dimensions of indirect cost recovery. Given the internal and external conflicts of interest involved and the potentially high cost of adhering strictly to rules and protocols, we have handled this issue loosely. Research incurs indirect costs, such as the costs of utilities, depreciation of facilities and equipment, administration, and various support services. These costs, although indirect, are real and must be recovered from some source. Most institutions, organizations, agencies, and private firms have indirect cost policies that guide decisions about what or how much indirect cost they will agree to pay when they sponsor research and how much they will try to recover from outside sponsors. Government-audited rates for many universities are between 40

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and 70% of modified total direct costs of projects. Private firms often have indirect cost rates exceeding 100%. For obvious reasons, universities do not ask donors and benefactors to pay additional sums to cover indirect costs incurred when gifts are used to support research. Universities have considerable discretion in their use of gifts. Donors may specify broad areas they wish their gifts to support but ordinarily would not specify in detail how the gift should be spent. In many cases, an institution can use gift income to pay for items usually classified as indirect costs. In the case of grant or contract research, however, activities, objectives, and associated direct costs are specified in proposals, agreements, and contracts. Money provided to cover direct costs cannot be used to pay indirect costs. Universities deem it appropriate in those situations to insist on full indirect cost recovery from sponsors. Intermediate indirect cost rates may be negotiated in individual cases, depending on how large the grants are, whether or not the university stands to gain royalties, the prospects for future gifts or grants, in-kind contributions by sponsors, etc. Institutions may establish relatively low “internal” indirect cost recovery rates for other public or quasi-public institutions, organizations, or agencies. A university researcher may encourage a sponsor to provide research support in the form of a gift rather than a grant or contract. In that situation, no indirect costs are recovered and the entire gift may go directly into the researcher’s program to cover the direct costs of research. There may be a tacit agreement between the researcher and the sponsor specifying the research to be done, the deliverables, and timeframes. Because the university does not know the tacit contract exists, it does not require indirect cost recovery. When these tacit contracts are funded with gifts, the indirect costs incurred by the research are shifted to other parts of the university’s budget. In effect, the money comes out of other research and education programs, without the approval of those managing, conducting, and sponsoring those programs. Increasingly, universities are imposing internal surcharges on gift accounts to help offset indirect costs incurred when gifts are used for research. Knowingly entering into such tacit contracts for the purpose of avoiding indirect costs is clearly unethical behavior on the part of both researchers and sponsors. It is ethically incumbent on researchers, sponsors, and others directly involved in negotiations on gifts, grants, and contracts to be open and honest about what is expected of each participant and what restrictions are being imposed on use of sponsors funds. The ethical gray areas include negotiated rates and situations in which sponsors make zero or inappropriately low indirect cost recovery a condition of their sponsorship. The most prominent example is the legislative mandate that the federal government will pay only 17% of direct costs to cover indirect costs associated with USDA competitive grants. The result of this policy is that other sponsors, particularly state governments,

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are required to share the unrecovered indirect costs of USDA competitively funded projects. Other federal research-sponsoring agencies pay full audited indirect cost rates. It is appropriate to ask if it is ethical for a government agency or institution to accept or provide grants with arbitrary limits on indirect cost recovery. It would be better if sponsors would simply look at the bottom line and decide if they are receiving the best value for their investment.

4. PeerReview Scientists are asked to provide peer reviews of proposals submitted to granting agencies and manuscripts submitted for publication in scientific journals. Program managers and journal editors are responsible for selecting reviewers with expertise and experience in the subject matter addressed by a specific proposal orjournal article. Reviewers are usually asked to decline the invitation to review a proposal or project if they think they do not have the required expertise, experience, time, or other resources required to generate a thorough, useful, and discriminating review. It is unethical for a scientist knowingly to undertake or complete a review if he/she lacks the necessary qualifications. It is also unethical, in my opinion, to provide a cursory, superficial review. In the gray area are reviews for which the reviewer is only partially qualified and reviews that are unnecessarily critical, laced with sarcasm and/or personal innuendos, or simply incompetent. Not surprisingly, the latter characteristics are often found in the same review.

5. Authorship and Shared Recognition It is clearly unethical for a scientist to claim to author something he/she did not author. This is plagiarism, which joins falsification and fabrication of data as the unforgivable sins of scientists. Authorship in this context requires an author to make a meaningful contribution to the research being reported. Gratuitous authorship, that is, including people in a list of authors for the sole purpose of rewarding them for past favors or attaching more credibility to research than it otherwise would have, is unethical. Intentionally refusing or failing to include or at least acknowledge an author who made a meaningful contribution is likewise unethical. Needless to say, the gray area is in the interpretation of meaningful in any specitic situation.

C. WHISTLE-BLOWING Perhaps this is best illustrated with an example. One of my colleagues is convinced that another colleague deliberately authored false statements in a peerreviewed article, intending to mislead the readers into placing more confidence in

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some data than is justified. He asked me if he should write a letter to the editor of the journal exposing this misconduct. I suggested that we hold a closed, internal hearing in which the accuser and the accused could make statements to a panel of their peers. That group could decide what steps to take next. I am concerned that such a serious accusation, publicized internationally before it can be thoroughly investigated, can devastate a person’s career, whether or not it is true. To further complicate matters, the accuser does not wish to confront the accused. This situation raises a number of the questions that are typical of the issues encountered in whistle-blowing. Is it ethical for this person who has become aware of serious misconduct not to reveal the misconduct? Is it ethical for the accuser to make accusations to others but refuse to confront the accused in an appropriate forum, thus giving the accused a chance to defend himself? Given a long history of personality conflict between these two individuals, are the accusations the result of an objective analysis by the accuser? Is it ethical for me, having been provided somewhat convincing evidence that some wrongdoing is involved, to let this matter drop without taking some kind of action to explore the situation further, even without the cooperation of the accuser? Is it ethical of me to reveal things that were revealed to me in confidence, knowing that the accuser does not wish this information revealed? The answer to all of these questions is no. I must weigh the potential good and bad consequences of the alternative courses of action. Some pertinent questions include the following: Could this apparent wrongdoing simply be a mistake that could be corrected by the accused writing a letter of explanation to the journal? What harm is done to this person and his students if he is guilty of wrongdoing, is not confronted, and is allowed to continue the wrongdoing? What effect would a serious, open confrontation on this issue have on morale and the image of our institution? Will having wrong information in the literature cause problems for other scientists or practitioners? How much administrative and faculty time can we afford to spend on this particular incident?

D. CONFLICTS OF INTEREST A potential conflict of interest arises when a scientist is in a position to use hisher position and the influence associated with it for personal gain beyond the contractual compensation associated with the position or to benefit disproportionately within hisher organization or system. Whether or not an actual conflict exists depends on several factors. It is unethical for a scientist to let outside relationships prevent himher from fulfilling contractual obligations to hisher employer. It is unethical for a scientist to attempt to manipulate competitive processes unfairly in hisher own favor or in favor of groups to which the faculty member is attached or obligated in some way.

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It is not unethical for a scientist to receive shares of royalties or other compensation as a result of contractual relationships between the investigator’s organization and other organizations. It is not unethical for a scientist to own, operate, or otherwise participate in a business, so long as the relationship does not prevent the scientist from meeting contractual obligations to his organization and moral obligations to its constituents. Employers may ask employees to reveal their relationships with outside organizations that compensate them more than a specified amount. This allows the employer to investigate and decide if a conflict of interest exists. Conflicts of interest are often subtle and virtually undetectable from the outside. Individuals know best when they have a conflict of interest. Ethical behavior includes revealing potential conflicts of interest and behaving in an even-handed and objective manner even when potential conflicts of interest exist. A person who has the level of intellectual honesty and integrity required to be a good scientist should be able to think and act objectively even when hisiher own interests are at stake. I have always been frustrated when conflict of interest rules require certain people on a proposal review panel to leave the room. The rules often preclude people from the same institution or even the same state as an applicant to refrain from participating in a review of the applicant’s proposal. In my experience, this usually has the effect of barring from the discussion the people who know the most about the proposal and the applicant. It also suggests that other panel members are so naive that they do not know when they are being manipulated. A problem is created in a proposal review process when more detailed information is available about some projects than others. This is given as another reason for barring those with a potential conflict from the review. An “ignorance is bliss” argument usually raises a flag with me, but I can understand the concerns of those who take this position.

E. INTELLECTUAL PROPERTY RIGHTS Increasingly, public institutions and agencies are expanding and improving systems to identify and protect intellectual property generated in their research operations. This represents a significant cultural as well as administrative change in many public institutions, particularly universities. A few decades ago, the most common attitude in universities and especially in colleges of agriculture was that intellectual property generated with public funds belonged to the public and should be freely available to any member of the public who wished to see or use it. Now, universities see their intellectual property as being owned by the public, but they feel a responsibility to manage it in the best interests of the public. Transferring intellectual property to the public domain, thus making it freely

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available to all members of the public, is not always in the public’s best interest. Sometimes, in order to assure that useful new information and technology will be commercialized and thus benefit the public, it is necessary to give some individual or group proprietary access to it. Often, intellectual property generated by public institutions and agencies takes the form of prototype products, services, or information. More investment, research and development, and marketing is often necessary before such prototype technology can be successfully commercialized. By licensing discoveries and inventions exclusively or semiexclusively to interested and capable firms or individuals, public institutions provide incentives for further investment in commercializing these developments. Public institutions use patents, copyrights, and trade secrets to protect intellectual property from being appropriated by unauthorized individuals. The management, including sale and licensing, of intellectual property involves several ethical issues.

1. Patent and Copyright Infringement It is illegal and unethical to commercialize patented material or duplicate, disseminate, or commercialize copyrighted intellectual property without the permission of the owner of the patent or copyright. This unethical behavior is similar to plagiarism. The law has never been specific on whether it is illegal to reproduce a patented item in order to conduct research on that item. Differences of opinion on whether patents or copyrights have been infringed upon are usually settled through litigation. Often the technical issues are quite complex. As more cases are litigated, attorneys and courts gain experience, and clearer precedents are set.

2. Ethics and Trade Secrets Until relatively recently, trade secrets were not protected, but laws now have words to the effect that if a secret is “sufficiently secret,” it is protected, much like a patent. Sufficiently secret means that the information or material has not been broadly disseminated. Also, those with whom it has been shared should know that they were not granted commercialization rights nor are they free to pass the information or materials to others. Trade secrets are not protected from the possibility that someone else may rediscover or reinvent the secret technology. Items of germplasm and other genetic material may be patented but more frequently are treated as trade secrets. Increasingly, institutions insist that people requesting and receiving germplasm from scientists sign material transfer agreements. These are usually papers acknowledging that the recipient has not been granted rights to commercialize the material. Recipients are not allowed to pass the material to others without permission of the owner of the trade secret.

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In some but not all universities, intellectual property generated by a faculty member belongs to the institution, unless and until the institution turns it back to the faculty member. An informal exchange of experimental material with a colleague in another organization may constitute an unauthorized release of intellectual property. Of course, such informal exchanges are very difficult to monitor. This situation is often frustrating to university scientists, who in the past may have freely exchanged material with others without giving much thought to the intellectual property dimensions of the exchanges. In the case of germplasm, so many undocumented exchanges have occurred in the past that establishing clear ownership of some materials would be virtually impossible. When dealing with trade secrets, the ethical course of action is not always clear. The principle is simple! Do not take things that do not belong to you without permission. However, as in many situations, the “devil is in the details.” Scientists who generate intellectual property are in the best position to know who owns it, that is, who really discovered or invented it. To a large extent, we depend on scientists’ honesty and integrity to resolve issues. Where there are strong differences of opinion, the issues may have to be resolved in a legal forum.

3. Nondisclosure Agreements In order to enable useful exchanges of proprietary information between organizations, it may be necessary for those participating in a discussion to sign nondisclosure agreements. Having signed such an agreement, one is legally and ethically constrained not to share the proprietary information with people who were not part of the discussion nor to use the proprietary information without permission of the owner. Problems may arise if the topic of the discussion is construed too broadly, and the agreement is seen as too restrictive by one or both parties. Problems are minimized if the proprietary information is clearly identified as it is discussed and is listed in a confidential written summary of the discussion. Adherence to nondisclosure agreements is hard to police. As in many situations described in this paper, honesty and integrity make nondisclosure agreements work and pave the way for productive cooperation in research.

F. ETHICAL TREATMENT OF ANIMALS INAGRONOMIC RESEARCH Animals are used in agronomic research, especially in grazing and feeding trials, metabolism and toxicity trials, and behavioral studies involving agronomic materials. There is a growing body of literature on animal behavior, animal welfare, and animal rights, which I make no attempt to report here. Suffice it to say

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that there is a considerable difference of opinion on what constitutes ethical treatment of animals. A basic philosophical issue is whether or not animals have inherent rights. People who believe animals have inherent rights oppose the confinement and use of animals. Although I respect this belief, I think that from a practical standpoint, whatever rights animals have are given them by humans. We clearly can abridge their rights, whatever those rights might be. Because we control animals, either directly or by default, we need to resolve ethical issues associated with the way we control them. For various evolutionary reasons, humans decided to domesticate and utilize animals for food and other purposes. Animal production by humans is a huge enterprise in the world. It is a safe assumption that it will continue long into the future. Growing affluence in the populous parts of the world is increasing the demand for animal products. Because there is a need to increase productivity of animal operations and improve quality, safety, affordability, and convenience of animal products, we will no doubt continue to conduct research involving animals. Circumstances have entrusted research animals to our care. In these circumstances, how should we treat them? As we consider the ethics of animal care, we cannot dismiss the animals as dumb brutes that are unaware of what happens to them. People who have been around farm animals know that some are intelligent, as animals go. I do not know how their intelligence differs qualitatively from human intelligence, but I am convinced it is different. Some farm animals are shrewd and most are intuitive. In various situations, they are sensitive, observant, perceptive, affectionate, nervous, stupid, violent, brutal, powerful, quick, and unpredictable. I think it is safe to say that they d o not think about rights. Although they may break free, I see no evidence that they understand the concept of freedom. Animals feel pain. They feel bad when they are sick. They are very upset when their young are threatened and when they are forcibly separated from their young. They are afraid of the unknown and much is unknown to them. The young play and the old are alert for danger, not unlike humans. They seem to live almost entirely in the present. They may not always be happy, but I d o not think they worry about the future. Very few domestic animals would ever live if they were not used by humans. Thus, human use does not disadvantage domestic animals. The use of domestic animals spares wild animals from being quickly hunted to extinction. The objective of most animal research conducted by or with agronomists is to improve crops as feed for animals, improve performance of animals consuming crops, or improve consumer products derived from animals that consume crops. The ultimate beneficiary of this research is the consumer. The health and well-

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being of commercial animals is usually improved through this research because healthy, comfortable animals tend to perform better. Some treatments imposed in animal trials may not be conducive to an animal’s health or well-being. For example, some experimental animals may be pastured on forages that may contain harmful alkaloids that may trigger disorders. This may be the only way to detect and understand toxicity problems and thus prevent such problems in commercial herds. Thus, a few animals are harmed so that many animals and people will not be harmed, which is a typical agricultural ethics situation. I think ethical behavior toward research animals spares them as much pain and discomfort as possible while gaining the necessary information. In some situations, pain, discomfort, and stress of research animals needs to be balanced against the value of the information in terms of avoidance of animal pain and discomfort, improved productivity and quality of other animals, and safety of consumers. Measures should be taken to minimize pain and discomfort and maximize animal well-being in each experiment without compromising the validity of the experiment design. This may involve using anesthetics or painkillers or substituting simulations where possible. It definitely involves refraining from unnecessarily harsh treatment of animals.

G. ETHICSAND COMPETITION Agronomic researchers constantly encounter situations involving competition. The competition is usually for resources, but may be for recognition, prestige, fame, self-satisfaction, and other intangibles. We live in a hierarchy of competition in which individual researchers compete for resources with other researchers within their departments or other organizational units. Departments compete within colleges or middle-level business units, which compete within each organization as a whole. Institutions and organizations compete for resources within states and nations. Nations compete in global arenas. Even so-called nonprofit organizations compete vigorously for resources. Competition can be very destructive, as in war. It can be very constructive, as in competition to serve clients and customers better. In a competitive framework, ethical behavior involves “playing fair.” Fair behavior is defined by predetermined rules and protocols. It is interesting that even in war there are conventions. Those who violate the conventions may be brought before some international tribunal to answer for their transgressions. In competitive situations, people employ strategies to try to gain competitive advantage. Some strategies are ethical and others are unethical. Research itself is a strategy by which one group may try to gain competitive advantage over other groups. Increasingly, strategy is seen as an important discipline in agriculture. As

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a discipline, its origins are in the military realm. Business strategy has been recognized as a discipline since the 1930s. Ethical behavior in agronomic research involves recognizing when it is appropriate to compete and when to cooperate. It also involves recognizing which strategies are appropriate and which are inappropriate in a competitive situation. In agronomic research, it is unethical to try to gain competitive advantage by subterfuge or deception. It is unethical to undermine the efforts of others to gain competitive advantage, but it is not unethical to try very hard through fair and honest means to exceed others in productivity and quality and in agronomic service and to be the first to discover new knowledge.

H. PERFORMINGTO SPECIFICATIONS In some research situations, particularly materials testing, the scope and scale of projects are specified in detail and agreed on in advance. Researchers estimate the cost of conducting the research and sponsors provide sufficient funds. Difficulties arise when the expectations of researchers and sponsors differ. Basically, participants need to keep their promises. Ethical behavior in this situation involves negotiating contracts and agreements openly, honestly, and thoroughly and then meeting commitments specified in the grants and contracts. Deliberately to underestimate or exaggerate what can be delivered in order to obtain a grant or contract is unethical. It is also unethical for sponsors to fail to meet their obligations. I have seen several situations in which researchers and sponsors had much different expectations as to the precision of experiments. Sponsors are particularly disturbed when a comparison of treatment means shows an advantage for their product, but the difference is found to be statistically nonsignificant. It is very important for researchers and sponsors to decide in advance what will be considered a meaningful difference among experimental treatments. Then experiments can be designed to detect differences of that magnitude. Because the cost of an experiment is proportional to the desired precision, sponsors can expect to pay more for greater levels of precision.

I. ETHICAL ISSUESASSOCIATEDWITH TECHNOLOGY TRANSFER Many agronomic researchers are also agents of technology transfer and face various related ethical issues. In an increasingly litigious society, technology transfer agents are concerned about liability. As technology becomes more complex, users may rely more heavily on information from various technology transfer agents and be more vulnerable to the agent’s mistakes.

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I believe that, apart from whether they are legally liable, technology transfer agents are morally obligated to provide information and assistance as honestly, thoroughly, and objectively as possible. Perhaps most important, they should not portray their knowledge as being greater than it is.

1. Transfer Agent’s Responsibility Technology transfer agents bear responsibility, either actual or perceived, for the outcome when clients follow their recommendations. This should not be a problem as long as they know the risks and uncertainties associated with using the technology in question and share that knowledge with clients. Limitations on responsibility and liability should be discussed and agreed on before a client accepts and acts on the recommendations of an agent. Such agreements, however, may not protect an agent if a client decides to sue. It is both practically and morally incumbent on both public and private research organizations to test new technologies and information under realistic conditions so as to determine in advance, as much as possible and practical, what should be expected when the product or practice is used on a commercial scale. Organizations should expect to bear legal, moral, and financial responsibility for bad outcomes achieved when a product or practice is used according to their recommendations and specifications and there are no mitigating circumstances. Users of a new technology also have a moral responsibility. Minimally, they should not seek restitution for problems caused when they misuse a product. Ordinarily, they should share or bear entirely the risk of unforeseen and unpredictable consequences beyond control of the supplier or technology transfer agent. Users may volunteer to bear the responsibility for consequences of using new prototype technology when they are given early or otherwise proprietary access to it. Often, the exact degree of responsibility or liability in situations like this is hard to determine. Suppliers, agents, and clients should participate honestly and fully in analyses of bad outcome situations. A fair outcome should be the objective, especially if strict adherence to the legalities of the situation does not lead to a fair outcome.

2. Testing and Comparing Products and Practices Agronomic researchers often test and compare alternative, competing products and practices. Unbiased comparisons, carried out under the wide range of conditions usually encountered in agricultural situations, play a very important role in helping practitioners select the best combination of products and practices for agricultural systems. The conclusions potential users draw from the tests and comparisons have important financial and perhaps other implications for both users and suppliers.

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Therefore, tests should be designed and comparisons made in the most thorough, objective, honest, and competent manner that is feasible under the circumstances. Reports should accurately and thoroughly depict the results of tests and comparisons. They should describe any circumstances that might render results unreliable or invalid. They should clearly describe the situations in which inferences from the tests and comparisons can and should be drawn and circumstances in which inferences would be inappropriate. Researchers must resist any tendency they might have, conscious or unconscious, to introduce bias into the tests and comparisons. It is obviously unethical for a researcher to falsify or fabricate test data or make other manipulations in order to cause tests and comparisons to favor one or the other of the items being compared. Likewise, it is unethical for suppliers to provide unrepresentative or deliberately misrepresented items for tests or use other means to bias comparisons. An APHIS official told me that data from tests of safety and efficacy submitted by private firms were more reliable than those submitted by universities. He attributed this difference to more careful and conscientious effort on the part of private firms, which he perceived as having a greater stake in the precision and accuracy of test results. This is a sad story if it is true. Marketing an unsafe or ineffective product will probably cause serious, direct legal and financial problems for a private firm. In my opinion, it is unethical for public-sector scientists to be less than rigorous and careful in safety and efficacy studies, even if they are not as likely to be held legally liable or reap other consequences of a defective product. It is unethical for researchers to misrepresent their ability and capacity to conduct tests and comparisons. Through honest communication and negotiation, researchers and sponsors of tests and comparisons can develop reasonable, shared expectations.

3. Advocating Products and Practices It is ethical for private-sector researchers to advocate their firms’ products and services as long as they do not misrepresent them. It is also ethical for publicsector researchers to advocate products that they have tested thoroughly and competently and that they honestly believe to be more effective than alternatives. It is clearly unethical for public researchers to advocate inferior products or practices in order to obtain kickbacks or other payments or rewards. It is appropriate for public-sector researchers to describe the relative merits of competing products and services in public forums, as long as they are scrupulously honest in reporting what they know and do not know about the products and services. Increasingly, public-sector researchers refrain from advocacy, choosing in-

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stead to simply present data and statistics or display experimental results and let the potential users draw their own conclusions. A particularly difficult ethical dilemma is posed when a researcher is convinced that a product or practice is defective, ineffective, or uneconomic and a powerful and influential constituency, such as a boss, sales manager, university administrator, or commodity group, perceives themselves as having a big stake in the product or practice. For example, having read about the failures of infant industry protection programs, I raised a question with a former director of agriculture in Illinois as to whether it was beneficial to continue the gas tax exemption for gasolene formulated with ethanol. I did not know the answer to the question but brought it up for discussion and study. Ethanol interests in Illinois learned of my question and interpreted it as criticism of the entire ethanol initiative. Several suggested that I should be fired. I was called to Springfield to answer accusations by about 50 hostile ethanol advocates. I appreciated the opportunity to discuss this issue frankly, and there were no serious repercussions. There have been several situations in my experience when enormous pressure was put on scientists to refrain from stating their views on some product or practice. Tenure is the only thing that prevented serious repercussions for some of the scientists who chose to speak out. In my view, this is still an excellent reason to perpetuate the tenure system. People who contract with universities for research should expect to hear what they need to hear, not necessarily what they want to hear. It is foolish and shortsighted for a group to try to suppress sound information about their favorite product or practice. In doing so, they delude both themselves and their customers and clients. Eventually, when the truth is evident, there is a day of reckoning, Suppressing the truth merely delays that day. Researchers should decline support from people who are not interested in the results of the research and who do not intend to use the results to improve their product or service. On the other hand, I believe it is unethical for a scientist to be insensitive to the possible consequences of various pronouncements about products and services. For example, unless an emergency exists, I think the scientist should share bad news first with sponsors of research or the suppliers of a faulty product or service. This provides the sponsors the opportunity to act responsibly, make adjustments, and be the ones to call the attention of their customers to the problem. This approach minimizes the likelihood that an entire product or service line will be jeopardized by overreaction of the public or a client group. When researchers make critical pronouncements to the public or to individual groups, they should focus on the perceived faults, provide unbiased information, and point up any weaknesses or possible weaknesses in the criticisms. They should keep criticisms focused only on those areas in which there is strong supporting information.

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J. ETHICSOF CONSULTING I believe that consultants work in the same ethical environment and face the same ethical decisions and dilemmas as public-sector scientists and extension people. Several of the ideas expressed in the rest of this section on technology transfer apply to consultants. Unlike private-sector scientists, it is not ethical for consultants to advocate certain products or services unless they are convinced that those products or services are truly superior to alternatives. Consultants are obligated to function in the best interests of their clients and to provide value in return for compensation. People vary in their ability and capacity to provide consulting services. It is unethical for consultants to misrepresent that ability and capacity to clients or to deliberately sabotage or undermine the efforts of competitors. It is also unethical for consultants to recommend that their clients engage in unethical behavior.

VII. ETHICAL IN RESEARCH ADMINISTRATION There are important ethical dimensions to many if not all administrative/ management activities and decisions. Administrative issues are important because they often directly affect peoples’ careers and lives. Administrative decisions often involve large sums of money and long-term commitments. Administrators are entrusted with other people’s money and careers and are expected to manage them ethically.

A. ETHICAL HIRINGAND TERMINATION It is in the best interest of both public and private institutions and organizations to attract and retain the best employees. In the university setting, it is particularly important to attract and retain outstanding faculty who can and will compete effectively for resources. This is because most institutions do not have sufficient institutional funds to mount research programs of the scale and scope required to satisfy fully the needs and desires of constituents and to capitalize fully on the knowledge and talent of its employees. The process of attracting, hiring, and retaining outstanding people is a competitive activity. The general ethical considerations associated with competition apply. It is ethical to compete vigorously within a system of rules andor codes that establish the boundaries of ethical behavior. It is unethical to misrepresent a position to a potential employee or to make

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promises of future support or other conditions that cannot be kept. It is unethical to misrepresent or otherwise deliberately and dishonestly undermine the efforts of competitors. Because funding is uncertain and the future of research operations is relatively unpredictable, not all tentative promises can be kept. As long as the risks are outlined as clearly as possible to prospective employees during the hiring process, unintentional breaking of tentative promises is not unethical. It is sometimes necessary to terminate employment of an employee because of wrongdoing or perceived deficiencies in hisher performance. This is a case of weighing the bad effects of termination on the employee with the bad effects on the organization and other employees if the employee is not terminated. Different weights are placed on these factors in different organizations. The process of termination should be as humane as possible. A stigma is attached to termination for cause, but its bad effects should be minimized. Open, honest, objective, and compassionate discussion of the situation with the person being terminated is of paramount importance. I maintain that when we deny tenure to a faculty member, we are remiss in having let the situation go too far. Most tenure denials can be predicted at least a year or two in advance. At that point, the person should be strongly encouraged to seek alternative employment, thus avoiding or minimizing any stigma that might be attached to denial of tenure. When a termination process is handled properly, it is possible for all parties to benefit. It is usually not beneficial for a person to hold a position for which he/she is not fully qualified or suited, intellectually or emotionally. It is far better to find a more satisfying, less stressful work environment. My experience is that people who are hired or retained as borderline cases generally remain at the borderline, which is not a good place to be in a hard-driving, competitive, organizational culture. It is not ethical, in my view, for employers knowingly to place people in that position or keep people who clearly cannot or do not meet specified standards of work quality and productivity. It is unethical for a prospective employee or an employee being terminated to misrepresent hisher qualifications andor past performance. Likewise, when there are appeals, administrators and employees involved are ethically constrained to present their cases as honestly and objectively as possible. Harassment by either side is clearly unethical.

B. NURTURING SCIENTISTS It is useful and practical for organizations that hire people into what are expected to be permanent positions to nurture their careers. This often involves assigning mentors from among successful, experienced employees. If career nurturing is successful, it will help employees and save the organization money and frustra-

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tion. Nurturing may involve counseling an employee to seek other, more suitable employment inside or outside the organization. Nurturing in the agronomic research situation is an ongoing process in which employees become well informed about what they have to do to meet expectations of peers, administrators, and other decision makers, be appreciated for their contributions, advance in the organization, and be appropriately compensated. Appropriate nurturing empowers employees and makes them more independent and interdependent. Nurturing helps avoid unnecessary terminations and the morale and productivity problems associated with them. People assigned as mentors take on an important responsibility. It is obviously unethical for a mentor to deliberately mislead an employee or administrators to whom they report concerning the employee’s performance. It behooves the mentor to study the mentoring process, be well informed about the employee’s work situation and other situations and conditions impinging on the employee’s work, and to communicate as objectively and honestly to the employee as possible.

C. FAIREVALUATION Agronomic researchers and administrators often find themselves in positions in which they must evaluate the performance of others or be evaluated themselves. Important decisions, including hiring and termination, promotion and tenure, and salary increments, hinge on evaluations.

1. Activity Reports People being evaluated often are asked to submit reports of their activities. They should be informed as to what criteria will be used to evaluate the reports. It is unethical to misrepresent performance on such reports or to wrongfully take credit for the accomplishments of others. These reports should be thorough, honest, and timely. Otherwise, those who submit them should not complain about the outcome. People evaluating others or gathering such evaluations should consider whether any conflicts of interest exist and make sure these do not affect evaluations and related decisions. Using one’s position as an evaluator to advance one’s own cause to the detriment of others, punish others for perceived wrongs or differences of opinion, or disable competitors are clearly cases of unethical behavior.

2. Letters of Recommendation, Support, and Evaluation Agronomic researchers and administrators are often asked to write letters of recommendation, support, and/or evaluation. Requests come from applicants and

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candidates for positions, promotion and tenure, and awards and from the people who are offering the positions, awards, etc. Candidates hope the letters will portray their cases in the best light, emphasizing their strong points. Prospective employers, award committees, and other evaluators hope the letters will be thorough, thoughtful, honest, and objective. They especially hope the letters will reveal any serious flaws in a prospect’s character or record that might not otherwise be evident. It is unethical to deliberately misrepresent or slant information in a letter of recommendation, either to favor or disfavor the candidate or to influence a competition in favor of or against some other candidate. Authors of such letters should be as honest and objective as possible, regardless of their relationship with the person being evaluated. They should portray their knowledge of that person accurately, especially not claiming to know more than they know. They should avoid rumor and innuendo and make sure the information they present is well documented. They should alert readers to possible conflicts of interest. There are situations in which supervisors actually misrepresent problem employees to other prospective employers in order to get rid of the problem employees. Besides being foolish because of its potential to backfire, this is unethical. In this situation, it is important for decision makers to take responsibility for their own mistakes and not to try to foist them on others.

3. Promotion Documents and Decisions In public research institutions, the process of determining if researchers should be promoted is often elaborate, especially when the granting of tenure is involved. The future of individuals, families, careers, institutions, and resources is at stake in these processes. Those who prepare promotion documents bear a heavy ethical responsibility both to candidates for promotion and to the institution or organization. They are obligated to present a thorough, well-documented case, which may or may not be supportive of promotion. Authors of promotion documents need to proofread carefully, adhere to format specifications, and provide clear, well-written narrative, where such is required. Otherwise, readers may be distracted from the substance of the document. Decision makers who review promotion documents are also ethically obligated to read and evaluate the documents thoroughly and objectively. Candidates are often asked to provide information for and even write portions of promotion documents, but they ordinarily do not bear final responsibility for either the information or the tone of documents. Nevertheless, they are morally obligated to provide accurate information and not to misrepresent their cases in any way. Integrity on the part of both candidates and decision makers is very important in these situations.

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4. Job Applications Candidates fill out applications, submit resumes, and are interviewed in their quest for positions. It is unethical for them to misrepresent their education and experience. They might be expected to present their case in the best light. If there have been major problems that are relevant to their future performance, however, they are morally obligated to reveal these problems. It is also wise for candidates to reveal major problems, lest prospective employers find out about them and believe they have been misled. When there are many qualified applicants for a position, prospective employers tend to scrutinize applications more closely, looking for anything that might help them decide among candidates. Unfortunately, but as one might expect, this scrutiny tends to focus on negative attributes. It is certainly appropriate for an applicant to describe any mitigating circumstances or contrary opinions regarding alleged negative attributes.

5. Evaluating Administrators/Managers Many organizations, especially universities, have processes for evaluating administrators periodically. Criteria for evaluation should be carefully developed and reviewed before any evaluation takes place. It is appropriate for the person being evaluated to review and comment on the criteria. Typically, a survey form or procedure is developed. It is sent to or employed with various groups. The groups are typically those that are exposed to and hopefully knowledgeable about the performance of the person in question. If administrators/managers have responsibilities outside the organization, it is important to obtain information from the outside groups with which the person works so as to achieve a balanced perspective on the person’s performance. The survey should be constructed and administered so as to gather information that is relevant, complete, accurate, and unbiased. Those being surveyed are obligated to answer survey questions as truthfully, thoughtfully, accurately, and objectively as possible. It is also incumbent on those being surveyed to disqualify themselves if there are insurmountable conflicts of interest or biases or if they are not well acquainted with the person’s position, responsibilities, and performance. The person being evaluated should be allowed to comment on any proposed survey instrument or procedure. Changes or different approaches recommended by that person should be considered before the final instrument or procedure is implemented. It is clearly unethical for any individual or group to use periodic review as a mechanism for launching a personal vendetta against an administratodmanager. Likewise, it is unethical for a person being evaluated to provide false information or to otherwise misrepresent hiskier performance.

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6. Equity and Merit Administrator/managers are often faced with balancing considerations of equity and merit. When determining salary increments, for example, administrators may decide to spread some of the resources evenly over a certain group of employees in order to treat them equitably. They may decide to reward merit without regard to equity. In the latter case, the organization may have decided that equity is adequately served if everyone has an equal opportunity to compete for the resources at hand. This situation reveals, perhaps more clearly than others, how people may differ in their concept of fairness. To many, fairness means to divide things equally among participants. To others, it means to divide things among people in proportion to the contributions made by individuals. The usual challenge is to arrive at some reasonable balance between achieving equity and rewarding merit. Equity/merit decisions involve many factors, including the type, culture, and history of the institution or organization; current and projected future financial state of the institution or organization; amounts of resources available for salaries, salary increments, bonuses, etc.; leadership style within the organization; messages administrators wish to send; and incentives they wish to provide. The ethical course of action for administrators in this situation is to make sure people being evaluated know what criteria will be used to judge merit. The criteria should be applied uniformly, fairly, and consistently. The interests of an organization’s clients, customers, and constituents should be taken into account.

D. HONESTCOMMUNICATION WITH CONSTITUENTS Administrator/managers often need to portray their organizations to groups of constituents. There is a natural tendency for valued employees to portray organizations in the best light. Sometimes an administrator wants constituent groups to respect and admire their organizations so the constituents will provide more support. Sometimes an administrator is a “true believer,” that is, really convinced that the organization is exceptionally good. This may be true or it may be that the administrator is too emotionally involved with the organization to make objective judgments about it. It is unethical to provide false or misleading information about one’s organization. In the long run, it is probably best to think of constituent groups as partners in this situation and to communicate with them as if they were partners. If there are problems within the organization or with the organizations’ relationships, it is best to share those problems with clients or constituents and, whenever possible, enlist

DON HOLT their aid in solving them. Relationships between organizations and their constituents should be characterized by ethical behavior, including honesty, fairness, and service.

ACKNOWLEDGMENTS I acknowledge Gary Shaw and Mary Scott (Scottie) Miller, former assistant directors of the Illinois Agricultural Experiment Station and Carol Neilson, secretary, for diligence in searching out and retrieving journal articles and other references on ethics for my use in this effort. Also acknowledged are the valiant and no doubt frustrating efforts of Greg McIsaac, Assistant Professor in the University of Illinois Department of Natural Resources and Environmental Sciences, to educate me on relevant environmental and social themes. Greg also provided several valuable references.

REFERENCES Atherton, L. (1961). “The Cattle Kings.” Univ. of Nebraska Press, Lincoln. Avery, D. T. (1995). “Saving the Planet with Pesticides and Plastic.” Hudson Institute, Indianapolis, IN. Brown, A. C. (1975). “Bodyguard of Lies.” Harper & Row, New York. Brown. L. R.. and Kane, H. (1994). “Full House: Reassessing the Earth’s Population Carrying Capacity.” Norton, New York. Busch, L. (1989). Irony, tragedy, and temporality in agricultural systems, or how values and systems are related. Agric. Hum. Values 6(4), 4-1 I . Covey, S. R. (1989). “The Seven Habits of Highly Effective People.” Simon & Schuster, New York. Elmore, R. W. (1996). Our relationship with the ecosystem and its impact on sustainable agriculture. J. Prod. Agric. 9,4245. Flora, C. B. (1986). Values and the agricultural cr Differential problems, solutions, and value constraints. Agric. Hum. Values 3(4), 16-23. Folwell, W. H. (1969).Communications must keep pace. Better crops with plant food LIII: I , pp. 20-23. (No. I.) Heichel, G. H. (1991). Ethical dimensions of agronomy. J. Agron. 20, 1-2. Holt, D. A. (1989). Presidential address: Change and stability in the American Society of Agronomy. Agron. J. 81,141-144. Monsen, E. R., Vanderpool, H. Y., Halsted, C. H., McNutt, K. W., and Sandstead, H. H. (1991). Ethics: Responsible scientific conduct. Am. J. Clin. Nurr: 54, 1-6. Panel on Scientific Responsibility and the Conduct of Research ( 1992). “Responsible Science: Ensuring the Integrity of the Research Process,” Vol. I. National Academy Press, Washington, DC. Ruttan, V. N. (Ed.) (1994). “Agriculture, Environment, and Health: Sustainable Development in the 21st Century.” Univ. of Minnesota Press, Minneapolis. Senge, P. M. (1990). “The Fifth Discipline.” Doubleday, New York. Solzhenitsyn, A. 1. (1973). “The Gulag Archipelago.” Harper & Row, New York. Thompson, P. 9 . (1988). Ethical dilemmas in agriculture: The need for recognition and resolution. Agric. Hum. Values 5 , 4 1 5 .

ARE AGROECOSYSTEMS SUSTAINABLE INSEMIARIDREGIONS? B. A. Stewart and C. A. Robinson Dryland Agriculture Institute West Texas A&M University Canyon, Texas 79016

I. Introduction 11. Agroecosystems 111. Semiarid Regions A. Aridity Index B. Length of Growing Period C. Example Locations IV The Issue of Sustainability A. Agroecosystem Processes B. Climatic Effect C. Soil Effect D. Socioeconomic Effect V. Technologies for Increasing Plant-Available Water A. Lengthening the Fallow Period B. Mulches C. Tillage D. Crop Calendars VI. Soil Organic Matter Maintenance VII. Summary References

I. INTRODUCTION Worldwide population and income growth are generating increasing demands for food and other agricultural products and will continue to do so into the next century. Most of the increased demand is in the developing countries, where it has been estimated that over the next 40 years, the demand for staple foods will grow at approximately 2.5% per year (World Bank, 1992). At the same time, there is a growing concern worldwide about the sustainability of agroecosystems. In 1800, the world’s population was approximately 1 billion people. It was 2.5 191 Advances in ARronyy. Volume 60

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billion in 1950,4.7 billion in 1983 (a 1.9% average annual increase), and is projected to reach 6.1 billion in 2000 (a 1.6% annual increase) (World Bank, 1986). Agricultural production, however, increased even faster, doubling between 1950 and 1980. This increased production resulted in about a 5% increase in per capita daily calorie consumption. People in developed countries consume approximately 3315 calories per capita compared to only 2180 in developing countries. Approximately 2300 is generally considered the minimum requirement (FAO, 198 1). From the beginning of agriculture until approximately 1950, increased food production resulted almost entirely from an expanded cropland base. Since 1950, however, the yield per unit of land area for major crops has increased dramatically. Much of the increase in yields is due to increased inputs of energy. Between 1950 and 1985, the farm tractor fleet quadrupled, world irrigated area tripled, and fertilizer use increased ninefold. Between 1950 and 1985, total energy used in world agriculture increased 6.9 times (a 5.7% average annual increase) (Brown and Postel, 1987). Some of the agroecosystems that propelled food and fiber production during this period are not being sustained. In addition, there are limited areas, particularly under favored environments, where further development can occur. Consequently, agroecosystems will become increasingly present on marginal lands in semiarid regions. Semiarid regions are characterized by insufficient precipitation, low soil fertility, and a rapid loss of arable land to soil degradation. An imbalance between natural resources, population, and basic human needs exists in these regions. The warm seasonally dry tropics occurring primarily in subSaharan Africa, Southwest and Southeast Asia, Central and South America, and Northern Australia contain more than 1.5 billion people (Technical Advisory Committee, 1990). Several of the countries in these regions have population growth rates among the highest in the world. Kanemasu et al. (1990) estimated that the semiarid tropics contained 13% of the world’s land and were inhabited by 15% of the world’s people but produced only 11% of the world’s food. There are growing concerns about the ability of the agricultural community to continue production of food and fiber at a rate sufficient to meet the demands of a growing population; this is particularly so if income growth results in dietary changes that require more grains for meat, eggs, and milk. Brown (1995) summarized the strategy developed at the 1994 Conference on Population and Development in Cairo. He concluded that their strategy reflects a sense of urgency-a feeling that unless population growth can be slowed quickly, it will push human demands beyond the carrying capacity of the land in many countries, leading to environmental degradation, economic decline, and social disintegration. Some important trends that Brown emphasized include a significant decline in the rate of irrigation development, a leveling off of worldwide fertilizer usage, and a slowing of cropland productivity.The world’s irrigated land in 1950 totaled 94 million hectares but increased to 140 million by 1960 (a 4.5% average annual increase), to 198 million hectares by 1970 (a 3.5% average annual in-

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crease), and to 271 million hectares in 1985 (a 2.1% average annual increase) (Rangely, 1985). Since 1985, however, the rate of expansion has slowed to less than 1% per year (Council for Agricultural Science and Technology, 1988). Worldwide, fertilizer use increased 10-fold between 1950 and 1989, when it peaked and then began to decline. During the following 4 years it fell 15%, with the decline concentrated in the former Soviet Union following the withdrawal of subsidies (Brown, 1995). The rise in rice yield per hectare, which halted in Japan a decade ago, is now slowing nearly everywhere, edging up only 2% from 1990 to 1994 (Brown, 1995). Another major concern in some countries, particularly China and India, is the loss of cropland to industrialization. All these factors are causing the agricultural community to question all agricultural ecosystems regarding their sustainability. The question of sustainability is especially relevant to agroecosystems in semiarid regions. Are such systems sustainable over the long term? The short answer is yes because there are many examples in which systems have been sustained for centuries. However, there are also many examples in which the development of agricultural ecosystems has led to severe soil degradation and a collapse of the agroecosystem. The objective of this treatise is to examine some of the more important soil-plant-water relationships in semiarid regions so that the steps required to sustain agroecosystems in these areas will be better understood.

II. AGROECOSYSTEMS Agroecosystems are ecological systems modified by human beings to produce food, fiber, or other agricultural products (Squires, 1991). In agricultural development, natural ecosystems are transformed for the purpose of food and fiber production. An agroecosystem is a complex of air, water, soil, plants, animals, microorganisms, and everything else in an area that people have modified for agricultural production. Crops and livestock are the major components of agroecosystems. An agricultural ecosystem such as a wheat field may be more productive than the native grass that originally occupied the same land. A greater percentage of the wheat produced is fit for human consumption than is the grass, but wheat is not self-sustaining and may be relatively vulnerable to external disturbances. Regular inputs of energy and nutrients are required to maintain production levels. For example, the soil may be too infertile for continuous crop growth unless it is fortified with fertilizers. Agroecosystems therefore tend to be less sustainable than natural ecosystems unless sustainability is very carefully designed into the system. Squires (1991) emphasizes that there are three qualities of agroecosystemsproductivity, stability, and sustainability. The productivity of an agroecosystem is not determined simply by the yield potential of the particular crop or livestock that

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is used. The yield that actually occurs depends on the climatic and nutritional (soil) environment, which is, in part, a consequence of how farmers manage. Productivity is therefore a consequence of the processes in the total interactive agricultural-environmental-social system. Stability is the reliability or consistency of farm production. It is important because people depend on a certain amount of production year after year. Fluctuation is a normal part of all ecosystems and agroecosystems are no exception. Agricultural production often fluctuates from year to year, and this is particularly true for agroecosystems in semiarid regions. Cropland in semiarid regions is often more marginal and precipitation is both lacking and highly sporadic. Although the risks of partial or complete crop failure are an unavoidable part of farming in semiarid regions, farmers place a high priority on minimizing risk. Sustainability concerns whether a given level of productivity can be maintained over the long term. Sustainability is defined as the ability of an agroecosystem to maintain productivity when subject to a major disturbing force (Squires, 1991). Salinity, toxicity, erosion, indebtedness, or declining market demand are examples of such forces. Sustainability thus determines the persistence or durability of an agroecosystem’sproductivity under known or possible conditions. Sustainability has a variety of measures associated with various measures of productivity. Some measures of sustainability can be high while others are low for the same agroecosystem. Although it is desirable for an agroecosystem to be high in all three qualitiesproductivity, stability, and sustainability-the three may conflict. An agroecosystem can often be managed in such a way that the short-term gains are very beneficial, but the long-term consequencesare devastating. Identifying and understanding the key relationships among these qualities are essential and require that appropriate questions be developed, asked, and answered. The key questions for semiarid regions are: (i) How can semiarid agriculture be intensified? (ii) Is this possible or desirable considering possible land degradation and other environmental consequences that threaten its long-term success? and, perhaps even more important, (iii) How far can agroecosystems be extended into a region of increasing marginality?

III. SEMLARTDREGIONS Semiarid is a comparative term implying a moisture state intermediate between truly arid conditions and others that are more humid. Semiarid regions have four unique characteristics. The first is that no growing season will have nearly the same amount, kind, or range of precipitation as the previous season; and the temperature average, range, and extremes will also be vastly different. Second, crops cannot be planned or managed the same from season to season. Third, and perhaps most important from a sustainability standpoint, the soil resource base and water

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holding capacity does not remain the same for any long period once an agroecosystem is introduced into a semiarid region. Fourth, abundant sunshine and cloud-free days induce rapid growth when moisture conditions are favorable, but these conditions cannot be sustained through the season, thereby demanding careful soil water management. Semiarid regions typically receive substantial precipitation for at least a few months of the year-enough to replenish soil water content to levels sufficient to produce amounts of biomass that far exceed those produced in arid regions. Dryland farming systems are common in semiarid regions. Dryland and rain-fed are often used synonymously, but they are considered vastly different by some workers. They both exclude irrigation, but beyond that they can differ significantly.Dryland agroecosystems in this paper are based on the concept presented by Stewart and Burnett (1987) that elucidates the differences between dryland and rain-fed systems. Stewart and Burnett stated that dryland agricultural systems emphasize water conservation, sustainable crop yields, limited inputs for soil fertility, and wind and water erosion constraints. Rain-fed systems, although they include dryland systems, can also include systems that emphasize disposal of excess water, maximum crop yields, and high inputs of fertilizer. Stewart and Burnett (1987) stressed that dryland farming in semiarid regions has been and will continue to be a high-risk undertaking and that the key to success is the use of systems and practices that take advantage of the favorable years.

A. ARIDITYINDEX The United Nations Conference on Desertification (UNESCO, 1977) defined bioclimatic zones based on the climatic aridity index: PIETP, where P is precipitation and ETP is potential evapotranspiration calculated by the method of Penman (Doorenbos and Pruitt, 1977),taking into account atmospheric humidity, wind, and solar radiation. The zones established by the conference were as follows: The hyperarid zone (PIETP = <0.03)-consisting of areas largely void of vegetation except for ephemerals and shrubs in riverbeds and that are virtually uninhabited. The arid zone (0.03 < PETP < 0.20)&comprising dryland areas with sparse perennial and annual vegetation utilized mainly in pastoral systems. The semiarid zone (0.20 < PIETP < OSO)-including steppe or tropical shrubland with a discontinuous herbaceous layer and increased frequency of perennials where dry land farming is widely practiced. The subhumid zone (0.50 < P E T P < 0,75)--characterized by more dense vegetation, where rain-fed farming is widely practiced with crops adapted to seasonal drought.

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B. LENGTHOF GROWINGPERIOD The Food and Agriculture Organization of the United Nations (FAO, 1978) used the growing period as the basis for assessing climatic resources in developing countries. The growing period is the number of days during a year when precipitation exceeds half the potential evapotranspiration, plus a period required to use an assumed 100 mm of water from excess precipitation (or less, if not available) stored in the soil profile. Any time interval during the period when water is available is excluded if the temperature is too low for crop growth (mean temperature below 6.5OC). Areas where precipitation never exceeds half the potential evapotranspiration are classified as dry with no growing period. An area having a growing period between l and 74 days is classified as arid, and areas with growing periods between 75 and 119 days are considered semiarid.

C. EXAMPLE LOCATIONS The average monthly precipitation, potential evapotranspiration, and half POtential evapotranspiration for three locations are presented in Fig. 1. All three locations are classified as semiarid by the aridity index (0.20 < P E T P < 0.50). However, by the FA0 growing period classification, only the Rajkot, India, location is classified as semiarid. The Amman, Jordan, location has a growing period in excess of 119 days, so this would be considered subhumid. Bushland, Texas, is classified as dry, with a 0-day growing period, because the average monthly precipitation never exceeds 0.5 ETP. These examples illustrate that there is still a lack of precision in defining and classifying climatic zones. Each classification scheme presented, as well as others in the literature, has advantages for specific purposes and locations; however, subjective judgment is required for their interpretation. The most important point, however, is to recognize and understand that there are very major differences in specific climates among semiarid regions and that each one must be addressed differently. Average precipitation values in semiarid regions can also be very misleading. For example, the average annual precipitation for Bushland, Texas, is 470 mm, but rainfall for any given year has been as low as 240 mm and as high as 830 mm. For a month, the variation can be even much greater. The average monthly precipitation for June is 76 mm, but the range has been from 1 to 248 mm, and less than 62 mm has been received in one-half of the years. In semiarid regions, there are more years below the average value than above because the high-rainfall years raise the average more than the low-rainfall years lower the average; thus, there is a degree of skewness inversely related to the amount of rainfall. The degree of skewness generally increases as the amount of annual precipitation for an area decreases. Mageed (1986) stated that in areas where annual precipitation amounts are 200-300 mm, the amount received in a given year ranges from 40% of the aver-

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age to 200%; for areas where the average is 100 mm per year, the range is from 30% of the average to 350%. This makes it imperative that more emphasis be placed on probabilistic analysis, with less attention given to average values when designing agroecosystems in semiarid regions. Kanemasu et al. ( 1990) have extensively reviewed the agroecological features

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of the semiarid tropics and presented strategies for identifying agroclimatic constraints for increased agricultural production. They concluded that the rainfall variability in the semiarid regions of the world is on the increase. They further concluded that the productivity of traditional agriculture in these areas is declining, and that the threat of serious food shortages is rising. At the same time, they proposed that by properly evaluating the agroclimatic resources and transferring and adapting improved technologies, the yet underexploited potential of semiarid tropical regions can be realized for sustained and improved agriculture.

Iv. THE ISSUE OF SUSTAINABILITY Until the past few years, sustainability was seldom mentioned in agricultural literature. Now, it is one of the most widely used words. High costs of irrigation development, escalating energy costs during the 1970%public concern over potential negative impacts of fertilizers and pesticides on water supplies, soil erosion, soil compaction, and salinity, as well as other concerns, have caused many people to question whether many of the current agricultural systems can be sustained. There are many different concepts of sustainability, but none is generally accepted. Sustainability, to many, conveys the idea of a balance between human needs and environmental concerns. The United States Congress defined sustainable agriculture in the 1990 farm bill as an integrated system of plant and animal practices having a site-specific application that will over the long term: satisfy human food and fiber needs; enhance environmental quality and the natural resource base upon which the agriculture economy depends; make the most efficient use of nonrenewable resources and on-farm resources and integrate, where appropriate, natural biological cycles and controls; sustain the economic viability of farm operations; and enhance the quality of life for farmers and society as a whole. Ruttan (1989) proposed, as a guide to research, that the definition of sustainability should include (i) the development of technology and practices that maintain and/or enhance the quality of land and water resources, and (ii) the improvement in plants and animals and the advances in production practices that will facilitate the substitution of biological technology for chemical technology. The first part of the Ruttan definition is similar to that of the United States Congress, but the key to the second part of the Ruttan definition is the emphasis on more reliance on biological versus chemical technology. Sustainability should be considered dynamic because, ultimately, it will reflect the changing needs of an increasing global population. The common thread among all definitions of sustainability, however, is that quality of the resource base should be enhanced.

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Francis and Youngberg (1990) stated that sustainable agriculture is a philosophy based on human goals and on understanding the long-term impact of human activities on the environment and on other species. Use of this philosophy guides our application of prior experience and the latest scientific advances to create integrated, resource-conserving, equitable farming systems. These systems reduce environmental degradation, maintain agricultural productivity, promote economic viability in both the short and long term, and maintain stable rural communities and quality of life. Despite the appeal of the philosophy for sustainability, there is often controversy about how best to achieve it. It is not known what methods and systems in diverse locations will really lead to sustainability, and this lack of knowledge often leads to conflict. Rodale ( 1988) listed seven reasons that sustainability has risen to importance on the farm policy agenda. The first is that there is currently enough food in the world. Surpluses and their timely distribution at an affordable price are more of a problem than shortages per se. Although many people are going hungry, it is not because of the inability to produce. Some policymakers are, however, wondering if this level of production can be sustained. The second reason is that nonrenewable resources are vitally important to the operation of the conventional system of agriculture in the United States and most other developed countries. Without adequate and low-cost supplies of oil, gas, phosphate rock, and other similar resources, U.S. agriculture would not be able to turn out large amounts of low-cost food and fiber. Third, high levels of production are having negative impacts on some systems. Soil erosion, degradation, and deforestation are widespread. This is particularly true in some developing countries where millions of people are literally trading soil for food. Even in the U.S. corn belt, there are approximately 2 tons of soil lost for every ton of corn produced and the loss ratio is even greater in wheat-producing areas (Larson et al., 1981). The fourth reason is that pollution problems can be traced to agriculture. Increasingly, residues of fertilizers and pesticides are being found in surface and underground water supplies. Fifth, world population continues to grow-in some places very rapidly. Although there is enough food in total today, there will be a much larger population to feed and clothe in future years. Sixth, society will possibly need agricultural production for energy and chemical feedstocks as well as for food. As fossil fuel reserves become more expensive, agriculture may have to meet new needs. Lastly, Rodale listed the concern for the family farm. He questioned whether the good life in rural areas can be sustained without a vibrant family farm sector. Sustainability clearly involves technical, economic, and social conditions. All three conditions are highly important and interrelated. It may be entirely possible to have an agroecosystem that is technically sustainable that is neither economically feasible nor socially acceptable. Herein, however, we will deal primarily with sustainability in regard to how agricultural practices and systems affect the sustainability of the soil resource base.

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A. AGROECOSYSTEM PROCESSES Whenever a natural ecosystem is transformed into an agroecosystem for the purpose of food and fiber production, there are several soil degradative processes set into motion. Hornick and Parr (1987) reported that most agroecosystems have degradation processes and conservation practices occurring simultaneously. The relation of soil productivity to soil degradation processes and soil conservation practices is illustrated in Fig. 2. As soil degradation processes proceed and intensify, there is a concomitant decrease in soil productivity and sustainability.Conversely, soil conservation and reclamation practices tend to increase soil productivity and sustainability. Therefore, the productivity level of an agroecosystem at any time is a result of the interaction of degradation processes and conservationheclamation practices, some of which are shown in Fig. 2. In natural ecosystems, sustainability is achieved through the efficient but delicate balance between all necessary inputs and outputs. Failure of agroecosystems to maintain this balance will ultimately lead to systems that cannot be sustained. The relation shown in Fig. 2 applies to all agroecosystems, although the importance of specific processes will vary substantially between agroecosystems.Also, the processes included in Fig. 2 are not all inclusive but are some of the most important and will be used for discussion. It is also important to recognize that some of the processes that are generally considered positive can, in some cases, result in soil degradation. For example, the use of organic wastes increases soil organic matter, improves soil structure, enhances soil water storage, and reduces erosion. In some circumstances, however, use of organic wastes results in accumulations of toxicants or nutrient depletion caused by increased leaching. The important point illustrated by Fig. 2 is that degradative processes and soil improvement processes always occur simultaneously, and the net result can be positive or negative. The negative impacts of soil degradation processes can become dominant. In such situations,agroecosystemscannot be sustained over the long term, and in ex-

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Figure 2 Relation of soil productivity to soil degradation processes and soil conservation practices (modified from Hornick and Pam,1987).

AGROECOSYSTEMS SUSTAINABLE IN SEMIARID REGIONS? 201 treme cases soil productivity can be reduced to zero. On the other hand, soil conservation practices rarely improve the productivity of cropland beyond the initial levels experienced when virgin land is cultivated. However, soil restoration practices, to be economically feasible and fonvard-looking, must, of necessity, be focused on improving the soil productivity beyond the initial level experienced after cultivation. Examples of such practices include adding fertilizers in areas where essential plant nutrients are deficient and installing water conservation practices to significantly reduce runoff. The most notable increases in dryland productivity generally occur when irrigation practices are established or when salt-affected areas are reclaimed.

B. CLIMATIC EFFECT The concepts illustrated in Fig. 2 apply to all climates and all levels of inputs. In every case, a truly sustainable agroecosystem results when the effects of conservation practices equal or exceed the effects of the soil degradation processes. Climate is often the most critical factor determining the sustainability of an agroecosystem. Stewart er al. (1991) presented a generalized view, shown in Fig. 3, of the effect of varying temperature and moisture regimes on the difficulty of achieving sustainability in an agroecosystem. As temperatures increase and the amounts of precipitation decrease, the development of sustainable cropping systems becomes more difficult. The reasons for these effects are readily apparent when the processes and practices presented in Fig. 2 are analyzed. The most serious degradation processes in semiarid regions are erosion and loss of soil organic matter. As temperatures increase, organic matter decline, particularly in frequently tilled soils, is greatly accelerated. The potential for wind erosion especially and also water erosion generally increases in warmer areas. These same degradation processes are also accelerated as the moisture regimes become more arid because, initially, there is a lower inherent organic matter level and less natural vegetation to prevent erosion. Not only do the soil degradation processes accelerate under hot and arid climatic regimes, but the benefits that can be derived from soil conservation practices in these regimes are fewer than those in the cooler and wetter areas. For example, the most important soil conservation practices to alleviate organic matter loss and control soil erosion usually involve crop residues, but the availability of crop residues decreases sharply in semiarid regions. Consequently, the negative processes shown in Fig. 2 can easily become much greater than the positive benefits and make the agroecosystem unsustainable. The relations presented in Fig. 3 do not apply to all climatic regimes. Soil degradation processes also accelerate under very high precipitation regimes due primarily to water erosion, nutrient depletion by leaching, and acidification. Cold conditions severely limit the choice of cropping systems and can also result in

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Figure 3 Generalized representation of the effects of temperature and precipitation on the difficulty of developing sustainable agroecosystems in semiarid regions (from Stewart e r a / . , 1991).

waterlogging. The relations represented in Fig. 3 should be restricted to moving from the more favored environments toward semiarid and arid regions.

C. SOILEFFECT The soil is also an important factor to consider in the development of sustainable agroecosystems.The generalized representation shown in Fig. 3 holds for all soils, but the potential rate and extent of soil quality decline depend on soil type. Pierce et al. (1983) developed a productivity index (PI) for assessing the effect of erosion on soil productivity. Briefly, the PI assumes that soil is a major determinant of crop yield because of the environment it provides for root growth. They evaluated the relative productive potential of soil by calculating a PI based on the available water capacity, resistance to root growth and development (bulk density), and adequacy of pH to a depth of 100 cm. A study by the National Research Council (1993) stated that the quality of a soil depends on attributes such as the soil’s texture, depth, permeability, biological activity, capacity to store water and nutrients, and the amount of soil organic matter. The report stressed that soils are living, dynamic systems that are the interface be-

AGROECOSYSTEMS SUSTAINABLE IN SEMIARID REGIONS? 203 tween agriculture and the environment. High-quality soils promote the growth of crops and make farming systems more productive. High-quality soils also prevent water pollution by resisting erosion, absorbing and partitioning rainfall, and degrading or immobilizing agricultural chemicals, wastes, or other potential pollutants. Soil degradation is an outcome of human activities that deplete soil and the interaction of these activities with natural environments. La1 and Stewart (1990a) stated that the three principal types of soil degradation are physical, chemical, and biological. Each type is made up of different processes, as illustrated in Fig. 4. Physical degradation leads to a deterioration of soil properties that can have a serious impact on water infiltration and plant growth. Wind and water erosion are generally the dominant physical degradation processes, but compaction is also a widespread concern in places where heavy machinery is commonly used. Hardsetting of a cultivated soil is also a process of compaction, but it results from wetting structurally weak or unstable soil rather than from the application of an external load. Laterization is the desiccation and hardening of exposed plinthitic material (material consisting of clay and quartz with other diluents; it is rich in sesquioxides, poor in humus, and highly weathered). Chemical degradation processes can lead to a rapid decline in soil quality. Nutrient depletion, acidification, and salinization are common soil degradation processes that have serious impacts on crop production. Chemical degradation is also caused by the buildup of toxic chemicals resulting from human activities. Biological degradation includes reductions in organic matter content, declines in the amount of carbon from biomass, and decreases in the activity and diversity of soil fauna. Biological degradation is perhaps the most serious form of soil degradation because it affects the life of the soil and because organic matter significantly affects the physical and chemical properties of soils. Biological degradation can also be caused by indiscriminate and excessive use of chemicals and soil pollutants. Biological degradation is also generally more serious in many semiarid regions, particularly in the tropics and subtropics, because of the prevailing high soil and air temperatures. Tillage also stimulates biological degradation because it increases the exposure of organic matter to decomposition processes.

D. SOCIOECONOMIC EFFECT Soil degradation is a complex phenomenon driven by strong interactions among socioeconomic and biophysical factors. These interactions, as perceived by La1 and Stewart (1990b), are illustrated in Fig. 5. Soil degradation is fueled worldwide by increasing human populations, fragile economies, and misguided farm policies. There is also often a conflict between short-term benefits and long-term consequences. This conflict is perhaps the most serious problem that must be addressed if truly sustainable agroecosystems are to be achieved.

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Acidifi- Sodication Toxic cation compounds

Figure 4 Qpes and processes of soil degradation (from La1 and Stewart, 1990a).

Soil degradation in semiarid regions can lead to desertification. Desertification, until recently, was generally considered to be primarily the result of prolonged drought. However, it has become apparent in recent years that human misuse of land is perhaps the basic factor, and drought is only an exacerbating factor (Dregne, 1989). In a paper prepared for the World Bank, Newcombe (1984) described the stages of land degradation that occur when natural forests are cleared and plowed as people seek new agricultural land. Nutrient cycling is drastically altered, and soil fertility declines following clearing. In the first stage, wood supplies remain plentiful, and gradual erosion is largely unnoticed. As the population increases, demand for wood increases for both construction and fuel, initiating a self-feeding cycle of degradation involving cutting wood from remnant forests to generate income, burning crop residues and dung for household fuel, reduced soil fertility, degraded soil structure due to residue and dung removal, and increased vulnerability to wind and water erosion. Eventually, dung and crop residues turn up in markets where only wood was previously sold. As a result of declining organic matter, the cropland becomes less productive, and crop yields prove barely sufficient even for subsistence. Eventually, dung becomes the main fuel sources in villages, and rural families use crop residues for cooking and for feeding livestock, which can no longer be supported by grazing land. In the final stage of degradation, crop failures become common even in normal seasons because topsoil and organic matter depletion have lowered the soil water-holding capacity. As a result, both food and fuel prices rise rapidly. Newcombe (1984) believes a critical point occurs in subsistence economies when more trees are cut for fuel than to make way for farmland. This is an excellent example of how short-term benefits can lead to

AGROECOSYSTEMS SUSTAINABLE IN SEMIARID REGIONS? 205 long-term disasters. This results in people moving to even more marginal areas. In essence, droughts seldom move to the people; rather, the people move to the droughts.

V. TECHNOLOGIES FOR INCREASING PLANT-AVAILABLE WATER Stewart and Burnett (1987) listed three components necessary for successful agroecosystems in dryland areas. These are (i) retaining the precipitation on the land, (ii) reducing evaporation, and (iii) utilizing crops that have drought tolerance and that fit the rainfall patterns. Although these components have been known for

*

4-b

Evapotranspiration

SOIL DEGRADATION 4-b

. Population density . Land: people ratio *

Land tenure systems

Figure 5 Interdependence of soil degradation on biological and socioeconomic factors (from La1 and Stewart, 1990b).

2 06

B. A. STEWART AND C. A. ROBINSON

centuries, new technologies and strategies have been developed that increase crop production in water-deficient areas. Some of these technologies and the principles on which they are based are presented in the following sections.

A. LENGTHENING THE FALLOW PERIOD One of the oldest, and most controversial, technologies for increasing plantavailable water is lengthening the fallow period. This is generally called summer fallow, defined as a practice wherein no crop is grown and all plant growth is controlled by cultivation or chemicals during a season when a crop might normally be grown. Proponents have emphasized the water-conserving, weed-controlling, and crop yield-stabilizing virtues, whereas critics have emphasized the inefficiency in soil water storage and the wind and water erosion and declining organic matter problems associated with fallow. Fallow efficiency is defined as the percentage of precipitation occurring during the fallow period that is stored in the soil profile at the end of the fallow period. Historically, fallow efficiencies have been in the range of 15-20%. The remainder of the precipitation is lost as runoff and evaporation and, on some soils, as drainage below the root zone. Stewart et al. (1 994) summarized long-term data from different cropping systems at Bushland, Texas, to illustrate the effect that lengthening the fallow period has on increasing soil water storage (Table I). The cropping systems were (i) continuous wheat, where winter wheat is seeded each year in October and harvested in late June or early July; (ii) wheat-sorghum-fallow (two crops in 3 years), where the land is fallowed for approximately 11 months following wheat harvest and the land is then seeded to grain sorghum in June and harvested in November and then fallowed for 1 1 months until the following October when the land is again seeded to winter wheat; and (iii) wheat-fallow (one crop in 2 years), where the land is fallowed for approximately 16 months from wheat harvest in July to October of the following year when wheat is seeded again. The amount of precipitation lost during the rotation as evaporation during the nongrowing (fallow) season ranges from 36% for the continuous wheat system to 61% for the wheat-fallow system. In the wheatfallow system, where the fallow period is 15 or 16 months, only approximately 15% of the precipitation that occurs during the fallow period is stored in the soil for later use by a growing crop. Even under the continuous wheat system, where the fallow period is only 3 or 4 months, the fallow efficiency is less than 20%. However, despite the very low efficiency of water storage during fallow periods, the wheat-fallow system is practiced widely because of the importance of sustaining crop yields under dry farming conditions. As illustrated in Fig. 1, there is no period during the year at Bushland, Texas, when average precipitation is as much as 50% of the potential evapotranspiration. Therefore, without a substantial amount of stored soil water present at time of seeding, the chance of a crop fail-

AGROECOSYSTEMS SUSTAINABLE IN SEMIARID REGIONS? 207 ure increases sharply. The annual precipitation in the area ranges from a low of approximately 50% of average to a high of approximately200% of average, and crop yields range from 0 to approximately three times the average. Again, referring to the data in Table I, soil water storage during a 4-month fallow period is only 37 mm compared to approximately 80 mm for an 11-month fallow period and almost 100 mm for a 15-month fallow period. The additional stored water leads to a significant increase in crop yield. Stewart and Steiner (1990) summarized long-term grain sorghum studies at Bushland and found that there is a threshold of approxi-

Table I Water Balance for Various Cropping Systems at Bushland, Texas" Continuous wheat One crop annually" (mm)

Precipitation Evapotranspiration Soil water change Evaporation (and runoff)

Wheat

Fallow

Total

256 293 -37

202

458 293

37 165

I65

Two crops in 3 years' (mm)

Precipitation Evapotranspiration Runoff Soil water change Evaporation

Wheat

Fallow

Sorghum

Fallow

Total

256 329 13 - 86

462

24 I 286 27 -72

416

1375 615 I08

25 86 35 I

43 72 30 1

652

One crop in 2 yearsd (mm)

Precipitation Evapotranspiration Soil water change Evaporation (and runoff)

Wheat

Fallow

Total

256 354 -98

660

916 354

98 562

562

"Adapted from 0. R. Jones (personal communication) and Johnson and Davis (1972). bF~llowperiod between crops is 3 or4 months. Runoff was not measured but would be minimal under annual cropping. 'Fallow periods between crops are approximately 1 I months. "Fallow period between crops is 15 or 16 months. Runoff was not measured but was a minor portion of the total.

B. A. STEWART AND C.A. ROBINSON

208

mately 1 10mm seasonal evapotranspirationrequired before any grain is produced, and for every mm of additional evapotranspiration, 15.5 kg ha-' of grain is produced (Fig. 6). This fact, coupled with the water use data for grain sorghum shown in Table I, suggests that grain sorghum yield would be 1.61 mg ha-' if only seasonal precipitation was available for evapotranspiration.The use of 72 mm stored soil water would increase the yield by 1.12 rng ha-'. Musick et al. (1994) developed a similar grain yield-seasonal evapotranspiration relation for winter wheat (Fig. 7) that indicates that about 200 mm of seasonal evapotranspirationis required before any grain is produced and that 12.2 kg ha-' of grain is produced for each mm above that threshold amount. The relations shown in Figs. 6 and 7 show the great importance of having substantial amounts of stored soil water at the time of seeding in areas where precipitation during the growing season is lacking. The relation between grain yield and evapotranspirationwill differ among semiarid locations because of climatic differences. The water balance values for annual cropping of winter wheat at three semiarid locations are presented in Table 11. The percentage of total rainfall that was used for seasonal evapotranspirationwas similar for all three locations-approximately 65%. Seasonal evapotranspiration is the combined loss of water from transpiration from the growing crop and evaporation from the soil surface during the period when the crop is growing. The fallow period is the time between harvesting the crop and seeding the subsequent

/

l0.OlI

7.5

-'

II

5.0-

Y = 0.0155X - 1.97

2.5-

0.00

100

200

300

400

500

600

700

800

SEASONAL EVAPOTRANSPIRATION - mm Figure 6 Relation between yield of grain sorghum and seasonal evapotranspiration at Bushland, TX (from Stewart and Steiner, 1990).

AGROECOSYSTEMS SUSTAINABLE IN SEMIAIUD REGIONS? 209

a

0

l RRl GAT ED DRYLAND

Y=

-2.52t0.0122 X R 2 = 0.74

SEb= t0.00054

200 400 600 , 800 SEASONAL EVAPOTRANSPIRATION, mrn Figure 7 Relation of wheat grain yield to seasonal evaportranspiration at Bushland, TX (from Musick et d.,1994).

crop. For the data presented in Table 11, evapotranspiration values were calculated by adding the growing season precipitationamounts to the change in the amount of available water held in the soil at seeding time and at harvest time. In all locations, soil water was decreased significantly during the growing season and increased during the fallow period. However, the change was considerably less for the Texas location. There was less precipitation at this location during the fallow period, and the potential evapotranspiration was very high, resulting in only 37 mm of storage compared to storage at the other two locations of 80 mm or more. The Texas location is the most arid of those presented in Table 11. Although total precipitation was more for the Texas site than for the China site, the amount of actual evapotranspiration was only 26% of the potential evapotranspiration for the Texas location compared to 56% for the China site. The China location had a much

Table II Water Balance Values for Annual Cropping of Wheat at Three Semiarid Locations" Texas Wheat Precipitation (mm) Evapotranspiration (ET)(mm) Soil water change Evaporation and runoff (mm) Potential evapotranspiration(PET)(mm) ETPET (%) PrecipitationlPET(%) ETlprecipitation (%) Yield Water use efficiency kg m-3

256 293 -37 I140 26

Shaanxi, China

Fallow

Total

202

458 293

37 165 740

165 1880

Wheat

181 264 -83 475 56

24 64 0.90 0.33

New South Wales, Australia

Fallow

Total

Wheat

213

394 264

280 360 - 80

83 130 408

130 883 45 67

1.25 0.47

"Adapted from 0. R.Jones, unpublished data, Lun et al. (1992). and Cornish and Pratley (1991).

Fallow

Total

280

560 360

80 200

200

64 2.40 0.67

AGROECOSYSTEMS SUSTAINABLE IN SEMIARID REGIONS? 2 1 I higher yield and a water use efficiency of 0.47 kg mP3 compared to 0.33 kg mP3 for the Texas location. The yield and water use efficiency values were low for both sites, but the Texas site values were extremely low. Water use efficiency values for wheat grown in humid regions or under irrigation often exceed 1.25 kg m-3and values as high as 1.9 kg mP3 are reported in the literature (Musick and Porter, 1990). The data presented in Figs. 6 and 7 and Tables I and I1 discussed previously clearly show the importance of stored soil water and explain why fallowing is so widely practiced even though the storage efficiency of precipitation occurring during the fallow period is very low. Perhaps the biggest concern about summer fallowing is its effect on soil degradation. Until herbicides became available in recent decades, tillage was the only means of controlling vegetative growth during the fallow period. Consequently, it was not uncommon for a field to be tilled 8-10 times during the fallow period. Intensive and frequent tillage buries most of the crop residues and hastens the decomposition of crop residues and soil organic matter. Cultivation increases biological activities, often as a result of better aeration. Cultivation also exposes fresh topsoil to rapid drying and after each drying a burst of biological activity occurs for a few days following rewetting (Allison, 1973). This is because the drying process releases organic compounds, probably from the breakdown of soil aggregates that are bound together by humic substances. Considerable organic nitrogen is mineralized as ammonia and later oxidized in large part to nitrates. Other nutrients are also made available from the decomposition of organic matter. This is particularly true for phosphorus because much of the phosphorus in soils is present in organic forms. The nutrients released as a result of tillage are readily available to growing plants and increased yields are generally obtained. Therefore, in addition to increasing water storage, summer fallowing increases available soil nutrients. However, unless the organic matter supply is replenished by plant residues or manures, the system is not sustainable. This is the situation for many soils of the world located in arid and semiarid regions and increased attention to the problem is critical. It is also the underlying principle that resulted in the infamous “Dust Bowl” that occurred in the U.S. Great Plains during the drought years of the 1930s and considered by many to be the worst ecological disaster ever exacerbated by man. The U.S. Great Plains region was largely settled in the early 1900s by farmers who migrated from the humid areas of the eastern United States and brought with them their clean-tillage tools and experiences. These worked well for the first few years after cropping began because the native soil organic matter content was high and the precipitation during the period of the “big plowout” was above average. However, when annual precipitation decreased to average and below, the annual net loss of soil organic matter accelerated and led to increased vulnerability to wind erosion. The moldboard plow, and many other intensive tillage implements, was developed in Europe, where soil organic matter content of soils is high and the organic matter level can be maintained at a high level because of relatively high precipi-

2 12

B. A. STEWART AND C. A. ROBINSON

tation amounts that produce large amounts of biomass and cool temperatures that slow the rate of decomposition. In arid and semiarid regions, high temperatures accelerate the rate of decomposition and the lack of precipitation severely limits biomass production so that organic matter loss can be rapid and severe. In certain situations, salinity problems can be very significant in semiarid regions. Summer fallow, particularly during years of above-average precipitation, can infiltrate more water than can be stored in the profile. This can result in substantial amounts of water moving through the profile removing nutrients and, if salts are present, they will be leached and cause saline seeps in certain situations. A saline seep occurs when water in excess of that required by plants percolates below the root zone and, upon encountering some type of barrier or restricting layer, movers laterally downhill and emerges in a seepage area, having picked up dissolved solids in transit. This has been a significant problem in parts of the northern Great Plains of the United States where spring wheat is the dominant cropping system (Halvorson, 1990). The length of the fallow period in a spring wheat-fallow cropping system is approximately 20 months during each 2-year cycle. Saline seep problems are also widespread in some semiarid regions of Australia (Sharma and Williamson, 1984). Summer fallowing has also been used extensively in Australia and China for increasing soil water storage. Cornish and Pratley (1991) stated that fallows have had a long and often sorry history in Australia. Summer fallowing in the United States was so successful when it was first introduced that the practice was imported by Australian farmers in the early 1900s. The primary practices involved deep plowing and frequent harrowing to produce a dust mulch. The plowing was thought to increase the water-holding capacity of the soil, whereas the dust mulch supposedly prevented water rising to the soil surface by capillary action and evaporating. Subsequent research showed that the major loss of water from soils was through transpiration by weeds and that the benefits of dust mulching were largely due to weed control. These technologies were used to extend the limits of wheat growing into the marginal 250- to 400-mm rainfall zone of the South Australian, Victorian, New South Wales, and Western Australian Mallee. Long fallow periods (15 months) were used and the frequent cultivation of these light-textured soils resulted in soil structural breakdown, fertility decline, and, ultimately, catastrophic erosion. By the mid- 1930s wheat farming in the Mallee was not a paying proposition and was replaced by sheep production. By the early 1950s wheat production occurred in rotation with pastures. Support for a 15-month fallow also waned as results showed that fallows of 8-11 months produced yields equal to those of a longer fallow. The cost in terms of loss of winter grazing and soil erosion was also significant. It was further shown that much of the benefit of fallowing was from nitrogen mineralization and not water conservation as originally supposed. Cornish and Pratley (1991) stated that two lessons emerged from the fallowing experiences in Australia. First, widely accepted farming practices can be based on poor foundations: Long fallowing very often gave no appreciable increase in water stor-

AGROECOSYSTEMS SUSTAINABLE IN SEMIARID REGIONS?

2 13

age over short fallows, whereas an unrecognized response to fallowing was nitrogen mineralization. It is therefore advisable to consider the outcome of farming practices in terms of specific effects on soils and plants and not yield alone. The second lesson is that practices can have effects well beyond those intended, emphasizing the need to understand how the specific effects interact with one another and the environment. Shengxiu and Ling (1992) summarized some of the results from the Loess Plateau region of China and concluded that fallowing was a good practice for the drylands. Summer fallowing was usually combined with summer deep plowing for controlling weeds, keeping the soil loose, and increasing soil infiltration. As a result, soil water was increased. In addition, available plant nutrients, especially nitrates, accumulated in the profile and stimulated growth of the subsequent crop. They did not stress the negative effects such as organic matter decline and deteriorating soil structure.This may be because Chinese farmers have historically used organic wastes on their fields and this may offset the otherwise negative effects. The steppe area of northern Kazakhstan is another region where fallow has been widely practiced. However, Souleimenov (1992) concluded that fallow in this region of about 350 mm annual precipitation was not justified. Research showed that the available water storage prior to seeding wheat was only slightly higher for the fallow fields than those for wheat after fallow or for continuous wheat. Fallow was adopted in this region in 1966 based largely on some selected data of the state farms for extremely dry years (1962, 1963, and 1965). The decision was also influenced by data and experiences from the Canadian prairies. Souleimenov (1 992) recommended that most fallow be discontinued with the more marginal lands being returned to grass and the better lands cropped annually. He also pointed out the benefits that such a system would have on the environment.Weed infestation, wind erosion, and other soil degradation processes have been widely experienced in the region where fallow systems were the dominant practice. The previous discussion clearly indicates that although summer fallow does increase soil water storage and greatly stabilizes grain yields and reduces risk, there are many disadvantages. The biggest disadvantage, particularly when tillage is used during the fallow period, is the decline in soil organic matter. A fallow system accelerates carbon oxidation, increases carbon removal by erosion, and adds less carbon back to the soil as crop residues. Fallow systems are best used on loam and clay loam soils; they are not effective on sandy soils because of low soil water storage capacities.

B. MULCHES The Dust Bowl of the 1930s in the U.S. Great Plains led to the development of stubble mulching. Stubble mulching uses V-shaped sweeps or blades that are pulled flat approximately 10 cm beneath the soil surface. This operation cuts plant

2 14

B. A. STEWART AND C. A. ROBINSON

roots and kills the weeds but does not invert the soil. Therefore, most of the crop residue is left on the surface where it can serve as a mulch to prevent wind and water erosion and slow evaporation losses. Only approximately 15-20% of the residue is buried by a sweep tillage operation, so there is substantial residue remaining on the surface even after three or four operations, which are commonly done between the time a crop is harvested and subsequent crop is seeded. At least this many tillage operations are performed when summer fallow is used and the length between crops is 11 months or longer. A rodweeder, a square rod approximately 25-mm thick that turns approximately 5-10 cm beneath the surface as it is pulled, is another tool that is sometimes used to kill weeds without intensively tilling the soil. A rodweeder operation can sometimes bury less than 10% of the crop residue present on the soil surface. In recent years, herbicides have been used to replace some tillage operations in reduced tillage systems and completely replace tillage when no-tillage systems are used. The sweep plow that was developed for stubble mulching is often referred to as “the plow that saved the plains” because it played a key role in controlling wind erosion that was so devastating during the Dust Bowl era. The Dust Bowl was one of the worst ecological disasters ever recorded. Although stubble mulching was developed to address the wind erosion problem, it soon became evident that mulches had beneficial effects on soil water storage. The increase in soil water storage generally is attributed to increased infiltration and reduced evaporation. Johnson et al. (1974) reported on studies from Bushland, Texas, conducted from 1942 to 1969 that compared stubble mulch tillage to one-way disk tillage (Table 111). For continuous wheat, which had only 3 or 4 months between crops, the plant-available soil water stored in 1.8 m profile at seeding time averaged 103 mm for the stubble mulch plots compared to 91 mm for the one-way disked plots. For the wheat-fallow system, which had 15 or 16 months between wheat crops, the plant-available soil water values at seeding time were 154 and 128 mm for the stubble mulch and one-way disk treatments, respectively. The yields were also increased approximately 100 kg ha-’ in both treatments as a result of the increased soil water storage. Unger (1978), also working at Bushland, Texas, on a clay loam soil found very significant increases in soil water storage when crop residues were maintained on the soil surface. The residues enhanced water infiltration and suppressed evaporation, thus providing more water for the subsequent crop (Table IV). This study was conducted on small plots and the straw was added from an outside source rather than being the residue from a crop produced on the plot. The straw rates ranged from 0 to 12 Mg ha-’. Under semiarid conditions, the amount of straw remaining after harvesting wheat would rarely exceed 5 or 6 Mg ha-’ and in many cases is only 1 or 2 Mg ha-’. Unger (1978) showed that even these small amounts, however, significantly increased water storage during fallow, grain yield, and water use efficiency (Table IV).

AGROECOSYSTEMS SUSTAINABLE IN SEMIARID REGIONS? 2 15 Table 111 Effect of Various Cropping Systems on Soil Organic Matter (SOM), Soil Water Content at Seeding (PAW), and Yield of Winter Wheat at Bushland, Texasa

Cropping system Continuous wheat” One-way disk tilled Subtilled with sweeps Wheat-fallow‘ One-way disk tilled Subtilled with sweeps Delayed subtilled with sweeps’

SOM 1941

SOM

(%)

(%)

Nitrate-N in 180cm” (kgha-I)

PAW in 180cm‘ (mm)

2.44 2.44

1.61 2.02

417 I79

91 I03

593 694

0-1915 0-2312

2.44 2.44 2.44

1.49 1.81 2.24

519 325 88

128 I54

944 1058 1038

0-2427 0-2589 c2440

1970

144

Average yield (kgha-I)

29-year yield range (kgha-I)

“Adapted from Johnson et al. (1994). ”Nitrate-N in 180 cm soil profile at end of experiment. “Average plant-available water in 180 cm soil profile at seeding time. %inter wheat seeded annually approximately October I and harvested approximately July I the following year. ‘Winter wheat seeded approximately October 1 every second year; approximately 15 months fallow between crops and yields shown must be divided by 2 to indicate annual land production. ’Tillage was delayed for approximately 10 months following wheat harvest; weeds and volunteer wheat were allowed to grow during the 10-month period.

Table IV Straw Mulch Effects on Soils Water Storage during an 11-Month Fallow, Water Storage Efficiency, and Dryland Grain Sorghum Yield at Bushland, Texasa Mulch rate (mg ha-‘) 0

I 2 4 8 12

Water storage”

Storage efficiency”

(mm)

(%)

72P 996 1006 1166 I39a 147a

22.6~ 31.16 3 1.4b 36.56 43.7~ 46.2a

Grain yield (Mg ha-I)

I .78c 2.416 2.606 2.986 3.68~ 3.99a

Total water use

(mm)

WUE‘ (kg m-’)

320 330 353 357 365 347

0.56 0.73 0.74 0.84 1.01 1.15

“Adapted from Unger (1978). ”Water storage determined to 1.8-mdepth. Precipitation averaged 318 mm. ‘Water use efficiency (WUE) based on grain produced, growing season precipitation. and soil water changes. “Column values followed by the same letter are not significantly different at the 5% level (Duncan’s multiple range test).

B.A. STEWART AND C. A. ROBINSON

2 16

Cornish and Pratley (1991), working on clay soils in Australia, found that plant residues on the soil surface increased fallow efficiencies from 21 to 29%. They attributed the increased soil water primarily to increased infiltration. When plant residues were not on the soil surface, there was a major increase in runoff due to crusting caused by raindrop action. They reported that approximately 4 Mg ha-' of wheat straw was sufficient to achieve the maximum benefit on infiltration. They further reported that the average grain yield of wheat in the region was 1.5 Mg ha-'. The straw to grain ratio for wheat is generally from 1.5 to 2, so the maximum benefit was probably not achieved in most years. Shengxiu and Ling (1992) summarized studies from China and results are similar to those reported previously. They concluded that straw mulch increased infiltration and reduced water loss by evaporation, thereby increasing water storage in both summer and winter. One of the studies they summarized was conducted by Siming et al. (1988) and some of their data are presented in Table V. Soil water storage was increased at all soil depths and with all levels of mulch. The researchers stated that in addition to increasing soil water storage, the mulch decreased bulk density and increased the number of earthworms and soil organic matter. Bulk density in the top 0-10 cm soil layer was 1.36 Mg mP3 when no mulch was present compared with 1.29 Mg mP3 with 4.5 Mg ha-' mulch and 1.23 Mg m-3 with 6.0 Mg- I ha-' mulch. The number of earthworms per square meter in the top 15 cm was 2 when no mulch was present but 12,32, and 34 when straw mulch was present at rates of 3.0,4.5, and 6.0 Mg ha-', respectively. Organic matter content in the top layer was 1.61% with no straw mulch and 1.67 and 1.76% with 4.5 and 6.0 Mg ha-', respectively. Soil temperature was also cooler in the summer and warmer in the winter when there was straw mulch on the surface. Although crop residues on the soil surface clearly reduce runoff and evapora-

Table V Amount of Water Stored in Different Soil Layers with Different Amounts of Straw Mulch" Mulch (kg ha-') 0 Depth (cm) 0-30 3ck100 100-200 0-200

3000

4500

6000

Amount of water stored (mm)

66. I 191.5 248.9 506.9

71.2 194.5 252.5 5 18.2

Note. Two years' average values in two locations.

"Adapted from Siming er al. (1988).

74.4 198.3 259.9 532.6

76.2 201.1 271.1 548.4

AGROECOSYSTEMS SUSTAINABLE IN SEMIARID REGIONS? 2 17

tion, it should be understood that these benefits diminish as the soil becomes wetter. Toward the end of a fallow, when the soil approaches maximum waterholding capacity, residues may have little effect. In fact, there may be more runoff from the mulched areas if the soil water content is substantially higher. If there are prolonged dry periods, the benefits of the mulch will diminish because evaporation from the mulched areas will continue at a slow rate and the soil will eventually dry to the same degree as the areas with no mulch.

C. TILLAGE The practice of tillage dates back to the beginning of history and was well established in Mesopotamia at least as early as approximately 2000 B.C. The first tillage tools were crude implements of stone, wood, and possibly bones and shells used by man to eliminate weeds and to chop or dig a few centimeters into the soil so seed could be placed into a favorable environment for germination. Later, animals were used to pull stick plows. Modem tillage systems had their origin in the 18th century when the moldboard plow was invented. A moldboard factory was opened in Scotland in 1760. Until recently, frequent and thorough cultivation was considered the mark of good farming. The seal of the U.S.Department of Agriculture has a moldboard plow as the focal point. Because the most prosperous farmers cultivated their soils very often, it was generally assumed that this was of major importance. Tillage was important because it controlled weeds and, more important, it resulted in the release of nutrients from the soil, mostly from soil organic matter. The traditional aims of tillage are to make efficient use of rainfall; to mineralize organic matter, mainly to release nitrogen; to prepare a seedbed suitable for sowing; and for crop sanitation including pest, disease, and weed control (Cornish and Pratley, 1991). Cook (1962) and Robinson et al. (1994) also discuss tillage goals and the effects of tillage on soil properties and crop production. Although the aims of tillage cannot be questioned, the extent that tillage is necessary can certainly be questioned and this is particularly so for semiarid regions. The most dominant constraint for successful agroecosystems in semiarid regions is the lack of water. Efficient capture and use of the limited precipitation are the starting points of any successful system. As already discussed in the previous sections, there is mounting evidence that reduced tillage and no-tillage systems result in more water storage and higher yields. Cornish and Pratley (1991) reported that when compared with a clean, cultivated fallow, fallow efficiencies improved with stubble mulching (but not stubble incorporation) and further improved with deletion of cultivation in a no-till fallow. They reported approximate fallow efficiencies of 19% when the straw was burned and the soil was tilled, 24% when the residue was mulched and the land was cultivated, 25% when the crop residue was burned but the soil was not tilled, and 29% when the residue was not burned and the soil was

2 18

B. A. STEWART AND C . A. ROBINSON

not tilled. These improvements in fallow efficiencies were for short fallows in the summer-rainfall zone of Australia where the fallow season is commenced with dry soil profiles. The soils are mostly heavy textured and self-mulching. Results for North America have shown increases in fallow efficienciesfrom approximately 20% when frequent and intensive tillage is used to approximately 30% when stubble mulching is used, and to approximately 50% when no-tillage is used. Greb et al. (1979) summarized more than 60 years of progress in wheat production in fallow systems in the central Great Plains, and the results are presented in Table VI. As the number of tillage operations was decreased, there were marked increases in the amounts of water stored during the fallow periods and dramatic increases in yields. These positive effects tend to accumulate because higher yields result in more residue and increased residue results in more water storage, which translates into higher yields, creating an upward spiral. Soil physical properties are generally also improved, and soil organic matter levels tend to increase. The more than 100% increase in grain yield was due to several improved technologies in addition to increased soil water storage. Greb (1979) attributed the credit of various technologies as follows: water conservation,45%; improved cultivars, 30%;improved harvesting equipment, 12%;better seeding equipment, 8%; and fertilizerpractices, 5%.The reason for the low impact of fertilizer was because of the long fallow period (16 months) between crops that resulted in sufficient min-

Table VI

Progress in Wheat-Fallow Systems at Akron, Colorado"

Years

Tillage

1916-1930

Maximum tillage: plow harrow (dust mulch) 1931-1945 Conventional tillage: shallow disk, rodweeder 1946-1960 Improved conventional tillage: begin stubble mulch in 1957 1961-1975 Stubble mulch: begin minimum tillage with herbicides in 1969 1976-1990 Minimum tillage (projected estimate): begin no-tillage in 1983 OAdapted from Greb et al. (1979).

Fallow water storage

Tillage operations (No.)

Wheat yield %of precip. (Mgha)

mm

7-10

102

19

I .07

5-7

118

24

1.16

4-6

137

27

1.73

2-3

157

33

2.16

0-1

183

40

2.69

AGROECOSYSTEMS SUSTAINABLE IN SEMIARID REGIONS? 2 19

eralization of nutrients. Although Greb gave water conservation only 45% of the credit, it is important to realize that without the increased water the other technologies would have been of little or no value. In semiarid regions, water is generally always the first limiting factor and must be addressed before other improved technologies can be of significance. Both burning and cultivation have been used historically to aid crop sanitation. Although both these practices have negative effects on water conservation, many advisers continue to recommend their use even in semiarid regions where water is so limiting. Farmers are often confused by conflicting information. Proponents of reduced tillage systems note that tillage reduces water conservation, enhances soil organic matter decline, increases wind and water erosion, and destroys soil structure. At the same time, farmers hear others suggest that the crop residues must be burned or buried to control insects and diseases and that tillage is necessary to control weeds. Cornish and Pratley (1991) have carefully considered this conflict in their excellent review and offered some new approaches. Although they state that burning and cultivation do play a part in crop sanitation, it is not always clear just how important they are. Some pathogens are controlled by tillage, but others may be encouraged. They further state, however, that it is very clear that crop rotations, careful weed management, genetically based resistance, and perhaps biological control will be determinants of success in any agroecosystem that places less emphasis on burning and cultivation. Cornish and Pratley (1991) call for a new approach for managing weeds. The usual approach to weed control aims at eradication, either by tillage or with herbicides. Of course, eradication is rarely feasible, either technically or economically. Successful implementation of reduced tillage or no-tillage practices depends heavily on a different approach to weed control. This is partly because less tillage is desired in the new systems and partly because of the cost and environmental concerns attached to the alternative-herbicides. The simple substitution of one herbicide treatment for one cultivation to control weeds is rarely economical or successful. Weed management is the new approach that is required. Weed management aims to prevent rather than eliminate weed problems by keeping the population of potential weeds so low that they cannot cause serious losses in future crops or pastures. An important feature of a weed management program is the flexibility to tackle weeds whenever the need arises and by the most appropriate means.

Traditional cropping patterns have evolved, in part, to match the growing patterns of common crops to average climatic patterns. However, many other factors besides climatic limitations to growth have determined the evolution of commonly used crop calendars. These include factors such as labor limitation, conflicting

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demands for inputs, unavailability of desirable crops or genotypes, and lack of markets. Objectives of designing an efficient crop calendar might include shifting cropping seasons to cooler, more humid periods of the year to improve the transpiration efficiency, seeding at times that avoid probable stress periods during anthesis of the crop, or manipulating the ratio of early season to late-season water use. A rather simple approach to matching a cropping pattern with the climate is shown for grain sorghum at Bushland, Texas, in Fig. 8. The solid line represents the 10-day rainfall amounts exceeded in 50% of the years, and the dashed line represents the number of weekly heat units (degrees Celsius above 0) exceeded in 50% of the years. Constraints for grain sorghum growth include cool spring and fall temperatures, late spring frosts, early fall frosts, low springtime rainfall, and high temperatures in mid-summer. Climatic characteristics that favor grain sorghum growth are reasonably reliable rains in late May and mid-August. The rectangular boxes represent a crop of grain sorghum seeded at four different times. The four divisions within each box represent the emergence, vegetative, reproductive, and grain-filling stages of crop production.

0

20

40

50

80 100 120 140 160 180 ZOO 220 240 260 280 300 320 340 360

0 EMERGENCE VEGETATIVE

DAYS

0 REPRODUCTION GRAIN F " G

Figure 8 Weekly heat units (above 0°C) were exceeded in 50%of the years and 10-day precipitation amounts were exceeded in 50%of the years at Bushland, Texas. The boxes represent grain sorghum seeded at four different dates (from Stewart and Steiner, 1990).

AGROECOSYSTEMS SUSTAINABLE IN SEMIARID REGIONS?

22 1

As indicated in Fig. 8, grain sorghum can be seeded in the Bushland, Texas, area over a fairly wide time period. Quimby et af. (1958) reported that the favorable seeding season in the area begins approximately May 15 but that higher yields of grain sorghum are produced from plantings made from June 10 to June 25. Field studies at Bushland suggest that the most favorable time to seed grain sorghum is approximately June 10 (Day 161). Soil moisture for seeding is usually favorable at that time because of the rains that usually occur in late May and early June, and the reproduction and grain-filling periods occur during a period when the temperatures are decreasing. The data in Fig. 8 clearly show that growing season precipitation decreases with time after May 24 (Day 144). However, there is also a decrease in the seasonal potential evapotranspiration for the later seeding dates, as indicated by the data for heat units. Stewart and Steiner (1990) used the SORKAM crop model and also concluded that this was the most optimum seeding date for grain sorghum. There are two apparent reasons why the increase in growing season rainfall with early seeding did not result in higher yields. The first was that the growing season potential evapotranspiration was higher for early seeded grain sorghum. A second reason was that the model simulations showed significant differences in the ratios of transpiration to evapotranspirationfor the various seeding dates and soil water levels at time of seeding. This indicates that a considerably higher percentage of the total evapotranspirationwas lost as evaporation from the soil for the earlier seeded than for the later seeded grain sorghum. An apparent reason for the higher loss of water by evaporation for the early seeded grain sorghum is that the soil temperature is considerably lower and the growth of the plants was slow, taking a long time for canopy development. The later seeded grain sorghum grew very rapidly in the early stages because of favorable soil water conditions and warm temperatures and full plant canopy developed quickly so that there was less evaporation from the soil surface. A greater proportion of the soil water was used for transpiration and a lesser amount lost by evaporation. The use of computer simulation models offers tremendous potential as a way to evaluate various crops and management practices. The ErosionProductivity Impact Calculator (USDA-ARS, 1990) is being used successfully by scientists in many countries throughout the world. The most efficient use of precipitation occurs when there is a crop growing at all times during the time that precipitation occurs. This is usually not feasible for grain crops except in areas where the precipitation occurs only during a short period of the year. There are areas, however, where multiple cropping can be used to take advantage of precipitation throughout the year. Double cropping is common in some humid regions and also in areas where irrigation is practiced. Double cropping of wheat and corn is successful in parts of the People’s Republic of China even in regions where the annual precipitation is less than 600 mm. Data from Gongxian, Henan Province, are presented in Table VII. Wheat is grown from October to early June and corn from mid-June to mid-September. The success of this cropping system depends on proper timing in taking advantage of the two sea-

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sons-a humid season when corn is grown and a semiarid season when the wheat is produced. The wheat is harvested only a few days prior to the start of the dominant rainy season. Corn is seeded directly in the standing wheat residue as soon as it is feasible after the wheat is harvested. At this time, the stored soil water has been depleted by the wheat crop and germination and emergence of the corn does not occur until the dominant rainy season begins. During the high rainfall period of July and early August, the emerging corn crop does not use large amounts of water so the soil profile is largely replenished with water. There is also substantial rainfall during the remainder of the corn growing season so the soil water storage at the end of the corn growing season remains high (TableVII). Following the harvest of the corn in mid-September, the soil is tilled and manure and other fertilizers are applied to the land in preparation for seeding wheat. The wheat is grown during the drier part of the year when the average precipitation is substantiallyless than one-half of the average potential evapotranspiration. Therefore, the wheat crop depends on soil water accumulated during the corn growing season for about one-half of the total amount of water used for evapotranspiration,and water stress is common during the latter stages of the wheat growing season. This example, however, shows that cropping systems can be designed in certain instances to utilize nearly all the precipitation during the growing season.

Table VII Relation between Yield and Growing Season Evapotranspirationfor Wheat and Corn in Northwest China”

Dates Wheat 16/10/83-31/5/84 7/10/84-30/5/85 11/10/85-1/6/86 8/10/8&1/6/87 18/10/87-1/6/88 Average Corn 716-81984 6/6/4/9/85 2/6/-10/9/86 516- 15/9/87 1216- I 2/9/88 Average

Available water at seeding (mm in 1.3 m)

Available water at harvest Evapotranspiration Rainfall Yield (mm in 1.3m) (mm) (mm) (Mg ha-’)

253 262 260 86 222 217

18 67 86 11 13 39

369 399 38 I 285 329 353

134 204 207 210 120 175

6.27 5.28 5.01 4.25 4.69 5.21

18 64 86 95 40 61

229 I77 80 I78 244 182

27 1 290 175 305 275 263

482 403 169 389 479 384

5.13 6.36 0.64 7.09 6.30 5.10

“Adapted from Zhu Zixi et al. (1994).

AGROECOSYSTEMS SUSTAINABLE IN SEMIARID REGIONS? 223 Tow and Schultz (1991) present a comparative analysis of dryland farming rotations occumng throughout the Australian cereal belt, from the southwest of Western Australia to north Queensland. The influences of rainfall amount and annual distribution and of soil type are explained. The analysis is an excellent example of how crop calendars can be designed as strategies for developing sustainable agroecosystems. One strategy that perhaps should be utilized more in water-deficient regions is the production of forages rather than grain crops. Forage production is not dependent on water being available late in the season as is the case for grain crops. In water-deficientregions such as Bushland, Texas, illustrated in Fig. 1, a forage crop can be grown each year, whereas it is necessary to use a fallow system to ensure successful grain production. This strategy can utilize a higher percentage of the precipitation for transpiration rather than water being lost as evaporation from the soil surface.

VI. SOIL ORGANIC MATTER MAINTENANCE Soil organic matter has over the centuries been considered by many as an elixir of life-in this case, plant life. Since the dawn of history, man has appreciated the fact that dark soils, found chiefly in river valleys and on broad level plains, are usually (but not always) productive soils. Man also realized at a very early date that soil color and productivity are commonly associated with organic matter derived mainly from decaying plant materials. A sustainable agroecosystem requires the conservation or enhancement of the soil resource base over the long term, and it is imperative that the organic matter content of soils be sustained. A decrease in soil organic matter content is an indicator of lower soil quality in most soils. This is because soil organic matter is extremely important in all soil processes-biological, physical, and chemical. Soil organic matter acts to store nutrients, improves nutrient cycling, increases the cation-exchange capacity, and reduces the effects of compaction. It builds soil structure and increases the infiltration of water. It serves as a buffer against rapid changes in pH and serves as an energy source for soil microorganisms. Organic matter tends to make very fine-textured soils behave like coarser-textured ones; the reverse is true for sandy soils. An annual loss of 1 or 2% of the organic matter in the surface 15 cm of topsoil by decomposition is not uncommon. In some climates, the loss rate can be considerably higher. Pieri (1995) summarized data from semiarid regions of Africa and reported that on very sandy soils, annual plowing with fertilizers led to an annual loss of 5% or more. Without exception, only the methods with manure application prevented a decline in soil organic matter. The effect of plowing was less

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clear but several of the reported studies did indicate that plowing increased the decline rate. Pieri (1995) proposed that there is a critical level for soil organic matter that is dependent on the soil organic matter%age and the sum of clay plus silt. He states that if the soil organic matter percentage falls below the critical level, the maintenance of soil structure is difficult to achieve. However, he disagrees with agronomists that argue that if soil organic matter is important in soil quality, then the higher soil organic matter content is, the better the soil is. Pieri states that in semiarid Africa, where there are so many technical and economic constraints to crop performance, it is fruitless to aim for a soil organic matter percentage above the critical value. Johnson er al. (1974) reported on a 29-year study at Bushland, Texas, where various cropping systems were compared for their effects on wheat production and soil organic matter maintenance (Table 111). They clearly showed that organic matter decline was increased when the length of the fallow period was increased or when tillage was intensified, and the greatest loss occurred when both circumstances were present. There was also a large accumulation of nitrate nitrogen in the soil profile for all treatments but it was particularly large for the intensively tilled fallow areas. The delayed subtilled plots, although not socially acceptable because of the uncontrolled weed production, had the smallest decline in soil organic matter and yielded about the same amount of wheat as the systems that controlled weed growth. Organic matter maintenance in semiarid regions is clearly one of the greatest constraints in the development of sustainable agroecosystems. This challenge is particularly great in many developing countries where the crop residues are so important as a source of animal feed and fuel for cooking. Whenever feasible, it is best to let animals graze the crop residues so the manure will be distributed over the area. When it is necessary to utilize the crop residues as animal feed away from the land, every attempt should be made to return manure to the land whenever feasible. Otherwise, the soil organic matter level will continue to decline to the point that long-term sustainability of the soil resource base will be threatened. Robinson er al. (1996) reported that the maintenance or enhancement of soil organic matter is proportional to the amounts of residues returned. Maintenance of soil organic matter is important to maintain yield potential (Bauer and Black, 1994).

MI. SUMMARY Achieving sustainable agroecosystems is the challenge of the coming century. With increasing population and improved living standards, the demand for food and fiber will force the development of agroecosystems into less favorable regions. There is often an imbalance between natural resources, population, and basic human needs in many regions and this is often particularly true for semiarid regions.

AGROECOSYSTEMS SUSTAINABLE IN SEMIARID REGIONS? 225 Agroecosystems in these areas can be developed and sustained, but careful management is required. The prevention of soil degradation is the first and most important issue that must be addressed in such areas. Soil degradation is a complex phenomenon. It is driven by strong interaction among socioeconomic and biophysical factors. It is fueled by increasing population, fragile economies, and poorly designed farm policies, and propelled by the fragility of the soil and harshness of the climate. Soil degradation can be subtle and slow until a certain threshold is reached, and then deterioration can occur quickly and, sometimes, irreversibly. Soil organic matter is significantly correlated with soil productivity. Maintaining soil organic matter, therefore, is of critical importance. This is a tremendous challenge in semiarid regions because insufficient precipitation seriously limits carbon inputs and the often warm conditions accelerate the decomposition of native soil organic matter during periods of favorable soil water conditions. Extensive tillage generally increases the rate of decomposition. There exists a considerable body of research knowledge and producer experiences. This information is sufficient in most cases to develop sustainable agroecosystems.The biggest challenge, however, is the implementation and execution of sound management plans. Sustainable systems must focus on long-term goals, but the reality is that short-term benefits and solutions almost always take precedence over long-term issues. Historically, agroecosystems have been developed for short-term benefits without a thorough analysis of what long-term consequences would result. Scientists, producers, policymakers, and governments must work together very closely in the future to meet the challenge of sustaining the natural resource base while producing adequate amounts of food and fiber.

REFERENCES Allison, F. E. (1973). “Soil Organic Matter and Its Role in Crop Production.” Developments Soil Science. Elsevier. Amsterdam, The Netherlands. Bauer, A,, and Black, A. L. (1994). Quantification of the effect of soil organic matter content on soil productivity. Soil Sci. Soc. Am. J. 58, 185-193. Brown, L. R. (1995).Nature’s limits. In “State of the World,” pp. 3-20. Norton, New York. Brown, L. R., and Postel, S. (1987).Thresholds of change. In “State of the World,” pp. 1-19. Norton, New York. Cook, R. L. (1962). “Soil Management for Conservation and Production.” Wiley, New York. Cornish, P. S., and Pratley, J. E. (1991). Tillage practices in sustainable farming systems. In “Dryland Farming-A Systems Approach” (V. Squires and P. Tow, Eds.), pp. 76-101. Sydney Univ. Press, South Melbourne, Australia. Council for Agricultural Science and Technology (1988). Effective use of water in irrigated agriculture, Report No. 11 3. Council for Agricultural Science and Technology, Ames, IA. Doorenbos, J., and Pruitt, W. 0. (1977). Crop water requirements. FA0 irrigation and drainage paper No. 24. Food and Agriculture Organization, United Nations, Rome. Dregne, H. E. (1989). Desertification of drylands. In “Proceedings of International Conference on Dry-

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land Farming, Amarillo/Bushland, Texas.” Texas Agricultural Experiment Station, College Station. Food and Agriculture Organization, United Nations (FAO) (1978). Report on the agro-ecological zones project. 1. Methodology and results for Africa. World Soils Resources Report No. 48. FAO, Rome. Food and Agriculture Organization, United Nations (FAO) (1981). Agriculture: Toward 2000. Main Report. FAO, Rome. Francis, C., and Youngberg, G. (1990). What is sustainable agriculture? Common misconceptions about sustainable agriculture, and historical developments relevant to the concept of sustainable agriculture. In “Sustainable Agriculture in Temperate Zones” (C. A. Francis, C. B. Flora, and L. D. King, Eds.), pp. 3-15. Wiley, New York. Greb, B. W. (1979). Technology and wheat yields in the Central Great Plains: Commercial advances. J. Soil Water Consen? 34,269-273. Greb, B. W., Smika, D. E., and Welsh, J. R. (1979). Technology and wheat yields in the Central Great Plains: Experiment station advances. J. Soil Water Consen! 34,264268. Halvorson, A.D. (1990). Management of dryland saline seeps. In “Agricultural Salinity Assessment and Management” (K. K. Tanji, Ed.), ASCE Manuals and Reports on Engineering Practice No. 7 I , pp. 372-392. American Society of Civil Engineers. New York. Hornick, S. B., and Pam, J. F. (1987). Restoring the productivity of marginal soils with organic amendments. Am. J. A/?.Agric. 2, 64-68. Johnson, W. C., and Davis, R. G. ( I 972). Stubble-mulch farming of winter wheat: A history of 28 years’ experience at USDA Southwestern Great Plains Research Center, Bushland, Texas. USDA Agricultural Research Report No. 16. USDA, Washington, DC. Johnson, W. C., Van Doren, C. E. and Burnett, E. (1974). Summer fallow in the southern Great Plains. In “Summer Fallow in the Western United States,” Conservation Research Report No. 17, pp. 86-109. Agricultural Research Service, USDA, Washington, DC. Kanemasu, E. T., Stewart, J. I., van Donk, S. J., and Virmani, S. M. (1990). Improving productivity in semiarid tropics. In “Dryland Agriculture: Strategies for Sustainability” (R. P. Singh, J. F. Pam, and B. A. Stewart, Eds.), pp. 273-309. Springer-Verlag. New York. Lal, R., and Stewart, B. A. (1990a). Soil degradation: A global threat. Adv. So;/Sci. 11. 13-17, Lal, R., and Stewart, B. A. (1990b). Need for action: Research and development priorities. Adv. Soil Sci. 11, 331-336. Larson, W. E., Walsh, L. M., Stewart, B. A., and Boelter, D. H. (Eds.) (1981). “Soil and Water Resources: Research Priorities for the Nation,” pp. 229. Soil Science Society of America, Madison, WI. Lun, S., Zhongmin, L., Xiping, D., and Yequan, X. (1992). Field water balance under the different crop rotations patterns in the Loess Plateau, China. In “Conservation Tillage Practices for Grain Farming in Semiarid Regions,” Proceedings International Symposium, July 7-9, (1992). Shortandy, Kazakhstan. CIS. Mageed, Y.A. ( 1986). “Anti-Desertification Technology and Management.” United Nations Environment Programme, Nairobi, Kenya. Musick, J. T., and Dusek, D. A. (1971). Grain sorghum response to number, timing, and size of irrigations in the Southern High Plains. Trans. Am. SOC.Agric. Eng. 14,401404. Musick, J. T., and Porter, K. B. (1990). Wheat. In “Irrigation of Agricultural Crops” (B. A. Stewart and D. R. Nielsen. Eds.), pp. 598-888. American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Madison, WI. Musick, 1. T., Jones, 0. R., Stewart, B. A,, and Dusek. D. A. (1994). Water-yield relationships for irrigated and dryland wheat in the U.S. Southern Plains. Agron. J. 86,980-986. National Research Council (1993). “Soil and Water Quality: An Agenda for Agriculture.” National Academy Press. Washington, DC. Newcombe, K. (1984). An economic justification for rural afforestation: The case for Ethiopia. Energy Depr. Paper No. 16. World Bank, Washington, DC.

AGROECOSYSTEMS SUSTAINABLE IN SEMIARID REGIONS? 227 O’Connell. P. F. (1991). Sustainable agriculture. In “Agriculture and the Environment. The (1991 Yearbook of Agriculture,” pp. 175-185. USDA, Washington, DC. Pierce, J. J., Larson, W. E., Dowdy, R. H., Graham, W. A. P. (1983). Productivity of soils: Assessing long-term changes due to erosion. J. Soil Water Consen! 3 8 , 3 9 4 . Pieri, C. (1995). Long-term soil management experiments in semiarid Francophone Africa. In “Soil Management: Experimental Basis for Sustainability and Environmental Quality” (R. La1 and B. A. Stewart, Eds.), pp. 225-266. CRC Press, Boca Raton, FL. Quimby, J. R., Kramer, N. W., Stephens, J. C., Lahr, K. A., and Karper, R. E. (1958). Grain sorghum production in Texas. Texas Agric. Exp. Stn. Bull. 912. Texas Agric. Exp. Stn., College Station. Rangely, W. R. ( I 985). Irrigation and drainage in the world. In “Proceedings, Water and Water Policy in World Food Supplies” (W. R. Jordan, Ed.), pp. 29-35. Texas A&M Univ., College Station. Robinson, C. A., Cruse, R. M., and Kohler, K. A. (1994). Soil management. In “Sustainable Agriculture Systems” (1. L. Hatfield and D. L. Karlen, Eds.), pp. 109-134. Lewis, Boca Raton, FL. Robinson, C. A,, Cruse, R. M., and Ghaffarzadeh, M. (1996). Cropping system and nitrogen effects on mollisol organic carbon. Soil Sci. Soc. Am. J. 60, 264-269. Rodale, R. (1988). Agricultural systems: The importance of sustainability. Phi Kappa Phi J. 2.6. Ruttan, V. W. (1989. Spring). Sustainability is not enough. Better Crops Plant Food, 6-9. Sharma, M. L., and Williamson, D. R. (1984). Secondary salinization of water resources in Southern Australia. In “Salinity in Watercourses and Reservoirs” (R. H. French, Ed.), pp. 571-582. Butterworth, Boston. Shengxiu, L., and Ling, X.(1992). Distribution and management of drylands in the People’s Republic of China. Adv. Soil Sci.%RR 18, 148-302. Siming, H., Chungfeng, Y., Juntong, S., and Huancheng, P. (1988). Stubble mulching in dryland on the Loess Plateau in China. Agric. Res. Arid Areas 3, 1-12. Souleimenov, M. K. (1992). Development of soil conservation farming practices for steppe areas of northern Kazakhstan. In “Conservation Tillage Practices for Grain Fanning in Semiarid Regions,” Proceeding International Symposium, July 7-9, (1992). Shortandy, Kazakhstan, CIS. Squires, V. R. ( 199I ). A systems approach to agriculture. In “Dryland Fanning-A Systems Approach” (V. Squires and P. Tow, Eds.), pp. 3-15. Sydney Univ. Press, South Melbourne, Australia. Stewart, B. A. (1989). Dryland fanning: The North American experience. p. 54-59. In “Challenges in Dryland Agriculture: AGlobal Perspective” (P. W. Unger, T. V. Sneed, W. R. Jordan, and R. Jensen, Eds.), pp. 54-59. Proceeding International Conference Dryland Fanning, AmarillolBushland, Texas, August 15-19, (1988). Texas Agric. Exper. Stn., College Station. Stewart, B. A,, and Burnett, E. (1987). Water conservation technology in rainfed and dryland agriculture. In “Water and Water Policy in World Food Supplies” (W. R. Jordan, Ed.), pp. 355-359. Proceedings of the Conference, May 2 6 3 0 , (1985). Texas A&M Univ., College Station. Stewart, B. A,, and Steiner, J. L. (1990). Water-use efficiency. Adv. SoilSci. 13, 151-173. Stewart, B. A,, Musick, J. T., and Dusek, D. A. (1983). Yield and water-use efficiency of grain sorghum in a limited irrigation-dryland system. Agron. J. 75,629-634. Stewart, B. A,, Lal, R., and El-Swaify, S. A. (1991). Sustaining the resource base of an expanding world agriculture. In “Soil Management for Sustainability” (R. La1 and F. J. Pierce. Eds.), pp. 125-144. Soil and Water Conservation Society, Ankeny, IA. Stewart, B. A,, Jones, 0. R., and Unger, P. W. (1993). Moisture management in semiarid temperate regions. p. 67-80. In “Agriculture and Environmental Challenges” (J. P. Srivastava and H. Alderman, Eds.), Proceedings of the 13th Agricultural Sector Symposium. The World Bank, Washington, DC. Stewart, B. A,, Zixi, Z., and Jones, 0. R. (1994). Optimizing rainwater use. In “Stressed Ecosystems and Sustainable Agriculture” (S. M. Virmani, J. C. Katyal, H. Eswaran, and I. P. Abrol, Eds.), pp. 253-265. Oxford & IBH, New Delhi, India. Technical Advisory Committee (1990). “Towards a Review of CGIAR Priorities and Strategies.” Technical Advisory Committee Secretariat, Rome.

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Tow, P. G. and Schultz, J. E. (1991). Crop and croppasture sequences. In “Dryland Fanning-A Systems Approach” (V. Squires and P. Tow, Eds.), pp. 55-75. Sydney Univ. Press, South Melbourne, Australia. United Nations Educational, Scientific and Cultural Organization (UNESCO) (1977). “World Map of Desertification,” NConf. 74/2. Food and Agricultural Organization, United Nations, Rome. United States Department of Agriculture, Agricultural Research Service. (USDA-ARS) ( 1990). EPIC-Erosioflroductivity Impact Calculator. Tech. Bull. 1768. USDA-ARS, Washington, DC. Unger, P.W. (1978). Straw-mulch rate effect on soil water storage and sorghum yield. Soil Sci. Soc. Am. J . 4 2 , 4 8 6 4 9 I . World Bank (1986). “The World Bank Atlas.” World Bank, Washington, DC. World Bank (1992). “World Bank Development Report.” World Bank, Washington, DC. Zixi, Z., Stewart, B. A., and Xiandun, F. (1994). Double cropping wheat and corn in a sub-humid region of China. Field Crops Res. 36,175-183.

Index A Activity reports, 186 Administrators, evaluating, 188 Agricultural research, see Ethics Agricultural themes, ethical dimensions, 155 Agriculture, sustainable, see Sustainability Agroecosystems, 191-225 climatic effect, 201-202 increasing plant-available water, 205-223 crop calendars, 2 19-223 lengthening fallow period, 206-213 mulches, 2 13-2 I7 tillage, 217-219 productivity, 193 socioeconomic effect. 203-205 soil degradative processes, 200-201 soil effect, 202-204 soil organic matter maintenance, 223-224 stability, 194 sustainability. 194 Alfalfa. salt tolerance, 95 Animakrop mix, changing, 62 Animal feeds, promoting more efficient use of nutrients, 62 Animal products, reducing consumption, 62-63 Animals ethical treatment in research, 177-179 integrating into cropping system, 64 waste management, 53 Aridity index, 195 Authorship, ethics, I73 Avocado, salt tolerance, 100- I0 I

B Bacteria, nutrient uptake stimulation, 19 Bermuda grasses, salt tolerance, 95 Berries, genetic variability and salt tolerance, 98-101 Boundaries, nutrient flows and cycles, 8-9

C Canola, salt tolerance, 92 Carbon, decomposition dynamics, 123, 127 Chinampas, 61 Chloride, toxicity in woody species, 98-101 Citrus, salt tolerance, 100 Clover introduction in Europe, 49 salt tolerance, 95 Commerce, ethics codes, 161 Communication, honest, with constituents, 189-190 Competition, ethics and, 179-180 Composting, low-input, on-farm, 141-144 Conflicts of interest, research ethics, 174-175 Consulting, ethics, 184 Copyright, infringement, 176 Corn relation of yield and growing season evapotranspiration, 222 salt tolerance, 90 Costs, indirect, recovery, ethics, 171-173 Cotton, salt tolerance, 91 Cover crops, 35-36 minimizing leaching losses, 59 Credibility, establishing and maintaining, researchers, 168-169 Cropping double, 221-222 pattern, matching with climate, 22G221 Crop residue burning, 141 chemical composition, 122 decomposition, modeling, 125-129 expert systems and erosion models. 126 RESMAN, theory in, 126-129 as nutrient cycling, 32-33 soil protection by, 122 surface managed, decomposition, 122-125 Crops calendars, plant-available water and, 219-223

229

230

INDEX

Crops (conrinued) management, practices and soil ecology, 20-2 1 mix, changing, 62 rotation, 34-35 substitution as method of dealing with salinity, 76

D Data analysis, ethics, 167-168 collecting and reporting, ethics, 167 Decomposition crop residue, modeling, 125-129 surface-managed crop residues, 122-125 Desertification, 204 Dust mulch. 212

E Ecosystem relations, 9-12 Energy, use and nutrient flows, 50-52 Environmental factor, residue decomposition, 127 Environmental stresses, interactions with salinity, 84 Ethical behavior, practical principles, 152-1 54 Ethical codes, as rules, 160 Ethical disputes, resolving, 156-157 Ethics, 149-1 90 choosing research subject matter, 154157 dimensions of agricultural themes, 155 resolving ethical disputes, 156-157 science paradigm criticism, 156 sustainable agriculture, 154-155 in conduct of research, 165- I84 authorship and shared recognition, 173 collecting and reporting data, 167 competition, 179-180 conflicts of interest, 174-175 consulting, 184 data analysis, 167-168 designing experiments, 166-167 drawing and reporting inferences, 168 establishing and maintaining credibility, 168- 169 ethical treatment of animals, 177-179 indirect cost recovery, 171-173 intellectual property rights, 175-177

peer review, 173 performing to specifications, 180 proposal budgets, 17I proposal preparation, 169-170 technology transfer, 180-183 topic selection, 166 whistle-blowing, 173-174 definitions, 151 difficultieswith utilitarian approach, 158-162 abiding by rules, 162 difficulty in evaluating outcomes, 158 ethics codes as rules, 160-162 evaluation of principles, 159-160 sea of uncertainty, 158-159 personal and group, 151-152 research administration, 184-190 activity reports, 186 equity and merit, 189 evaluating administratorsand managers, 188 hiring and termination, 184-185 honest communication with constituents, 189-1 90 job applications, 188 letters of recommendation,support, and evaluation, 186- I87 nurturing scientists, 185-186 promotion documents and decisions, 187 scientific misconduct, 152 world food situation and, 162-165 driving forces, 162-164 message for agronomists, 165 moot questions, 164-165 sources of research support, 165 Experiments designing, ethics, 166167 drawing and reporting inferences, 168 Expert system, residue decomposition models, 126

F Fairness, 153, 189 Fallow period efficiency and tillage, 218 lengthening, 206-2 13 mulch and plant-availablewater, 214-217 Fertilizers overuse, 2-3 utilizing more efficiently, 60

23 1

INDEX Field crops, genetic variability and salt tolerance, 9 1-92 Field screening techniques, salt tolerance, 103 Flow nutrients, nearby, agricultural use, 61 Food, consuming local produce, 64 Forages, genetic variability and salt tolerance, 94-95 Fruits, genetic variability and salt tolerance, 98-101 Fungi, nutrient uptake stimulation, 19

G Genes, salt tolerance, 101-102 Geologic deposits, nutrient dynamics, 50 Grains, genetic variability and salt tolerance, 88-91 Grasses, genetic variability and salt tolerance, 94-95

H Harvest, nutrient loss, 24-26 Heritability, salt tolerance, 103 Hiring, ethical, 184-185 Honesty, 152-153 Human waste, land application, 53

I Immobilization, inorganic nutrients, 18-19 Inorganic nutrients, immobilization, 18-19 Integrity, 153 Intellectual property rights, ethics and, 175-177 Ion accumulation, salt tolerance and, 86 selectivity, salt tolerance and, 85-86 Irrigation, increasing salinity of lands and, 76

J Job applications. I88

K Kentucky bluegrasses, salt tolerance, 95

L Land, increased yield, 192 Letters of recommendation, support, and evaluation, 186-187 Lettuce, salt tolerance, 98 Linseed, salt tolerance, 93

M Managers, evaluating, 188 Manure nutrient flow, 40 utilizing more efficiently, 60 Melon, salt tolerance, 97-98 Merit, rewarding, equity and, 189 Military, ethics codes, 161-162 Mineralization, 14-15 soil organic matter, 18 Modeling, salt tolerance, 107 Molecular biology, salt tolerance, 106-107 Mulches, increasing plant-available water, 21 3-2 I7

N Nitrate leaching, 26.42 Nitrogen decomposition dynamics, 123, 127 fixation, 19 by symbiotic and nonsymbiotic organisms, 6M1 recommendations, 30-3 1 Nondisclosure agreements, 177 No-till cropping systems, 121-144 deleterious rhizobacteria for weed control, 137-141 domination by fungi and earthworms, 136 low-input, on-farm composting, 141-144 root-microbial relationships, 129-1 37 Nutrient cycle, 8 crop residues, 32-33 ecology, 17-2 1 efficiency, 9-10 plant strategies, 10 simplified managed system, 10-1 1 simplified natural system, 9-10 Nutrient dynamics, 1-66: see also Soil-plant system definitions, 7-9

232

INDEX

Nutrient dynamics (continued) energy use and nutrient flows, 50-52 farm-level changes, 60-62 farm-scale cycling and Rows, 3 8 4 7 to and from farms, 4 0 4 3 between farms, 43-44 nutrient exports > imports, 44-46 nutrient exports < imports, 46 nutrient exports = imports, 46-47 within-farm, 3 9 4 0 at field level, 23-38 changes in nutrient, 36-37 changing to biologically based nutrient sources, 37-38 cover crops, 35-36 crop residues, 32-33 crop rotation, 34-35 inadvertent nutrient losses, 26-27 nutrient additions, 27-3 1 nutrient losses, 24-27 pastures, 35 tillage systems, 33-34 field-level changes, 58-60 finite geologic deposits, 50 harvest removal, 24-26 historical overview, 5-7 increasing soil nutrient availability, 5 9 4 0 influences on flow patterns, 54-56 intercontinental flows, 49-50 landscape position, 12-13 possible changes in large-scale flows, 52-54 seasonal patterns, 12-1 3 societal-level changes, 6 2 4 5 spatial cycle and ecosystem relations, 9-12 spatial scale of changes and time needed to complete, 57-58 utilizing fertilizers and manures more efficiently, 60 utilizing more efficiently taken up nutrient sources, 60-6 I watersheds, 4 7 4 8 Nutrient flow, 8 ecology, 17-2 1 field, changes in, 3 6 3 7 patterns, potential implications, 44-45 Nutrient leaks, 62-63 Nutrients added, quantity, 29-3 I application timing and methods, 28-29 balances, mixed crop and livestock farm, MI

degree of synchronization between availability and uptake needs, 58-59 enhancing uptake efficiency, 58-59 management issues, 2-4 sources biologically based, 37-38 soluble, sparing use, 61 transformations, ecology, 17-2 I transporting back to farmland, 64-65 Nuts, genetic variability and salt tolerance, 98-101

0 Oil seed crops, genetic variability and salt tolerance, 92-93 Organic matter maintenance, semiarid regions, 223-224 semiarid regions, 21 1 Ornamentals, genetic variability and salt tolerance, 101 Osmotic adjustment, salt tolerance and, 87

P Pastures, 35 Patent, infringement, 176 Peer review, ethics, 173 Performing to specifications, I80 Phosphorus, solubility, 19 Plant-animal-human trophic pyramid, segment separation, 6 3 4 5 Plant nutrition, 13-17 Potato, salt tolerance, 92 Practices advocating, 182-183 testing and comparing, 181-182 Products advocating, 182-1 83 testing and comparing, 18 1-1 82 Promotion, documents and decisions, 187 Proposals budgets, ethics and, 17 1 preparing, ethics and, 169-1 70

R Research, see Ethics RESMAN, theory used in, 126-129 Rhizobacteria, deleterious, 129-1 32 weed control, 137-141

INDEX Rice, salt tolerance, 90-91 Root environment, optimizing, 59 Root-microbial relationships, 128-1 37 deleterious rhizobacteria, 129-132 root zone temperature and, 133 Ruminant livestock, biological nitrogen fixation, 4041

S Safflower, salt tolerance, 92-93 Salinity problems, semiarid regions, 212 Salt, accumulation, 86 Salt stress, short- and long-term effects, 85 Salt tolerance, 75-108 breeding methods, 101-105 field screening techniques, 103 genes for tolerance, 101- 102 heritability, 103 selection methods. 104-105 crop species, 77 genetic variability, 88-101 field crops, 91-92 fruit, nuts, and berries, 98-101 grains, 88-9 I grasses and forages, 94-95 oil seed crops. 92-93 ornamentals, 101 vegetable crops, 95-98 in low-yielding varieties, 81 measurement, 79-80 mechanisms, 84-88 ion accumulation, 86 ion selectivity,85-86 organic solutes, 87 osmotic adjustment, 87 water use efficiency. 87-88 modeling, 107 molecular biology, 106-107 rationale for breeding for, 77-78 selection for. 78-84 environmentalinteractions, 84 growth stage, 82-83 specific ion tolerance, 83-84 yield and productivity, 80-82 tissue cultures, 105-106 Science, pure, ethics codes, 161 Science paradigm. criticism, 156 Scientific misconduct, 152 Scientists, nurturing. 185-1 86

233

Seasonal patterns, nutrient dynamics, 12-13 Selection methods, salt tolerance, 104-105 Semiarid regions, 194-198 aridity index, 195 characterization, 192 example locations, 196-198 length of growing period, 195-196 soil organic matter maintenance, 223-224 Soil chemical properties, 21 degradation, interdependenceon biological and socioeconomicfactors, 203, 205 effect on sustainability,202-204 erosion, 26 fertility, maintaining long-term, 15-17 management, practices and soil ecology, 20-2 1 nutrients, increasing availability,5 9 4 0 nutrient stocks, 13-17 organic matter depletion, 3 dynamics, 17 maintaining high levels, 15-16 mineralization, 18 physical properties, 2 1-22 Soil-plant system, 13-23 biological, chemical, and physical interactions, 22-23 ecology of nutrient flows, transformations, and cycles, 17-21 maintaining long-term soil fertility, 15-1 7 satisfying short-term fertility needs, 14-15 simplified nutrient cycle, flows, and transformations, 14 Soil resources, wasteful use, 62 Solutes, organic, salt tolerance and, 87 Sorghum relation of yield and seasonal evapotranspiration, 208 salt tolerance, 90 Soybean, salt tolerance, 92 Spacial scale, 9-12 Stocks, 7-8 Straw mulch, 215-216 Stubble mulching, 213-215 Sugar beet, salt tolerance, 91-92 Summer fallow, 206.21 1-213 Sunflower, salt tolerance, 93 Sustainability,4, 56-57, 194, 198-205 climatic effect, 201-202

2 34

INDEX

Sustainability (conrinued) definition, 198 ethics and, 154-155 reasons for importance in policy agenda, 199 socioeconomic effect, 203-205 soil effect, 202-203

T Technology transfer, ethical issues, 180-1 83 Technology transfer agents, responsibility, 181

,

Temperature function, residue decomposition, 128 Termination, of employees, 185 Tillage, plant-available water and, 217-219 Tillage systems, 33-34 Tissue cultures, salt tolerance, 105-106 Tomato, salt tolerance, 97 Trade secrets, ethics and, 176177 Transformations, 8 Trophic pyramid, 5

V Vegetable crops, genetic variability and salt tolerance, 95-98

W Water, plant-available, technologies for increasing, 205-225 crop calendars, 2 19-223 lengthening fallow period, 206-213 mulches, 2 13-2 17 tillage, 217-219 Water function, 127 Watersheds, nutrient dynamics, 47-48 Water use, efficiency and salt tolerance, 87-88 Weed control, deleterious rhizobacteria, 137-14 1 Wheat relation of yield and growing season evapotranspiration, 222 relation of yield to seasonal evapotranspiration, 208-210 salt tolerance, 89-90, 102 Wheatgrass, salt tolerance, 94 Whistle-blowing, ethics, 173-174 Winter wheat deleterious rhizobacteria effect, 129-132 high crown set, 141-142 rhizoplane populations of inhibitory pseudomonads, 134-1 35 yield from fields inoculated with rhizobacteria, 138 World food, ethics and, 162-165

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