The Biogeochemistry of the Amazon Basin

THE BIOGEOCHEMISTRY OF THE AMAZON BASIN

This page intentionally left blank

The Biogeochemistry of the Amazon Basin

Edited by MICHAEL E. MCCLAIN Florida International University, USA REYNALDO L. VICTORIA Universidade de Sao Paulo, Brazil JEFFREY E. RICHEY University of Washington, USA

OXFORD UNIVERSITY PRESS

2001

OXFORD UNIVERSITY PRESS

Oxford Athens Chennai Kolkata Nairobi

New York Auckland Bangkok Bogotá Buenos Aires Cape Town Dar es Salaam Delhi Florence Hong Kong Istanbul Karachi Kuala Lumpur Madrid Melbourne Mexico City Mumbai Paris Sao Paolo Shanghai Singapore Taipei Tokyo Toronto Warsaw

and associated companies in Berlin Ibadan

Copyright © 2001 by Oxford University Press, Inc. Published by Oxford University Press, Inc. 198 Madison Avenue, New York, New York 10016 Oxford is a registered trademark of Oxford University Press. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior permission of Oxford University Press. Library of Congress Cataloging-in-Publication Data The biogeochemistry of the Amazon Basin / edited by Michael E. McClain, Reynaldo L. Victoria, Jeffrey E. Richey. p. cm. Includes bibliographical references. ISBN 0-19-511431-0 1. Biogeochemistry—Amazon River Wathershed. I. McClain, Michael E. II. Victoria, Reynaldo L. III. Richey, Jeffrey Edward, 1946QH112 .B56 2000 577'.14'09811—dc21 00-041642

9 8 7 6 5 4 3 2 1 Printed in the United States of America on acid-free paper

Acknowledgments This book represents the efforts of many individuals and many projects. We collectively thank the individuals and funding agencies that made this work possible. The Editors' own contributions were made possible with support from the National Science Foundation Division of Environmental Biology and International Programs, the NASA Earth Observing System, the FundaCão de Amparo a Pesquisa do Estado de São Paulo, the Inter American Institute for Global Change Research, and the Andrew W. Mellon Foundation. We would particularly like to thank the reviewers of individual chapters: Dominique Arrouays, Robert Berner, Vladimir Elias, Christian Feller, Augusto Cesar Franco, Sana Gardescu, Jean Loup Guyot, Steven Hamilton, Robert Harrison, Dominique Irvine, Rod Keenan, George Klinge, Kirsten Laarkamp, Ariel Lugo, Flavio Luizao, Luiz Martinelli, Ernesto Medina, Michel Meybeck, Jose Natalino Silva, Jason Neff, Christopher Neill, Jorge Paolini, Walter de Paula Lima, Kathleen Ruttenberg, Miguel Simao Neto, Morgan Smidt, Boris Volkoff, Michael Williams, and Summer Wilson We would also like to thank Nancy Blanton, Cristina Dumlao, and Madelyn Mateo for final preparation of the manuscript.

This page intentionally left blank

Contents 1

2

3

4

5

6

7

8

9

The Relevance of Biogeochemistry to Amazon Development and Conservation Michael McClain

3

General Characteristics and Variability of Climate in the Amazon Basin and its Links to the Global Climate System Jose A. Marengo & Carlos A. Nobre

17

The Atmospheric Component of Biogeochemical Cycles in the Amazon Basin Paulo Artaxo

42

Soil versus Biological Controls on Nutrient Cycling in Terra Firme Forests Elvira Cuevas

53

Nutrient Cycling as a Function of Landscape and Biotic Characteristics in the Cerrados of Central Brazil M. Haridasan

68

Linking Biogeochemical Cycles to Cattle Pasture Management and Sustainability in The Amazon Basin Moacyr B. Dias-Filho, Eric A. Davidson & Claudio J. Reis de Carvalho

84

Nutrient Considerations in the Use of Silviculture for Land Development and Rehabilitation in the Amazon Florencia Montagnini

106

Extractive Reserves and Participatory Research as Factors in the Biogeochemistry of the Amazon Basin Foster Brown , Karen Kainer & Eufran do Amaral

122

The Recovery of Biomass, Nutrient Stocks, and Deep-Soil Functions in Secondary Forests Daniel Nepstad, Paulo R. S. Moutinho & Daniel Markewitz

139

Contents

viii

10

The Interface Between Economics and Nutrient Cycling in Amazon Land Development Carl F. Jordan

156

11

Carbon Storage in Biomass and Soils Martial Bernoux, Paulo M. A. GraCa, Carlos C. Cerri, Philip M. Fearnside, Brigitte J. Feigl & Marisa C. Piccolo

12

Terrestrial Inputs to Amazon Streams and Internal Biogeochemical Processing Michael E. McClain & Helmut Elsenbeer

185

Geo-ecological Controls on Elemental Fluxes in Communities of Higher Plants in Amazonian floodplains Maria Tereza F. Piedade, Martin Worbes & Wolfgang J. Junk

209

Biogeochemistry of Amazon Floodplain Lakes and Associated Wetlands John M. Melack & Bruce R. Forsberg

235

13

14

165

15

Organic Matter and Nutrients in the Mainstem Amazon River Allan H. Devol & John I. Hedges

275

16

Trace Elements in the Mainstem Amazon River Patrick T. Seyler & Gerald R. Boaventura

307

17

Biogeochemical Processes on the Amazon Shelf: Changes in Dissolved and Paniculate Fluxes During River/Ocean Mixing David J. DeMaster & Robert C. Aller

328

Contributors Andrea S. Alechandre, Department of Agrarian Sciences, Federal University of Acre, Rio Branco, AC 69.915, Brazil; [email protected] Robert C. Aller, Marine Sciences Research Center, SUNY at Stony Brook, Stony Brook, NY 11794-5000, USA; [email protected] Eufran do Amaral, Center for Agroforestry Research, Brazilian Agricultural Research Agency (CPAF/EMBRAPA), Rio Branco, AC, Brazil; [email protected] Paulo Artaxo, Instituto de Fisica, Universidade de São Paulo, Rua do matao, Travessa R, 187, CEP 05508-900, Sao Paulo, Brazil; [email protected] Martial Bernoux, Institut de Recherche pour le Développement (IRD), Centro de energia Nuclear na Agricultura (CENA), University of Sao Paulo (USP), Laboratorio Biogeoquimica do Solo, Caixa Postal 96, 13400-970 Piracicaba, SP, Brazil; [email protected] Geraldo R. Boaventura, Universidade de Brasilia, Instituto de Geociências Campus Universitário Darcy Ribeiro, ICC-Central CEP: 70910-900 Brasilia, Brazil; [email protected] I. Foster Brown, Woods Hole Research Center, Federal Fluminense University, Federal University of Acre, Parque Zoobotanico, Rio Branco, AC 69.915, Brazil; [email protected] Cláudio Jose Reis de Carvalho, Laboratorio Ecofisiologia Vegetal, Embrapa Amazonia Oriental, C. Postal 48, 66017-970 Belém, PA, Brazil; [email protected] Carlos C. Cerri, Centro de energia Nuclear na Agricultura (CENA), University of Sao Paulo - USP, Lab. Biogeoquimica do Solo, Caixa Postal 96, 13400-970 Piracicaba, SP, Brazil; [email protected] Elvira Cuevas, Centro de Ecologia, Instituto Venezolano de Investigaciones Cientificas, Apartado 21827, Caracas 1020-A, Venezuela; [email protected] Eric A. Davidson, The Woods Hole Research Center, P.O. Box 296, Woods Hole, MA 02543-0296, USA; edavidson® whrc. org David J. DeMaster, Deptment of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695, USA; [email protected]

x

Contributors

Allan H. Devol, School of Oceanography, Box 357940, University of Washington, Seattle, WA 98195, USA; devol@u. washington.edu Moacyr B. Dias-Filho, Laboratorio Ecofisiologia Vegetal, Embrapa Amazônia Oriental, C. Postal 48, 66017-970 Belém, PA, Brazil; [email protected] Helmut Elsenbeer, Department of Civil and Environmental Engineering, University of Cincinnati, P.O. Box 210071, Cincinnati, OH 45221, USA; [email protected] Philip M. Fearnside, Institute Nacional de Pesquisas da Amazonia (INPA), Coordenadoria de Pesquisas em Ecologia, Caixa Postal 478, 69011-970 Manaus, Amazonas, Brazil; [email protected] Brigitte J. Feigl, Centro de energia Nuclear na Agricultura - CENA, University of Sao Paulo (USP), Laboratorio Biogeoquimica do Solo, Caixa Postal 96, 13400-970 Piracicaba, SP, Brazil; [email protected] Bruce R. Forsberg, Instituto Nacional de Pesquisas da Amazonia (INPA), Coordenadoria de Pesquisas em Ecologia, Caixa Postal 478, 69011-970 Manaus, Amazonas, Brazil; [email protected] Paulo M. A. Graca, Instituto Nacional de Pesquisas da Amazonia (INPA), Coordenadoria de Pesquisas em Ecologia, Caixa Postal 478, 69011-970 Manaus, Amazonas, Brazil; [email protected] M. Haridasan, Departmento de Ecologia, Instituto de Ciências Biológicas, Universidade de Brasilia, 70919970 Brasilia, DF, Brazil; [email protected] John 1. Hedges, School of Oceanography, Box 357940, University of Washington, Seattle, WA 98195, USA; [email protected] Carl F. Jordan, Institute of Ecology, University of Georgia, Athens, GA 30602, USA; cfjordan® arches, uga. edu Wolfgang J. Junk, Max-Planck-Institut fur Limnologie, Arbeitsgruppe, Tropenokologie, Postfach 165, 24302 Plon, Germany; [email protected] Karen A. Kainer, Tropical Conservation and Development Program, Center for Latin American Studies, University of Florida, P.O. Box 115531, Gainesville, FL 32611, USA; [email protected] Daniel Markewitz, University of Georgia, Daniel B. Warnell School of Forest Resources, Athens, GA, 30602, USA; [email protected] Jose A. Marengo, Centro de Previsao de Tempo e Estudos de Clima (CPTEC), Institute Nacional de Pesquisas Espaciais (INPE), Cacheoira Paulista, Sao Paulo, Brazil; [email protected] Michael E. McClain, Department of Environmental Studies, Florida International University, Miami, FL 33199, USA; [email protected]

Contributors

xi

John M. Melack, Donald Bren School of Environmental Science and Management, University of California, Santa Barbara, CA 93106-5131, USA; [email protected] Florencia Montagnini, Area of Management and Conservation of Forests and Biodiversity, Centre Agronómico Tropical de Investigacion y Enseñanza (CATIE), 7170 Turrialba, Costa Rica; [email protected] Paulo R. S. Moutinho, Instituto de Pesquisa Ambiental da Amazonia (IPAM), Eneas Pinheiro 1424, 66087-430 Belém, Para, Brazil; Daniel Nepstad, The Woods Hole Research Center, P.O. Box 296, Woods Hole, MA 02543-0296, USA; [email protected] Carlos A. Nobre, Centre de Previsão de Tempo e Estudos de Clima (CPTEC), Instituto Nacional de Pesquisas Espaciais (INPE), Cacheoira Paulista, Sao Paulo, Brazil; [email protected] Marisa de Cassia Piccolo, Centro de energia Nuclear na Agricultura (CENA), University of Sao Paulo USP, Lab. Biogeoquimica do Solo, Caixa Postal 96, 13400-970 Piracicaba, SP Brazil; [email protected] Maria Tereza F. Piedade, Instituto Nacional de Pesquisas da Amazonia (INPA), C.P. 478, 69011-970 Manaus, AM, Brazil; [email protected] Jeffrey E. Richey, School of Oceanography, Box 357940, University of Washington, Seattle, WA 98195, USA; [email protected] Patrick T. Seyler, Département Milieux et Environnement, Institut de Recherche pour le Developpement, 213, Rue Lafayette, 75480 PARIS Cedex 10, France; [email protected] Reynaldo L. Victoria, Centro de energia Nuclear na Agricultura (CENA), University of Sao Paulo (USP), Caixa Postal 96, 13400-970 Piracicaba, SP, Brazil; [email protected] Martin Worbes, Forstbotanisches Institut, AuJSenstelle: Busgenweg 2, 37077 Gottingen, Germany ; [email protected]

This page intentionally left blank

THE BIOGEOCHEMISTRY OF THE AMAZON BASIN

This page intentionally left blank

1 The Relevance of Biogeochemistry to Amazon Development and Conservation Michael E. McClain

To read the press of recent years, one might imagine that the fate of the world rests in the hands of those who would develop the Amazon basin. Waves of incoming colonists are blamed for the bulk of the deforestation and development (Schomberg 1998), but Asian logging firms, multinational oil companies, and gold miners are also portrayed as destructive agents hacking down the forest, systematically undermining its biodiversity, and severely contaminating its myriad ecosystems (Althaus 1996, Ferreira 1996, James 1998). The effects of these varied threats are regularly broadcast in alarming tones. Rueters News Service warned in January 1998 that "Brazil's Amazon rain forest, the world's richest trove of biological diversity and source of much of the Earth's oxygen, continues to be ravaged" (Craig 1998). And, in April 1999, a writer for the Associated Press communicated the "fear" of unspecified scientists that "damage to the rain forest... could throw the Earth's climate out of balance" (Donn 1999). Clearly, the fate of the Amazon and the implications of its fate to the overall Earth system are topics of enormous scientific and popular interest. While there is little disagreement that the complete destruction of Amazon forests would be catastrophic, what about partial deforestation of the region? How much, and which parts, of the Amazon can be converted to sustainable human land

uses without compromising the ecological integrity of the conserved areas? How might this development impact regional climate, adjoining coastal systems, and overall global processes? Answers to these volatile questions remain elusive and seemingly endless strands of controversy swirl about them. At the heart of the matter, yet largely beyond the public discussion, are biogeochemical cycles that support and regulate the functioning of the Amazonia's biological systems. Moreover, it is the incomplete understanding of these cycles that promotes uncertainty and feeds the controversy. The purpose of this book is to present a coherent assessment of our current understanding of the biogeochemical functioning of the Amazon basin. Although it is surely presumptuous to assume that this presentation will shed sufficient light on the uncertainties to eliminate the current controversies, we hope that it will provide a basis for lifting the discussion to a higher level. Cycles of carbon, nitrogen, phosphorus, oxygen, and many other bioactive elements link the region's flora and fauna to associated cycles of water, soil, and seasonal energy fluxes. Research into the biogeochemistry of Amazonia's natural ecosystems has revealed an unprecedented level of sophistication of adaptive mechanisms within the flora and fauna for conserving and accessing nutrients. Replacement of native vegetation with poorly

4

Michael E. McClain

adapted crops and pasture grasses often of global greenhouse gases. Recent articles leads to problems of nutrient availability and have highlighted the Amazon's role as a carultimately to crop failure and pasture degra- bon dioxide sink (Prentice and Lloyd 1998), dation. Sustainable agrarian development in and modeling results suggest that Amazon Amazonia is thus intimately related to funda- vegetation may account for between 5% and mental biogeochemical processes and to our 35% of the terrestrial sink for excess CO2 in understanding of those processes. the atmosphere (Tian et al. 1998). Flora and fauna composing the expansive Conversely, during El Nino years when the aquatic ecosystems exhibit a similar level of Amazon is drier than normal, these same adaptive sophistication, which enables a rich modeling results suggest that Amazon diversity of organisms to flourish in spite of vegetation may serve as a net source of CO2 modest nutrient inputs from lowland terrestri- to the atmosphere. The Amazon is also a al systems. The bounty of aquatic resources significant source of other greenhouse gases and water itself provides many essential serv- such as methane and nitrous oxide. When ices to the people of the Amazon. The these gaseous emissions are combined with region's natural fisheries provide a large pro- strong energy transfers linked to evaporation portion, and sometimes the majority, of ani- over the basin, the role of the Amazon in mal protein consumed by inhabitants. Fish are regional and global climate grows well also a valuable source of income to fishermen. beyond the areal coverage of the basin. By River and lake water satisfies nearly all of the considering the quantitative impact that water supply needs of Amazon peoples, Amazon development may have on these including drinking, cooking, bathing, and emissions and energy transfers, one begins to waste removal. While little water is used for understand the "fear" voiced in the irrigation, river channels and lakes are impor- opening paragraph. tant avenues of transportation and shipping. Each year, the Amazon river discharges Water bodies also provide abundant opportu- nearly 20% of the total volume of water delivnities for recreation and are enjoyed in this ered to the oceans by rivers. In fact, the way by most of the region's inhabitants. annual discharge of the Amazon is greater With each of the above-mentioned uses of than the combined annual discharge of water, quality (or chemical purity) is crucial. Earth's next eight largest rivers! AccompanyThis is especially true because water is ing this huge volume of water are generally not treated prior to human use in correspondingly large loads of salts, metals, the Amazon. Threats to water quality include organic compounds, and sediments. These excess erosion and nutrient runoff from inputs to the Atlantic Ocean take on both poorly managed fields and pastures, metal regional and global significance by fueling pollution from mining and other industrial coastal productivity and significantly impactactivities, and organic pollution from the ing the global cycles of certain elements. discharge of untreated human and animal wastes. The capacity of the river system to Biogeochemical Cycles in the assimilate these threats varies as a function of Atmosphere - Global Linkages discharge and biogeochemical processes operating to eliminate harmful conditions Biogeochemical cycles in the Amazon (McClain 2001). basin are linked to global cycles through On a global scale, and as Earth's largest exchange with the atmosphere and river conterminous area of wet tropical forest, the discharge into the Atlantic Ocean. Of these Amazon is viewed as a significant regulator two, exchange with the atmosphere is by far

The Relevance of Biogeochemistry to Amazon Development and Conservation

the most interactive and variable on seasonal and inter annual time scales. Furthermore, it is through atmospheric exchanges that the Amazon interacts directly with the global climate system. In chapter 2, Jose Marengo and Carlos Nobre examine the nature of climatic variability over the Amazon and the influence of the basin within the global climate system. Over most of the Amazon temperatures are remarkably uniform and do not vary appreciably with season. The Andean Amazon is of course an important exception, as temperatures there vary over large ranges. Precipitation, however, varies significantly with space and time throughout the basin and stands as the most important climatic variable in explaining the nature, distribution, and magnitude of certain biogeochemical processes. Variability in rainfall is tied to large-scale circulation patterns of air masses in the region and the interactions of these air masses with ocean surface waters. The tight coupling of these processes is evidenced by the profound impacts that El Nino Southern Oscillation (ENSO) events have upon rainfall in the basin. Overall, the Amazon experiences reduced rainfall during ENSO events, although certain areas experience elevated rains. The impact of such anomalies on land and aquatic biogeochemical cycles is likely to be significant but has not been adequately assessed as yet. The general reduction in rainfall across the Amazon during ENSO events raises the specter that global warming will produce similar reductions. This threat is amplified by the results of a growing number of largescale modeling efforts, which consistently show a decrease in precipitation due to deforestation. The combined effects of these disturbances could significantly alter the cycling of water and associated elements in the Amazon. In addition to climatic influences on land and aquatic biogeochemical cycles, there are also exchanges of bioactive elements

5

between the Amazon surface and atmosphere. These exchanges take the form of gases and aerosols, which have direct impacts on regional and global climate. In chapter 3, Paulo Artaxo discusses the forms and dynamics of these exchanges. Each year, Amazon forests and savannas emit large quantities of aerosols and associated bioactive elements to the atmosphere. In general, higher concentrations of bioactive elements are found in the coarser fraction of aerosols, which are recycled within the region itself. The finer fraction, however, may be transported for thousands of kilometers and thus exit the Amazon region. Atmospheric transport out of the region, although still poorly quantified, probably represents a significant loss of nutrients from the basin. During the dry season, when slash burning is most practiced, concentrations of aerosols in the less than 10 urn fraction increase by an order of magnitude to values ranging from 400-500 pg rrr3. Under certain circumstances of enhanced convection, these aerosols may rise to altitudes approaching 10 kilometers, where transport velocities are greatly accelerated and their distribution may be global. The biogeochemical roles of rainfall, aerosol, and gas emissions converge in the form of wet deposition, which is well documented as a significant source of nutrients and organic matter to Amazon surface systems (Andreae et al. 1990, Lesack and Melack 1996, Williams et al. 1997a). Important components of wet deposition are organic acids, which control rain pH values, and essential nutrients like phosphorus (P) and potassium (K).

Biogeochemical Cycles on Land Coping with Nutrient Scarcity According to an assessment by Ozorio de Almeida and Campari (1995), the bulk of deforestation in the Brazilian Amazon is carried out by people moving within the basin, as opposed to newly arriving colonists.

6

The internal movement is prompted by a loss in soil fertility, which drives families to abandon their pastures and fields and to clear new land elsewhere. An essential element in reducing deforestation is thus improving management of cultivated land, thereby extending its use and reducing the rate of land abandonment. Widespread use of fertilizers and highly mechanized agriculture is impractical for most people in the Amazon. So, improved land management for nutrient conservation must make maximal use of natural mechanisms of nutrient recycling and conservation. Management practices imported to the region regularly fail because they are adapted to conditions different than those present in the Amazon. New practices must be developed that are best suited to Amazon nutrient and soil/climate conditions. The luxurious growth which typifies Amazon forest is a clear indication that natural vegetation is well adapted to nutrient/soil/climate conditions of the region. Thus, important insights to sustainable land use are likely to be found by carefully examining the functioning of these systems.

Forests and savannas In chapter 4, Elvira Cuevas examines the mechanisms through which terra firme (or nonflooded) forests of the Amazon satisfy their nutrient needs on the generally nutrientpoor soils that cover greater than 80% of the basin. On these soils, forest growth is limited mainly by P, calcium (Ca), and magnesium (Mg), as evidenced by significant correlations between forest productivity measures and abundances of these elements (Cuevas and Medina 1986, Cuevas and Medina 1988). Strong and persistent leaching processes have severely depleted many Amazon soils in Ca and Mg and have produced conditions under which mineral forms of P are bound to iron and aluminum oxides and generally

Michael E. McClain

unavailable to plants. Under these conditions, the key to sustained nutrient flows seems to lie in organic pools of these elements and the vegetation's adaptations to access these pools. Along a toposequence in the upper Negro basin of Venezuela, Tiessen et al. (1994 a, b) documented that greater than 70% of available P occurred at the surface and in the upper organic horizons of the soil column. Of this P, 50% occurred in paniculate organic matter with a mean residence time of 4 years. The remaining mineral-associated P cycled over much longer time periods or was permanently sequestered in lateritic nodules. Amazon forest plants access organic pools of P, Ca, and Mg through surface mats of fine roots and symbiotic interactions with mycorrhizae fungi. Root mats in Amazon forests on average account for 30% of the total fine root mass, and root mat thickness is inversely correlated with nutrient availability. Mycorrhizae fungi stimulate phosphatase production in the vicinity of fine roots and thus enhance the availability of PO4 for uptake by fine roots. In fact, the combined action of surficial fine roots and mycorrhizae fungi has been shown to retain nearly 100% of added 32P in uptake experiments in the upper Negro basin (Cuevas and Medina 1988). It is important to note here that this emphasis on recycling means that the productivity of Amazon forests is linked to the rate of nutrient remineralization rather than the standing stock of nutrients at anytime. Furthermore, these recycling mechanisms are a direct adaptation to the humid conditions of the Amazon, where extreme leaching potential necessitates the concentration of fine roots at the surface and abundant water makes this root allocation scheme functionally possible. In the more arid savanna regions of the Amazon basin and Brazilian Cerrado, the problem of nutrient depleted soils is compounded by seasonal water shortages. As discussed by M. Haridasan in chapter 5, savanna vegetation is thus of greatly reduced

The Relevance of Biogeochemistry to Amazon Development and Conservation

biomass, with correspondingly reduced rates of nutrient cycling. Very little research has been conducted into the biogeochemical processes which link vegetation productivity to nutrient pools, but one especially important consideration is the role of fire in injecting periodic pulses of available nutrients into the system. Trees of the Cerrado are well adapted to the frequent fires and are seldom killed. Total vegetation cover responds quickly after fires and generally recovers with a few months of the fire event (Haridasan, this volume).

Pastures, plantations, and extractive reserves The conversion of Amazon forest to other land uses sends a shock through the ecosystem which significantly impacts underlying biogeochemical cycles. Deforestation accompanied by slash removal and burning initially releases large quantities of carbon and nitrogen to the atmosphere and decreases the standing stocks of these elements. Mineral nutrients such as P, Ca, Mg, and K are concentrated in ashes and are returned to the soil column as a pulse accompanying infiltrating rainwater. These transformations are most dramatic and enduring in the conversion of forest to pasture, where grasses replace trees and cattle replace forest animals. The \videspread creation of pastures in the Amazon has met with limited success because pasture productivity generally declines within three to five years of establishment, leading to expensive rehabilitation or, more likely, pasture abandonment. The key to extended pasture productivity lies in better management of nutrient cycles and therefore pasture fertility. In chapter 6, Moacyr Dias-Filho, Eric Davidson, and Claudio Reis de Carvalho discuss the role of biogeochemical cycles in regulating pasture productivity. The biogeochemistry of P is shown to be of particular

7

importance, as this element is most limiting. Of the approximately 200 kg ha'1 of P measured in the soils of one pasture in the eastern Amazon, less than 1% is thought to be available to plants. Because only a very small pool of P is available to maintain pasture grasses, and fertilization is economically difficult, management techniques which are normally ignored become important. For example, the animals themselves become a key link in the cycle which replenishes available P. Instead of returning to the soil surface as litterfall, as would be the case in natural Amazon forests, P taken up by plants is returned to the soil surface mainly as animal feces. This, in turn, leads to problems of P distribution, as cattle tend to deposit feces in concentrated areas rather than uniformly across the pasture (Buschbacher 1987). Cattle also alter nutrient cycles by compressing the soil surface and promoting overland hydrological flows and greater losses of P. Pasture creation impacts carbon (C) and nitrogen (N) cycles by reducing standing stocks and cycling rates of these elements. The main loss of C and N of course occurs in the aboveground biomass. In fact, C and N stocks may actually increase in pasture soils, but such increases are unpredictable and dependent on factors such as native soil fertility, fertilization, climate, fire frequency, and grazing intensity (Dias-Filho et al., this volume). Although standing stocks may rise or fall, research from the eastern and western Amazon shows that cycling rates consistently decrease. Net N mineralization and nitrification rates in pastures were found to range from less than 15% to 40% of rates measured in adjacent primary forest (unpublished data from Verchot and Davidson, presented in Dias-Filho et al., this volume). Similar patterns were observed for emissions of NO and N2O gases. These sharply reduced rates of N cycling raise the question of whether N might become limiting under certain conditions in Amazon pastures or secondary forests.

Michael E. McClain

8

As described by Florencia Montagnini in chapter 7, large-scale conversion of forest to silvicultural plantations has been carried out with generally disappointing results in the Amazon. Expansive stands of commercial species are highly susceptible to pests and disease, and in the Amazon they are dependent on additions of fertilizers to sustain multiple rotations. Thus it appears that such large-scale silviculture is unlikely to become a widespread use of Amazon land. Smaller scale mixed plantations, however, have a far greater potential as a sustainable land use and moreover as a tool in the rehabilitation of degraded landscapes. Such small-scale plantations place lesser demands on nutrient supply mechanisms and conserve a greater percentage of the system's natural nutrient conservation processes. Recognizing the special importance of organic nutrient pools in Amazon ecosystems, plantations can be used to produce mulch for effective nutrient application to crops or other managed lands. Depending on the nutrient content of different mulches and their biodegradability, specific needs can be satisfied. For example, quickly decomposing mulch may be added to short rotation crops to provide a pulse of nutrients to stimulate growth, or more slowly decomposing mulch may be applied to facilitate a more continuous source of nutrients. An especially valuable role for small plantations is the rehabilitation of degraded sites (Montagnini, this volume). Planting species with high nutrient use efficiencies will more quickly replenish organic matter levels in degraded soils. The form of Amazon land use which least impacts natural biogeochemical cycles is extractive reserves. In chapter 8, Foster Brown, Karen Kainer, and Eufran do Amaral describe how a typical extractivist household "stores" more than 50,000 Mg of carbon in their managed forest. A large portion of this carbon would have otherwise been released to the atmosphere as CO2 if the forest had

been cut. Because the forest remains intact, so do other components of the natural biogeochemical cycles of C, N, P, Ca, Mg, and other bioactive elements. Although extractive reserves are suitable for only 25% of the Brazilian Amazon, they could serve an important role in reducing emissions of C and N to the atmosphere.

Secondary forest As a result of the high rates of land abandonment in the Amazon, a great deal of land is undergoing forest regeneration. The rate of recovery of secondary forest is partially controlled by biogeochemical controls linked to soil fertility, and the forest itself plays a role in larger biogeochemical cycles by sequestering C and N within its increasing biomass, thus partially offsetting emissions from deforested areas. In chapter 9, Daniel Nepstad, Paulo Moutinho, Daniel Markewitz and Elizabeth Cheng calculate that carbon sequestration in second growth forests amounts to approximately 10% of that released through deforestation. These authors also raise the very intriguing issue of deep roots and their role in accessing nutrients for secondary forest growth. The significant investment made in superficial roots by trees of Amazon forests is a clear indication of the importance of nutrient recycling from organic pools at the soil surface. However, research from the central and eastern Amazon has shown that trees in seasonally dry forests also have roots extending to at least 18 m depth (Nepstad et al. 1994). While the main function of these roots appears to be the uptake of deep soil water and groundwater, there is also potential for these roots to access deeper nutrient pools in the soil column. Nepstad et al. (this volume) elaborate on this issue by demonstrating that secondary forests growing in the eastern Amazon have P and K nutrient needs that cannot be satisfied by available stocks in the

The Relevance of Biogeochemistry to Amazon Development and Conservation

upper soil levels. They also show that secondary forests may re-establish deep root systems (6 m deep) within 16 years. They further show that abundant fine roots infected with mycorrhizal fungi characterize the deep roots of secondary forest. These circumstantial data suggest that secondary forests are mining nutrient reserves in deeper soils, but conclusive data are still unavailable.

Economic consequences of nutrient scarcity on land Sustainable development in the Amazon hinges upon many factors which are ecological, physical, cultural, and economical. Among the most practical aspects of biogeochemical cycles in the Amazon are those that exhibit direct economical ties. In chapter 10, Carl Jordan discusses the interacting forces that bridge the gap between biogeochemical science and development economics. He recounts the dismal performance of such grand projects as the Jari plantation and the reported failure of as much as 85% of the ranches in a section of the eastern Amazon. Although intensive agriculture, characterized by application of fertilizers and pesticides, has been shown to be profitable in select parts of the Amazon, in general the inhabitants of the region must look to alternative means of economic development. Jordan asserts that production systems in the Amazon must be designed in such a way to mimic the structure of the original forest and its natural nutrient conserving mechanisms.

Carbon storage The consequences of deforestation, land management, and secondary growth extend beyond regional issues of conservation and sustainable land use to global issues of climate change. With CO2 and CH4 levels continuing to rise in the atmosphere, many global change scientists view the Amazon as

9

a key variable in the global carbon budget. Will the Amazon's forests act as a sink for atmospheric CO2, or will deforestation and burning of the region's forests only exacerbate the problem? Answers to these questions depend on the course of development (or conservation) in the region, but they also depend on exactly how much carbon the region's biota and soils can effectively hold. In chapter 11, Martial Bernoux, Paulo Graca, Carlos Cerri, Philip Fearnside, Brigitte Feigl, and Marisa Piccolo provide a detailed assessment of the quantity of carbon stored in soils and biota of the Brazilian Amazon basin. In total, the Brazilian Amazon is estimated to store more than 120 Pg (1015g) of reduced carbon, of which 34% is held in soils and 66% is held in aboveground vegetation. Of the several natural and managed vegetation types in the Amazon, natural forest is the most important C repository, accounting for 97% of the aboveground C. These estimates are likely to improve as more data are collected and better techniques of extrapolation are developed. However, it is already clear that the Amazon is a major reservoir of reduced carbon on the globe.

Biogeochemical Cycles in Aquatic Systems - The Balance of Water Quality Water is abundant in the Amazon, and nearly all living things in the basin are somehow dependent on this abundance. From the standpoint of development and conservation, the most crucial issue in water management is water quality. Clean water is fundamental to the maintenance of the Amazon's unique aquatic ecosystems and to the health of its people, who rely on surface water to satisfy their household water needs and to provide fish and other aquatic plants and organisms for their nutritional needs. Water quality is intricately linked to biogeochemical reactions such as the dissolution and decomposition of

10

organic wastes, the transport and scavenging of heavy metals, and the regulation of proper nutrient levels. These processes are similarly linked to hydrological variability in the system, which determines transport conditions and dilution through mixing. The continued development of the Amazon basin will increase demands placed on the region's surface water resources. In light of the limited economic resources of the nations composing the basin, it is likely that development will proceed without the implementation of adequate pollution control measures. For most activities, pollution "control" will be left to the natural assimilation capacity of the surface water system (McClain 2001). This is especially true in connection with the diffuse development activities in rural areas and small towns. Consequently, there is an acute need to better understand how the river system processes and transports organic matter, nutrients, and metals.

Michael E. McClain

this region, thus the influence of upland forests is reduced (McClain and Elsenbeer this volume). The generally greater relief of the western Amazon Andean foothills forms narrower stream valleys with less pronounced riparian wetland zones and therefore greater connectivity between streams and upland forests. The strong heterotrophic nature of Amazon streams makes them efficient at retaining both nutrients and organic matter input from adjoining lands. However, the capacity of these streams to assimilate contaminant levels of these same compounds is likely to be small. The greatest buffering capacity of Amazon stream systems likely lies in the riparian forest/wetland systems that adjoin them. Research by McClain et al. (1994) and Williams et al. (1997b) has demonstrated the ability of processes in the riparian zones of central Amazon watersheds to strip nitrate from upland groundwaters in forested and deforested settings. In the undisturbed setting Streams of the Barro Branco stream, nitrate levels were reduced by 80% in comparison with Streams are perhaps the most sensitive levels in upland groundwater entering the components of the surface water system riparian zone (McClain et al. 1994). Williams to processes on land due to their small et al. (1997b) found that groundwater nitrate size and limited dilution capabilities. In concentrations were elevated in groundwater chapter 12, Helmut Elsenbeer and I examine following cutting and burning of the forest, the role of streams in processing material and but upon passing through the riparian zone solute inputs from the Amazon landscape. levels decreased significantly. The same Detailed studies of biogeochemical processes authors reported that intact riparian forest in Amazon streams have been limited to only effectively buffered the stream from erosive a few sites, but results from these sites clear- inputs and their associated particulate nutrily indicate that inputs to streams from forests ent and organic matter inputs. vary as a function of soils, geomorphology, and rainfall characteristics. In the nutrient Lakes and floodplains depleted soils of the central Amazon, inputs of ions to the stream systems are reduced More than 1 million square kilometers and a significant proportion of ions, includ- of the Amazon basin may be classified as ing Ca, Mg, and K, may derive from vegeta- floodplain, wetland, or lake. These environtion sources via throughfall and overland ments range from narrow, sporadically flow of storm waters. Organic matter and flooded lowlands bordering streams to inorganic nitrogen also appear to derive massive, regularly flooded plains bordering largely from riparian forests and wetlands in the basin's largest rivers. Nearly 9000 lakes,

The Relevance of Biogeochemistry to Amazon Development and Conservation

11

covering an average maximum area of 67,900 control on species distributions is the km2, occur as integrated components of the biogeochemical nature of the river water and massive floodplains bordering the mainstem sediments. Distinct types of floodplain forest Amazon and its major tributaries (Sippel et al. develop adjacent to Whitewater rivers 1992). These varied aquatic habitats play a (varzea) and blackwater rivers (igapo). pivotal role in many ecological, as well as Piedade et al. show that geoecological biogeochemical, processes in the region. variables impact both the magnitude and From a practical standpoint, floodplains and timing of elemental fluxes in floodplains. their associated lakes provide essential Leaves of flooded forest in varzea are signifbreeding and feeding areas for many of the icantly enriched in nutrients in comparison to Amazon's most economically important fish leaves of flooded forest in igapo, and when species (Goulding 1980). Along Whitewater combined with productivity data these foliar rivers rich with Andean sediment, floodplains nutrient levels indicate more efficient nutrient also provide fertile soils for crops and use in igapo forest. By taking a more inpasture. Finally, these environments emit depth look at N pools and dynamics of the quantities of methane that are significant in varzea forest, Piedade et al. conclude that the global atmospheric methane budget and approximately 200 kg ha'1 of N (and associrelevant to studies of climate change (Bartlett ated nutrients) are input to the floodplain in the form of fresh leaf matter annually. The and Harriss 1993). In chapter 13, Maria Teresa Piedade, Martin annual pulse of nutrient input is even more Worbes, and Wolfgang Junk examine the dramatic in floodplain herbaceous communistrong links between ecological aspects of ties, which turnover completely each year. A the higher plants and elemental fluxes on stand of the grass Echinochloa polystachya Amazon floodplains. Distribution, species was found to take up 377 kg ha'1 of N from composition, biomass, and primary produc- river water and to return it to the soil surface tivity constitute fundamental ecological as fresh organic litter. Corresponding inputs parameters that exert marked control over of P and K were 51 and 1136 kg ha'1, respecelemental fluxes. Piedade et al. show that tively. Transformations such as these in nutrithese parameters are in turn linked to inter- ent forms are essential in maintaining the acting geomorphological and hydrological overall fertility of the environment and also factors. The most influential combined effect feeding the food chains which support of geomorphology and hydrology is period commercially important fish species. of inundation, which corresponds to the In chapter 14, John Melack and Bruce number of days per year that a given point is Forsberg provide a quantitative assessment of submerged by the oscillating flood waters. the role of floodplain lakes in regional cycles Species of the flooded forest are organized of C, N, and P. Floodplain lakes were found into zones which relate directly to period of to be important centers of organic carbon inundation. Species distributions throughout production and delivery to the river system. these zones vary as a function of both the The combined primary production of macrodegree of stress imposed and the adaptive phytes, forests, periphyton, and plankton strategies employed by the trees. Within the associated with floodplain lakes is estimated herbaceous community, distribution patterns at 117 Tg C yr1, of which only 24% is also vary according to the balance between remineralized in the lakes. As a result, an floodpulse-induced stresses and adaptive estimated 90 Tg C yr1 are delivered to the strategies, but additional factors linked to river system by continual and seasonal physical stability are also important. A final exchanges. This input alone amounts to

12

more than twice the annual export of organic C in the river system. Given that the river also receives organic C inputs from vast upland forests and other parts of the floodplain, these new data suggest that consumption of organic C is likely to be far higher in the river's mainstem than had been previously thought (see next section). Cycling of N and P in these lakes is closely correlated with that of organic C. According to Melack and Forsberg, productivity in floodplain lakes may be limited by either N or P in conjunction with spatial and temporal differences in the supplies of these nutrients across the spectrum of lakes. Melack and Forsberg discuss an array of anthropogenic impacts on Amazon floodplain lakes, including mining, hydroelectric dams, mercury pollution, and deforestation of adjoining uplands. Each of these activities upsets normal cycles of important elements, either by poisoning organisms which carryout the cycling (mining wastes and mercury pollution), altering the cycle of flooding to which organisms are adapted (dams), or otherwise changing the natural rates and forms of element inputs (dams, mining, and deforestation). Specific impacts of these activities are still mostly unstudied in the Amazon, but in light of the crucial role floodplain lakes play in larger elemental cycles and associated ecosystem products (for example, fisheries), negative impacts there are likely to propagate to much wider areas.

The mainstem river and its estuary The flow of water, solutes, and particulate matter in the mainstem Amazon river is the product of integrated hydrological, biological, physical, and biogeochemical processes across the entire basin. Of the many natural features of the Amazon, none more completely integrates these processes, because there is no corner of the basin that

Michael E. McClain

does not contribute some proportion of its water and associated load to the river system. The condition of the river may thus be viewed as a kind of proxy for the condition of the larger basin. As with aquatic resources throughout the Amazon, water quantity is not a problem, with the exception of occasional extreme flooding events. The main issue is water quality and the maintenance of water quality within ranges appropriate to support natural aquatic ecosystems and the water needs of the human population. Water quality in the mainstem Amazon remains excellent, with the exception of isolated sites of relatively small-scale contamination near the river's big cities. Thus biogeochemical investigations of the river enjoy the luxury of focusing mainly on natural, or background, compositional patterns and dynamics. In chapter 15, Alan Devol and John Hedges examine the relative role of upstream sources and in-channel processing in determining the dynamics of C, N, P, and oxygen (O) in the mainstem river. The major proportion of organic matter transported in the mainstem is refractory material of upstream origin, which appears to pass through the mainstem rather conservatively. Heterotrophic metabolism is fueled by a smaller, more labile pool of organic matter derived from local sources such as the floodplain (see Melack and Forsberg, above). Because the Amazon mainstem is almost entirely heterotrophic, the foodweb of the river's myriad aquatic organisms is based on the input of energy and nutrients from this more labile organic matter pool. Nearly all fine particulate forms of C, N, and P in the mainstem derive from Andean environments many thousands of kilometers upstream. This is a striking example of the interconnectivity and interdependency of processes across the Amazon. In an especially interesting application of biogeochemical investigative techniques, Devol and Hedges use rather sophisticated

The Relevance of Biogeochemistry to Amazon Development and Conservation

molecular and isotopic biomarkers to illuminate the finer details of organic matter dynamics in the mainstem. Sources of the main organic matter pools (dissolved, fine particulate, and coarse particulate) are identified using concordant data on elemental ratios, carbon stable isotope ratios, and relative abundances of specific compounds derived from lignin molecules. These data suggest that terrestrial vegetation is the predominant source of organic matter (OM) in the river, as opposed to plankton or floodplain grasses. Furthermore, subcomponents of the lignin molecular fraction point specifically to leaves as the source instead of the woody portions of trees. The course and degree of degradation within the OM fractions is shown to be quite distinct based on abundances of carbohydrates, amino acids, and lignin fractions. Dissolved OM is the most highly degraded fraction, while coarse particulate OM retains molecular signatures that are closest to fresh leaf material and fine particulate OM lies in between. When analyzed more closely, the consistent patterns in elemental and molecular variability between the fractions suggest that differential partitioning (or sorption) may account for final separation of refractory molecules into the fine particulate or dissolved phases. The transport dynamics implied by the foregoing data are confirmed by abundances of bomb carbon-14 in the different fractions. Devol and Hedges synthesize the above results into an elegant conceptual model explaining the processing of organic matter carried by the Amazon mainstem. The model, which they refer to as "Regional Chromatography", follows the processing from production of leaf litter in the basin's forests until the arrival of the final refractory OM in the mainstem river. Within the model, coarse particulate OM is transported to the river system via quick overland hydrological flowpaths or direct litterfall, whereupon it undergoes rapid decomposition but still

13

retains an appreciable proportion of its original biogeochemical signature upon reaching the mainstem. Conversely, fine particulate OM and dissolved OM are assumed to emanate from the soils after partitioning into their respective fractions, where movement to the river system occurs either as soil erosion (fine particulate OM) or subsurface runoff (dissolved OM). Although field investigations undertaken in upland portions of the basin to test the predictions of the Regional Chromatography model have not always yielded supporting data (McClain et al. 1997), the model continues to provide an internally consistent and conceptually appealing explanation for mainstem OM compositional patterns. An additional set of elements which play important roles in reactions ranging from biogeochemical to purely geochemical are trace metals. In chapter 16, Patrick Seyler and Geraldo Boaventura present a comprehensive survey of trace metal forms and fluxes in the mainstem Amazon and its major tributaries. Trace metal abundances between the mainstem and its tributaries vary as a function of the source rocks occurring in the basins and in function with the reactivity of the metals. Concentrations of most metals in the Amazon are comparable to those in other major rivers of the world, and the Andes stand out as the dominant source area for most metals transported by the river. In the Amazon mainstem, trace metals are transported primarily in particulate form. Transport in the dissolved phase predominated only in the Negro River, where many metals were complexed with dissolved organic matter. Seyler and Boaventura present new data illustrating the temporal variability of trace metal fluxes in the main river system. They identified four discrete patterns of variability which reflect the source, process of mobilization, and in-channel reactivity of the metals measured.

14

Currently, trace metal pollution is not a problem in the Amazon River, with the exception perhaps of localized small-scale occurrences. However, the basin includes several sites of elevated metal releases linked to gold and manganese ore mining and industrial discharges from the large cities. Seyler and Boaventura discuss the likelihood of contamination from these sources. Moreover, they come to the provocative conclusion that elevated fluxes of manganese, copper, vanadium, arsenic, and nickel may already be detectable at the river's mouth due to anthropogenic activities in the basin. Possible metal contamination should be taken seriously in the Amazon, as even small amounts of the more toxic metals may lead to widespread adverse effects in the river's aquatic systems. Whereas the Amazon River is the recipient of energy and nutrients provided by its upland basin and flood plain, it is an important provider of these things to the estuarine and coastal zone of the western Atlantic. In chapter 17, David DeMaster and Robert Aller examine the interactive physical and biogeochemical processes which regulate biological productivity on the Amazon shelf. In budgets calculated from recent data, they demonstrate that the river is the most important source of silica (Si) to the shelf area and therefore strongly controls production within the diatom community. Further-more, the bulk of this Si is not retained on the Amazon shelf but is instead transported offshore where it contributes to the nutrient needs of open-water plankton. The river is also an important source of P and N, but oceanic sources, and especially internal recycling, account for the largest inputs of these elements. Approximately 60% of riverine paniculate organic C is remineralized on the shelf, mostly on the seafloor. This is a small fraction of heterotrophic activity, however, as more labile marine particulate organic C is greatly preferred by heterotrophic organisms.

Michael E. McClain

By analyzing the relationship between C and O budgets, DeMaster and Aller conclude that biological production is approximately balanced by consumption, such that the shelf area exhibits no obvious autotrophic or heterotrophic character.

The Challenge for Today and the Future As the preceding paragraphs highlight, and as the following chapters will detail, biogeochemical cycles are at the heart of many of the key issues facing the Amazon basin. Both sustainable development and preservation of the region's unique ecosystems depend on our understanding of fundamental processes such as the balance of nutrients in land and aquatic ecosystems, the assimilation capacity of contaminants in these same ecosystems, and ultimately the tolerance of Amazon ecosystems to specific development pressures. The basin's singular importance in the larger Earth system means that the consequences of poor decisions in the Amazon will be felt in adjoining regions and perhaps even on the global scale. Amazon biogeochemists face the challenge of advancing understanding at a rate greater than the advancing development. In other words, if sound scientific understanding is to guide sustainable develop in the Amazon, then research must move ahead of development. This is a formidable challenge due to the large size of the basin and the relatively small research budget devoted to Amazon research by countries composing the basin and the international scientific funding agencies. The challenge is made more difficult by the unique environmental circumstances found in the Amazon, and thus the limitations against applying models borrowed from more intensively studied systems outside the tropics. Many of the most notorious failures to date in the development of the Amazon have come from applying models of land and

The Relevance of Biogeochemistry to Amazon Development and Conservation

water management borrowed from temperate zones. Whether these models assume an unrealistic level of soil fertility, unachievable financial resources, or other conditions taken for granted in temperate zones, they are predisposed to failure. Most biogeochemists working in the Amazon today understand that the key to sustainable development is to understand the integration of physical and biogeochemical factors responsible for the luxurious natural ecosystems of the basin and then to emulate

15

or attempt to preserve these factors in development programs. There are currently several ambitious research programs that aim to do just that, and the leaders of many of these are the authors of the following chapters. I invite you the reader to delve into the volume of good research presented in this book and to make use of it in the manner that best suits your own professional position and that best enables you to join in our efforts to guide Amazon conservation and development into the future along a sustainable path.

Literature Cited Althaus, D. 1996. "Amazon's empty legacy/Big oil responds to environment/Toll on rain forest, culture still unacceptable to critics." Houston Chronicle (TX), December 15, p. 27. Andreae, M. O., R. W. Talbot, H. Berresheim, and K. M. Beecher. 1990. "Precipitation chemistry in Central Amazonia." Journal of Geophysical Research 95(D10): 16987-16999. Bartlett, K. B., and R. C. Harriss. 1993- Review and assessment of methane emissions from wetlands. Chemosphere 26: 261-320. Burns, M. K. 1998. "Biological diversity being eroded in Amazon: Even in an Ecuadorean national park, where rain forest species are supposed to be protected, colonization and poaching threaten the delicate ecosystem." The Sun (Baltimore, MD), April 23, p. 2A. Buschbacher, R. 1987. "Cattle productivity and nutrient fluxes on an Amazon pasture." Biotropica 19: 200-207. Craig, J. 1998. Ravaging of Brazil's Amazon continues - survey, Reuters, January 26. Cuevas, E., E. and Medina. 1988. "Nutrient dynamics within Amazonian forests. II. Fine root growth, nutrient availability and leaf litter decomposition." Oecologia 76: 222235. Donn, J. 1999- Report: Amazon Rain Forest Fading, The Associated Press News Service, April 8, 1999. Ferreira, L. 1996. "Asian logging companies turning to Amazon rain forest," Kyodo News International, Inc., June 4. Gomez, R. 1994. "Contaminacion Ambiental en la Amazonia Peruana." Informe Tecnico de Avance of the Institute de Investigaciones de la Amazonia, Peruana, Iquitos. Goulding, M. 1980. The Fishes and the Forest. University of California Press, Los Angeles. James, I. 1998. The Endangered People of Peru, St. Petersburg Times (FL), July 5, 1998 p. ID .

Lesack, L. F. W., and J. M. Melack. 1996. "Mass balance of major solutes in a rainforest catchment in the Central Amazon: Implications for nutrient budgets in tropical rainforests." Biogeochemistry 32: 115-142. McClain, M.E. 2001. "Ecohydrology as a tool in the sustainable development of large tropical rivers". In: Ecohydrology From Concepts to Application. Zalewski, M. (Cambridge University Press, New York). McClain, M. E., J. E. Richey, J. A. Brandes, and T. P. Pimentel. 1997. "Dissolved organic matter and terrestriallotic linkages in the central Amazon basin of Brazil. "Global Biogeochemical Cycles 11: 295-311. McClain, M. E., J. E. Richey, and T. P. Pimentel. 1994. "Groundwater nitrogen dynamics at the terrestrial-lotic interface of a small catchment in the Central Amazon basin." Biogeochemistry 27: 113-127. Nepstad, D. C., C. R. de Carvalho, E. A. Davidson, P. Jipp, P. Lefebvre, G. H. Negreiros, E. D. da Silva, T. Stone, S. Trumbore, and S. Vieira. 1994. "The role of deep roots in the hydrological and carbon cycles of Amazonian forests and pastures." Nature 372: 666-669. Ozorio de Almeida, A. L., and J. S. Campari. 1995. Sustainable Settlement in the Brazilian Amazon. Oxford University Press, New York. p. 189. Prentice, I. C., and J. Lloyd. 1998. "C-quest in the Amazon basin." Nature 396: 619-620. Schomberg, W. 1998. Brazil says farmers are destroying the Amazon, Reuters, March 3. Sippel, S. J, S. K. Hamilton, and J. M. Melack. 1992. "Inundation area and morphometry of lakes on the Amazon River floodplain, Brazil." Archiv fur Hydrobiologie 123: 385-400. Tian, H., J. M. Melillo, D. W Kicklighter, A. D McGuire, J. V. K. Helfrich, B. Moore, and C. J. Vorosmarty. 1998. "Effect of interannual climate variability on carbon storage in Amazonian ecosystems." Nature 396: 664-667.

16 Tiessen, H., E. Cuevas, and P. Chacon. 1994a. "The role of soil organic matter in sustaining soil fertility." Nature 371: 783-785. Tiessen, H., P. Chacon, and E. Cuevas. 1994b. "Phosphorus and nitrogen status in soils along a toposequence of dystrophic rainforests on the upper Rio Negro." Oecologia 99: 145-150. Williams, M. R., T. R. Fisher, and J. M. Melack. 1997a.

Michael E. McClain "Chemical composition and deposition of rain in the central Amazon, Brazil," Atmospheric Environment 31, No. 2: 207-217. Williams, M. R., T. R. Fisher, and J. M. Melack. 1997b. "Solute dynamics in soil water and groundwater in a central Amazon catchment undergoing deforestation." Biogeochemistry 38: 303-335.

2

General Characteristics and Variability of Climate in the Amazon Basin and its Links to the Global Climate System Jose A. Marengo and Carlos A. Nobre

The Amazon region is of particular interest because it represents a large source of heat in the tropics and has been shown to have a significant impact on extratropical circulation, and it is Earth's largest and most intense land-based convective center. During the Southern Hemisphere summer when convection is best developed, the Amazon basin is one of the wettest regions on Earth. Amazonia is of course not isolated from the rest of the world, and a global perspective is needed to understand the nature and causes of climatological anomalies in Amazonia and how they feed back to influence the global climate system. The Amazon River system is the single, largest source of freshwater on Earth. The flow regime of this river system is relatively unimpacted by humans (Vorosmarty et al. 1997 a, b) and is subject to interannual variability in tropical precipitation that ultimately is translated into large variations in downstream hydrographs (Marengo et al. 1998a, Vorosmarty et al. 1996, Richey et al. 1989a, b). The recycling of local evaporation and precipitation by the forest accounts for a sizable portion of the regional water budget (Nobre et al. 1991, Eltahir 1996), and as large areas of the basin are subject to active deforestation there is grave concern about how such land surface disruptions may affect the water cycle in the tropics (see reviews in Lean et al. 1996).

Previous studies have emphasized either how large-scale atmospheric circulation or land surface conditions can directly control the seasonal changes in rainfall producing mechanisms. Studies invoking controls of convection and rainfall by large-scale circulation emphasize the relationship between the establishment of upper-tropospheric circulation over Bolivia and moisture transport from the Atlantic ocean for initiation of the wet season and its intensity (see reviews in Marengo et al. 1999). On the other hand, Eltahir and Pal (1996) have shown that Amazon convection is closely related to land surface humidity and temperature, while Fu et al. (1999) indicate that the wet season in the Amazon basin is controlled by both changes in land surface temperature and the sea surface temperature (SST) in the adjacent oceans, depending if the region is north-equatorial or southern Amazonia. A better understanding of rainfall variability and its mechanisms in Amazonia requires a clear documentation of the major elements of the warm season precipitation regime, within the context of the annual cycle, and longer time scale variations such as interannual and interdecadal. Rainfall variability in Amazonia has been the subject of several studies regarding physical causes, seasonal variations, and links to the Southern Oscillation (SO), and to SST conditions in the tropical Atlantic (see reviews in Ropelewski

18

Jose A. Marengo and Carlos A. Nobre

and Halpert 1987, 1989; Marengo 1992; Rao (Fig. 2.1c) show a persistent thermal lowet al. 1993, Nobre and Shukla 1996, Guyot et pressure system between 202 and 30s S over al. 1997, Marengo et al. 1998a, b). The low SO the Gran Chaco and Pampean Sierras, related phase, which is associated with the El Nino to the maximum of cloudiness over central phenomenon, is related to negative rainfall Amazonian and the Peruvian-Bolivian anomalies in northern and central Amazonia Altiplano. At this time of the year, cold fronts and anomalously low river levels in the that enter northern Argentina and southern Amazon River, while the high SO phase Brazil are frequently accompanied by (related to the La Nina phenomenon) features enhanced deep convection over western and anomalously wet seasons in northern and southern Amazonia (Garreaud and Wallace central Amazonia. Tendencies toward drier 1998) and by increased southward moisture conditions were observed during the El Nino flux from lower latitudes. It has been sugevents of poor rainy seasons of 1925-26, gested that a strong low-level jet east of the 1982-83 and more recently during 1997-98, Andes (Nogues-Paegle and Mo 1997) while wetter conditions were observed enhances this moisture flux. Another feature during the La Nina years of 1988-89 and 1995- is the strong and persistent northeasterly flow 96. The drought of 1998 in north and central from the North Atlantic into the Amazon Amazonia is being considered as the most basin, associated with the increased pressure over the tropical North Atlantic. In this region intense of the last 118 years. On longer time scales, studies have docu- there is an indication of the presence of a mented long-term variations in rainfall in the low-level northerly jet, with a maximal wind basin, associated with possible trends or cycles speed on the order of 15 m/s at 850 mb, at in both rainfall and river levels (Rocha et al. approximately 17s S and 62Q W, that is 1989, Chu et al. 1994, Dias de Paiva and Clarke responsible for the transport of water vapor 1995, Marengo et al. 1998a, Marengo et al. and heat to the region of Paraguay and 2001a, Zhou and Lau 1999), as well as climat- northern Argentina from the Amazon. At upper levels (Fig. 2.la), the elevated teric tendencies in the water balance and moisture transport in the basin (Costa and Foley rain of the Andes and the latent heat released 1999). However, no significant trends have through deep cumulus convection during the been detected in the rainfall regime of the warm season give rise to this upper-troposAmazon region, or on the discharges of the pheric high. The causes of the Bolivian high are therefore strongly related to latent heat Amazon river and its tributaries. release over the regions of precipitation (Figueroa et al. 1995, Lenters and Cook 1995, Seasonal Variation of Climate Seluchi et al. 1998). To the east of the Bolivian high (downstream), there is an in Amazonia upper-level cold trough sitting off the east cost of Brazil. That trough is associated with Atmospheric circulation and convection descending motion and is part of the mechaThe near surface and upper air circulation nisms that lead to the very low rainfall over the Americas, as deduced by the National observed in that region (Moura and Shukla Centers for Environmental Prediction (NCEP) 1981). Southern Amazonia and the Bolivian reanalysis (Kalnay et al. 1996), are shown in Altiplano are strongly heated during the Fig. 2.1a-d for the austral summer (December- austral warm season resulting in enhanced February) and winter (June-August)]. The near tropospheric zonal temperature gradients surface circulation patterns in austral summer and enhanced upper-tropospheric meridion-

General Characteristics and Variability of Climate in the Amazon Basin and its Links to the Global Climate System

Figure 2.1 a-b Upper-level circulation (200 hPa) over South America. (a) Austral summer (December-February), (b) austral winter (June-August).

Figure 2.1 c-d Near surface (850 hPa) over South America. (c) Austral summer (December-February), (d) Austral winter (June-August).

19

Jose A. Marengo and Carlos A. Nobre

20

al flows in the vicinity of both coasts of South America. The austral wintertime upper-level circulation (Fig. 2.1b) over South America is characterized by weak winds over the tropics, while the subtropical westerly jet is stronger and located more equatorward than in summer, in agreement with the descending branch of the Hadley-type circulation over that area. At lower levels, a northward-displaced near-equatorial lowpressure trough and SST maximum characterize the circulation (Fig. 2. Id). A northward cross-equatorial flow turning clockwise is found over the tropical eastern Pacific and Atlantic Oceans (Fig. 2.1b). Near the surface, during wintertime, surges of cold high-latitude air are known locally as "friagens." These events move across southeastern Brazil and the Amazon region from the south, greatly modifying the atmospheric structure and climatic conditions. Friagens can produce severe frosts in the coffee growing areas of southern Brazil and substantial cooling in the Amazon basin (Seluchi and Nery 1992, Marengo et al. 1997 a,b, Marengo 1999). The cold air heading these thrusts of high-latitude air may reach as far north as the equator. The events are relatively common in the Amazon region during the winter

Figure 2.2 Annual rainfall distribution in Amazonia [meters] (Marengo 1995b).

(May-September). Gulf et al. (1996) note that there were 6 such events in 1992 and 9 in 1993 while 14 events were reported during the winter of 1994. Summertime convection shows a maximum over southern Amazonia which extends to the southeast. This represents the South Atlantic Convergence Zone (SACZ) and is consistent with the low-level convergence shown in Fig. 2.1c. More information on the SACZ can be found in Nogues-Paegle and Mo (1997). During winter, the convection bands move northward towards the Northern Hemisphere, determining the rainy season in northern South America.

Rainfall The Amazon region is characterized by a strong annual cycle of rainfall, whose maximum follows the meridional displacement of solar heating to a first approximation (Rao and Hada 1990, Figueroa and Nobre 1990, Marengo 1995b). Southern Amazonia has distinct dry and rainy seasons, with a maximum of rainfall occurring in summer. In the northern and central regions there is almost no dry season, but approaching the equator there are distinct maximum in spring and "suppressed" maximum in fall. During the austral spring precipitation increases over the Amazon basin, and bands of precipitation develop linking tropical convection and precipitation activity in the lower extratropics of this region. Together with the OLR fields, rainfall displays a similar seasonal cycle. The heaviest rainfall and lowest OLR occurs over southern Amazonia. Annual variation of rainfall is linked to annual changes in large-scale upper-air circulation, and surface oceanic conditions have implicated the Atlantic, at least indirectly, as being responsible for interannual rainfall variability in the Amazon. For example, Marengo (1992) found more than average rainfall throughout the northern side of the

General Characteristics and Variability of Climate in the Amazon Basin and its Links to the Global Climate System

basin in southern summer in times of enhanced northeast trades due to increased moisture flux from the Atlantic. Strong Atlantic trades bringing anomalous moisture into the Amazon are associated with a southward displaced ITCZ, which is in turn related to an anomalous distribution of Atlantic SST (Moura and Shukla 1981, Nobre and Shukla 1996). Although the timing of the annual cycle of rainfall is largely controlled by the sun, rainfall in different parts of the basin is triggered by different rain producing mechanisms (Garreaud and Wallace 1997, 1998, Fisch et al. 1998). These mechanisms can be grouped into the following categories: * Diurnal deep convection resulting from surface warming, which is most prevalent in central Amazonia.

21

* Deep convection in northern Amazonia related to the ITCZ-like convection and moisture transported from the Atlantic. This depends on the meridional SST gradient of the tropical North Atlantic. * Instability lines originating near the mouth of the Amazon River that move inland and may even reach the Andes (Cavalcand 1982, Cohen et al. 1995) can initiate rainfall in northern Amazonia. * Convective activity at the meso scale and large scale, associated with the penetration of frontal systems in south/ southeast Brazil that can reach western southern Amazonia. As a consequence of recent international field campaigns if the Amazon basin (TRMM-

Figure 23 Annual cycle of rainfall in Amazonia (Marengo and Hastenrath 1993). Scale is indicated on the lower left side.

22

LBA and LBA-WET AMC, Marengo 2001 b) during the summer of 1999, a unique data set and knowledge was provided to allow, among other things, the description and analysis the diurnal and day-to-day variations of rainfall in southwestern Amazonia. By means of a high resolution time and space rainfall data, it was possible to get a better documentation of the diurnal cycle of rainfall in this part of the Amazon, and together with other observations such as radar and lighting and soundings there was possible to identify several episodes of rainfall and relate them to different rainfall mechanisms. Rainfall in the region shows clearly a preference for

Jose A. Marengo and Carlos A. Nobre

maximum rainfall intensity between 1200 and 1600 LST, and sometimes 1800 LST, while in some periods, a second maximum shows up between 0000 and 0400 LST. However, the more frequent maximum rainfall is detected at late afternoon. Annual rainfall in northern South America varies greatly, from less than 400 mm in northeast Brazil and the Caribbean coast of South America to more than 3000 mm in the upper watershed of the Rio Negro. Fig. 2.2 shows three centers of abundant rainfall in the Amazon basin. One is located in northwest Amazonia, with more than 3600 mm per year. Another region with abundant rainfall is

Figure 2.4 Daily Precipitation at Manaus, Brazil (03Q 50'S, 602 Ol'W) during two extremes of the Southern Oscillation

General Characteristics and Variability of Climate in the Amazon Basin and its Links to the Global Climate System

the central part of Amazonia around 50Q S, with 2400 mm per year. A third center is found close to the mouth of the Amazon river near Belem, with more than 2800 mm per year. This is in agreement with the circulation patterns over the Amazon basin and their variations as shown in Fig. 2.1c and 2.Id. In the upper Rio Negro basin, in northwestern Amazonia, rainfall is abundant throughout the year, reaching its maximum in April-June (Fig. 2.3), while southern Amazonia's rain peaks earlier (JanuaryMarch). The extreme high and localized values of precipitation in narrow strips along the eastern side of the Andean slopes are thought to be due to upglide condensation and a rain shadow effect on the lee side, so the localized maximum is due to the easterly winds being lifted when they flow over the Andes. The coastal maximum is caused by nocturnal convergence between the tradewind and the land breeze. The transition between the wet and dry seasons is short in Amazonia. The onset of the wet season is a relatively fast one that typically occurs within the within the period of a single month and may sometimes occupy only a single 5-day period. The transition to the dry season is on average, longer than a month. There have been relatively few efforts devoted to understanding wet season onset in the Amazon basin. Kousky (1988) determined the climatological onset, where onset progresses from northwest to southeast, although his criteria were never met in the extreme northwest. Withdrawal of the rainy season occurs toward the northwest. Marengo et al. (2001b) identified the onset and demise of the rainy season, with the onset dates progressing southeastward from near mid-September in the north and west to the beginning of October in the southeast. Near the mouth of the Amazon, onset occurs almost at the end of December, while north of the equator rainfall is apparently tied to Northern Hemisphere summer and does not

23

occur until mid-April. The withdrawal of the rainy season progresses toward the north, but moves slower and more systematically than does onset.

Figure 2.5 Rainfall and river variability in Amazonia, (a) Rainfall in the northern Amazonia, (expressed by the NAR index, Marengo [1992]), (b) discharges of the Amazon River at Obidos, (c) Solimoes river at Manacapuru, (d) Tapajoz River at Santarem, (e) Tapajos River at Itauituba (Marengo and Hastenrath 1993).

24

A very interesting aspect of the intraseasonal variability in the Amazon region and other tropical regions is the so-called veranico, or midsummer droughts, which occur during the austral summertime rainy season. The dry pells last between 5 days and 3 weeks and have a strong effect on agriculture if they occur near the beginning of the rainy season. Figure 2.4ab shows the daily rainfall from July 1988 to June 1989 (La Nina year) and from July 1982 to June 1983 (El Nino year), respectively, for Manaus. On average, the rainy season in Manaus starts in September, reaches its maximum in February-April and minimum in JuneAugust, when the end of it is detected. During the El Nino 1982-83, deficient rain during

Jose A. Marengo and Carlos A. Nobre

November 1982, January, February, and beginning of March 1983 was related to 3 extended veranicos of more than 15 days each.

Impacts of the Southern Oscillation in the Hydrometeorology of Amazonia Ropelewski and Halpert (1987, 1989), Kiladis and Diaz (1989) and Rogers (1988) have studied the anomalous patterns of rainfall all around the world during both extremes of the SO. As a result of very sparse data coverage in Amazonia, no clear signal of rain has been identified for this region, showing only negative rainfall anomalies over the mouth of the Amazon River. This was

Figure 2. 6 Rainfall anomalies during summer (DJF) and autumn (MAM) for El Nino 1972-73 (a),(b) and El Nino 1982-83 (c),(d). [Source: CPTEC].

General Characteristics and Variability of Climate in the Amazon Basin and its Links to the Global Climate System

observed for the El Nino 1982-83, while for the El Nino 1972-73 positive rainfall anomalies for the same region were reported. ENSO processes appear dominantly as interannual fluctuations in most climate records, at least in part because historical records are short and decadal fluctuations in them are few. However, at least some suggestions of long-term variations in the character of ENSO are evident. During this century, some decades have yielded frequent and more vigorous ENSO episodes while others have had relatively weak and infrequent episodes. Most notably, the time interval from the 1930s through the 1960s had weak ENSO variations relative to the first decades of the century and the decades beginning with the 1970s. Some decades (1950s and 1970s) seem to favor cold tropical episodes whereas others favor warm episodes [e.g. 1980s and 1990s] (Dettinger et al. 2000). On multiscale hydrologic variability associated with ENSO, Dettinger et al. (2000) indicated that influences are significant in several regions. Most notably, the Amazon basin (especially in the northern drainage basin) is dry in terms of both precipitation and flows during El Nino and La Nina years (Richey et al. 1989a). El Nino influences of Amazon streamflows are replaced by approximate mirror images during years of La Nina, although precipitation differences are not strictly mirror images (See their Fig. 1). Streamflow is not exactly symmetric in its response to the positive and negative SO conditions. Analyses of hydrometeorological series in Amazonia revealed the coherence of rainfall anomalies across northern part of the basin. For the period 1967-87, Marengo and Hastenrath (1993) compared the variability of rainfall in the northern portion (expressed by the NAR index) of the basin and data from the Solimoes, Tapajos, and Santarem rivers (Fig. 2.5a-e). This work showed that rainfall and river discharge anomalies in extreme years, such us the anomalously dry years 1979-80 and

25

1982-83 (El Nino) are mutually consistent, while the rest of the years do not show consistency. In fact, the river data show a period of relatively large discharges between 1972 and 1979 at the Solimoes basin, also observed at the Tapajos River and the main Amazon stream. For the whole basin, rainfall anomalies from the El Nino events 1972-73 and 1982-83 are depicted on Fig. 2.6a-d. The rainfall anomalies in the Amazon basin show a different behavior. At the end of 1972 (Fig. 2.6a), negative rainfall anomalies were observed over western Amazonia and near the mouth of the Amazon River, where anomalies reached -25 mm. Later, at the beginning and during summer and fall (Fig. 2.6b), positive rainfall anomalies (+100 mm) are shown in western and southern Amazonia extending towards the mouth of the Amazon. During the El Nino 1982-83, the anomalies in rainfall were distributed differently than on the 1972-73 event, in terms of intensity and location. At the end of 1982 and the beginning of summer 1983 (Fig. 2.6c) large negative rainfall anomalies occurred over central and eastern Amazonia, while small positive anomalies occurred over western Amazonia. During the fall (Fig. 2.6d), large negative rainfall anomalies are observed over western and central Amazonia, and especially the mouth of the Amazon river. At the end of the rainy season large negative rainfall anomalies are observed over "western Amazonia. Regarding streamflow data, the records of the Amazon, Negro, Xingu, and Tocantins rivers (Fig. 2.7a-d) are displayed for 2 El Nino years [1982-83 and 1986-87] and 2 La Nina years [1975-76 and 1988-891- Extreme years were chosen since they may show better the associations between rainfall and rain in their basins. The year before the peak of El Nino, the river discharge at Obidos are anomalously high, while on the year of El Nino the discharges are lower than average (Fig. 2.7a). During La Nina years, the discharges at Obidos are more than 7000 mVs above normal. At Manaus (Fig. 2.7b), the main

26

Jose A. Marengo and Carlos A. Nobre

Figure 2.7 Variations of river discharge/level in Amazonia during 2 El Nino (EN) and La Nina (NN) events (Marengo et al. 1998b).

difference with Obidos is the lack of anomalously large values on the year before the peak of the event. The El Nino in 1983 showed river levels below average, while during the peak of the Nino 1987 and the two La Nina year values larger than average were observed. At Belo Monte and Tucurui (Fig. 2.7c, d) the signal is mixed. The records for the El Nino year 1982-83 are larger than average, while for the other El Nino year 1986-87 the discharges are similar or just a bit larger than average. The discharges during the La Nina years show values that are near average. In principle, the records at Obidos should reflect the rainfall regimes from the Negro River, together with the Solimoes, Madeira and Purus. Chu (1982) found that two thirds of water level variability of the Amazon River at Manaus can be explained by the volume of

the Solimoes, while the rest comes from the Rio Negro basin. The large volumes of the Solimoes may upset the gauging values in lower courses of tributaries, producing what is known as the backwater effect (Meade et al. 1991, Guyot el al. 1997, Richey, personal communication 1998). The lower slope of the Solimoes and the large extension of the basin determine this effect that certainly would affect the discharges along the main stream, as well as the water levels measured in Manaus and in Iquitos (Marengo 1999)- This indicates that for some basins, the assumption that the discharges measured on large basins represent the hydrometeorological conditions at those sites is no longer valid. As seen in the previous section, with regard to El Nino/La Nina, if the main discharge is reduced during El Nino years, then so would tributary discharge be reduced. Conversely,

General Characteristics and Variability of Climate in the Amazon Basin and its Links to the Global Climate System

the opposite effect would prevail during La Nina. During the recent El Nino 1997-98, relatively dry conditions were observed during most of the summer of 1998 in central, western, and northern Amazonia. Rainfall over northern and central Amazonia from September to December 1997 (the beginning of the rainy season) showed negative anomalies up to 200 mm or more. Rainfall anomalies up to 200 mm below the normal were observed in Brazilian Amazonia especially to the north and western sections at the beginning of summer, while in the middle of summer negative rainfall anomalies were concentrated in the extreme west and northern sections of the basin. The unusually long, dry season in northern Amazonia during 1997 and the drought during summer 1998 produced an increase in burnings and forest fires, and the main cities in Amazonia were affected by the smoke of those fires.

Recent studies by Marengo (1999) for the Peruvian Amazon show negative water level anomalies for the Amazon river at Iquitos during very strong El Nino events, although the river flow measured at Iquitos is derived from such a large upstream area that it appears relatively insensitive to the El Nino/La Nina cycle in comparison with rainrelated anomalies at Iquitos. For the Colombian Amazon, observations indicate that ENSO's effect on river discharges occurs progressively later for rivers toward the east of the country and northern South America (Poveda and Mesa 1997). The impacts of La Nina are more pronounced than those of El Nino; however, there are instances in which rainfall anomalies are not associated with extreme phases of the ENSO, and vice versa. The 1982-83 and 1997-98 El Nino events are the strongest on record but did not produce intense dry anomalies. During 1957-60 Colombia experienced one of the longest dry

Table 2.1 Various estimates of the annual water budget of the Amazon basin. Precipitation Evapotranspiration Study (mm/yr) (mm/yr) Baumgartner and Reichel (1975) Villa Nova et al. (1976) Marques et al. (1977) Marques et al. (1980) Jordan and Heuveldop (1981) Leopoldo et al. (1982) Franken and Leopoldo (1984) Shuttleworth (1988a, b) Vorosmarty et al. (1989) Russell and Miller (1990) Nizhizawa and Koike (1992) Matsuyama (1992) Marengo et al. (1994) Costa and Foley (1999) Oki et al. (1999) Zeng (1999) Marengo et al. (2001 a)

27

2170 2000 2083 2328 3664 2076 2510 2636 2260 2010 2300 2153 2888 2166 2076 2044 2146

Source: Matsumyama 1992, Marengo et al. 1994, Costa and Foley 1999.

1185 1080 1000 1261 1905 1676 1641 1319 1250 1620 1451 1139 1616 1366 1023 1879 1581

Streamflow (mm/yr) 985 920 1083 1067 1759 400 869 1317 1010 380 849 849 1272 1106 1053 365 931

28

Jose A. Marengo and Carlos A. Nobre

Foley 1999). Several of these results represent the characteristics of the hydrological cycle near Manaus. Nishizawa and Koike (1992) slightly overestimated the annual evapotranspiration ratio, since both evapotranspiration and drainage are overestimated. Differences in results are due to different areas considered for the basin, different precipitation networks and methods of assessment (isolated stations, Manaus- only rain, the EOS-DNAEE network) and the methods used to determine the annual water balance, either using the ET = P - R approximation, or using the water vapor flux convergence equation, for the entire Amazon basin, or near Manaus only. For more discussion of these vales, see Matsuyama (1992). Hydrology has emphasized the movement Water Balance of Amazonia of water in the terrestrial system, considering forcings such as precipitation and potential The water balance of the Amazon basin is evapotranspiration and responses such as of great importance, due to the presence of streamflow and storage. Regional water balthe world's largest hydrographic system. ances are calculated based on streamflow There is a concern that large-scale land use data, observed precipitation, and evapotranchanges may significantly change the flow spiration measurements. In this context, regimes of rivers within the region and the water that evaporates from land surface is land-atmosphere exchange of moisture. The lost to the system if it is advected out of the lack of continuous precipitation and evapo- prescribed region by atmospheric motion, ration data, and of measurements of river but recycled in the system if it falls again as discharge along the Amazon and its main precipitation (Brubaker et al. 1993). Studies tributaries has forced many scientists to use on recycling of water in the hydrological indirect methods for determining the water cycle of the Amazon basin have been perbalance for the region. Early studies by Salati formed on the last two decades by Molion and Marques (1984) attempted to quantify (1975), Lettau et al. (1979), Salati (1987), the components of the water balance: Salati et al. (1979), Salati and Voce (1984), average precipitation was estimated as and Eltahir and Bras (1993). 11.9 x 1012 m3 yr1 (Villa Nova et al. 1976); The moisture balance in the Amazon discharge of the Amazon at Obidos was region indicates a substantial role of evapoestimated as 5.5 x 1012 m3 yr1, from Oltman (1967); and evapotranspiration was estimated by the Penman method as 6.4 x 1012 m3 yr1. Table 2.2 Estimates of water vapor in The latter is in good agreement with the dif- Amazonia during the ABLE-2B experiment.

seasons on record, but the 1957-58 El Nino was not particularly marked either in intensity or duration. One of the rainiest years on record was 1971, which was accompanied by only a moderate La Nina effect. Clearly, other factors affect Colombian hydrology besides ENSO, and their dependence is probably nonlinear. During the recent E Nino 1997-98, deficient rainfall in Northern Amazonia, including Iquitos, Peru and the State of Roraima, Brazil, was observed during most of the rainy season. Droughts that lead to forest fires were detected in 1997/98, as it were observed during other very strong ENSO events of: 1911/12, 1925/26, 1982/83.

ference between precipitation and river discharge. The results of other studies of the annual water budget in Amazonia are listed in Table 2.1 (compiled from Matsuyama 1992, Marengo et al. 1994, and Costa and

Precipitation Water vapor divergence Evapotranspiration Source: Rocha 1992

295.8 mm/month 187.7 mm/month 108.1 mm/month

General Characteristics and Variability of Climate in the Amazon Basin and its Links to the Global Climate System

transpiration (Lettau et al. 1979, Salati et al. 1979, Marques et al. 1980). The aerological estimates of the evapotranspiration over the eastern and central parts of the basin averaged 3 to 3-5 mm/day. More recent estimates based on ABLE-2B and FLUAMAZON estimated the evapotranspiration as high as 5 mm/day with significant transient variations (Nobre et al. 1991). During highly active precipitation episodes moisture convergence can account for 70 to 80% of precipitation. However, in monthly or longer-term averages evapotranspiration is responsible for approximately 55% of precipitation. The high variability of the moisture balance components has been identified in the aerological method (Rocha 1992) and satellite derived estimates (Cutrim et al. 1995). During the rainy season in Amazonia, water vapor flux transported across the Andes is small compared with the incoming flux, and the water vapor convergence area is connected with the SACZ. Conversely, during the dry season the water vapor transported across the Andes is as large as the incoming flux near the equator. Estimates of the water balance for the ABLE-2B period (April 13-May 13, 1987) using the divergence equation are shown in Table 2.2. The approach of the water balance by meteorology has focused more on the dynamics and thermodynamics of the atmosphere at larger scales. Moisture evaporated from the land surface can act not only as a source of water for later precipitation but can also change the thermodynamic structure of the atmosphere, altering circulation and convection. For the Amazon basin, meteorological studies of the water balance have ranged from large-scale atmospheric water budgets based on the ECMWF reanalysis or FGGE gridded data (Matsuyama 1992, Oki et al. 1995, Rao et al. 1996), and previously based on climatic observations of precipitation, a few

29

radiosonde stations, and river discharges (Marques et al. 1977, 1979a, b, 1980a, b, Chu, 1982, Marengo et al. 1994). Rao et al. (1996) used the ECMWF data from 1985-89 and their results suggested that (a) the Amazon basin is the principal source of moisture for central Brazil for the period September-February, and (b) the water vapor flux from the equatorial Atlantic associated with tradewinds is the main moisture source for the Amazon basin. However, the interannual variations of these fluxes and their associations with El Nino/La Nina have not been studied yet. It is also clear that a variety of human activities can act to modify various aspects of surface hydrologic systems. For example, changes in land cover can significantly affect the surface water and energy balance through changes in net radiation, evapotranspiration, and runoff (Costa and Foley 1999). However, because of the intricate relationships between the atmosphere, terrestrial ecosystems, and surface hydrological systems, it is still difficult to gauge the importance of human activities in the Amazonian hydrologic cycle. The availability of worldwide gridded reanalysis (NCEP, ECMWF, NASA-DAO) has allowed assessments of the water vapor transport into Amazonia, and in fact some recent works have shown contradictory results in terms of trends in input moisture into the Amazon basin. For instance, Costa and Foley (1999) identified a statistically significant decreasing trend in the atmospheric transport of water both into and out of the Amazon basin, based on 20 years (1976-96) of the NCEP reanalysis. On the other hand, Curtis and Hastenrath (1999) have identified statistically significant upward trends of lower tropospheric convergence, upward flow, convergence of atmospheric water vapor transport, and precipitable water over the Amazon basin, based on the analysis of 40 years (1958-97) of the NCEP reanalysis.

30

More recently, Marengo et al. (2001a) assesses the components of the water budget in the Amazon basin and their time and space variations, by using a combination of hydrometeorological observations and moisture fluxes derived from the NCEP/ NCAR reanalyses, for the period 1970-98. Their results show that there is a seasonality and interannual variability of the water balance that varies across the basin. Because of its larger size, southern Amazonia dominates the seasonal cycle of the water balance of the entire region, while at interannual time scales rainfall anomalies in northern Amazonia modulate the water budget of the entire basin. This is due to a relatively stronger links between the hydrometeorology of northern Amazonia and the tropical Pacific interannual variability. In the entire Amazonia, precipitation exceeds evaporation representing a sink of moisture (P>E). Our estimates of the Amazon region's water balance do not show a closure of the budget, with an average imbalance of almost 44%, meaning that some of the moisture that converges in the Amazon region is not unaccounted for. The imbalance is larger in southern Amazonia as compared to northern Amazonia. Large uncertainties are detected in the moisture convergence fields derived from the reanalyses (sometimes on the order of 40% around the mean values), and in the precipitation based from observations due to sampling problems. However, variability in the moisture convergence and observed rainfall and runoff matches quite well. The cycling ratio exhibits a slight and non-significant negative trend, which is steeper in northern Amazonia, showing that for the period of the study there are no signals of an intensification of the hydrological cycle (and higher cycling rates) in northern Amazonia, while southern Amazonia exhibits a systematic increase in cycling ratio.

Jose A. Marengo and Carlos A. Nobre

Long-Term Climate Variability and Change in Amazonia The Amazon humid forest plays an important role in the water cycle and water balance of much of South American. Several model studies and field experiments show that large part of the rainfall in the region originates as water recycled in the forest. However, under the current rate of deforestation of no more than 10% per year, rainfall and streamflow observations across the basin do not exhibit any significant trends yet. The Amazon basin has experienced increased deforestation in time, and the resulting possible changes in regional and global climate have motivated a host of climate model experiments (Zeng 1999). Possible scenarios include a reduction in precipitation and evapotranspiration rates and an increase in regional air temperatures. However, the southern portions of the basin (which have the highest rates of deforestation) actually do not show reduced precipitation. For a review on the results of several experiments using different models, please refer to Fisch et al. (1998) and Gash and Nobre (1997). It is also clear that a variety of human activities can act to modify various aspects of surface hydrologic systems. For example, changes in land cover can significantly affect the surface water and energy balance through changes in net radiation, evapotranspiration, and runoff. However, because of the intricate relationships between the atmosphere, terrestrial ecosystems, and surface hydrological systems, it is still difficult to gauge the importance of human activities in the Amazonian hydrologic cycle. Previous studies have considered the long-term variability of precipitation in Amazonia, either based on records at individual stations, several stations with short time series (Rocha et al. 1989, Marengo 1992, Dias de Paiva and Clarke 1995, Chu et al.

General Characteristics and Variability of Climate in the Amazon Basin and its Links to the Global Climate System

31

Table 2.3 Deforestation rates in Brazilian Amazonia (km2/ yr). State Acre Amapa Amazonas Maranhao Mato Grosso Para Rondonia Roraima Tocantins Amazonia % /yr

78-88* 620 60 1510 2450 5140 6990 2340 290 1650 21130 0.54

88-89 540 130 1180 1420 5960 5750 1430 630 730 17860 0.48

89-90 550 250 520 100 4020 4890 1670 150 580 13810 0.37

90-91 380 410 980 670 2840 3780 1110 420 440 11130 0.30

91-92 400 36 799 135 4674 3787 2265 281 409 13786 0.37

92-94** 482 0 370 372 6220 4284 2595 240 330 14896 0.40

94-95 1208 9 2114 1745 10391 7485 4730 220 797 29059 0.81

95-96 433 0 1023 1061 6543 6135 2432 214 320 18161 0.51

Source: PRODES/INPE. * Average for the period 1978-88. ** Average for the period 1992-94.

1994), or based on discharges of the Amazon rivers and its tributaries (Gentry and LopezParodi 1980, Marengo et al. 1998a, Marengo 1995b, Guyot et al. 1997). These studies identified short periods of low-frequency variation, meaning upward or downward trends in natural climate variability rather than an effect of global climate change. Zhou and Lau (1999) have investigated the interannual and decadal variation of summer rainfall in Amazonia and found that the ENSO influence is dominant in the interannual scale, confirming earlier results by Marengo (1992). On the decadal and longer time scale the footprints of SST variation are found in the rainfall observation. The decadal mode shows a meridional displacement of the precipitation belt over the equatorial Atlantic, which may be attributed to the dipole pattern of anomalous SST across the equator. This SST dipole and its influence on circulation and interdecadal rainfall variations in Northeast Brazil have been discussed by Nobre and Shukla (1996). Regarding interdecadal circulation changes in the tropical South Atlantic, studies by Wagner (1996) have identified a substantial

warming trend in the surface waters during the austral summer months of February and March. As a consequence of the strengthened interhemispheric SST gradient in this season, the position of the ITCZ was displaced southward, and rainfall in northeast Brazil increased over 1951-90 with a statistically significant tendency. Since the North Atlantic is a source of moisture for northern Amazonia, this southward displaced ITCZ would also imply a systematically increased moisture flux from the Atlantic into Amazonia. Interdecadal and long-term variability of rainfall in Amazonia have been studied by Marengo et al. (2001 d). They based the analysis on the Indices of rainfall for Northern Amazonia NAR and southern Amazonia SAR, defined in Marengo (1992), and updated and upgraded. The NAR and SAR time series available since the late 1920's have shown a non-significant decreasing trend, and a positive trend significant at 5% level, respectively (Fig. 2.8a, b). However, the most remarkable feature is the presence of alternating phases or cycles of relatively wetter and drier periods rather than any unidirectional trends.

32

The maximum values of NAR were during the anomalously wet period of the late 1960's and the minimum was on the second half of the 1990's . SAR shows an opposite pattern. For NAR, the periods around 1945-50 and 1975-80 were transitional periods leading to relatively wetter and drier conditions, respectively. The relatively wet period from the late 1960's and middle 1970's reported in earlier publications (Marengo et al. 2001 c) is consistent with a period with positive NAR (Fig. 2.8 a), and Rio Negro water levels at Manaus. A period with negative NAR indices has been identified from the 1980's and extending into the present. On the other hand SAR shows such periods of transition a bit earlier (194045) for the first one and almost coinciding for

Jose A. Marengo and Carlos A. Nobre

the second one on the second half of 1970s. Since approximately 1992 both NAR and SAR series have been decreasing. There is a period between the end of the 1960's to the beginning of the 1970's where NAR shows opposite tendencies. For the rest of the period, NAR, together with the discharges of the Amazon River at Obidos and the rio Negro in Manaus (not shown) exhibit similar variability in time. The 10-year centered moving average curves in Fig. 2.8a, b are superimposed on the interannual variability of these hydrometeorological and circulation indices. The curves show that on decadal time scales the most notable variation identified in the

Figure 2.8 Trends and cycles in Northern Amazonian Rainfall as represented by the NAR index (Marengo 1999), for the period 1929-96.

General Characteristics and Variability of Climate in the Amazon Basin and its Links to the Global Climate System

33

Table 2.4 Comparisons of GCM simulations of Amazon deforestation experiments. Results show the differences between deforested minus control run. Experiment

AE*(mm/day)

AT*(K)

AP*(mm/day)

AC*(mm/day)

CCM, BATS Dickinson, Henderson-Sellers (1988) Dickinson and Kennedy (1992) Henderson-Sellers et al. (1993) Hahman and Dickinson (1995) Zeng et al. (1996) Hahmann and Dickinson (1997)

-0.5 -0.7 -0.6 -0.4 -2.0 -0.4

+3.0 +0.6 +0.5 +0.8 +1.0

0.0 -1.4 -1.6 -0.8 -3.1 -1.0

+0.5 -0.7 -1.0 -0.4 -1.1 -0.6

GENESIS Costa and Foley (2000)**

-0.4

+3.5

-0.4

0.0

UKMOGCM Lean, Warrilow (1989) Lean and Rowntree (1993) Lean, Rowntree (1997) Lean et al. (1996)

-0.6 -0.6 -0.8 -0.8

+2.0 1.9 +2.3 +2.3

-1.3 -0.8 -0.3 -0.4

-0.7 -0.3 +0.5 +0.4

COLA GCM, SiB, SsiB Shukla et al. (1990), Nobre et al. (199D -1.4

+2.5

-1.8

-0.4

Dirmeyer, Shukla (1994) Sud et al. (1990)

+2.0

-0.7 -1.5

-0.3 -0.3

+3.0

-0.3 -0.7 -1.5

-0.1 0.3 -0.3

+3.8 +0.1

+1.0 -0.5

+3.7 -0.1

-0.4 -1.2

GLA GCM, SiB

Sud et al. (1996a) Sud et al. (1996b) Walker et al. (1995)

-0.2 -1.0 -1.2

LMD GCM, SECHIBA Polcher, Laval (1994a) Polcher, Laval (1994b)

-2.7

-0.4

* AE is change in evapotranspiration (mm/day), AT is the change in surface air temperature (aK), AP is the change in precipitation (mm/day), AC is the moisture convergence, calculated as the difference of AP and AE (AC=AP-AE). ** Combined effects of deforestation and double atmospheric CC^-

northern Amazonia is a 2 decade-long contrasting variability, with changes in sign around the 1940's and the 1970's. The change appears to have been associated with weakening of ENSO and, possibly, a weakening of connections between its atmospheric and oceanic components

(Dettinger et al. 2000, Marengo et al. 2001d). These associations are more apparent during the period since the middle 1970s when there were more moderate to strong El Nino events. The variability of NAR and SAR reflects the ENSO effects at interannual scales, and on decadal scales with less rain in

34

Jose A. Marengo and Carlos A. Nobre

northern Amazonia in a time when more Oki et al. 1995, Costa and Foley 1999). To frequent and strong ENSO events occurring relate GCM source runoff to the available hydrographic data, runoff routing models since mid 1970's. Guyot et al. (1997) show that the Solimoes were developed and applied to regional or river in Manacapuru exhibit positive long- continental scale. The scheme of Vorosmarty term trends, together with the Madeira river at et al. (1989) was applied with water balance Porto Velho and Xingu river in Altamira. models for several worldwide large basins. These results are consistent with the positive Marengo et al. (1994) applied the river trends of the Rio Negro river in Manaus routing scheme developed by Miller et al. (Marengo 1995a), but none of those trends (1994) to the Amazon basin and its main subbasins and identified the impacts of the reach statistical significance. Regarding tendencies in air temperature, parameterization of land surface interactions Sansigolo et al. (1992) assessed tendencies on the model generated runoff, as well as the in several major cities in Brazil and identified size of the basin. positive trends significant at 95% in Belem, Many areas of Amazonia and the Pantanal while nonsignificant trends were identified for region, when viewed at GCM grid resoluManaus and Cuiaba. More recently, Victoria et tions, have greater than 10% open-water al. (1998) used the records of 17 stations coverage. The low resolution of global GCMs spread across the Amazon region for the cannot resolve even large features such as period 1918-95 to try to identify signals of the Amazon floodplain or the Pantanal, global warming in Amazonia. They found an which may have significant impacts on increasing trend (+0.63SO per century that regional energy and water fluxes between is slightly higher than the trend found for land and the atmosphere. Patchiness in the the Southern Hemisphere (+0.562C per pattern of vegetation and deforestation, century). The results of 2 nonparametrical sta- evidenced in satellite images over Rondonia, tistical tests showed that the observed trend is also impossible for low-resolution GCMs to started to be significant in the late 1960s to the represent. Higher resolution will allow better early 1970s. However, a conclusive relation- representation of these areas in the models. ship between warming trends and deforesta- Coupling to river flux models over regions tion in Amazonia is difficult to establish with and subbasins also requires higher resolutions than traditional GCMs can supply. the data available. Higher-resolution regional modeling will Climate and Hydrological Modeling greatly facilitate our hydrologic studies.

in Amazonia

Overview of experiments on Climate models have been used to simulate Amazon deforestation

the behavior of large-scale hydrologic systems such as the Amazon basin in response either to regional deforestation or doubling CO2 (see reviews in Marengo et al. 1994 and Costa and Foley 1999). In addition, several authors have simulated hydrological processes and river discharge within atmospheric general circulation models (GCM) (Vorosmarty et al. 1989, Miller et al. 1994, Marengo et al. 1994, Sausen et al. 1994,

Regarding Amazon deforestation, the largest deforestation rates are observed in southwestern and eastern Amazonia, the so-called Deforestation Arc. The Institute Nacional de Pesquisas Espaciais (INPE) of Brazil developed the Deforestation Project (in Portuguese, PRODES Programa de Monitoramento de Desflorestamento na Amazonia) based on LANDSAT images for

General Characteristics and Variability of Climate in the Amazon Basin and its Links to the Global Climate System

monitoring deforestation in the Brazilian Amazon. The estimates of deforestation rates indicated in Table 2.3 show an increase of 33% in the annual deforestation rate, varying from 0.30% in 1991 to 0.40% in 1994. In 1997 the rate increased to 0.81%. Deforestation was greater on the Brazilian states of Para, Mato Grosso, Rondonia and Acre, while the deforestation rates decreased in Amazonas, Amapa, Roraima, Maranhao, and Tocantins. Maps derived from LANDSAT products indicate that deforestation is increasing in the savanna region while smaller in the tropical rain forest. Atmospheric GCMs simulations have suggested a possible change in the regional and global climate as the results of tropical deforestation (see reviews in Zeng 1998). Under a hypothesized Amazon basin deforestation scenario, almost all models show a significant reduction in precipitation and evapotranspiration , and most found a decrease in moisture convergence. The picture becomes more complicated when one examines the sensitivities to changes in individual land surface properties such as albedo and roughness length (Dirmeyer and Shukla 1994, Hahmann and Dickinson 1997). Different experiments emphasize the importance of different aspects of the land atmospheric systems. Many more issues arise, such as the roles of moist convection (Polcher and Laval 1994a, b), the effects of a warmer ground (Eltahir and Bras 1993), the effects of a more realistic surface representation (Lean et al. 1996), and the combined effect of deforestation and increased CO2 in the atmosphere (Costa and Foley, 2000). Previous modeling studies with GCMs have suggested that large-scale changes in Amazonia land surface cover may affect the regional climate. Sensitivity studies with these models have shown that realistic descriptions of the land surface are critical to these results. For example, the predicted climatic effects of large-scale deforestation

35

can be highly dependent on albedo, surface roughness, or soil hydraulic properties. The spatial variation of these and other properties are not well established for Amazonia. Experiments by Lean and Rowntree (1993) have shown similar results, but differing in the rate of change. Eltahir and Bras (1993) have studied the sensitivity of regional climate deforestation in the Amazon basin, finding decreases up to 10% in precipitation for deforestation in patches of 250 km on a side in a mesoscale model. Dirmeyer and Shukla (1994) used the SiB model coupled to the COLA GCM and found that the change in climate, particularly in precipitation, is strongly dependent on the change of surface albedo that accompanies deforestation. Polcher and Laval (1994a, b) and Polcher (1995) used the French LMD model coupled to the SECHIBA land surface parameterization in an experiment on Amazonian and African deforestation. They found that the model produces a reduction in evaporation and an increase of soil temperature and, in contrast to similar experiments, a significant increase of moisture convergence was obtained. This increase is attributed to an enhancement of the convective activity of the ITCZ. Walker et al. (1995) and Sud et al. (1996a, b) studied the impact of in situ deforestation in Amazonia on regional climate. They have found that evapotranspiration decreases and land surface outgoing longwave radiation and sensible heat flux increase, thereby warming and drying the planetary boundary layer. A decrease in precipitation is also found, consistent with the reduction in atmospheric flux of moisture convergence. Their results are consistent with earlier simulation studies of deforestation. Rahman and Dickinson (1995) and Zeng et al. (1996) used the GCM with BATS and a primitive equation model, respectively. Zeng and collaborators showed a much-weakened Atlantic Hadley-Walker circulation as a result of a strong positive

36

Jose A. Marengo and Carlos A. Nobre

feedback loop in the atmospheric circulation However, both processes work to warm the and the hydrological cycle. Continuous Amazon basin. Deforestation results in an deforestation suggests that the replacement increased surface temperature, largely of forest by grassland may be able to sustain because of decreases in evapotranspiration. dry climate. Table 2.4 summarizes the results The combined effect of deforestation and of simulations of Amazon deforestation double CO2 is an increase in temperature on experiments. In some of the model experi- the order of 3-5QC. ences (Lean and Rowntree 1997) relative changes in the magnitude of rainfall and Conclusions evapotranspiration indicate that increased moisture convergence partially compensates Regional studies discussed in this chapter for the reduced evaporation, in contrast to clearly demonstrate the temporal and spatial many previous deforestation experiments. dependence of Amazon rainfall on variations The wide disparity of results indicated in in the meridional positions of the ITCZ and Table 2.4 may be partially explained by the regional features such as the sea-breeze specification of vegetation and soil character- interactions near the mouth of the Amazon, istics between experiments and the differing the convection forced by the Andes in model resolutions. The availability of western Amazonia, and the effects of cold extensive measurements taken during the fronts organizing summertime convection in ABRACOS field campaigns (Lean et al. 1996, southern Amazonia. These also determine Gash et al. 1996, Gulf et al. 1996, Gash and different diurnal cycles of precipitation Nobre 1997) for paired sites of forest and across the basin. Equatorial Amazonia does cleared areas provides a major opportunity to not experience a dry season, as southern or introduce some consensus across the breadth eastern Amazonia does. Southern Amazonia of experiments. Gash and Nobre (1997) using rainfall peaks in early summer, while central the ABRACOS measurements of surface Amazonia rainfall peaks in late summer. climate demonstrated the local-scale, meso- Consequently, the season of largest river scale and large-scale climatic impacts of discharges occurs in early winter, a few Amazonian deforestation. The difference in months after the peak of the rainy season in radiation and energy balance between northern and western Amazonia. Daily forest and clearing produces higher air variability of rainfall is characterized by the temperatures in the clearings, particularly in presence of dry spells (veranicos) during the dry season. In areas of substantial defor- the rainy summer season extending between estation, higher sensible heat fluxes from the November and February. Northern Amazonia cleared forest produce deeper convective exhibits longer and more intense veranicos boundary layers, with differences in cloud during El Nino years, as compared to cover being observed and meso-scale circu- short and less intense veranicos during La lations being predicted. Nina years. A recent study by Costa and Foley (2000) The tropical Atlantic is the main source of examined the combined effects of deforesta- moisture for the Amazon via the tradewinds, tion and doubled atmospheric CO2 concentra- and the intensity and timing of this transport tions on the climate of Amazonia. He found is dependent on the interannual variability of that the effects of deforestation and increasing tropical Atlantic circulation and SST meridCO2 concentrations on evapotranspiration and ional gradients. Furthermore, signals of the precipitation tend to counteract one another. tropical Pacific in Amazonia are observed

General Characteristics and Variability of Climate in the Amazon Basin and its Links to the Global Climate System

especially during very strong El Nino events, such as 1982-83 and in 1997-98. These two events featured large negative rainfall anomalies in the northern and central parts of the basin, due to a suppressed convection and relatively strong subsidence in the region. The strongest signal of the SO is detected in northern Amazonia, while rain in southern Amazonia is not affected by the extremes of the SO. Anomalously low rainfall favors the propagation of forest fires in northern Amazonia, and also causes problems with the generation of hydroelectricity and transportation due to very low levels in the main Amazonian rivers. Recent studies have shown a decadal variability in rainfall and riverflows in Amazonia, which is consistent with similar time scale behavior in Northeast Brazil, southern Brazil and Northern Argentina, and which is coherent with decadal scale variability in SST in the tropical and the southern Atlantic. On longer time scales, no unidirectional tendencies in rainfall and rivers have been found in Amazonia, as one would expect from increased deforestation in the basin and as is predicted by model

37

experiments on regional deforestation. Most of the climate models show similar tendencies even though magnitudes on change differ. Analysis of climatic data from forest and recently deforested areas in Amazonia has corroborated changes in air temperature, evaporation, albedo, and humidity due to the removal of the forest. A positive trend in air temperature has been identified from observations, even though these records are sometimes not long enough or do not cover enough of the region to affirm that a signal of global warming or deforestation is detected in Amazonia. The motivation for large-scale studies, such as the Large Scale Biosphere Atmosphere Experiment in Amazonia (LBA), is the need to understand the different terms in the atmospheric and surface water, carbon and energy budget on a basin-wide scale. These must be understood both as they are now and as they may evolve in the future in response to vegetation change and human-induced climate change. This new understanding will be developed using a host of new observational evidence gathered in Amazonia and hydrological and GCM models.

Literature Cited Baumgartner, A., and R. Riechel. 1975. The World Water Balance. Elsevier, New York, pp. 464. Brubaker, K., D. Entekhabi, and P. Eagleson. 1993. "Estimation of continental precipitation recycling." Journal of Climate 6: 1077-1089. Cavalcanti, I. F. A. 1982. "Urn estudo sobre interacoes entre sistemas de circulacao de escala sinotica e circulacoes locais." Relatorio Tecnico INPE 2492—TDL/097, p. 113 Chu, P. S. 1982. Diagnostics of climate anomalies in tropical Brazil. Ph.D. Dissertation. Department of Meteorology, University of Wisconsin, Madison. USA. Chu, P. S., -P. Yu, and S. Hastenrath. 1994. "Detecting climate change concurrent with deforestation in the Amazon basin: which way has it gone?" Bulletin American Meteorological Society. 75: 579-583. Cohen, J. C. P., M. A. F. Silva Dias, and C. A. Nobre. 1995. "Environmental conditions associated with Amazonian Squall Lines: a case study." Monthly Weather Review. 123(11): 3163-3174.

Costa, M. H., and J. Foley. 1999. "Trends in the Hydrological Cycle of the Amazon basin." Journal of Geophysical Research. 104: 14189-14198. Costa, M., Foley, J. 2000. "Combined effects of deforestation and double atmospheric CO2 concentrations on the climate of Amazonia." Journal of Climate. 13:18-34. Gulf, A. J., Esteves, A. Marques Filho, and H. da Rocha. 1996. "Radiation, temperature and humidity over forest and pasture in Amazonia." In: Amazonian Deforestation and Climate, eds. J. Gash, C. Nobre, J. Roberts, and R. Victoria, pp. 175-192. Curtis, S., and S. Hastenrath, 1999. Trends of upper-air circulation and water vapour over equatorial South America and adjacent oceans. International Journal of Climatology 19: 863-876. Cutrim, E., D. Martin, and R. Rabin. 1995. "Enhancement of cumulus clouds over deforested lands in Amazonia." Bulletin American Meteorological Society 76: 1801-1805.

38

Dettinger, M., D. Cayan, G. McCabe, andj. Marengo. 2000. "Multiscale ENSO Variability and Hydrological Systems." In: El Nino: Multiscale Variability and its Impacts on Natural Ecosystems and Society, eds. Henry Diaz, Vera Markgraf (Cambridge University Press), pp. 113-148 Dias de Paiva, E. M. C, and R. Clarke. 1995. "Time trends in rainfall records in Amazonia." Bulletin American Meteorological Society 75: 579-583. Dickinson, R., and A. Henderson-Sellers. 1988. "Modeling tropical deforestation: A study of GCM land-surface parameterization." Quarterly Journal Royal Meteorological Society 114: 439-462. Dickinson, R., and P. Kennedy. 1992. "Impacts on regional climate of Amazon deforestation." Geophysical Research Letters. 19: 1947-1950. Dirmeyer, P., and J. Shukla. 1994. "Albedo as a modulator of climate response to tropical deforestation." Journal of Geophysical Research 99: 20863-20877. Eltahir, E., and R. Bras. 1993. "On the response of the tropical atmosphere to large-scale deforestation." Quarterly Journal Royal Meteorological Society 119: 779-783. Eltahir, E. 1996. "Role of vegetation in sustaining large-scale atmospheric circulations in the tropics." Journal of Geophysical Research 101: 4255-4268. Eltahir, E., and J. Pal. 1996. "Relationships between surface conditions and subsequent rainfall in convective storms." Journal of Geophysical Research 101: 26237-26245. Figueroa, N., and C. Nobre. 1990. "Precipitation distribution over central and western tropical South America." Climanalise. 5: 36-40. Figueroa, N., P. Satyamurty, and P. L. Silva Dias. 1995. "Simulations of the summer circulation over the South American region with an eta coordinate model." Journal of Atmospheric Science 52: 1573-1584. Fisch, G., J. Marengo, and C. Nobre. 1998. "Uma revisao geral sobre o Clima da Amazonia." Acta Amazonica. 28: 101-126. Franken, W., and P. Leopoldo. 1984. "Hydrology of catchment areas in Central-Amazonian forest streams." In: The Amazon, Limnology and Landscape Ecology of a Mighty Tropical River and its Basin, ed. H. Soili (W. Junk, Dordrecht, The Netherlands), pp. 501-519. Fu, R., B. Zhu, and R. Dickinson. 1999. "How do atmosphere and land surface influence seasonal changes of convection in the tropical Amazon?" Journal of Climate 12: 1306-1321. Garreaud, R., and J. M. Wallace. 1997. "The diurnal march of convective cloudiness over the Americas." Monthly Weather Review 125: 3157-3171. Garreaud, R., and J. M. Wallace. 1998. "Summertime incursions of midlatitude air into subtropical and tropical South America." Monthly Weather Review 126: 27143-2733. Gash, J., and C. Nobre. 1997. "Climatic effects of Amazonian deforestation: Some results from ABRACOS." Bulletin American Meteorological Society 78: 823-830. Gash, J., C. Nobre. J. Roberts, and R. Victoria. 1996. "An overview of ABRACOS." In: Amazonian Deforestation and Climate, eds. J. Gash, C. Nobre, J. Roberts, and R. Victoria (John Wiley & Sons, New York), pp. 1-14.

Jose A. Marengo and Carlos A. Nobre

Gentry, A., and J. Lopez-Parodi. 1980. "Deforestation and increased flooding in the upper-Amazon." Science 210: 1354-1356. Guyot, J.-L., J. Callede, M. Molinier, W. Guimaraes, and E. de Oliveira. 1997. "La variabilite hydrologique actuelle dans le bassin de 1'Amazone. Seminario Internacional Consequencias climaticas e Hidrologicas del Evento El Nino a Escala Regional y local." Memorias Tecnicas. 26-29 November Quito, Ecuador, pp. 285-293. Hahmann, A., and R. Dickinson. 1995. "Performance and sensitivity of the RCCM2/BATS model to tropical deforestation over the Amazon Basin." Conf. proc. XXI IUGG general Assembly, Boulder, CO. Hahmann, A., and R. Dickinson. 1997. "RCCM2 BATS model over tropical South America: application to tropical deforestation." Journal of Climate 10: 1944-1964. Hastenrath, S. 1995. "Climate and Dynamics of the Tropics." Kluwer. p. 256. Henderson-Sellers, A., R. Dickinson, T. Durbidge, P. Kennedy, K. McGuffie, and A. Pitman. 1993. "Tropical deforestation: modeling local to regional scale climate change." Journal of Geophysical Research 98: 7289-7315. Jordan, C., and J. Heuveldop. 1981. "The water balance of an Amazonian rain forest." Acta Amazonica 11: 87-92. Kalnay, E., and coauthors. 1996. "The NCEP/NCAR Reanalysis Project." Bulletin American Meteorological Society 77: 437-471. Kiladis, G. N., and H. F. Diaz. 1989- "Global climatic anomalies associated with extremes in the Southern Oscillation." Journal of Climate. 2: 1069-1090. Kousky, V. E. 1988."Pentad outgoing longwave radiation climatology for the South American sector." Revista Brasileira Meteorologia. 3: 217-231. Lean, J., and D. Warrilow. 1989. "Climatic impact of Amazon deforestation." Nature 342: 311-313. Lean, J., and P. Rowntree. 1993. "A GCM simulation of the impact of Amazon deforestation on climate using an improved canopy representation." Quarterly Journal Royal Meteorological Society. 119: 509-530. Lean J., and P. Rowntree. 1997. "Understanding the sensitivity of a GCM simulation of Amazonian deforestation to specification of vegetation and soil characteristics." Journal of Climate 6: 1216-1235. Lean, J., C. Bunton, C. Nobre, and P. Rowntree. 1996. "The simulated impact of Amazonian deforestation on climate using measured ABRACOS vegetation characteristics." In: Amazonian Deforestation and Climate, eds. J. Gash, C. Nobre, J. Roberts, and R. Victoria (John Wiley & Sons, New York), pp. 549-576. Lenters, J., and K. H. Cook. 1995. "Simulation and diagnosis of the regional summertime precipitation climatology of South America." Journal of Climate 8: 2988-3005. Leopold, L. 1962. "Rivers," American Scientist 50: 511-537. Leopoldo, P., W. Franken, E. Matsui, and M. Roibeiro. 1982. "Estimativa da evapotranspiracao da floresta Amazonica de terra firme." Acta Amazonica 12: 23-28. Lettau, H., K. Lettau, and L. Molion. 1979. "Amazonia's hydrologic cycle and the role of atmospheric recycling in assessing deforestation effects." Monthly Weather Review. 107: 227-238.

General Characteristics and Variability of Climate in the Amazon Basin and its Links to the Global Climate System Marengo, J. 1992 "Interannual variability of surface climate in the Amazon Basin." International Journal of Climatology. 12: 853-863. Marengo, J. 1995a. "Variations and Change in South American Streamflow." Climate Change 31: 99-117. Marengo, J. 1995b. "Interannual variability of deep convection in the tropical South American sector as deduced from ISCCP C2 data." International Journal of Climatology 15: 995-1010. Marengo, J. 1999- "Factores medio-ambientales del area de estudio: Climatologia en la Zona de Iquitos. Parte III." In: Geoecologia y Desarrollo de la Zona de Iquitos, Peru, eds. R. Kalliola, and S. Flores University of Turku, Finland, pp.35-55 Marengo, J., and S. Hastenrath. 1993. "Case studies of extreme climatic events in the Amazon basin." Journal of Climate 6: 617-627. Marengo, J., J. Miller, G. Russell, C. Rosenzweig, and F. Abramopoulos. 1994. "Calculations of river-runoff in the GISS GCM: impact of a new land surface and runoff routing model in the hydrology of the Amazon River." Climate Dynamics 10: 349-361. Marengo, J., A. Cornejo, P. Satyamurty, C. A. Nobre, and W. Sea. 1997a. "Cold waves in the South American continent. The strong event of June 1994." Monthly Weather Review 125: 2759-2786. Marengo, J., C. Nobre, and A. Gulf. 1997b. "Climatic impacts of Friagens in forested and deforested areas of the Amazon Basin." Journal of Applied Meteorology 36: 1553-1566. Marengo, J., Liebmann, B. Kousky, V., Filizola, N., Wainer I, 2001a. "On the onset and end of the rainy season in the Brazilian Amazon Basin." Journal of Climate, 14: 833-852. Marengo, J., Nobre, C. A., Sanches, M. B. 2001b. "On the atmospheric water balance and moisture cycling in the Amazon basin. Characteristics and space-time variability." Submitted to Journal of Hydrometeorology. Marengo, J., Fisch, G., Vendrame, I., Cervantes, P., 2001c. "On the diurnal and day-to-day variability of rainfall in Southwest Amazonia during the TRMM-BA and LBAWET AMC campaign of the summer of 1999." Submitted Journal of Geophysical Research. Marengo, J, Bhatt, U., Cunningham, C., 2001d. "Decadal and multi-decadal climate variability in the Amazon basin." Submitted International Journal of Climatology. Marengo, J., 1999. "Factores medio-ambientales del area de estudio: Climatologia en la Zona de Iquitos. Parte HI." In Geoecologia y Desarrollo de la Zona de Iquitos, Peru. R. Kalliola, S. Flores Eds. University of Turku, Finland, pp. 35-55. Marengo, J., J. Tomasella, Uvo. 1998a. "Long-term streamflow and rainfall fluctuations in tropical South America: Amazonia, Eastern Brazil and Northwest Peru." Journal of Geophysical Research 103: 1775-1783. Marengo, J., C. A. Nobre, and G. Sampaio. 1998b. "On the associations between hydrometeorological conditions in Amazonia and the extremes of the Southern Oscillation." In: extended abstracts of Memorias Tecnicas, Seminario internacional Consecuencias climaticas e hidrologicas

39

del evento El Nino a escala Regional y Local. 26-29 November 1997, Quito, Ecuador, pp. 257-266. Marengo, J., B. Liebmann, V. Kouksy, N. Filizola, and I. Wainer. 1999. "On the onset and end of the rainy season in the Brazilian Amazon Basin." Journal of Climate. Submitted. Marques, J., M. dos Santos, N. Villa Nova, and E. Salati. 1977. "Precipitable water and water wapor flux between Belem and Manaus." Acta Amazonica 7: 355-362. Marques, J., M. dos Santos, and E. Salati. 1979a. "O armazenamento atmosferico de vapor dagua sobre a regiao Amazonica." Acta Amazonica 9: 715—721. Marques, J., M. dos Santos, and E. Salati. 1979b. "O Campo de fluxo de vapor de agua sobre a regiao Amazonica." Acta Amazonica 9: 701-713. Marques, J., M. dos Santos, and E. Salati. 1980a. "Calculo de evapotranspiracao na bacia Amazonica a traves do metodo aerologico." Acta Amazonica 10: 357-361. Marques, J., M. dos Santos, and E. Salati. 1980b. "A divergencia do campo do fluxo de vapor dagua e as chuvas na regiao Amazonica." Acta Amazonica 10: 133-140. Matsuyama, H. 1992. "The water budget in the Amazon River Basin during the FGGE Period." Journal of Meteorological Society Japan 70: 1071-1083Meade, R., J. Rayol, S. Conceicao, and J. Natividade. 1991. "Backwater effects in the Amazon river basin of Brazil." Environmental Geological Science 18: 105-114. Miller, J., and G. Russell. 1992. "The impact of global warming on river runoff." Journal of Geophysical Research 97: 2757-2764. Miller, J., G. Russell, and G. Caliri. 1994. "Continental scale river flow in climate models." Journal of Climate7: 914-928. Milliman, J., and R. Meade. 1983. "World-wide delivery of river sediment to the oceans." Journal of Geology 91: 1-21. Molion, L. 1975. A climatonomic study of the energy and moisture fluxes of the Amazon basin with consideration on deforestation. Ph.D. Dissertation, Department of Meteorology, University of Wisconsin, Madison, p. 190. Moura, A. D., and J. Shukla. 1981. "On the dynamics of the droughts in Northeast Brazil: observations, theory and numerical experiments with a general circulation model." Journal of Atmospheric Science 38 (12): 2653-2673. Nace, R. I. 1972. "World hydrology: status and prospects," in IAHS-UNESCO-WMO-ed., World Water Balance 1: 1-10. Nizhizawa, T., and Y. Koike. 1992. Amazon-Ecology and Development, Iwanami, Tokyo, p. 221. Nizhizawa, T, and M. Tanaka. 1983. "The annual change in the tropospheric circulation and the rainfall in South America." Archives of Meteorological Geophysical Bioklimatologie Series 33: 107-116. Nobre, C., P. Sellers, and J. Shukla. 1991. "Amazonian deforestation and regional climate change." Journal of Climate 4: 957-988. Nobre, C. A., J. C. Gash, R. Hutjes, D. Jacob, A. Janetos, P. Kabat, M. Keller, J. Marengo, J. R. McNeal, P. Sellers, D.

40

Wickland, and S. Wofsy. 1996. The Large Scale Biosphere-Atmosphere Experiment in Amazonia (LBA). Concise Experimental Plan. Compiled by the LBA Science Planning Group. Staring Center-DLO, Wageningen, The Netherlands. Nobre, P., and J. Shukla. 1996. "Variations of sea surface temperature, wind stress, and rainfall over the tropical Atlantic and South America." Journal of Climate 9: 2464-2479. Nogues-Paegle, J., and K. M Mo. 1997. "Alternating wet and dry conditions over South America during Summer." Monthly Weather Review 125: 279-291. Oki, T., K. Musiake, H. Matsuyama, and K. Masuda. 1995. "Global atmospheric water balance and runoff from large river basins." Hydrological Processes 9: 655-678. Oki, T., 1999. The global water cycle. Chapter 1.2, Global Energy and Water Cycles, Eds. K. Browning and R. Gurney, Cambridge University Press, pp. 10-27. Oilman, R. 1967. "Reconnaissance investigations of the discharge and water quality of the Amazon," In: Atas do Simp, sobre Biota Amazonica, Rio de Janeiro, CNPq, vol. 3, pp. 163-185. Polcher, J. 1995. "Sensitivity of tropical convection to land surface process." Journal of Atmospheric Science 52: 3143-3161. Polcher, J., and K. Laval. 1994a. "The impact of African and Amazonian deforestation on tropical climate." Journal of Hydrology 155: 389-405Polcher, J., and K. Laval. 1994b. "A statistical study of the regional impact of deforestation on climate in the LMD GCM." Climate Dynamics 10: 205-219. Poveda, G., and O. Mesa. 1997. "Feedbacks between hydrological processes in tropical South America and large-scale ocean atmospheric phenomenon." Journal of Climate 10: 2690-2702. Rao, V. B., M. C. Lima, and S. H. Franchito. 1993. "Seasonal and interannual variations of rainfall over eastern Northeast Brazil." Journal of Climate 6: 1754-1763. Rao, V. B., and K. Hada. 1990. "Characteristics of rainfall over Brazil: Annual variations and connection with the Southern Oscillation." Theoretical and Applied Climatology 42: 81-91. Rao, V. B., I. Cavalcanti, and K. Hada. 1996. "Annual variation of rainfall over Brazil and water vapor characteristics over South America." Journal of Geophysical Research 101: 26539-26551. Richey, J., C. Nobre, and C. Deser. 1989a. "Amazon River discharge and climate variability: 1903 to 1985." Science 246: 101-103. Richey, J. E., L. A. K. Mertes, T. Dunne, R. L. Victoria, B. L. Forsberg, A. C. N. S. Tancredi, and E. Oliveira. 1989b. "Sources and routing of the Amazon River flood wave. Global Biogeochem." Cycles 3: 191-204. Rocha, R. P. 1992. "Simulacao numerica de sistema de mesoescala sobre a America do Sul." Tese de Mestrado. IAG/USP. Rocha, H., C. Nobre, and M. Barros. 1989. "Variabilidade natural de longo plazo no ciclo hidrologico da Amazonia." Climanalise 4(12): 36-42. Rogers, J. C. 1988. "Precipitation variability over the

Jose A. Marengo and Carlos A. Nobre Caribbean and tropical Americas associated with the Southern Oscillation." Journal of Climate 1: 172-182. Ropelewski, C., and M. Halpert. 1987. "Global and regional scale precipitation patterns associated with the El Nino-Southern Oscillation." Monthly Weather Review 115: 1606-1626. Ropelewski, C., and M. Halpert. 1989. "Precipitation patterns associated with the High Index of the Southern Oscillation." Journal of Climate 2: 268-284. Russell, G., and J. Miller. 1990. "Global river runoff calculated from a global atmospheric general circulation model." Journal of Hydrology 117: 241-254. Salati, E. 1987. "The forest and the hydrological cycle." In: The Geophysiology of Amazonia, ed. R. E. Dickinson (Wiley, New York), pp. 273-296. Salati, E., and P. Voce. 1984. "Amazon basin: A system in equilibrium," Science 225: 128-138. Salati, E., and J. Marques. 1984. "Climatology of the Amazon region." In: The Amazon: Limnology and Landscape Ecology of a Mighty Tropical River and its Basin., ed. H. Sioli (W. Junk, Dordrecht, The Netherlands). Salati, E., and P. B. Vose. 1984. "Amazon Basin: A system in equilibrium." Science 225: 129. Salati, E., A. DaH'Ollio, E. Matsui, and J. Gat. 1979. "Recycling of water in the Amazon basin: An isotopic study." Water Resource Research 15: 1250-1258. Sansigolo, C., R. Rodrigues, and Etchichury. 1992. "Tendencias nas temperaturas medias do Brasil." Anais do VII Congresso Brasileiro de Meteorologia. Vol. 1, 367-371. Sausen, R., S. Schubert, and L. Dumenil. 1994. "A model of river runoff for use in coupled atmosphere-cean models." Journal of Hydrology 155: 337-352. Seluchi, M., and J. Nery. 1992. "Condiciones meteorologicas asociadas a la ocurrencia de heladas en la region de Maringa." (in Spanish). Revista Brasileira de Meteorologia 7: 523-534. Seluchi, M., Y. V. Serafini, and H. Le Treut. 1998. "The impact of the Andes on transient atmopsheric systems: A comparison between observations and GCM results." Monthly Weather Review 126: 8905-912. Shukla, J., C. Nobre, and P. Sellers. 1990. "Amazonia deforestation and climate change," Science 247: 1322-1325Shuttleworth, W. J. 1988a. "Evaporation from Amazonian rainforests." Philosophical Transactions of Royal Society of London B233. 321-346. Shuttleworth, W. J. 1988b. "Macrohydrology—the new challenge for process hydrology." Journal of Hydrology 100: 31-56. Sud, Y., Y. Sellers, Y. Mintz, G. Chou, G. Walker, and W. Smith. 1990. "Influence of the biosphere on global circulation and hydrologic cycle- a GCM simulation experiment." Agricultural and Forest Meteorology 52: 133-180. Sud, Y, R. Yang, and G. Walker. 1996a. "Impact of in situ deforestation in Amazonia on the regional climate: General circulation model simulation study." Journal of Geophysical Research 101: 7095-7109. Sud, Y., G. Walker, J.-H. Kim, G. Listen, P. Sellers, and W. Lau. 1996b. "Biogeophysical consequences

General Characteristics and Variability of Climate in the Amazon Basin and its Links to the Global Climate System of a Tropical Deforestation Scenario: A GCM simulation study." Journal of Climate 9: 3226-3247. Victoria, R., L. Matinelli, J. Moraes, M. Ballester, A. Krusche, G. Pellegrino, R. Almeida, and J. Richey. 1998. "Surface air temperature variations in the Amazon region and its border during this century." Journal of Climate 1: 1105-1110. Villa Nova, N., E. Salati, and E. Matsui. 1976. "Estimativa da evapotranspiragao na bacia Amazonica." Acta Amazonica 6: 215-228. Vorosmarty, C, C. Willmott, B. Choudhury, A. Schloss, T. Stearns, S. Roberson, and T. Dorman. 1996. "Analyzing the discharge regime of a large tropical river through remote sensing, ground-based climatic data, and modeling." Water Resource Research 32: 3137-3150. Vorosmarty, C. J., K. Sharma, B. Fekete, A. H. Copeland, J. Holden, J. Marble, and J. A. Lough. 1997a. "The storage and aging of continental runoff in large reservoir systems of the world." Ambio 26: 210-19Vorosmarty, C. J., M. Meybeck, B. Fekete, and K. Sharma. 1997b. "The potential impact of neo-Castorization on sediment transport by the global network of rivers." In: Human Impact on Erosion and Sedimentation, eds. D. Walling, and J.-L. Probst (IAHS Press, Wallingford U.K.), pp. 261-72.

41

Vorosmarty, C., B. Moore, A. Grace, L. Gildea, J. Melillo, B. Peterson, E. Rastetter, and P. Steudler. 1989. "Continental scale models for water balance and fluvial transport: An application to South America." Global Biogeochemistry 3: 241-265. Wagner, R. 1996. "Decadal-scale trends in mechanisms controlling meridional sea surface temperature gradients in the tropical Atlantic." Journal of Geophysical Research 101: 16683-16694. Walker, G., Y. Sud, and R. Atlas. 1995. "Impact of the ongoing Amazonian deforestation on local precipitation: A GCM simulation study." Bulletin American Meteorological Society 76: 346-361. Zeng, N., 1999"Seasonal cycle and interannual variability in the Amazon hydrologic cycle." Journal of Geophysical Research. 104, 9097-9106. Zeng, N. 1998: "Understanding climate sensitivity to tropical deforestation in a mechanistic model." Journal of Climate 11: 1969-1975. Zeng, N., R. Dickinson, and X. Zeng. 1996. "Climatic impact of Amazon deforestation—A mechanistic study." Journal of Climate 9: 859-883. Zhou, J., and K.M. Lau. 1999. "Interannual and decadal variability of summer rainfall over South America." Journal of Climate (accepted).

3 The Atmospheric Component of Biogeochemical Cycles in the Amazon Basin Paulo Artaxo

Tropical forests, with their high biological activity, have the potential to emit large amounts of trace gases and aerosol particles to the atmosphere. The accelerated development and land clearing that is occurring in large areas of the Amazon basin suggest that anthropogenic effects on natural biogeochemical cycles are already occurring (Gash et al. 1996). The atmosphere plays a key role in this process. The tropics are the part of the globe with the most rapidly growing population, the most dramatic industrial expansion and the most rapid and pervasive change in land use and land cover. Also the tropics contain the largest standing stocks of terrestrial vegetation and have the highest rates of photosynthesis and respiration. It is likely that changes in tropical land use will have a profound impact on the global atmosphere (Andreae 1998, Andreae and Crutzen 1997). A significant fraction of nutrients are transported or dislocated through the atmosphere in the form of trace gases, aerosol particles, and rainwater (Keller et al. 1991). Also the global effects of carbon dioxide, methane, nitrous oxide, and other trace gases have in the forest ecosystems a key partner. The large emissions of isoprene, terpenes, and many other volatile organic compounds could impact carbon cycling and the production of secondary aerosol particles over the Amazon region. Vegetation is a natural source of many types of aerosol particles that

play an important role in the radiation budget over large areas (Artaxo et al. 1998). There are 5 major reservoirs in the Earth system: atmosphere, biosphere (vegetation, animals), soils, hydrosphere (oceans, lakes, rivers, groundwater), and the lithosphere (Earth crust). Elemental cycles of carbon, oxygen, nitrogen, sulfur, phosphorus, and other elements interact with the different reservoirs of the Earth system. The carbon cycle has important aspects in tropical forests due to the large amount of carbon stored in the tropical forests and the high rate of tropical deforestation (Jacob 1999). In Amazonia there are two very different atmospheric conditions: the wet season (mostly from November to June) and the dry season (July-October) (see Marengo and Nobre, this volume). Biomass burning emissions dominate completely the atmospheric concentrations over large areas of the Amazon basin during the dry season (Artaxo et al. 1988). In the wet season, a very clean atmosphere shows very low concentrations for most of trace gases and aerosol concentration (Artaxo et al. 1990).

Trace Gases Relevant to the Biogeochemical Cycles in Amazonia Nitrogen and oxygen account for 99% of the Earth's atmosphere. Their concentrations have stayed nearly constant over the

The Atmospheric Component of Biogeochemical Cycles in the Amazon Basin

43

last several hundred million years. Anthro- tant source of CO2, and the average soil pogenic influences are altering the concen- respiration rate depends on soil moisture tration of most of the other relevant gases, and temperature. Biomass burning influences which occur in only trace amounts. Carbon significantly soil respiration rates for the dioxide (CO2) is chemically inert in cerrado and savanna ecosystems (Matson the troposphere, with a lifetime of about and Harriss 1995). 105 years. Methane (CH4) has an atmospherMethane (CH4) has important soil, water, ic lifetime of about 9-6 years and is chemi- and vegetation emissions, and it is the second cally active and emitted in large amounts most important greenhouse gas after CO2. from the flooded forest, from swamps, and Biomass burning of cerrado and tropical forest through biomass burning. Carbon monoxide is an important global source of methane. The (CO) is emitted through incomplete combus- CH4 emission rates for soils depend strongly tion and in soil processes, whereas oxides of on soil moisture. Large positive and negative nitrogen (NO, NO 2 , N2O) are also emitted as fluxes were observed for savannas in Africa, a consequence of biogenic processes in soils depending on flooding condition and ecosysand through biomass burning (Warneck and tem type. Termite and enteric fermentation in Zellner 1999). The tropics present the highest large herbivores are also a significant source of concentrations of the radical OH, because of methane (Jacob 1999). the high water vapor and UV radiation Globally, the oxides of nitrogen, NO (nitric (Andrea and Crutzen 1997). As OH is respon- oxide), NO2 (nitrogen oxide), and N2O sible for most of the removal mechanism of (nitrous oxide), are key species involved in the O3, CH4, CO, and other trace gases from the chemistry of the troposphere and stratosphere. atmosphere, it influences significantly tropi- NO and N2O are produced mostly by microcal trace gas concentrations (Jacob 1999, bial soil activity, whereas biomass burning is Brasseur et al. 1999). Tropical savannah soils also an important source of NO. Nitric oxide is are important sources of many trace gases a species involved in the photochemical prosuch as CO2, CH4, N2O, and NO (Sanhueza duction of ozone in the troposphere, is et al. 1994). After emissions some of these involved in the chemical production of nitric gases are chemically transformed under the acid, and is an important component of acid precipitation. Nitrous oxide plays a key role in very active tropical atmosphere. Carbon dioxide (CO2) is the most impor- stratospheric ozone depletion and is an important trace gas in the forest ecosystem, and it tant greenhouse gas, with a global warming is subject to strong and complex biogeo- potential more than 200 times that of CO2. Carbon monoxide (CO) strongly influences chemical interactions (Houghton et al. 1998). CO2 participates in respiration and photo- the concentration of the radical OH in the synthesis and it is the driving force behind tropical atmosphere. CO oxidation can lead to most of the biological processes in the soil either production or destruction of ozone, and vegetation. Several short-term recent depending on the NOX mixing ratio. Tropical experiments show an accumulation of car- soils are either a sink or a weak source of CO, bon in the forest ecosystem. More long-term where photochemical oxidation of methane measurements and better integration of car- and other hydrocarbons and biomass burning bon flux measurements with other ecosystem emissions are the predominant CO sources. Tropospheric ozone (O3) is a gas that has and climatic measurements are needed to better address the issue of whether tropical no direct emission sources and is the third forest is a sink or a source of CO2 to the most important greenhouse gas after CO2 atmosphere. Soil respiration is also an impor- and CH4. The lifetime of O3 in the

44

Paulo Artaxo

atmosphere is on the order of days to weeks, observed in regions very far from the emisleading to a highly variable temporal and sions, such as tropical Pacific and southern spatial distribution. The chemical precursors Atlantic oceans. of O3 are hydrocarbons (including CH4 and nonmethane hydrocarbons-NMHCs), CO, The Role of Aerosols in Biogeochemical and the oxides of nitrogen (NOX). NOX play Cycles in Amazonia a particularly important role in the ozone budget, since their abundance determines Aerosol particles are important to the if the photochemical oxidation of hydrocar- Amazon ecosystem because of several bons and CO results in net O3 production or roles they play in the atmosphere and in destruction. During the biomass burning biogeochemical cycles. Aerosol particles are period in Amazonia, ozone concentrations responsible for the airborne transport of reach 60-90 ppb and may actually become phosphorus, calcium, sulfur, nitrogen comtoxic to vegetation. Concentrations of ozone pounds, and other essential nutrients. in Amazonia during the wet season are very Aerosols also act as cloud condensation low, in the range of 8-12 ppb at midday. nuclei affecting the cloud formation mechaNMHCs are an important emission nisms (Rogers et al. 1992). Aerosols affect category for tropical forests. Guenther et al. regional and global climate through their (1995) estimate that vegetation is the source radiative properties, reducing the amount of of over 90% of all NMHC in the global atmos- solar radiation that reaches the ecosystem. phere and Amazonia is the largest contribu- The forests emit large amounts of aerosol tor. The reason for this large natural particles to the atmosphere as part of their biogenic emission is still unknown. This natural metabolism. During the wet season, category includes isoprene, terpenes, and when no biomass burning occurs, aerosol hundreds of volatile organic compounds. concentrations for particles less than 10 pm There are large variations in emission rates are about 10-20 pg/m3. Aerosol particles in for different plant species and different land- forest regions are also formed as secondary scapes. Some recent studies suggest that a products from natural emissions of biogenic plant emits isoprene as part of the leaf tem- hydrocarbons, such as terpenes. A large perature regulation mechanism. Terpenes are fraction of the aerosol mass (70-85%) is in the know to form aerosol particles via efficient form of organic matter, transporting mechanisms and are thus responsible for a significant amounts of carbon in the form of significant part of the total organic aerosol many different organic compounds. mass. Several NMHCs have mass fractions in Vegetation has long been recognized as an the aerosol and gas phases (Hewitt 1998, important source of both primary and Andreae and Crutzen 1997). secondary aerosol particles. Forest vegetation Biomass burning is an important global is the principal global source of atmospheric source of hydrogenated species (e.g. CH4, organic particles, and in a tropical forest NMHCs, CH3C1, and CH3Br), as well as other natural vegetation plays a major role in trace gases such as CO (Crutzen and Andreae airborne particle concentrations. Only a few 1990). These species are produced predomi- studies of natural biogenic aerosols from nantly in smoldering conditions characterized vegetation in tropical rain forests have been by insufficient O2 supply. Methyl bromide undertaken (Artaxo et al. 1988, 1990, 1994, and methyl chloride are important in stratos- 1998, Echalar et al. 1998). Natural biogenic pheric ozone depletion. Ozone secondarily aerosols consist of many different types of produced by biomass burning emissions is particles, including pollen, spores, bacteria,

The Atmospheric Component of Biogeochemical Cycles in the Amazon Basin

algae, protozoa, fungi, fragments of leaves, excrement, and fragments of insects. This aerosol component is mainly in the coarsesize fraction (dp > 2 pm). The mechanisms of particle emission are still not well

45

understood, but probably include mechanical abrasion by wind, biological activity of microorganisms on plant surfaces and forest litter, and plant physiological processes such as transpiration and guttation. These process-

Table 3.1 Elemental composition of natural biogenic particles in the Amazon basin*. Cuiaba Wet Season Coarse Mode Fine Mode

Alta Floresta Wet Season Coarse Mode Fine Mode Mean

Standard

Mean

3.5

16.4

-

9.4 105

3.9

4.6 170 2.3 94 8.0 2.1 5.5 0.53 24

0.41 110 23 57 7.4 130 1.3 74 7.2 1.7 3.2 0.46 23

0.72 33 96 2.3 170 1.03 80 12.0 3.4 0.49 1.2 0.57

1.2

1.7

0.69 1.19

0.99 0.72 0.12 0.10 0.08 0.16 0.42 0.43 0.14

Variable

Mean

Standard

Mass** BC** Mg Al Si P S Cl K Ca Ti V Cr Mn Fe Ni Cu Zn Ga Ge As Se Br Rb Sr Zr Au Pb

5.5 0.66

103 27 74

0.10 0.12 0.19 0.12 1.37 0.53 0.21 0.81 0.18 0.52

.68

0.13 0.30

103 230 270 86 140 41 270 44 17 1.05 2.21 3.0 190 0.49 1.2 2.3 0.59 0.233 0.24 0.21 0.68 0.80 0.67 0.90 0.29 1.0

240 230 56 103 24 180 28 15 0.76 -

3.1 190

Mean

Standard

1.8

11.7

0.33

-

5.7 270 590 13 39 9.1 100 120 26 1.4 0.88 2.5 470 0.40 3.5 8.3 -

Standard

30 67 1.4 110 0.93 44 6.5

2.6 0.43

2.3

1.16

3.4 790

82

60

0.19 0.73

1.6

2.0

0.76 0.015 0.17 0.30 0.46 0.54 0.54 0.29 0.18 1.4

-

-

0.10 0.72 1.1 0.062 0.30 0.21 0.14 0.055 -

1.24

0.86

0.104 0.64 0.32 0.16 0.104

36 1.6

0.37

0.48 1.7

-

407 970 18 65 14.3 200 180

0.55 2.4 4.5 0.68 0.91 0.96 1.44 3.8

0.51 0.68 0.68 0.92 -

3.0

* Averages and standard deviations are presented for the atmospheric concentration (in ng m~3) of the participate mass, black carbon, and aerosol trace elements in samples collected in Alta Floresta from August 92 to March 95, and in Cuiaba from July 90 to August 95. ** Aerosol mass and black carbon (BC) are expressed in ug m~3.

46

es may generate particles containing biogenic-related elements such as Na, Mg, P, S, K, Ca, Zn, and Rb. Also, a significant fraction of aerosols is comprised of secondary aerosols, formed by gas-to-particle conversion of organic and sulfur-related natural biogenie gases. These biogenic particles are mostly submicrometer in size. Bacteria in forested areas were found in the size range of 0.5 to 2.5 pm. Biological activity of microorganisms on leaf surfaces and forest litter results in airborne particles. Windblown pollens certainly contribute to coarse particles in forested areas. The transpiration of plants can lead to migration of Ca, SO4, Cl, K, Mg and Na to the atmosphere. The biogenic-related elements (K, P, S, Zn, Rb, and others) are essential to superior plants. They are present in the fluids circulating in the plant and are released from the leaves to the atmosphere. Natural biogenic aerosol particles emitted by plants play an important role in nutrient cycling in tropical ecosystems. Tropical ecosystems maintain a delicate nutrient balance characterized by intense internal recycling and depend on atmospheric input of certain mineral nutrients to fulfill certain nutrient requirements. It has been shown that biogenic particles may influence cloud properties (Schnell 1982) and can act as cloud condensation nuclei, potentially affecting cloud formation mechanisms and cloud dynamics. These particles can travel for long distances (Talbot et al. 1990, Artaxo et al. 1998). Table 3.1 shows the average elemental composition of natural biogenic aerosol particles collected in two background sampling stations in Amazonia (Artaxo et al. 1998, Echalar et al. 1998). The first site (Alta Floresta) is a forest site, and the second (Cuiaba) is a cerrado vegetation site. The average aerosol mass and black carbon concentration is also presented in Table 3.1. It is possible to observe that most of the elements occur predominantly in the coarse

Paulo Artaxo

mode, with sulfur being the only exception. The aerosol mass for particles less than 10 pm is 15-20 pg/m3, a very significant aerosol mass. Phosphorus is strongly present in the coarse mode with more than 90% of P concentrations in particles larger than 2 pm. This means that the cycling of phosphorus is mostly local or regional. Figure 3.1 shows the time series of aerosol mass concentrations for Alta Floresta from 1991 to 1998. Alta Floresta is in the northern part of Mato Grosso state and is a place where heavy biomass burning occurs from July to October. The large increase in aerosol concentrations during the biomass burning period is remarkable. Background aerosol concentrations of about 10-20 pg/m3 of particles less than 10 pm in size during the wet season can increase to 400-500 pg/m3 during the dry season. Even if nutrients comprise only a small fraction of this aerosol fraction, it can represent a significant mass fraction of the elements that are lost from the ecosystem. The natural biogenic aerosol component is comprised of particles that can travel for thousands of kilometers and has an atmospheric residence time of 2-10 days. The composition of these natural biogenic aerosol particles can be observed in Table 3.1, for particles collected in Alta Floresta and Cuiaba and for the fine and coarse mode fractions. We observe that phosphorus, for example, shows concentrations of about 90 ng/m3 in Alta Floresta, with 80% of this concentration in the coarse mode fraction. The coarse fraction has mostly local or regional impact. In contrast, Table 3.2 shows the average elemental concentration for aerosols during the biomass burning period. Concentrations presented in Table 3-2 come from samples collected in the INPE Bandeirante aircraft during the SCAR-B experiment and also ground-based concentrations for Alta Floresta and Cuiaba. Sulfur can reach an average of 1000 ng/m3 during

The Atmospheric Component of Biogeochemical Cycles in the Amazon Basin

47

Alta Floresta Aerosol Mass Concentration

Fig. 3.1 Time series of PM10 aerosol mass concentrations in pg/m for Alta Floresta from 1992 to 1998. Aerosol mass concentration are for particles less than lOum. the dry season, compared with 300 ng/m3 et al. 1994, Echalar et al. 1995, Andreae et al. during the wet season. For the past 20 years 1998). Trace gas emissions in the burning of most of the tropical forest areas have been African savannah and the Brazilian cerrado under strong pressure by rapid change of are also similar (Scholes et al. 1996, Scholes land cover, with forest and adjacent savannas and Andreae 2000). The pyrogenic particles being cleared, most of the time through the are efficient cloud condensation nuclei use of fire, and converted to pasture and (CCN) (Rogers et al 1992). The abundance of agricultural fields at a substantial rate (Artaxo CCN in the dry season over Amazonia and Africa certainly has a strong effect on cloud et al. 1998). Pyrogenic and natural biogenic emissions formation and precipitation patterns, even to the atmosphere in the Amazon basin may for regions far from biomass burning, but this have an impact on the global tropospheric issue has not yet been properly addressed. The aerosols emitted during biomass chemistry, because this region exhibits intense convective activity that injects gases burning can travel long distances. Figure 3.2 and aerosols to high altitudes where they can shows a three-dimensional trajectory analysis be transported over long distances (Andreae for aerosol emissions during the dry season et al. 1996, 1998, Echalar et al. 1998). (Freitas et al. 1997). From each biomass Elemental concentrations in aerosols from burning spot, the air masses that transport Amazonia and Africa are very similar (Artaxo the aerosol particles and trace gases stay at

48

Paulo Artaxo

Table 3.2 Elemental composition of biomass burning aerosols in the Amazon basin measured during the SCAR-B experiment and comparison with long-term ground-based measurements at Alta Floresta and Cuiaba. SCAR-B Mean

Standard Deviation

SCAR-B Minimum

SCAR-B Maximum

Alta Floresta Dry Season

Cuiaba Dry Season

Na

95.4

59.9

30.0

659

385

_ -

2292 3126

1808 2104

196 350 713

270 2356 7297 8673

.

Mg AI Si P S Cl K Ca Ti V Cr Mn Fe Cu Zn Rb Sr Br Ga As Zr Mass* BC*

140

136 900

559

122 1130

Element

(ng nr3)

1198 222

1581 1251 121 3.67 5.14 67.7 1132 2.90 10.7 8.82 16.4 13.1 0.49 0.21 6.10 107 5.49

229 1076

1134

123 3.43 1.86 64.4

985 1.71

6.3 5-32 14.2 8.7 0.34 0.16 3.69 60.7 3.92

(ng nr3)

3.50 59.0 27.6 137.3 36.2 9.29 0.54 2.29 10.8 76.9 0.64 1.92 1.55 1.44 2.40 0.01 0.041 2.08 8.05 0.23

(ng nr3)

4333 1291 5581 5722

533 14.2 8.16 322 4327 8.77 27.7 25.9 59.5 36.9 1.46 0.85 15.7 297 17.5

(ng nr3)

1130 2540 2620

125

1590 270 162 7.1 41.6 18.8 1623

7.9 13.1

(ng m-3)

1342 2930 21.7 560 36 1020 489 130.8 6.37

4.6 16.5 1990 5.91 11.85

10 5.4

-

19.0 2.36

6.1 -

2.7 8.2 81.2

5.73

3.34

5.86 49-1

2.6

3

Aerosol mass and black carbon (BC) are expressed in pg m' .

low altitude (below 3000 meters) in the Amazon region. When an air mass reaches the Andes, it will rise to higher altitudes because of orographical effects and will reach the Pacific Ocean in only a few days. In the south of Brazil, when Amazon-derived air masses encounter cold fronts they can rise to very high altitudes of about 10 kilometers where atmospheric transport is fast and very efficient. High ozone and aerosols in the South Atlantic were observed during August-

September as a result of biomass burning in Amazonia and Africa (Andreae et al. 1996). Talbot et al. (1990) and Swap et al. (1995) have shown the chemistry changes associated with long-range transport of aerosols in the Amazon basin. Also, the Amazon region could receive a large influx of trace elements from Sahara dust and tropical Africa (Swap et al. 1995). The magnitude of this flux that can carry essential nutrients to the Amazon basin is still unknown.

The Atmospheric Component of Biogeochemical Cycles in the Amazon Basin

The Role of Wet Deposition in Amazonia The Amazon basin is characterized by high rainfall rates, and wet deposition could be significant in the biogeochemical cycles of several nutrients (Andreae et al. 1990). The biogeochemistry of trace elements in the atmosphere is influenced by different sources of gases and aerosols. During a rain event, these gases and aerosols are incorporated into raindrops, and a significant amount of these aerosols are cloud processed, and incorporated into cloud droplets from the very beginning. During the Amazonian wet season, rainwater chemistry is dominated by natural biogenic emissions, consisting mostly of

49

organic acids, potassium, phosphorus, and other components (Lesack and Melack 1996). In the wet season, average pH is about 5.0 (Williams et al. 1997, Andreae et al. 1990). During the biomass burning period, rainwater pH decreases to about 4.5, mostly because of enhanced concentrations of organic acids, ammonia, and nitrates. Williams et al. also measured DOC (Dissolved Organic Carbon) at average values of 138 and 238 pM for the dry and wet seasons, respectively. The annual volume weighted mean of 159 pM implies an annual deposition of about 0.4 moles C nr2. The acidity was basically determined by the concentrations of organic acids. The concentration of acetate was 9-3 peq H and formate was 2.9 peq H. The average conductivity was

Fig. 3.2 Three dimensional forward air mass trajectories starting from biomass burning spots on August 22, 1985, during the SCAR-B experiment. Shades of gray indicate elevation in meters.

50

Paulo Artaxo

Table 3.3 Rainwater composition at Lake Calado, West of Manaus. Volume Weighted Mean ((eqH) Solute

H pH NH4 Na K Ca Mg Cl SO4 NO3 P04 TDP (/tM)*

Annual

Dry Season

17.0

4.7 3.0 2.4 0.8 2.4 0.9

Wet Season

Annual

Dry Season

Wet Season

31.7

11.2

46.8

24.7

22.1

4.5 7.4 3-4

5.0 1.2 2.1 0.7 2.4 1.0 4.6

8.2 6.7 2.3 6.6 2.5

-

1.6 3.3

5.4

5.8 2.6 0.9 1.9 0.5 3.5 2.2 5.0 0.05

0.04

0.4

0.3

2.0 4.2

1.2 2.4 0.6 4.5 2.8 6.4

0.03 0.25

0.06 0.47

4.6

Deposition (meq nr2)

0.02 0.15

12.6 11.5 0.09

0.7

2.4 4.1 1.4 4.7 2.0 9.1 3.2

6.5

Source: Adapted from Williams et al. 1997. * TDP is the total dissolved phosphorus.

6.5 pScrrr1, with larger conductivity during the dry season. Williams et al. (1997) observed that solute deposition occurred in proportion to total rainfall, suggesting that there was a rather constant source of aerosols in the atmosphere throughout the year. Andreae et al. (1990), during two campaigns in the 1987 wet season and the 1985 dry season during the ABLE experiments, measured similar rainwater concentrations. Significant marine influences were observed in the rainwater collected near Manaus, indicating the long-range transport of marine aerosols within the Amazon basin. A large fraction of the sulfate content of wet season rain appears to be of marine origin. The emissions of biogenic sulfur and nitrogen species from the rain forest ecosystem and their subsequent oxidation in the atmosphere appear to make only a small contribution to the deposition flux of sulfate and nitrate in central Amazonia. Forti et al. (1991) analyzed samples of rainwater and throughfall during ABLE-2B for a site near Manaus (Reserva Ducke). The throughfall was enriched 40-90% compared with the rainwater for Mg, K, Na, Ca, Cl, and SO4. Absorption of NH4+ by the

vegetation was also observed during the dry period. These values are similar to rainwater composition in Africa (Freydier et al. 1998), where a pH value of 5.0 was observed. In the tropical forest of Costa Rica, Eklund et al. (1997) also identified organic acids as the major determinant of precipitation pH, but they observed a higher marine aerosol component than that in Central Amazonia. Measurement of total nutrient deposition requires studies integrating rainfall, throughfall, and stemflow in the wet component, as well as aerosol dry deposition. No studies in Amazonia have yet integrated all these components. Throughfall studies indicate a large enrichment of potassium, on the order of 2.7 to 10.2, leached from plant leaves (Holscher et al. 1998). This large variation in potassium enrichment could be caused by different floristic composition and precipitation patterns.

Conclusions The atmosphere in tropical forests is a very active part of biogeochemical cycles that

The Atmospheric Component of Biogeochemical Cycles in the Amazon Basin

include the soil, vegetation, and hydrological pathways. Trace gases, aerosol particles, and rainwater interact with the system in a variety of ways. Atmospheric transport extends local or regional processes to far larger scales, reaching even to global scales. Integrated studies involving wet and dry deposition fluxes for key nutrients in Amazonia and other tropical areas are needed. Also the issue of how CCN affects cloud formation mechanisms in the dry season must be more thoroughly investigated to assess the impact of anthropogenic influences in tropical forests. A flux of trace elements and nutrients from Africa to Amazonia and from Africa to the Indian Ocean and Australia has been well

51

documented qualitatively for many years. Efforts in quantifying these fluxes are very important for improving our understanding of how tropical ecosystems use and exchange key nutrients. Acknowledgements: I thank Andi Andreae for providing very useful unpublished manuscripts, and for very useful discussions over the last two years in the chapter's theme. Thanks are due to Alcides C. Ribeiro, Ana L. Loureiro, Tarsis Germano, and LAMFI staff for assistance during aerosol sampling and PIXE analysis. This work was financed through grant 97/11358-9 from FAPESP-Fundacao de Amparo a Pesquisa do Estado de Sao Paulo.

Literature Cited Andreae, M. O., R. W. Talbot, H. Berresheim, and K. M. Beecher. 1990. "Precipitation chemistry in Central Amazonia." Journal of Geophysical Research 95, D10: 16987-16999. Andreae, M. O., J. Fishman, and J. Lindesay. 1996. "The Southern Tropical Atlantic Region Experiment (STARE): TRansport and Atmospheric Chemistry near the EquatorAtlantic (TRACE-A) and Southern African Fire/ Atmosphere Research Initiative (SAFARI): An introduction." Journal of Geophysical Research 101: 23519-23520. Andreae, M. O., and P. J. Crutzen. 1997. "Atmospheric aerosols: biogeochemical sources and role in atmospheric chemistry." Science 276: 1052-1058. Andreae, M. O. 1998. "Feedbacks and interactions between Global change, atmospheric chemistry and the biosphere." Paper presented at the Workshop on GeosphereBiosphere Interactions and Climate. Pontifical Academy of Sciences, Vatican City, 9-13 November 1998. Andreae, M. O., T. W. Andreae, H. Annegarn, F. Beer, H. Cachier, W. Elbert, G. W. Harris, W. Maenhaut, I. Salma, R. Swap, F. G. Wienhold, and T. Zenker. 1998. "Airborne studies of aerosol emissions from savanna fires in southern Africa: 2. Aerosol chemical composition." Journal of Geophysical Research 103: 32119-32128. Artaxo, P., H. Storms, F. Bruynseels, R. Van Grieken, and W. Maenhaut. 1988. "Composition and sources of aerosols from the Amazon Basin." Journal of Geophysical Research 93: 1605-1615. Artaxo, P., W. Maenhaut, H. Storms, and R. Van Grieken. 1990. "Aerosol characteristics and sources for the Amazon Basin during the wet season." Journal of Geophysical Research 95: 16971-16985.

Artaxo, P., F. Gerab, M. A. Yamasoe, andj. V. Martins. 1994. "Fine Mode Aerosol Composition in Three Long Term Atmospheric Monitoring Sampling Stations in the Amazon Basin." Journal of Geophysical Research 99, Dll: 22857-22868. Artaxo, P., E. T. Fernandes, J. V. Martins, M. A. Yamasoe, P. V. Hobbs, W. Maenhaut, K. M. Longo, A. Castanho. 1998. "Large Scale Aerosol Source Apportionment in Amazonia." Journal of Geophysical Research 103, D24: 31837-31848. Brasseur, G., J. J. Orlando, and G. S. Tyndall. 1999. Atmospheric Chemistry and Global Change-Topics in Environmental Chemistry, Oxford University Press, New York. Crutzen, P., and M. O. Andreae. 1990. "Biomass burning in the tropics: Impact on atmospheric chemistry and biogeochemical cycles." Science 250: 1669-1678. Echalar, F., A. Gaudichet, H. Cachier, and P. Artaxo. 1995. "Aerosol emissions by tropical forest and savanna biomass burning: characteristic trace elements and fluxes." Geophysical Research Letters 22: 3039-3042. Echalar, F., P. Artaxo, F. Gerab, M. A. Yamasoe, J. V. Martins, K. M. Longo, W. Maenhaut, and B. N. Holben. 1998. "Aerosol composition and variability in the Amazon basin." Journal of Geophysical Research 103, D24: 31849-31866. Eklund, T. J., W. H. McDowell, and C. M. Pringle. 1997. "Seasonal variation of tropical precipitation chemistry: La Selva, Costa Rica." Atmospheric Environment 31, 23: 3903-3910. Freitas, S. R., K. M. Longo, M. A. F. Silva Dias, and P. Artaxo. 1997. "Numerical modeling of air mass trajectories from the

52

biomass burning areas of the Amazon basin." Annais da Academia Brasileira de Ciencias 68: 193-206. Forti, M. C., and L. M. Moreira-Nordemann. 1991. "Rainwater and through/all chemistry in a Terra Firme' rain forest: Central Amazonia," Journal of Geophysical Research 96: 7415-7421. Freydier, F., B. Dupret, andj. P. Lacaux. 1998. "Precipitation chemistry in intertropical Africa." Atmospheric Environment 32: 749-765. Gash, J. H. C., C. A. Nobre, J. M. Roberts, and R. L. Victoria. 1996. "Amazonian Deforestation and Climate, John Wiley & Sons, England. Guenther, A., C. N. Hewitt, D. Erickson, R. Fall, C. Geron, T. Graedel, P. Harley, L. Klinger, M. Lerdau, W. A. McKay, T. Pierce, B. Scholes, R. Steinbrecher, R. Tallamraju, J. Taylor, and P. A. Zimmerman. 1995. "Global model of natural volatile organic compound emissions." Journal of Geophysical Research 100: 8873-8892. Holscher, D., T. D. de A. Sa, F. F. Moller, M. Denich, and H. Folster. 1998. "Rainfall partitioning and related hydrochemical fluxes in a diverse and in a mono specific secondary vegetation stand in Eastern Amazonia." Oecologia 114: 251-257. Hewitt, C. N. 1998. Reactive Hydrocarbons in the Atmosphere, Academic Press, San Diego. Houghton, R. A., E. A. Davidson, and G. M. Woodwell. 1998. "Missing sinks, feedbacks, and understanding the role of terrestrial ecosystems in the global carbon balance." Global Biogeochemical Cycles 12: 25-34. Jacob, D. 1999. Introduction to Atmospheric Chemistry, Princeton University Press, New Jersey. Keller, M., D. J. Jacob, S. C. Wofsy, and R. C. Harriss. 1991. "Effects of tropical deforestation on global and regional atmospheric chemistry." Climatic Change 19: 139-158. Lesack, L. F. W., and J. M. Melack. 1996. "Mass balance of major solutes in a rainforest catchment in the Central

Paulo Artaxo Amazon: Implications for nutrient budgets in tropical rainforests." Biogeochemistry 32: 115-142. Matson, P. A., and R. C. Harriss. 1995. Biogenic Trace Gas Emissions, Blackwell, Oxford, p. 384. Rogers, C. E, J. G. Hudson, B. Zielinska, R. J. Tanner, J. Hallett, and J. G. Watson. 1992. "Cloud condensation nuclei from biomass burning." In: Global Biomass Burning: atmospheric, climatic, and biospheric implications, ed. J. Levine (MIT press, Cambridge, MA). Sanhueza, E., L. Cardenas, L. Donoso, and M. Santana. 1994. "Effect of plowing on CO2, CO, CH4, N2O, and NO fluxes from tropical savannah soils." Journal of Geophysical Research 99: 16429-16434. Schnell, R. C. 1982. "Kenyan lea litter: A source of ice nuclei." Tellus 34: 92-95. Scholes, M., and M. O. Andreae. 2000. "Biogenic and pyrogenic emissions from Africa and their impact on the global atmosphere." Ambio 29:23-29. Scholes, R. J., D. Ward, and C. O. Justice. 1996. "Emissions of trace gases and aerosol particles due to vegetation burning in southern-hemisphere Africa," Journal of Geophysical Research 101: 23677-23682. Swap, R., M. Garstang, S. Macko, P. Tyson, W. Maenhaut, P. Artaxo, P. Kallberg, and R. Talbot. 1996. "The long-range transport of southern African aerosols to the tropical South Atlantic." Journal of Geophysical Research 101: 23777-23792. Talbot, R. W., M. O. Andreae, H. Berresheim, P. Artaxo, M. Garstang, R. C. Harriss, K. M. Beecher, and S. M. Li. 1990. "Aerosol chemistry during the wet season in central Amazonia: The influence of long-range transport." Journal of Geophysical Research 95: 16955-16970. Warneck, P., and R. Zellner. 1999. Global Aspects of Atmospheric Chemistry, Dietrich Steinkopf, Germany. Williams, M. R., T. Fisher, and J. M. Melack. 1997. "Chemical composition and deposition of rain in the central Amazon, Brazil." Atmospheric Environment 31, 2: 207-217.

4 Soil versus Biological Controls on Nutrient Cycling in Terra Firme Forests Elvira Cuevas

Terra firme forests are those that by definition are not permanently or seasonally flooded (terra firme meaning "firm terrain"). This type of forest encompasses the Amazon and Orinoco basins, stretching from the lower slopes of the Andes, east to the Guianas, and south to about 15°S in western Brazil and northern Bolivia (Richards 1996). Structural and compositional variability in these forests in the Amazon basin is very wide as a result of climate differences and geomorphological position. The region is not climatically uniform; the central and much of the southern parts have less and more seasonal rainfall than the eastern and western parts (Walsh 1996). These differences have direct and indirect ecological significance, as phenology and biological processes related to nutrient availability will be strongly influenced by both factors (Cuevas and Medina 1986, 1988, 1990, Medina and Cuevas 1989). Periods of two or more consecutive dry days are ecologically significant in a humid area such as San Carlos de Rio Negro, in the northern part of the Amazon, because of low water retention capacity in the widespread sandy soils. In lower geomorphological positions, dry spells of 5-10 days may result in fluctuations of the water table from 0.4-1.0 m (Herrera 1977, Bongers et al. 1985). In areas with a more strongly seasonal climate, roots have been found extending to 18 m depth (Nepstad et al. 1995). This may explain the

presence of evergreen forest in the seasonally dry eastern Amazon. Structure and physiognomy of terra firme forests is very similar throughout Amazonia, but floristically it is quite variable due to different compositions in the subbasins of the Amazon's major tributaries. These subbasins are located within geochemical regions that can be differentiated based on the physicochemical properties of drainage waters (Sioli 1975, Fittkau 1971, Fittkau et al. 1975). Blackwater rivers, such as the Rio Negro, drain mostly sandy podsolized soils low in most essential nutrients for plant growth. They are characterized by a high content of humic acids, which remain dissolved because of the predominant low concentrations of polyvalent cations, mainly Ca2+ and Mg2+. Whitewater rivers, on the other hand, have a higher sediment content and are relatively rich in cations because they drain geologically younger Andean areas. Clearwater rivers, which are common in the central Amazon region, also drain nutrient-poor soils. On a small-scale basis, the composition of mixed forests is related to environmental factors such as topography, drainage, and soil characteristics (Medina and Cuevas 1989, Steege 1993, Duivenvoorden and Lips 1995, Brouwer 1996, Richards 1996). The history of erosion and redeposition in the Amazon basin has led to an extensive leaching of nutrients, particularly the common

54

cations K+, Ca2+, and Mg2+ and inorganic phosphate reserves (Fittkau et al. 1975, Furch and Klinge 1989). This impoverishment is particularly intensive in the central and northern areas of the basin characterized by annual rainfall around or above 2000 mm (Salati et al. 1978, Matsui et al. 1982). Terra firme forests growing on Oxisols/ Ultisols seem to be primarily limited by phosphorus (Vitousek 1984, Vitousek and Sanford 1986, Cuevas and Medina 1986, Cuevas and Medina 1988), while at the same time being tolerant of high soil concentrations of mobile aluminum (Sprick 1979, Sobrado and Medina 1980). Cuevas and Medina (1988) showed that terra firme forests can also be limited by calcium and magnesium, which corroborated the hypothesis of Furch and Klinge (1989) who measured low Ca and Mg contents in biotic and abiotic compartments in terra firme forests of Amazonia. Plant adaptation to these conditions requires highly efficient uptake and/or utilization of nutrients, especially phosphorus, calcium and magnesium (Marschner 1995). Biomass development (Jordan 1985) and regeneration capacity (Uhl 1987) are not always clearly associated with differences in soil fertility, but they are certainly related to rates and patterns of nutrient cycling (Vitousek and Sanford 1986, Medina and Cuevas 1989, Tiessen et al. 1994b). On poor soils, nutrients may cycle without substantial losses from the system (Baillie 1989, Burnham 1989). In such dystrophic systems, organic matter and particularly the forest litter mat may play an essential role in conserving nutrients for sustaining forest production (Stark and Jordan 1978). Nutrient recycling and supply is tightly regulated by largely biotic processes, such as retranslocation or resorption prior to leaf abscission (Medina and Cuevas 1989, Cuevas and Medina 1990). Nutrients are also reutilized from decomposing residues through rapid mineralization and uptake by a dense

Elvira Cuevas

fine-root mat, or ectorganic horizon, associated with the litter layer (Cuevas and Medina 1986, Cuevas and Medina 1988). In addition, nutrient cycling and turnover seem to be tied to organic matter dynamics (Tiessen et al. 1994a), illustrating again the importance of biological processes as the main regulators of nutrient availability. Understanding the changes to soil biogeochemical cycles that influence soil fertility after clearing is a key condition for predicting ecological consequences of deforestation and for implementing effective management schemes (see Dias Filho et al., this volume). A central question to be addressed then is "what role does organic matter dynamics play in biogeochemical cycling in terra firme forests?"

Soils under Terra Firme Forests The Amazon region is dominated by old, highly weathered, leached soils due to large areas of tectonically and geomorphologically stable land surfaces (Baillie 1996). The main soils developed under these conditions, which account for around 70% of the Amazon basin (Richter and Babbar 199D, can be grouped under modal kaolisols (Baillie 1996), which include the ferralitic soils or ferralsols (FAO-UNESCO 1974), or Oxisols and Ultisols (Soil Survey Staff 1975). The classification is based on the importance of iron and aluminum in the elemental composition or of sesquioxides and kanditic aluminosilicates in the clay minerals (Bleeker 1983). Because of the advanced weathering state of these soils, quartz is the only primary mineral remaining which forms a significant sand fraction and gives a coarse texture to the soils. All other primary minerals have been weathered to low-activity, secondary clay minerals with few permanent negative charges (Buol 1985). Labile aluminum occurs either in the soil solution or adsorbed onto cation exchange complexes. These soils are acidic (pH 3-5.5), with a considerable

Soil versus Biological Controls on Nutrient Cycling in Terra Firme Forests

depletion of basic cations and very low phosphorus availability due to the high P fixing capability of the clays and co-precipitation with labile forms of aluminum and iron. Because of the weathering conditions and the predominance of low-activity clays, these soils have very low cation availability ( K+, Ca2+, and Mg2+). Clay kaolisols can also be present in some areas. pH, exchangeable base status, and available phosphate contents are still low by global standards but are slightly higher than in modal kaolisols. The ranges, however, overlap considerably and aluminum is still the dominant exchangeable cation in the soil. Total contents of potassium, magnesium, and to a lesser extent, phosphorus, are considerably higher, mostly held in non-exchangeable forms by the nonkandite aluminosilicates (Baillie 1996). The remaining, nonkaolisol, soils underlying terra firme forests are mostly podzols and white sands. These developed from very sandy and quartzose parent materials on gently sloping tops of old alluvial terraces, and on plateaux and gentle dip slopes. These soils have limited available moisture, drainage, and aeration. They also have limited supplies of all nutrients, especially nitrogen, and can have strong to moderate acidity. For a detailed description of these soils and the others mentioned above please refer to Baillie (1996). The differences in the soils presented above are related mainly to variations in their parent materials and thus could reflect the geological structure of the region. A good example is the study by Duivenvoorden and Lips (1995) in the middle Caqueta region of the Colombian Amazon. These authors distinguished between two groups of welldrained upland soils: soils of the Ali-Acrisol group occurring on clayey sediments of Andean provenance (clay kaolisols) and soils of the Acri-Ferrarsol group occurring on Palaeozoic sandstone plateaus and on deposits derived from the Precambrian Shield

55

(modal kaolisols). The parent material of the first group is less weathered and still contains appreciable amounts of clay minerals other than kaolinite, such as vermiculite (18%) and illite (10%). This is evidenced by high total Mg, K, and Na concentrations, low exchangeable bases, and higher exchangeable acidity. The second group, on the other hand, derives from highly weathered parent material with low levels of nutrient reserves, low exchangeable bases and exchangeable acidity, and an absolute dominance of kaolinite (over 95%) in the clay fraction. The conditions expressed above, which impose substrates with low to very low nutrient availability, especially the lithogenic ones, will determine nutrient limitations to plant growth. Vegetation modifies these constraints through its capacity to accumulate and concentrate carbon and nutrients.

Species Richness and Composition in Terra Firme Forests Species richness and composition can be related to soil properties (Richards 1996). Steege (1993) found that species distribution patterns and population structure in Guyana were related to soil type; however, soil drainage played an important role in species occurrence. Eperua rubiginosa, which can be found in higher numbers growing on soils with impeded drainage conditions, had a high presence on poorly drained sandy Ferralsols. The study by Duivenvoorden and Lips (1995) in the Colombian Amazon showed that patterns of tree species composition in terra firme forests depended significantly on soil properties even though the edaphic component explained only a small fraction of the variance. There were, however, no significant differences with respect to tree species density (Table 4.1). On the other hand, when density of tree species was analyzed in proportion to stem density, there was a higher species density in the relatively

56

Elvira Cuevas

richer soils. This corroborates what Baillie et al. (1987) showed for a mixed Diptercarp forest wherein properties determined by lithology of the parent material were more important for species distribution than properties such as organic matter and exchangeable cations. In fact, organic matter and exchangeable nutrient patterns are likely the products of species distributions through differing rates and amounts of nutrient return to the forest floor via litterfall and root residues. These plant inputs control the formation of soil organic matter and the magnitude of variation of the superficial soil contents of nutrients (Lugo et al. 1990, Cuevas and Lugo 1998, Tiessen et al. 1994b).

Soil Nutrients and Biomass Aboveground biomass of terra firme forests can differ by as much as 250 Mg ha"1 (Table 4.2). Medina and Cuevas (1996) proposed that differences in the range of aboveground biomass calculated by Brown and Lugo (1992) could result from the variability of landscapes, but also that they could be a result of different stages of succession after partial exploitation in the past. When taking into consideration the broad spectrum of soils and climate variation present in the locations in Table 4.2 and in the sites conidered by Brown and Lugo (1992), it is

likely that biomass differences could be explained by differences in water and nutrient availability, although successional stage can not be ruled out. The differences in amounts and distribution of total biomass reinforce the hypothesis of nutrient availability, especially P, being the overriding determinant for biomass development. Forests with similar amounts of aboveground biomass can differ up to 100 Mg ha'1 in total biomass when the belowground component is also measured (Table 4.2). The biomass contribution of roots can vary between 9 to 20% in those forests. Root/shoot ratios can range between 6 and 33, suggesting that belowground biomass may be allocated to increase nutrient and/or water uptake. Regulation of the root/shoot ratio according to growing conditions is well known (Russell 1977), as there is a functional balance between the two. Growth and maintenance of roots can exert a strong drain of carbon from the aboveground part (Marschner 1995), thus along a water and/or nutrient availability gradient, aboveground biomass will be limited towards the extreme of availability, resulting in higher carbon allocation belowground. This certainly seems to be the case presented in Table 4.2 as the terra firme forests from Venezuela tend towards the extreme of limited soil nutrient availability (Medina and Cuevas 1989).

Table 4.1 Tree species density and cumulative amount of tree species (DBH > 10 cm) in 0.1 ha nlots in well-drained uplands from the middle Caaueta areas. Soil type Ali-Acrisol Acri-Ferralsol Tree species/0.1 ha-1 Tree species/stem density Cumulative No. Tree species/19 plots of 0.1 ha Cumulative No. stems/ 19 plots of 0.1 ha Source: Duivenvoorden and Lips 1995 * n = 21 plots in the Ali-Acrisol soil, 19 in the Acri-Ferralsol soil. " Denotes significant differences between soil types.

39 ± 8.3 0.58 ± 0.09** 431 ± 9** 1307 ± 18**

37.9 ± 9.2 0.50 ± 0.10 379 1430

Soil versus Biological Controls on Nutrient Cycling in Terra Firme Forests

57

Table 4.2 Belowground and aboveground biomass, percent contribution of belowground and root/shoot ratios in terra firme forests. Location

Belowground Mg ha'*

Aboveground Mg ha"1

Total Mg ha"1

% Roots

66

407

473 541

14

6

Klinge 1976

108

20

25

Russell 1983

542

20

25 19 24

Poels 1987 Uhl and Jordan 1984

Brazil Brazil Surinam Venezuela Venezuela

108

433 434

49 57

261

310

16

234

292

20

Venezuela

61 61

185

246

25

335

396

15

423 30

465 28

9 28

Venezuela Venezuela CV (%)

42 37

Here, not only is total biomass less than in other forests, but root/shoot ratios are also considerably higher. In contrast to shoot growth, root growth is much less inhibited under P deficiency (Marschner 1995). This can be construed as a mechanism to maximize soil P uptake, especially when the element is strongly limiting for growth or when ratios among specific soil resources are unusually skewed (Lajtha and Harrison 1995). In the toposequence of San Carlos de Rio Negro, Tiessen et al. (1994b) indeed found that distributions of carbon, nitrogen, and phosphorus of total plant biomass (above- plus belowground, Medina and Cuevas 1989) follow those of soil C, N,

Root/Shoot

Source

Saldarriaga 1985 Sanford 1989 Sanford 1989 Buschbacher 1984

33 18 10 44

and available P (Table 4.3). The terra firme forests at the top and midslope had considerably lower biomass P than the Amazon Caatinga forests in the lower slopes, which related to the very low P availability of these soils (shown both in absolute amounts and in the extreme skewness of the N/P ratios of the soil). Both biomass and soil N were higher in these two sites. Available P from leaf litter and the ectorganic horizon in the terra firme forests and in leaf litter and humic horizon in the Tall Amazon Caatinga accounted for 71, 85, and 72%, respectively, of the total available P. This illustrates that available P may be closely related to the amount of organic residues present, and to

Table 4.3 Comparison of nitrogen and available phosphorus in the soil and total biomass (aboveplus belowground) for three slope positions in a toposequence in San Carlos de Rio Negro. Forest Type

Soil Type

Slope Position

SoilN (gm-2)

Biomass N (gm-2)

Soil Resin P Biomass P (gm-2) (g m-2)

Terra firme Terra firme Tall Amazon Caatinga

Plinthic hapludox Plinthic hapludox Tropaquod

Top Middle Lower

869 1333 414

1485 1817 1145

2.6 4.0 5.5

Sources: Soil data from Tiessen et al. 1994b; biomass data from Medina and Cuevas 1989.

48 55 101

58

their turnover. In situations such as this, where organic matter mineralization is the most important process providing available P, the involvement of the mineral soil is minimized. Tiessen et al. (1994a), working in the same terra firme forest, estimated a soil organic matter budget down to 60 cm depth which showed the following distribution of carbon, nitrogen, and phosphorus: 60% of C, 65% of N, and 50% of P occurred in particulate organic matter with a mean residence time of less than 4 years; 27% of C, 29% of N, and 33% of organic P were mineral-associated and had a mean residence time near 50 years. The remainder of the organic matter and associated nutrients were sequestered in lateritic nodules (Fig. 4.1). Thus, ecosystem stability is dependent on the maintenance and cycling of the faster pool of organic matter in the system.

Elvira Cuevas

Productivity and Nutrient Fluxes in Terra Forests Ecosystem productivity is an index integrating the cumulative effects of many processes and interactions which proceed simultaneously in the system. Whether or not a relation exists between soil fertility and forest productivity is still under discussion (Proctor 1992, Silver 1994, Thompson et al. 1992). There is, however, no doubt that soil fertility influences input/output budgets, and that soil fertility influences the rate of recovery of the forest ecosystem after disturbance (Bruijnzeel 1992). Measurement of litterfall and its associated fluxes of nutrients is a practical way to evaluate production capacity and availability of nutrients in tropical forests. Mass and nutrient fluxes of litterfall in terra firme forests can vary considerably according to specific loca-

Fig. 4.1 Organic matter stocks in soil and litter layer under a terra firme forest in San Carlos de Rio Negro, Venezuela (data from Tiessen et al. 1994a).

Soil versus Biological Controls on Nutrient Cycling in Terra Firme Forests

59

Table 4.4 Mass and nutrient fluxes in leaf litter from terra firme forests growing on Ultisols/Oxisols. Site

Mass (Mg ha-1 yr1)

N

P

K (kg ha-1 yr--1)

Ca

Mg

Source

5.4 6.1

73.2 97.6

1.1 1.0

22.7 13.4

8.9 12.2

12.4 7.3

Brower 1996 Duivenvoorden and Lips 1995

6.8

87.7

0.8

19.7

10.9

8.8

5-4

83-7

0.8

7.8

5.4

4.3

7.6

121.1

2.1

15.4

13.0

5.2

7.0

85

1.8

9

12

7

Guama, Para, Brazil Tucurui, Para, Brazil Manaus, Brazil

7.3 4.8 5.6

122.6 95.5 84.0

3.0 2.4 1.7

12.4 17.8 11.2

22.6 35.5 11.2

20.4 11.0 11.2

Central Amazonia, Brazil Maraca, Roraima, Brazil

5.4 6.3

97.2 81.9

1.1 3.7

8.1 29.6

20.5 46.6

9.7 17.

Average sd CV (%) Spodosols/Entisols*

6.2

93.6 15.8 17

1.8 1.0

15.2 6.8

53

45

18.1 12.6 70

10.4

0.9 15

Average sd CV (%)

4.8 0.6 10

49.8 13.4 30

1.8

13.1 8.2 60

24.8

9.6 2.3 20

Mabura Hill, Guyana Aracuara, Colombia

San Carlos de Rio Negro, Venezuela

0.9 50

15.9 60

Cuevas and Medina 1986 Medina and Cuevas 1989 Klinge 1977 Silva 1984 Klinge and Rodrigues 1968a,b Luizao 1989 Scott et al. 1992

4.9 47

* Data from Brower 1993, Luizao 1995, Duivenvoorden and Lips 1995, Cuevas and Medina 1986, Herrera 1979.

tions and sites. The range of mass values presented in Table 4.4, 4.8 - 7.6 Mg ha"1 yr1, are very similar to that reported by Vitousek and Sanford (1986) for forests growing on infertile Oxisols/Ultisols worldwide. No relationship has been found between annual rainfall and annual leaf litterfall, which indicates that other factors, such as nutrient availability, play an important role in the productivity of these forests. As with mass, nutrient inputs into the system from leaf litterfall vary considerably; however, the degree of variation is higher for lithogenic elements such as P, K, Ca, and Mg than for N. The coefficient of variation of total and N mass are similar (15 and 17%, respectively) indicating the biogenic input of this element and the relatively small variation among sites.

Potassium and Mg also have a similar degree of variation, around 45%, whereas P and especially Ca are the elements with the higher degree of variation in these forests (53 and 67%, respectively). The Maraca site in Brazil is cycling 7 times more Ca and 4 times more P via leaf fall than the poorest site in Colombia. Conversely, N is 1.3 times higher in the poorest Colombian site than in the Maraca site. In other words, nutrient fluxes other than N seem to be reflecting the variable lithology and degree of weathering of the parent material, while N varies according to the organic matter dynamics of the system. When tierra firme forests growing on Ultisols/Oxisols are compared with forests growing on Spodosols/Entisols, only one specific difference is noted (Table 4.4). The

60

range of values for leaf fall mass and nutrient fluxes overlap between the two groups, with the exception of nitrogen, which is considerably lower in Spodosols/Entisols. Nitrogen fluxes to the forest floor in terra firme forests are twice the amount in forests growing on white sands. Average P fluxes are similar despite the fact that the organic matter flux is higher in terra firme forests growing on Ultisol/Oxisols. Using an index of nutrient use efficiency (mass of litter produced/mass of nutrient in leaf litter, Vitousek 1984), there are contrasting patterns for the two groups according to soil type and nutrient availability (Table 4.5). Nitrogen use efficiency is higher in Spodosols/Entisols (96 v 64, respectively) while P use efficiency is higher in Ultisols/ Oxisols (4096 v 2067, respectively). This reflects the relative impoverishment in available P and N enrichment in the soil and vegetation of terra firme forests (Cuevas and

Fig. 4.2 Distribution of dry mass/P ratios and P/N ratios from terra firme forests growing on Ultisols/Oxisols (references in Table 4.4). MFS=Medium Fertility Soil

Elvira Cuevas

Medina 1986, 1988, 1990, Medina and Cuevas 1989, Tiessen et al. 1994a, Duivervoorden and Lips 1995). Phosphorus/nitrogen ratios of leaf litterfall are inversely related to P use efficiency as measured by the organic matter/P ratio (Fig. 4.2). There is no relationship between P/N (or N/P) ratios and the efficiency of N use in these forests. This indicates that P/N ratios are mostly regulated by the availability of P and not of N, as the latter is not limiting for these types of forests (Cuevas and Medina 1988, Medina and Cuevas 1989). The same general relationship was found by Medina and Cuevas (1994) when taking into consideration a wider set of data encompassing humid tropical forests growing on soils of contrasting fertility. The forests from Colombia, Guyana, and Venezuela have high P use efficiency and low P/N indicating strong P limitation; the Brazilian forests range from low to medium P use efficiency suggesting a higher variability in P availability in these systems. The Maraca forest, on the other hand, reflects the effect of seasonality in enhancing nutrient availability. Complex nonsclerphyllous forests can develop in apparent nutrientdeficient soils (Thompson et al. 1992), where fine litter production is among the highest reported for tropical forests (8.85-9-52 Mg ha-iyr1) (Scott et al. 1992). Phosphorus use efficiency and P/N ratio of leaf litterfall are in the medium range, indicating higher P availability in the soil. The lack of correspondence between leaf chemistry and soil nutrients may be attributed to the strong seasonality of rainfall, which causes pulses in decomposition of soil organic matter and subsequent nutrient release (Chapin and Bieleski 1982). Fungal and other microbial biomass fluctuate rapidly in response to wetting and drying cycles, even in nonseasonal tropical forests, leading to net nutrient immobilization followed by pulses of mineralization (Lodge 1993). Thus, the rate of nutrient circulation in

61

Soil versus Biological Controls on Nutrient Cycling in Terra Firme Forests

Table 4.5 Ratio of mass and nutrients circulated in litterfall as an index of nutrient use effciency (Vitousek 1984). Site Mabura Hill, Guyana Aracuara, Colombia

Mass/Nitrogen

Mass/Phosphorus

74

4909 6100 8500 6750 3619 3889 2433 2000 3294 4909 1703 4373 48

62

78 64

San Carlos de Rio Negro, Venezuela Guama, Para, Brazil Tucurui, Para, Brazil Manaus, Brazil Central Amazonia, Brazil Maraca, Roraima, Brazil

63 82 60 50 67 56 77

—Average —CV (%)

67 15

Sources: Sources of data are presented in Table 4.4.

the forest floor, rather than the absolute amounts, explains the relatively high production rates, and high fluxes of P in litterfall.

Fine Roots and Nutrient Cycling All terra firme forests develop an ectorganic horizon composed mostly of a root mat, of variable thickness, on top of mineral soil. The thickness of the root mat, made up of apogeotropically growing fine roots (Sanford 1987), can vary from a few centimeters to 80 cm depending on site characteristics and nearness to tree trunks and can make up an average of 30% of total fine-root biomass in these systems (Stark and Spratt 1977). The proportion of fine roots in the ectorganic horizon can vary from 12 to 54% of total fineroot biomass depending on the sites studied (Table 4.6). The absence of a superficial root mat at the Roraima site can be accounted for by increased nutrient availability due to seasonal pulses in the system. Sanford (1989) found that terra firme forests in San Carlos de Rio Negro have 95% of total root biomass in the upper 20 cm of mineral soil and in the root mat, of which 48% is in the surface root

mat alone. Ageotropism, or lack of growth toward the soil, can result from the very low Ca levels in the soil system, as Ca is responsible for the gravitropic response in plant roots (Bennet et al. 1990). Production of roots on top of the mineral soil has been explained as a consequence of the low nutrient availability in Amazon forests (Herrera et al. 1978, Cuevas and Medina 1983, Medina and Cuevas 1989). Vertical root distribution results from differential nutrient availability in the soil profile (Berish 1982, Berish and Ewel 1988). Shallow rooted systems may be a result of litter and soil organic matter production and decomposition rates in systems where nutrient input from litter exceeds that of nutrient release by soil weathering, as is the case of Ca, Mg, and P in terra firme forests (Medina and Cuevas 1989). In the Middle Caqueta region of Colombia, for example, Ca and Mg concentrations in the L and F layers are between 15 and 20 times higher than in the mineral soil (Duivenvoorden and Lips 1995). Soils under terra firme forests can vary in the amount and availability of Ca and Mg as previously indicated. Cuevas and Medina

62

Elvira Cuevas

Table 4.6 Total mass ± s.d. (live + dead) of fine roots (< 5 mm diameter) in ectorganic horizons and mineral soils at 0-40 cm depth in terra firme forests. Site Middle Caqueta area, Colombia!* Soils Ali-Acrisol group Soil Acri-Ferralsol group San Carlos de Rio Negro, Venezuela2** Dark red concretionary ferralsol Red-yellow concretionary ferralsol Manaus, Brazil3*** Roraima, Brazil4**** * ** *** *"*

Ectorganic horizon Mineral Soil (g/m2) (g/m2)

Total (g/m2)

Proportion in Ectorganic (%)

200 ±130 600 ± 380

1500 ± 450 1900 ± 730

1700 ± 530 2500 ± 730

12 ± 7 23 ± 14

2940 ± 2650

3070 ± 1580

6010

49

5750 ± 4680

4950 ± 2920

10700

54

500 0

900 700

1400 700

36 0

Duivenvoorden and Lips 1995, Sanford 1989, Klinge 1976, Thompson et al. 1992

(1986) proposed that terra firme forests were primarily limited by Ca, Mg, K, and P. In the same system, fine-root production in the root mat was significantly increased in growth cylinders with added Ca or P (Cuevas and Medina 1988). The role of Ca and Mg on fine root growth was confirmed by measuring root growth inside of decomposition bags, where root biomass was weakly and negatively correlated with N but positively and strongly correlated with Ca and Mg (Cuevas and Medina 1988). Duivenvoorden and Lips (1995) found that the amount of fine roots in the very superficial F horizon in the ectorganic layer was significantly higher in Ali-acrisols and Acri-ferralsols when compared to forests in relatively richer soils, such as those on well-drained floodplains or in white sand soils of the same area. There were no significant differences in N and P concentrations among the different areas; however, Ca and Mg had significantly lower concentrations in the acrisols and ferralsols. Fine roots in the F horizon of these

two soils are associated with proportional decreases of Ca and Mg concentrations from the L to F horizon. These results correspond to the those from litterbag studies in San Carlos de Rio Negro, Venezuela, where attachment of fine roots had a positive effect upon Ca and Mg mobility in surface root mats (Cuevas and Medina 1988). Magnesium deficiency can severely impair root growth (Marschner 1995), impacting not only the acquisition of Mg but also other mineral nutrients and of water. This may also reduce drought resistance and adaptation to nutrient poor sites. Calcium is essential for the strengthening of cell walls and plant tissues. It is also essential for root extension, as root growth is dependent upon exogenous Ca supply (Marschner 1995). An exploring root system with a large surface area (which may include the mycorrhizal component) is important in water- or P limited situations, whereas such properties are less important for mobile ions, such as nitrate. These conditions suggest that the nutrient saving mecha-

Soil versus Biological Controls on Nutrient Cycling in Terra Firme Forests

nism of fine-root accumulation in very superficial soil horizons is primarily directed at conservation of Ca and Mg. In areas with a more strongly seasonal climate, deeper root distribution up to 18 m can be found (Nepstad et al. 1995). This deeper distribution is not only important in accessing water but also nutrients resulting from weathering, such as Mg (Brouwer 1996).

Role of Symbiotic Interactions in Nutrient Cycling The presence and amount of active roots have a direct effect on microbial activities and nutrient dynamics via rhizosphere interactions (Newman 1985). Areas rich in organic matter have a higher proportion of labile nutrients which can be absorbed by fine roots either directly or via mycorrhizal symbiosis (Janos 1983, Hogberg 1986, Alexander 1989, St. John 1983, Lodge 1993, among others). The degree to which this mycorrhizal symbiosis is relevant to the cycling of nutrients within the ecosystem depends on the inherent fertility of the underlying mineral soil. Salcedo et al. (1991), working in an Atlantic coastal forest in Recife, Brazil, showed that phosphorus from the litter/fermentation layer is cycled back to the vegetation via mycorrhizae-mediated mechanisms. However, 6l% of the added 32P moved down to the mineral soil, where P in the soil solution is controlled by microbial biomass activity. In contrast, Stark and Jordan (1978), working on a P deficient upland terra firme forest in San Carlos de Rio Negro, Venezuela (Cuevas and Medina 1988), found that nearly 100% of the added $2P was retained in the root mat associated with the litter layer, with less than 0.1% moving down to the surface of the mineral soil. A superficial and dense root distribution is an important adaptation of plants on soils that show high P fixation, when water is

63

not a limiting resource. A root/litter mat allows direct uptake of mineralized P by the vegetation, thereby shunting nutrient cycling via mineral soil-microbial biomass mediated processes (Tiessen et al. 1994b). In terra firme forests with higher nutrient availability, the microbial biomass component in the soil plays a more important role in nutrient cycling (Salcedo et al. 1991, Luizao et al. 1992, Feigl et al. 1995). The presence of a root mat or ectorganic layer (Stark and Spratt 1977, Duivervoorden and Lips 1995, Luizao 1995) of varying thickness increases the surface area for absorbing nutrients mineralized from decomposing litter, the principal source of nutrients in these systems (Medina and Cuevas 1989, Tiessen et al 1994a). Haynes (personal communication) measured values of 20 and 2 me/100 g dry weight for the cation exchange capacity and anion exchange capacity of roots in a Terra firme forest in San Carlos de Rio Negro. Retention of cations and anions from decomposing litter could be a consequence of the ion exchange capacity of the root mat. This mechanism is similar to those operating in other forest ecosystems, even on sites richer in nutrients. However, the process of nutrient capture may be especially effective in terra firme forests as a result of very high root densities in the upper soil layers, or above the soil, and the occurrence of heavy mycorrhizal infection throughout (Went and Stark 1968, Janos 1983, St. John 1980, St. John and Uhl 1983, Caceres 1989, Moyersoen 1993). In addition, vesiculararbuscular mycorrhizal fungi (VAM) stimulate phosphatase production in the extended rhizosphere, thus enhancing P availability via organic P mineralization (Jayachandran et al. 1992). Medina and Cuevas (1994) analyzed available data for potential mineralizaton of nitrogen and nitrification rates from tropical forests. Nitrogen mineralization rates in

64

Elvira Cuevas

estimated the degree of change in topsoil C due to slash-burn cultivation on a red-yellow concretionary sandy ferralsol in San Carlos de Rio Negro, Venezuela (Table 4.7). After three years, which is the average period of crop production before site abandonment, 81% of the C in the organic layer was lost. This quantity represented only 29% of the C in the whole soil down to 15 cm depth. Under the strong leaching regime of San Carlos, mineralization of organic N provides a steady supply for the vegetation via the rapid turnover of litter and organic matter (Cuevas and Medina 1986, 1988, Tiessen et al. 1994a). Tiessen et al. (1994b) characterized the P status and P transformations in both coarse and fine soil fractions in the same toposequence under mature forests and showed that the litter and root mat layers above the tierra firme soils maintained higher total P levels than the underlying mineral soil. In the organic mats, 50-60% of the P is in inorganic forms, which maintain high levels of available (resin extractable) P. As a result, the litter mats account for over 70% of the total available P in the top 72 cm of soil. Of the total P, about 8% is resin extractable, reflecting in part the high contribution and P availability in the Mechanisms of Sustainability organic layer. A factor analysis showed that available P may be closely related to the The intricate mechanisms of forest adapta- amounts of organic residues present, and by tion and the importance of biotic recycling of implication to their turnover. nutrients raise the question, what role does The natural rates of nutrient cycling are the mineral soil play in the nutrient transfor- accelerated under agriculture, where over half mations of the ecosystem? In regions of the of the organic-bound nutrients will be minerAmazon basin with soils of higher clay and alized in around 2 years. The remainder will nutrient contents, crop production can be contribute small amounts of nutrients for a fursustained with relatively low-input technolo- ther 25 years (Tiessen et al. 1994a). Thus, agrigies (Sanchez and Benitez 1987). Agricultural culture is not sustainable without nutrient use of other more sandy, low-nutrient sites is inputs beyond 3 years, although the release of limited to small (< 1 ha) slash-burn sites with remaining nutrients can provide for the a 3-5 year cultivation cycle. The end of the reestablishment of secondary successions. cycle usually coincides with the disappear- Tiessen et al (1994b) proposed that secondary ance of the ectorganic horizon, indicating succession will be as much P as N-limited, that mineral soils contribute little to sustained since much of the natural P availability is crop production. Tiessen et al. (1994a) controlled by organic matter dynamics.

Amazon terra firme forests are lower than those measured on richer soils in Costa Rica and Barro Colorado, Panama. In general all the organic N mineralized is subsequently nitrified, leaving little or no free NH^ Under natural conditions, however, NH4 could be incorporated into biomass, immobilized in the soil, or taken up by the vegetation, resulting in a reduction of nitrification rates. In relatively infertile soils of the Amazon basin, nitrogen mineralization rates are much higher in the upper soil organic layer (including root mat if present) than in the mineral soil underneath (Vitousek and Matson 1988, Montagnini and Buschbacher 1989). While almost 100% of the N mineralized is also nitrified, the organic layer nitrification amounts to only 25-39% in Brazil and 44-51% in Venezuela. Mycorrhiza in general enhances ammonium uptake (Alexander 1989). The high amount of fine roots, coupled with the very high micorrhizal association in the ectorganic layer and the superficial organic layer in these forests, allows the incorporation of the mineralized nitrogen, thus preventing significant losses of this element.

Soil versus Biological Controls on Nutrient Cycling in Terra Firme Forests

65

Table 4.7 Changes in topsoil carbon on cultivation in a typical 3 yr slash and burn site on a Red-yellow concretionary sandy ferralsol in San Carlos de Rio Negro, Venezuela. C Content Native (g nv2) Organic layer Whole soil (-15 cm) Coarse C, root C Root C alone

7000 5100 2800 500

C Content after 3 Years 1300

3600 ND 500

C Loss

Mean Residence Time C Loss (yr)

(g m-2)

C Loss (%)

5700 1500

81 29

2 9

0

0

0.4

Source: Tiessen et al. 1994.

Secondary forest growth shows rates of productivity similar to or greater than those of intact mature forest and can reestablish relatively high soil organic matter levels (Uhl 1987), although basal area and total biomass similar to the mature forest are reached only after 190 years in areas like San Carlos de Rio Negro (Saldarriaga et al. 1988). Saldarriaga (1987) studied 23 sites near San Carlos, the oldest of which had been abandoned for 80 years. He found that K recovers most quickly, followed by P. The recovery of N stocks to the level of the original forest was estimated to be around a hundred years. On relatively richer soils, recovery of nutrient stocks will happen more quickly. In areas with high rainfall and limited stabilization of organic and other materials by silts and clays (10% of soil weight in the case of San Carlos), fertilization is not a viable option even for cash crops, unless a successful litter-organic matter cycle can be estab-

lished via alternative management techniques. In less leached areas fertilization might be an option; however, the potential for crop failure is still there due to the effect of droughts and the rapid degradation of the soil organic matter. Both scenarios make the management of organic matter an essential condition in crop sustainability. We can then consider that the quantification of rates of nutrient cycling associated with soil organic matter turnover under primary and secondary forests could be a predictive tool for evaluating the potential of soils for agriculture and for subsequent forest recovery. The development of easily measured indicators for nutrient cycling, and the evaluation of nutrient cycling and organic matter turnover in natural and derived systems, are essential for decisions concerning land use and management. Measurements of static nutrient pools are simply inadequate.

Literature Cited Alexander, I. 1989. "Mycorrhizas in tropical forests." In: Mineral Nutrients in Tropical Forests and Savanna Ecosystems, ed. J. Proctor (Blackwell Scientific Publications, Oxford), pp. 169-188. Baillie, I. C. 1989. "Soil characterisitcs and classification in relation to the mineral nutrition of tropical wooded ecosystems." In: Mineral Nutrients in Tropical Forests and Savanna Ecosystems, ed. J. Proctor (Blackwell Scientific Publications, Oxford), pp. 15-26.

Baillie, I. C. 1996. "Soils of the humid tropics." In: The Tropical Rain Forest. Second Edition, ed. P.W. Richards (Cambridge University Press, Cambridge), pp. 256-286. Baillie, I. C., P. S. Ashton, M. N. Court, J. A. R. Anderson, E. A. Fitzpatrick, and J. Tinsley. 1987. "Site characteristics and the distribution of tree species in Mixed Dipterocarp Forest on tertiary sediments in Central Sarawak, Malaysia." Journal of Tropical Ecology 3: 201-220.

66 Bennett, R. J., C. M. Bree, and V. H. Bandu. 1990. "A role for Ca2+ in the cellular differentiation of root cap cells: a re-examination of root growth control mechanisms." Environmental Experimental Botony 30: 515-523. Berish, C. 1982. "Root biomass and surface area in three successional tropical forests." Canadian Journal of Forest Research 12: 699-704. Berish, C., and J. J. Ewel. 1988. "Root development in simple and complex tropical successional systems." Plant Soil 106: 73-84. Bleeker, P. 1983- Soils of Papua New Guinea. Australian National University Press, Canberra. Bongers, F., D. Engelen, and H. Klinge. 1985- "Phytomass structure of natural plant communities on spodosols in southern Venezuela: the bana woodland." Vegetation 63: 13-34. Brouwer, L. C. 1996. Nutrient cycling in pristine and logged tropical rain forest. A study in Guyana. Tropenbos Guya Series 1. Thesis Universiteit Utrecht. The Netherlands. Brown, S., and E. Lugo. 1992. "Above-ground biomass estimates for tropical moist forests of the Brazilian Amazon." Interciencia 17: 8-16. Bruijnzeel, L. A. 1992. "Managing tropical forest watersheds for production: where contradictory theory and practice co-exist." In: Wise Management of Tropical Forests, eds. F. R. Miller, and K. L. Adam (Oxford Forestry Institute, Oxford), pp. 37-75. Buol, S. W. 1985- "Mineralogy classes in soil families with low activity clays." In: Mineral Classification of Soils, ed. J. A. Kittrick (Spec Publ 16. Soil Sci. Soc. Amer. and Amer. Soc. Agr. Madison, Wisconsin), pp. 169-178. Buschbacher, R. J. 1984. Changes in productivity and nutrient cycling following conversion of Amazon forest to pasture. PhD Thesis, University of Georgia, Athens, GA. Burnham, 1989. "Pedological processes in temperate and tropical soils." In: Mineral Nutrients in Tropical Forests and Savanna Ecosystems, ed. J. Proctor (Blackwell Scientific Publications, Oxford), pp. 27-41. Caceres, A. 1989- Las micorrizas vesiculo-arbusculares en un bosque humedo tropical y su evolucion luego de la perturbacion (conuco) y la sucesion por 60 anos en San Carlos de Rio Negro, T.F. Amazonas. Masters Thesis. Institute Venezolano de Investigaciones Cientificas. Caracas, Venezuela. Chapin, F. S., and R. L. Bieleski. 1982. "Mild phosphorus stress in barley and a related low-phosphorus-adapted barley grass: Phosphorus fractions and phosphate absorption in relation to growth." Physiology Plant. 54: 309-317. Cuevas, E., and E. Medina. 1983. "Rootproduction and organic matter decomposition in a terra firme forest of the upper Rio negro basin." In: International Symp. Root Ecol. and its Practical Applications, ed. L. Kutschera (Gumpenstein, Austria), pp. 653-666. Cuevas, E., and E. Medina. 1986. "Nutrient dynamics within Amazonian forest ecosystems. I. Nutrient flux in fine litter fall and efficiency of nutrient utilization." Oecologia 68: 466-472. Cuevas, E., and E. Medina. 1988. "Nutrient dynamics within Amazonian forests. II. Fine root growth, nutrient availability and leaf litter decomposition." Oecologia 76: 222-235.

Elvira Cuevas Cuevas, E., and E. Medina. 1990. "Phosphorus/nitrogen interactions in adjacent Amazon forests with contrasting soils and water availability." In: Phosphorus Cycles in Terrestrial and Aquatic Ecosystems, eds. H. Tiessen, D. Lopez-Hernandez, and I.H. Salcedo (Proc. SCOPE-UNEP Regional Workshop 3: South and Central America. Saskatchewan Institute of Pedology. Saskatoon, Canada), pp. 83-94. Cuevas, E., and A. E. Lugo. 1998. "Dynamics of organic matter and nutrient return from litterfall in stands of ten tropical plantation species." Forest Ecology and Management. 112: 263-279Duivenvoorden, J. F., and J. M. Lips. 1995. A land-ecological study of soils, vegetation, and plant diversity in Colombian Amazonia. The TROPENBOS Foundation. Wagenigen, The Netherlands. FAO/UNESCO. 1974. Soil Map of the World 1: 5 000 000. I. Legend. UNESCO, Paris. Feigl, B. J., G. P. Sparling, D. J. Ross, and C.C. Cent 1995. "Soil microbial biomass in Amazonian soils: evaluation of methods and estimates of pool sizes." Soil Biology and Biochemistry 27: 1467-1472. Fittkau, E. -J. 1971. Okologische Gliederung des AmazonasGebietes auf geochemischer Grundlage. Miinsterige Forschungen zur Geologic und Palaontologie 20/21: 35-50. Fittkau, E.-J., W. Junk, H. Klinge,. and H. Sioli. 1975"Substrate and vegetation in the Amazon." In: Berichte der Internationalen Vereinungung fur Vegetationskunde, ed. R. Tiixen (J. Cramer. Vaduz, Liechstenstein), pp. 73-90. Furch, K., and H. Klinge. 1989. "Chemical relationships between vegetation, soil and water in contrasting inundation areas of Amazonia." In: Mineral Nutrients in Tropical Forest and Savanna Ecosystems, ed. J. Proctor (Blackwell Scientific Publications, Oxford), pp. 189-204. Herrera, R. 1977. "Correlation with vegetation types." In: Soil and terrain conditions in the International Amazon Project at San carlosde Rio Negro, Venezuela, ed. E. F. Briinig (Trans. International MAB/IUFRO Workshop on Tropical Rainforest Ecosystems Research. Hamburg, West Germany), pp. 182-188. Herrera, F., C. F. Jordan, H. Klinge, and E. Medina. 1978. "Amazon ecosystems. Their structure and functioning with particular emphasis on nutrients." Interciencia 3: 223-232. Hogberg, P. 1986. "Soil nutrient availability, root symbioses and tree species composition in tropical Africa; a review." Journal of Tropical Ecology 2: 359-372. Janos, D. P. 1983. "Tropical mycorrhizas, nutrient cycles and plant growth." In: Tropical Rain Forest: Ecology and Management, eds. S. L. Sutton, T. C. Whitmore, and A. C. Chadwick (Blackwell Scientific Publications, Oxford), pp. 327-345. Jayachendran, K., A. P. Schwab, and B. A. D. Hetrick. 1992. "Mineralization of organic phosphorus by vesiculararbuscular mycorrhizal fungi." Soil Biology and Biochemistry 24: 897-903Jordan, C. F. 1985. Nutrient Cycling in Tropical Forest Ecosystems. (John Wiley & Sons, Chichester). Klinge, H. 1973. "Root mass estimation in lowland tropical rain forests of central Amazonia, Brazil. I. Fine root mass-

Soil versus Biological Controls on Nutrient Cycling in Terra Firme Forests es of a pale yellow latosol and a giant humus podzol." Tropical Ecology 14: 29-38. Lajtha, K., and A. F. Harrison. 1995. "Strategies of phosphorus acquisition and conservation by plant species and communities." In: Phosphorus in the Global Environment, ed. H. Tiessen (John Wiley & Sons, Chichester), pp. 139-147. Lodge, D. J. 1993. "Nutrient cycling by fungi in wet tropical forests." In: Aspects of tropical Mycology, eds. S. Isaac, J. C. Frankland, R. Watling, and A. J. Whalley (Cambridge University Press, Cambridge), pp. 37-58. Lugo, A. E., E. Cuevas, and M. J. Sanchez. 1990. "Nutrients and mass in litter and top soil of ten tropical tree plantations." Plant Soil 125: 263-280. Luizao, F. J. 1989- "Litter production and mineral element input to the forest floor in a Central Amazonian forest." Geojournal 19: 407-417. Luizao, F. J. 1995. Ecological studies in contrasting forest types in central Amazonia. Ph.D. Thesis. Stirling University, U. K. Luizao, R. C. C., T. A. Bonde, and T. Roswall. 1992. "Seasonal variation of soil microbial biomass-the effects of clearfelling a tropical rainforest and establishment of pasture in the central Amazon." Soil Biology and Biochemistry 24: 805-813. Marschner, H. 1995. Mineral Nutrition of Higher Plants. Academic Press. London. Matsui, E., E. Salati, M. N. G. Ribeiro, C. M. dos Reis, A. C. S. N. Tancredi, and J. R. Gat. 1982. "Precipitation in the Central Amazon Basin: the isotopic composition of rain and atmospheric moisture at Belem and Manaus." Acta Amazonica 13: 307-369. Medina, E., and E. Cuevas. 1989. "Patterns of nutrient accumulation and release in Amazonian forests of the Upper Rio Negro basin." In: Mineral Nutrients in Tropical Forest and Savanna Ecosystems, ed. J. Proctor (Blackwell Scientific Publications, Oxford), pp. 217-240. Medina, E., and E. Cuevas. 1994. "Mineral nutrition: humid tropical forests." Progress in Botany 55: 115-129Medina, E., and E. Cuevas. 1996. "Biomass production and accumulation in nutrient-limited rain forests: implications for responses to global change." In: Amazonian Deforestation and Climate, eds. J. H. C. Gash, C. A. Nobre, J. M. Roberts, and R.L. Victoria (John Wiley & Sons, Chichester), pp. 221-239Moyersoen, B. 1993- "Ectomicorrizas y micorrizas vesiculoarbusculares en Caatinga Amazonica del Sur de Venezuela." Scientia Guaianae 3. Caracas.

67

Montagnini, F., and R. Buschbacher. 1989. "Nitrification rates in two undisturbed tropical rain forest and three slash-and-burn sites of the Venezuelan Amazon." Biotropica 21: 9-14. Nepstad, D. C., C. R. De Carvalho, E. A. Davidson, P. H. Jipp, P. A. Lefebvre, G. H. Negreiros, E. D. da Silva, T. A. Stone, S. E. Trumbore, and S. Vieira. 1995. "The deep-soil link between water and carbon cycles of Amazonian forests and pastures." Nature 372: 666-667. Newman, E. I. 1985. "The rhizosphere: carbon sources and microbial populations." In: Ecological Interactions in the Soil, ed. A. H. Fitter (Blackwell Scientific Publications, Oxford). Poels, R. L. H. 1987. Soils, water and nutrients in a forest ecosystem in Suriname. Ph.D. Thesis, Agricultural University Wagenigen, The Netherlands. Proctor, J. 1992. "Soils and mineral nutrients: what do we know, and what do we need to know, for wise rain forest management?" In: Wise Management of Tropical Forests. Oxford Forestry Institute, eds. F. R. Miller, and K. L. Adam (Oxford), pp. 27-35. P. Chacon, and E. Cuevas 1994b. "Phosphorus and nitrogen status in soils along a toposequence of dystrophic rainforests on the upper Rio Negro." Oecologia 99: 145-150. Thompson, J., J. Proctor, V. Viana, W. Milliken, J. A. Ratter, and D. A. Scott. 1992. "Ecological studies on a lowland evergreen rainforest on Maraca Island, Roraima, Brazil. I. Physical environment, forest structure and leaf chemistry." Journal of Ecology 80: 689-703. Uhl, C. 1987. "Factors controlling succession following slash-and-burn agriculture in Amazonia." Journal of Ecology 75: 377-407. Uhl, C., and C. F. Jordan. 1984. "Succession and nutrient dynamics following forest cutting and burning in Amazonia." Ecology 65: 1476-1490. Vitousek, P. M. 1984. "Litterfall, nutrient cycling, and nutrient limitation in tropical forests." Ecology 65: 285-298. Vitousek, P. M., and P. A. Matson. 1988. "Nitrogen transformations in a range of tropical forest soils." Soil Biology and Biochemistry 20: 361-367. Vitousek, P. M., and R. L. Sanford, Jr. 1986. "Nutrient cycling in moist tropical forest." Annual Review of Ecological Systems 17: 137-167. Walsh, R. P. D. 1996. "Climate." In: The Tropical Rain Forest. Second Edition, ed. P. W. Richards (Cambridge University Press, Cambridge), pp. 159-205. Went, F., and N. Stark. 1968. "Mycorrhiza." BioScience 18: 1035-1039.

5 Nutrient Cycling as a Function of Landscape and Biotic Characteristics in the Cerrados of Central Brazil M. Haridasan

The cerrados of central Brazil have long been designated savannas without sufficient understanding of the structure and functioning of the different vegetation forms in the region. Excessive emphasis on identifying similarities with other savannas in Africa and Australia, and even within South America outside Brazil, prevented researchers from recognizing the cerrados' special features and interdependence among themselves in the landscape where they occur. The more extensive cerrado sensu stricto on dystrophic soils, and to a lesser extent the gallery forests known locally as matas de galeria or matas ciliares, dominated the attention of most botanists and other researchers (Ratter and Dargie 1992, Furley 1992, Furley and Ratter 1988, Furley et al. 1992). Even with increasing interest in biodiversity and ecosystem functioning, very little ongoing research is reported on nutrient cycling from the cerrado region (Solbrig et al. 1996). Information available in the literature is restricted to isolated attempts to describe and quantify specific processes like litterfall and decomposition (Peres et al. 1983), rainwater composition (Schiavini 1983, Delitti 1984), soil fertility gradients (Lopes and Cox 1977), leaf nutrient concentrations (Haridasan 1987, 1992, Araujo and Haridasan 1988), primary productivity of the ground layer (Batmanian and Haridasan 1985, Meirelles and Henrique 1992), effects of burning (Coutinho 1990,

Kauffman et al. 1994, Miranda et al. 1996c) and activities of soil fauna (Constantino 1988, Egler and Haridasan 1987, Oliveira Jr. 1985) at specific sites within a particular vegetation. Results of long duration experiments from permanent plots or watersheds are not yet reported in the literature. Very little information is available on the food webs or the role of fauna in nutrient cycling. Research on specific processes like CO2 emission on an ecosystem basis is quite recent (Miranda et al. 1996a, b, Mier et al. 1996). The following discussion is therefore restricted to the occurrence of different vegetation forms in the cerrado region and environmental factors affecting their distribution and functioning in relation to nutrient availability and nutrient cycling processes. One of the difficulties in getting information on research already carried out in Brazil is that the dissertations of graduate students in the universities are seldom published in indexed journals. There has been no attempt to compare the cerrados with other savannas in South America, other continents, or Amazon forests because the information available from the cerrados on quantitative aspects of nutrient cycling in natural ecosystems is very meager (Baruch et al. 1996). There are several important studies reported from the Amazon region on nutrient concentrations in plants, litter decomposition, and hydrological cycles over the last couple of

Nutrient Cycling as a Function of Landscape and Biotic Characteristics in the Cerrados of Central Brazil

decades (Vitousek and Sanford 1986, Proctor 1989, Jordan, 1987, Medina and Cuevas 1996, Klinge et al. 1995) but there are no studies of all pools and flows, especially those involving fauna and foodwebs.

69

periods. The density, height, canopy cover, and species composition of trees vary with soil depth, presence of concretions and soil fertility. As slope increases and deep Latosols give way to Entisols or Inceptisols, tree density and biomass diminish, giving rise to Geology and Geomorphology campo sujo (a low tree and shrub savanna) or campo limpo on nutrient-poor parent The modern Brazilian landscape began as material. Deciduous forests occur on lime a vast peneplain produced by denudation outcrops and well-drained calcium-rich soils. between the late Cretaceous and mid- Gallery forests occur on dissected terrain Tertiary. Uplifting and polycyclic stream along water courses on lower pedi-plains incisions during the later Tertiary dissected and young Pleistocene landforms (Emmerich the upland. Present-day plateaus (known 1990). When the water table is present near locally as chapadas) are the end products the surface, cerrado woodlands give way to of this old cycle of erosion (Cole 1986) and campo limpo (Fig. 5.1) and often to buritizais their surfaces are remnants of the original (Fig. 5.2). A few tree species of the cerrado South American land surface (King 1956, like Curatella americana L. and Byrsonima Braun 1971). The hilly lands represent a crassifolia (L.) Kunth, however, seem to tolnew cycle of dissection. Intensive weathering erate poor drainage conditions and occur in of the Tertiary/Quaternary sediments on so-called hydrologic (Ratter 1992) or hyperthese plateaus gave rise to different soils seasonal or semiseasonal (Sarmiento 1992) of the Oxisol order, depending on the local savannas, where lateral roots of trees are relief and drainage conditions (Curi and often restricted to upper soil layers not Franzmeier 1984, Macedo and Bryant 1987, reached by seasonal water tables (Dubs Rodrigues and Klamt 1978). Soils on 1992). Mesophytic forests also occur on hill slopes are generally shallow Entisols slopes on dissected surfaces, probably or Inceptisols. because of higher availability of moisture in Geomorphological evolution of the landscape played the most important role in the distribution of vegetation forms in the cerrado region (Cole 1986, 1992, Emmerich 1990). Topography, water table and parent material determine the boundaries among vegetation forms ranging from cerradao (a dense woodland) and cerrado sensu stricto (an open woodland) on well-drained soils to campo limpo (an open grassland), gallery forests and buritizais (swamps with the predominance of the burity palm, Mauritia viniferd) on poorly drained soils. The open woodland form of cerrado sensu stricto Figure 5.1 Campo /zmpo-cerrado sensu stricto occurs generally only on well-drained soils of transition on a Typic Quartzipsamment. Water peneplain or pediplain plateaus, because table near the soil surface is responsible for most of the woody species of the cerrado do the absence of cerrado tree species in the not tolerate waterlogging for extended campo limpo in the foreground.

70

the surface layers during the dry season due to subsurface seepage from the surrounding areas and release of nutrients through weathering of underlying parent material (Burnham 1989, Cole 1986). Thus, in any watershed of the cerrado region the landscape consists of a mosaic of soil and vegetation forms on different erosion surfaces. It is only reasonable to assume that past processes such as erosion and pediplanation played an important role in creating gradients in soil fertility and vegetation biomass within such watersheds. For example, organic matter accumulates in waterlogged and poorly drained soils but decomposes relatively rapidly in well-drained deep latosols. Besides retarding aerobic microbial decomposition processes, waterlogging also reduces the activities of soil fauna like termites and ants. Intense leaching of soluble nutrients like K and Mg from the plateaus benefited vegetation in the low-lying terrain. These gallery forests then act as sinks for leached nutrients, trapping them within their biomass and preventing their loss from the ecosystem as a whole (Ponomareva and Dokuchayev 1984).

M. Haridasan

The most important influence of geology on the vegetation of cerrado is the association of deciduous forests with limestone outcrops and the Ca-rich, mesotrophic soils derived from them (Ratter 1992, Ratter and Dargie 1992). Otherwise, cerrado vegetation is characterized by uniform edaphic conditions occurring over vast extensions of old planation surfaces with relict Latosols developed from underlying sandstone formations (Cole 1986). Sandy soils (Quartzipsamments) extend over 30 million hectares in the cerrado region, supporting mainly the cerrado sensu stricto vegetation when there is no waterlogging. Approximately 50% of the cerrado region belongs to the Precambrian, 30% to the Cretaceous, and 20% to other geological systems (Parada and Andrade 1976). The Precambrian system can further be subdivided into the Inferior Precambrian period, dominated by rocks of medium to high metamorphism (slate, phyllite, gneiss, and quartzite), and the Superior Precambrian, dominated by rocks of low metamorphism (metasedimentary rocks). The limestones which occur in the cerrado region (the Araxa, Arai, and Tocantins groups) belong to the Inferior Precambrian system. The superior Precambrian system (Bambui group) includes some of the phosphatic rocks encountered in the cerrado region of central Brazil, as in Patos de Minas, as well as limestones which are often dolomitic.

Climate

Figure 5.2 Gallery forest (to the left in the background) and the buritizal (to the right) in a cerrado landscape. The burity palms occur where drainage is impeded and in the gallery along the water courses.

Climate has a profound influence on the nature of native cerrado sensu stricto vegetation, though it is not the unique determinant of its origin and maintenance. Average annual temperatures range from 18 to 262C in the cerrado region (Eiten 1972). Average temperatures of the hottest and coldest months are only slightly different in the core region but daily variations,

Nutrient Cycling as a Function of Landscape and Biotic Characteristics in the Cerrados of Central Brazil

especially during the dry season, can be greater than the seasonal variation. The lowest minimum temperatures as well as the largest variations in temperature occur in the southern regions of the cerrado in Sao Paulo state. Day length varies very little throughout the year. Air humidity is high during the wet (rainy) season but can be very low during the dry season. Potential evapotranspiration is in the range of 1700-1800 mm, and water deficits affect plant growth only during the prolonged dry season in the core region. Thus climatic conditions are totally favorable for the maintenance of evergreen vegetation forms in the cerrado region. The main growth-limiting factor is the low availability of soil water in the upper layers of welldrained soils on upland plateaus during the dry season. The consequences of low availability of soil water are the deciduousness of many tree and shrub species and the low primary productivity of grasses in the dry season. Differential growth rates and leaf production during the dry and wet seasons among tree species are well documented in the literature. Litter accumulation and decomposition are

71

also affected by seasonal variations in humidity (Schiavini 1983). The lack of organic matter accumulation in well-drained soils is a direct consequence of the favorable conditions for decomposition throughout the year. Research on physiological aspects of plant metabolism in native plants as related to water and nutrient stress is meager.

Soils The different physiognomic forms of vegetation in the cerrado region are a direct consequence of the edaphic gradients associated with geomorphological variations. The distribution of soils in the cerrado region is shown in Table 5.1. Deep, well-drained latosols are most common on plateaus and in valleys when slope does not exceed 8%, except when the parent material is predominantly quartzitic in nature. Sandy soils, classified as areia quartzosa in the Brazilian system of soil classification (Typic Quartzipsamments in Soil Taxonomy), extend over 34 million ha or 20% of the cerrado region. Ultisols make up less than 5% of the total area. Fertile soils, defined as

Table 5.1 Distribution of soils in the cerrado region of central Brazil (Hoeflich et al. 1977)

Soil classification Soil Taxonomy

Brazilian system

FAO

Latossolos Vermelho-Amarelo Vermelho Escuro Latossolo Roxo

Ferralsols Acric Ferralsols Orthic Ferralsols Rhodic Ferralsols Arenosols Plinthic Acrisols Lithosols

Areia Quartzosa Lateritas hidromorficas Litossolo Podzolicos Vermelho Amarelo Distrofico Orthic Acrisols Eutrofico Ferric Luvisols Total

M ha

Area % Total

Oxisols Acrustox Haplustox Haplustox Psamments Plintaquults Lithic Dystropepts

94.5

69.7 17.9 6.9 34.3 17.0 15.1

56 41 11 4 20 10 9

Ustalts Ustalts

2.1 7.0

1 4

175.0

100

72

mesotrophic soils and identified on the basis of base saturation (greater than 50%) in the Brazilian system account for about 1% of the area. The nonexistence of a hierarchical scheme of soil classification in the Brazilian system makes it difficult to establish clear-cut soil-vegetation associations from surveys such as those of RADAMBRASIL. While such surveys describe variations in soils and vegetations separately and use natural vegetations to subdivide broad soil classes, they do not establish any correlations between soil properties and vegetation parameters (Haridasan 1993). For example, a soil survey report for the Federal District in central Brazil (EMBRAPA 1978) identifies four vegetation forms (forests, cerradao, cerrado, and campo sujo) associated with dark red latosols (Latossolo Vermelho Escuro) without discussing variations in soil properties which cause this gradient in vegetation. Botanists and ecologists, on the other hand, often discuss gradients in vegetation and soilavailable nutrients across different classes of soils (for example, Oxisols versus Entisols) without paying attention to important factors that place these soils in different categories at the highest level. Most comparisons of cerrados and other savannas in the literature focus on cerrado sensu stricto on deep latosols, but these comparisons have several limitations. These comparisons do not, for example, give proper attention to variations in geomorphological aspects, water table fluctuations or physical properties of soils. Attempts to correlate vegetation parameters with chemical properties of soils, especially analytical data on available nutrients in the surface layers are more common but should be viewed with caution for several reasons. First of all, such correlations by themselves do not establish a cause-and- effect relationship between soil properties and vegetation parameters. More often, the vegetation

M. Haridasan

should be affecting the available nutrient status of the surface soil through litterfall than the other way around. Secondly, attempts to establish a general deficiency parameter are inadequate unless specific conditions are defined under which individual nutrients become limiting. Another common error is to attribute undue significance to small, though consistent, quantitative variations in analytical parameters. Ratter and coworkers placed due emphasis on the influence of high calcium levels in the soil on the cerrado vegetation when they concluded that the floristic composition and phytosociological parameters of cerradoes and cerrados on more fertile soils can be quite different from those on dystrophic soils (Ratter 1971, 1992, Ratter et al. 1977, 1978). Many species occur only on dystrophic soils, others only on mesotrophic soils. Even when species indifferent to soil fertility occur in different communities their relative importance can vary to a great extent (Araujo and Haridasan 1988). Nutritional adaptations of such species and communities have received very little attention for any useful discussion at a community or ecosystem level (Haridasan 1992). What seems rather obvious at present is that calcareous soils generally have enough nutrients to permit transfer of large quantities of nutrients to plant biomass during the rainy season when the vegetation is green. We have discussed in an earlier publication the difficulties in comparing nutritional adaptations of native plant species in different communities (Araujo and Haridasan 1988). Occurrence of species exclusive to each locality or soil, differences in distribution of common species among sites, distinct characteristics of functional groups such as leguminous species, calcifugous or calcicole species, and aluminium accumulators make such comparisons on a community level very difficult. Without such details, it is difficult to define ecosystem

Nutrient Cycling as a Function of Landscape and Biotic Characteristics in the Cerrados of Central Brazil

processes which will determine nutrient pools and fluxes. Whatever data we have generally available in most publications are mean concentrations of litter or leaves of all species in an area. For example, it is not all species which contribute to fixation of atmospheric nitrogen and contribute to a general improvement of soil fertility. Calcicole species can probably survive well only on soils with neutral or alkaline pH. The ecological significance of the differences in relative importance of functional groups such as Al-accumulators or Leguminosae in native communities is rarely discussed in the literature (Haridasan and Araujo 1988).

Natural Vegetation Among the different physiognomic forms of vegetation occurring in the cerrado region, the most extensive is the cerrado sensu stricto. The floristic composition of different vegetation forms, geographic distribution of species and their relationship to Amazonian forests are well discussed in the literature. However, variations in phytosociology on regional and local levels have been reported on only more recently (Ratter and Dargie 1992). Detailed studies on soil-plant associa-

73

tions involving phytosociology of native species and environmental gradients in soil properties are also recent (Haridasan and Araujo 1988, Araujo 1992, Haridasan et al. 1997, Silva Junior 1995, Oliveira-Filho et al. 1989). The following discussion is restricted to only those aspects which are known to affect nutrient cycling directly or indirectly without going into details regarding the structure and functioning or determinants of the cerrado vegetation.

Tree biomass and nutrient pools Probably the most important and characteristic aspect of the cerrado sensu stricto vegetation, as far as nutrient cycling is concerned, is its low biomass. Very few measurements are available of the biomass of the different components—trees, shrubs and the ground layer—of the cerrado vegetation. Silva (1990), based on measurements of the weight of all individual trees with a minimum of 5 cm girth at a height of 30 cm in three 20 m x 30 m plots, determined the live aerial biomass of trees alone to be 21.38 Mg ha"1 (Table 5.2). These measurements were carried out by felling all trees in three 20 m x 30 m plots at the end of the dry season. Since some of the trees shed leaves during

Table 5.2 Live aerial biomass (dry weight) of the different components of the trees (girth > 5cm at 30 cm height) of a cerrado sensu stricto on a dark red latosol (Haplustox) at the Fazenda Agua Limpa in Brasilia.

Source: Silva, 1990

Tree Component

Dry weight (Mg ha-1)

Trunk Thick branches Fine branches Leaves Fruits Total

9.40 6.58 4.27 1.04 0.09 21.38

M. Haridasan

74

Table 5.3 Comparison of the aerial biomass (dry matter) and nutrient stocks of trees of a cerrado sensu stricto in Brasilia and an Amazonian seasonal varzea forest in Ilha de Marchantaria. Cerrado sensu stricto*

Vegetation

Amazonia 1**

Amazonia 2** 254.6

9.9

97.5 38.2

N

not reported

387.1

1107.2

P

4.9

K

30.5 24.9 12.0

31.5 665.6 805.8 163.0

67.1 1806.2 2988.2

Biomass, Mg ha'1 2

21.4 1

Basal area, m ha'

95.7

Proportion Amazonia 1 Cerrado

Proportion Amazonia 2 Cerrado

4.55 3.86

11.90 9.67

6.43 21.82 32.36 13.58

13.69 59-22 120.00 28.48

Nutrients, kg ha'1

Ca Mg

341.7

* Silva 1990. ** Klinge et al. 1995.

the dry season, the dry weight of leaves could be an underestimate. A comparison of these data with those of an Amazonian forest shows that the aerial biomass of the trees of a cerrado sensu stricto in central Brazil may be only 8 to 22% of that of an Amazonian forest, and the basal area only 10 to 26% (Table 5.3). This difference in biomass reflects directly on the nutrient pools in the biomass. A comparison of the data reported by Klinge et al. (1995) for the aboveground biomass and nutrient stock in two inundation forests in the Ilha de Marchantaria with the data for a cerrado sensu stricto from central Brazil (Silva 1990) illustrates how nutritionally poor the cerrado is in quantitative terms. The proportions of stock of essential nutrients in the tree biomass of cerrado are: 7 to 16% for P, 1.7 to 4.6% for K, 0.83 to 3.09% for Ca, and 3-5 to 7.4% for Mg. Thus Ca, K, and Mg seem to be much more deficient in the cerrados than P. We have no corresponding data for the stock of nutrients in the root biomass of trees for comparison among the two ecosystems. This comparison is only illustrative of two specific sites. Estimates of aboveground biomass for the Amazonian forests may vary

considerably depending the site and methodology (see Cuevas, this volume; Brown and Lugo 1992; Fearnside 1992). Similarly, any comparisons on a regional scale with cerrados are complicated by the large variety of the vegetation forms. Abdala et al. (1997) estimated the total aboveground live biomass of trees of another cerrado sensu stricto on a dark red latosol to be 22.9 Mg ha-1 (Table 5.4). This estimate was based on the measurements of basal areas and plant height and estimates of cylindrical volume of individual species from two 20 m x 50 m plots. The biomass of shrubs totaled 3-1 Mg ha"1; grasses contributed 4.13 Mg ha'1, or 74%, of the total biomass of the herbaceous layer (5.58 Mg ha"1). Litter was estimated at 5.19 Mg ha'1. Their estimate of belowground biomass of living and dead roots of this vegetation was 41.1 Mg ha'1 to a depth of 6.2 m. Thus the belowground biomass was slightly greater than that aboveground (39.8 Mg ha"1 including litter and dead trees). The belowground component should therefore exercise an enormous influence on nutrient cycling, but we have no estimates of the nutrient content of the root biomass or the rate of root

Nutrient Cycling as a Function of Landscape and Biotic Characteristics in the Cerrados of Central Brazil

decomposition. This is in addition to the organic matter component of the soil, which was estimated at 642 Mg ha'1 in the top 6.2 m of soil. The proportion of root biomass as compared to live aerial biomass in the cerrado is thus much higher than in tropical forests where root mass varies anywhere from about one-thirtieth to one-half of the aerial biomass (Cuevas this volume, Medina and Cuevas 1996). Castro (1995) established the variation in aerial and belowground biomass along a vegetation gradient from campo limpo to cerrado sensu stricto near Brasilia. The author's estimates of the aerial biomass of two cerrado sensu stricto sites show how variable such estimates can be. Her estimate of 12.9 Mg ha'1 for tree biomass amounted to only 58% of that measured by Silva (1990) and 54% of the estimate of Abdala et al. (1997). The author established this relationship by estimating aerial biomass from measurements of diameter and height using equations of biomass for tropical forests. However, her measurements of belowground biomass confirm that root biomass (46.6 to 52.9 Mg ha'1) exceeds the aerial biomass

75

(24.8 Mg ha'1) in a cerrado sensu stricto. In the campo limpo, which is devoid of trees, the aerial biomass (5.5 Mg ha"1) amounted to only 34% of the root biomass (16.3 Mg ha'1 ) in the top 2 m of soil. This proportion was 31% in the campo sujo with its sparse distribution of trees and 47.9 to 53-3 in the cerrado sensu stricto. Thus the different cerrado vegetation types showed a higher root/shoot ratio than many tropical forests.

Ground layer biomass and nutrient pools Batmanian and Haridasan (1985) estimated the primary production of the herbaceous layer of a cerrado sensu stricto to be 3.27 Mg ha'1 yr1 using the peak biomass method, with the grasses contributing 68 to 78% of the live biomass of the herbaceous layer. Thus, the transfer of nutrients from soil or litter to live biomass should be of the order of 20 kg ha'1 yr1 of N, 2 kg P, 20 kg K, 10 kg Ca, and 3 kg Mg. Even allowing for the higher estimates of primary production and live biomass of the herbaceous layer from other studies, these amounts are insignificant when

Table 5A Partitioning of biomass in a cerrado sensu stricto on a dark red latosol near Brasilia in central Brasil (Abdala et al. 1998) Component Trees (girth > 6cm at 30 cm) Stems and branches Leaves Dead trees Shrubs (girth < 6cm at 30 cm) Leaves Stems and branches Herbs Grasses Non-grasses Litter Total aboveground biomass Roots Total plant biomass

Dry weight, Mg ha'1

21.7 1.2 3.0 0.3 2.8 4.1 1.5 5.2 39.8 41.1 80.9

76

M. Haridasan

Table 5.5 Aerial biomass and nutrient stock of the ground layer vegetation of a cerrado sensu stricto in Brasilia.

Biomass, kg ha"1 Nutrient concentrations, mg g*1 P K Ca Mg Nutrient stock, kg ha"1 P K Ca Mg

Grasses

Nongrasses

778

663

0.53 5.79 1.85 1.09

0.53 5.38 6.35 1.72

0.412 4.505 1.439 0.848

0.351 3.567 4.210 1.140

Source: Based on data for unfertilized plots in Villela and Haridasan 1994

compared to the uptake by any field crop or the amounts of fertilizer application in agriculture (see for example, Fassbender et al. 1985). Thus it is evident that the native grasses and herbaceous species are adapted for survival under low soil fertility conditions. However, this does not imply that they are not capable of responding to higher availability of nutrients through fertilizer applications. The estimate of Villela and Haridasan (1994) for the biomass of the herbaceous layer of unirrigated plots of a cerrado sensu stricto in Brasilia was only 1441 kg ha"1. Grasses contributed 54% of the biomass of the herbaceous layer (Table 5.5). The nutrient concentrations and stock of Ca and Mg were higher in the nongrasses, while the stock of K was higher in the grasses. Monitoring any alteration of the total nutrient pool or fluxes due to fire or disturbance becomes a difficult task because of the very small quantities involved. Irrigation during the dry season could increase the live aerial biomass of the grasses in a cerrado sensu stricto, and liming could increase the uptake of Ca by the herbaceous layer. Haridasan et

al. (1997) reported that liming resulted in a linear increase in the concentration of Ca in the biomass of grasses and 5 herbaceous species of the ground layer of the same cerrado, proportional to the dosage of liming and the concentrations of N, P, and K increased generally with the application of fertilizers. These results indicate a general deficiency of nutrients in the soils of the cerrado and the capacity of the native species to respond to improvement in soil fertility. However, quick recovery of the native vegetation after fires should be in part due to the capacity for growth under low nutrient requirements.

Nutrient Fluxes The processes which contribute to transfers of nutrients among different pools in a natural ecosystem include uptake by plants, transfer to primary and secondary consumers, return to the organic and inorganic pool in the soil through litter of plant origin and animal excreta and dead bodies, decomposition of roots, rainfall, and fixation of

Nutrient Cycling as a Function of Landscape and Biotic Characteristics in the Cerrados of Central Brazil

77

Table 5.6 Production of litter and their nutrient concentrations in different physiognomic forms of vegetation in the cerrado region of central Brazil. Vegetation type Reference Cerrado Peres et al. (1983) Schiavini (1983) Delitti (1984) Pompeia (1989) Cerradao Peres et al. (1983)

Production (Mg ha-1 y-D

N (mg g-1)

(mg g'1)

2.1

8.5 (5.8-10.6) 13.4 14.6

0.6 (0.5-0.7) 0.3 0.6 0.5

1.6 (0.6-2.8)

8.2 (5.8-11.8)

0.6 (0.5-0.7)

2.4 3-2

7.8

atmospheric nitrogen (Fassbender 1985). The present author is not aware of any estimates of the transfer of nutrients to the animal foodweb from plant biomass or the size of the animal pools in natural cerrado ecosystems. Also lacking are studies on decomposition rates of roots. Whatever information is available on production and decomposition of litter and rainfall inputs are reviewed here. The role of fauna like termites and ants in recycling organic matter and altering soil properties creating gradients in fertility have been very little investigated (Egler and Haridasan 1987). The higher levels of available nutrients in abandoned termite mounds, for example, could facilitate the germination and early growth of tree seedlings during their disintegration. The termites could also play an important role in the disintegration and decomposition of dry litter in "welldrained soils.

Litter production and decomposition

P

K Ca Mg (mg g-1) (mg g-1) (mg g-1) 3.2

2.0 2.2 2.1

(2.1-4.4) 1.5 6.4 8.3

1.2 (0.7-1.9) 1.2 1.7 1.9

1.6 (0.8-2.8)

3.4 (2.1-4.4)

1.4 (0.7-1.9)

comparable in spite of the higher production in the cerradao (7.8 Mg ha^yr"1). Thus the flow of nutrients from the plant biomass to the soil surface could be about 3.7 times higher in the cerradao than the cerrado. Their results of similar nutrient concentrations in the litter in cerrado and cerradao were consistent with the observations of Ribeiro (1983) who compared the leaf concentrations of nutrients of individual tree species of a cerrado and cerradao on dystrophic soils. The N concentrations (8 mg g'1) reported by Peres et al. (1983) were considerably lower than the values (1.34 mg g"1) reported by Delitti (1984) for a cerrado in the Sao Paulo state of Brazil. Available reports indicate wide variations in the estimates of litter decomposition rates probably because of the differences in methodology and the ambients where the studies were conducted. Long-term studies are lacking for definite conclusions.

Rainfall inputs Estimates of litter production in the cerrado sensu stricto varies from 2.1 to 3.2 Mg ha'1 yr1 (Table 5.6). Peres et al. (1983) concluded that the concentrations of nutrients in the litter in a cerrado, and cerradao on dystrophic soils of similar fertility near Brasilia, were

Because cerrado soils and native vegetation are generally deficient in nutrients, even the small quantities of nutrients in the rainfall might be an important source for the recuperation of the native vegetation after

M. Haridasan

78

perturbations and fires (Coutinho 1979, 1990, Pivello-Pompeia and Coutinho 1992). However, data available so far are too few for any definite conclusions.

Mineral nutrition of native plants vis-a-vis nutrient cycling and ecosystem functioning Very few ecosystem studies emphasize interspecific variations in the uptake and translocation of nutrients. It has been shown that nutrient concentrations in leaves and other components do not reflect the nutrient levels in the soil because of species-specific uptake (Ernst and Tolsma 1989, Araujo and Haridasan 1988). While specific physiological adaptations of native species such as nitrogen fixation and mycorrhyzal associations have received research attention, the relative contributions of such functional groups in different native communities have seldom been quantified. For example, Haridasan (1987) hypothesized that Al-accumulating species could have a competitive advantage in dystrophic soils where Al saturation of soils could be a limiting factor for plant growth. In many situations of extreme stress in the cerrado region, Al-accumulating species show relatively high importance values. Among the 28 species occurring at 49 or more of the 96 sites of cerrado throughout Brazil with information available on phytosociological parameters of woody species, Ratter et al. (1996) cites 4 known Al-accumulators, Qualea grandiflora, Q. parviflora, Q. multiflora, and Salvertia convallariodora. Vochysia thyrsoidea Pohl, an Al-accumulating species very common on deep well-drained latosols, tolerates waterlogging on one extreme and resists drought well on shallow sands of quartzitic parent material at the other extreme. Another Al-accumulating species, Vochysia divergens, occurs as monodominant species in seasonally flooded forests of the Pantanal region (Nascimento and Cunha 1989).

Disturbances and nutrient cycling Besides deforestation and land clearing for agricultural crops, pasture, and silvicultural plantations, large areas of cerrados are destroyed by mining activities which include the complete removal of topsoil. Spontaneous revegetation or colonization by native species often depends on the depth of topsoil removal and the physical and chemical properties of the remaining subsoil. Even when there are no physical limitations and only a few centimeters of soil are removed, regeneration of secondary vegetation often takes several years. In many places vegetative propagation from deep roots is a source of reestablishment of shrubs and tree species in the absence of seed propagation and seedling establishment. More often soil compaction and depauperate subsoil are the main restrictions for natural recolonization, even by grass and herbaceous species from surrounding native vegetation. De-compaction and manuring results in spontaneous recolonization in such cases. In a recent field experiment we have been able to demonstrate that lack of nutrients is indeed the most limiting factor preventing the colonization of degraded areas in the latosol region of cerrados (Leite et al. 1994). Addition of organic matter supplements like castor bean cake and peat resulted in spontaneous invasion and establishment of herbaceous and shrub species providing hundred percent vegetative cover within a couple of years in an area which had not shown any signs of recovery even after 30 years of abandonment. Inputs through rainfall and other sources were not sufficient to produce any significant impact on the establishment of secondary vegetation though such inputs may be significant in natural ecosystems.

Fire in the cerrados Among the natural disturbances that the cerrado sensu stricto suffers periodically,

Nutrient Cycling as a Function of Landscape and Biotic Characteristics in the Cerrados of Central Brazil

Figure 5.3 A cerrado in Brasilia during the dry season showing the fuel load in the herbaceous layer.

79

a predominance of non-grasses but within a year the original proportion of grass biomass may be restored (Batmanian and Haridasan 1985). Several dormant species of the ground layer produce flowers and seeds only after a fire. Kauffman et al. (1994) estimated the fuel loads along a vegetation gradient from campo limpo to cerrado sensu stricto near Brasilia. In the cerrado only 27% of the fuel load of 10 Mg ha*1 was comprised of graminoids; the remainder was deadwood and leaf litter. They estimated the nutrient pools in combustible components in the cerrado sensu stricto to be 54.7 kg ha'1 N, 13.8 kg ha-1 K, 3.5 kg ha-1 P, and 30.5 kg ha'1 Ca. They concluded that the total biomass of the herbaceous layer of the cerrados was similar to that of other savanna ecosystems. The authors concluded that any loss of N due to fire was negligible compared to the N pool in the soil. Biological N fixation and precipitation inputs would compensate for such losses. Similarly, precipitation inputs would compensate for the loss of P, K and Ca (Schiavini 1983, Coutinho 1979, Pivello-

none is more important than fire (Eiten 1972, Coutinho 1982), which is more frequent during the long dry season when the herbaceous layer dries out. The intensity of the fire depends on the quantity of fuel material accumulated at the soil surface. Quantity of fuel material is, in turn, dependent on the period intervening since the last fire, the primary production of the ground layer, and the quantity of litter produced by the shrubs and trees (Fig. 5.3). These fires often totally destroy the aerial portion of the ground layer vegetation but seldom kill any tall trees, though the high temperatures of the flames may result in trees shedding most of their leaves a few days after the fire (Fig. 5.4). Cerrado trees and shrubs are able to survive the frequent fires because of several adaptive strategies such as thick, corky barks and the ability to sprout adventitious buds from deeper lying tissues such as wood cambium. What is impressive is the ability of the ground layer vegetation to sprout within a Figure 5.4 A cerrado sensus stricto in Brasilia few weeks following the fire, often providing immediately after a fire during the dry season. total vegetative cover within three to four The ground layer is completely burnt while months (Fig. 5.5). During the first few leaves of the trees have dried due to heat months the ground layer biomass may show from the fire and will shed in a few days.

80

M. Haridasan

Pompeia and Coutinho 1992) within one to three years.

Soil erosion and nutrient cycling The great susceptibility of latosols to erosion following removal of native vegetation and soil compaction is an important aspect to be considered in nutrient loss studies. Accelerated erosion and the formation of wide deep gullies alter the landscape and bring the unwelcome consequences of sedimentation and destruction of water courses and reservoirs. Ross (1992) concluded that rates of soil erosion after clearance of native vegetation were up to eight times higher than those after partial clearance and under forest in Roraima in the Amazon region. Soil erosion resulted in accelerated rates of litter removal from the soil surface and significantly higher losses of nitrogen, calcium and potassium from the soil nutrient pool. Similar consequences can be expected in the cerrados. Devoid of natural vegetation, the soils of the cerrado region are highly susceptible to erosion by water. High rainfall intensities favor severe erosion of all forms in unprotected soils. The consequences forn nutrient and water cycling are several. First, the removal of the weathered well-structured surface soil, which is relatively richer in nutrients than the underlying material, leaves behind compact impermeable nutrient-poor regolith. This compacted surface is incapable of supporting spontaneous recolonization by native species. The removal of porous top soil also implies a loss of water storage capacity, resulting in excessive runoff during the rainy season and drought during the dry season. Construction of roadways and mining activities often result in the formation of long, wide and deep gullies affecting the whole drainage pattern of entire watersheds because of the high intensity rains in the region.

Figure 5.5 Sprouting of the ground layer vegetation of campo sujo on sandy soil within a month after the fire during the dry season.

Conclusions Richter and Babbar (199D concluded their review on soil diversity in the tropics stating, "to speak carelessly about 'tropical soil' greatly oversimplifies the complexity and diversity of ecosystems in this 5-billion hectare region. As soil diversity at all spatial scales (from region to microsite) is better documented, understanding and use of tropical ecosystems can only improve." To speak carelessly about "savannas", similarly, oversimplifies the complexity and diversity of ecosystems in the tropical region. Documentation of spatial variability of determinants such as geomorphological features, soil properties, water regimes, flora and fauna and ecosystem processes is still too sparse to define and understand different facets of nutrient cycling in Amazon savannas and the cerrados of central Brazil. Generalizations based on information from isolated site-specific studies may not be totally valid on an ecosystem basis because of these variations. Future research on nutrient cycling in cerrados should emphasize the specific adaptations of native communities and individual species

Nutrient Cycling as a Function of Landscape and Biotic Characteristics in the Cerrados of Central Brazil

or functional groups as components of these natural ecosystems. These adaptations are crucial to overcoming often extreme stress conditions, including nutrient deficiency and alu-

81

minum toxicity in strongly acid soils and water deficits during the dry season. Any research on nutrient pools and fluxes involving primary and secondary consumers will be practically new.

Literature Cited Abdala, G. C., L. S. Caldas, M. Haridasan, and G. Eiten 1998. Below-ground organic matter and root-shoot ratio in a cerrado in central Brazil. Brazilian Journal of Ecology (1998) 2:11-23 Araujo, G. M. 1992 Comparacao da estrutura e do teor de nutrientes nos solos e nas folhas de especies arboreas de duas matas mesofilas semideciduas no Triangulo Mineiro. Ph. D. Thesis. Universidade Estadual de Campinas, Campinas, Brazil. Araujo, G. M., and M. Haridasan. 1988. A comparison of the nutrient status of two forests on dystrophic and mesotrophic soils in the cerrado region of central Brazil. Communications in Soil Science and Plant Analysis 19:1075-1089. Baruch, Z., A. J. Belsky, L. Bulla, A. C. Franco, I. Garay, M. Haridasan, P. Lavelle, E. Medina, and G. Sarmiento. 1996. Biodiversity as regulator of energy flow, water use and nutrient cycling, in O. T. Solbrig, E. Medina, and J. F. Silva, editors. Biodiversity and savanna ecosystem processes - a global perspective. Ecological Studies, Vol. 121. Springer-Verlag, Berlin, Heidelberg. Batmanian, G. J., and M. Haridasan. 1985. Primary production and accumulation of nutrients by the ground layer community of a cerrado vegetation of central Brazil. Plant and Soil 88: 437-440. Brown, S., and A. E. Lugo. 1992. Aboveground biomass estimates for tropical moist forests of the Brazilian Amazon. Interciencia 17(Jan-Feb):8-18. Braun, O. P. G. 1971. Contribuicao a geomorfologia do Brasil. R. bras. Geogr. 32(3):3-39. Burnham, C. P. 1989.Pedological processes and nutrient supply from parent material in tropical soils. Pages 2741. in J. Proctor. Mineral nutrients in tropical and savannah ecosystems. Blackwell, Oxford. Castro, E. A. 1995. Biomass, nutrient pools and response to fire in the Brazilian cerrado. Master's thesis. Oregon State University. Corvallis. Cole, M. M. 1986. The savannas: Biogeography and Geobotany. Academic Press. London. Cole, M. M. 1992. Influence of physical factors on the nature and dynamics of forest-savanna boundaries. Pages 63-75. in P. A. Furley, J Proctor, and J. A. Ratter, editors Nature and Dynamics of Forest-Savanna Boundaries. Chapman and Hall, London. Constantino, R. 1988. Influencia da macrofauna na dinamica de nutrientes do folhedo em decomposicao em cerr do sensu stricto. Master's thesis. Departamento de Biologia Vegetal, Universidade de Brasilia, Brasilia, Brazil. Coutinho, L. M. 1979. Aspectos ecologicos do fogo no cerrado. III. A precipitacao atmosferica de nutrientes minerais. Revista Brasileira de Botanica 2:97-101. Coutinho, L. M. 1982. Ecological effects of fire in Brazilian cerrados. Pages 273-291. in B. J. Huntley and B. H.

Walker, editors. Ecology of tropical savannas. Ecological Studies, Vol. 42. Springer-Verlag, Berlin. Coutinho, L. M. 1990. Fire in the Ecology of Brazilian Cerrado. Pages 82-105. inj. G. Goldammer, editor. Fire in the tropical biota: Ecological processes and global challenges. Ecological Studies, Vol. 84. Springer-Verlag, Berlin. Curi, N., and D. P. Franzmeier. 1984. Toposequence of oxisols from the Central Plateau of central Brazil. Soil Sci. Soc. Am. J. 48:1245-1248. Delitti, W. C. 1984. Aspectos comparatives de ciclagem de nutrientes na mata ciliar, no campo cerrado e na floresta implantada de Pinus elliotti Engelm. var. elliottii (Mogi-Guacu, SP). Ph. D. thesis, Universidade de Sao Paulo, Sao Paulo, Brazil. Dubs, B. 1992. Observations on the differentiation of woodland and wet savanna habitats in the Pantanal of Mato Grosso, Brazil. Pages 431-449. in P. A. Furley, J Proctor, andj. A. Ratter, editors. Nature and Dynamics of Forest-Savanna Boundaries. Chapman and Hall, London. Egler, I., and M. Haridasan. 1987. Alteration of soil properties by Procornitermes araujoii Emerson (Isoptera, Termitidae) in latosols of the cerrado region of central Brazil. Pages 280-308. in J. J. San Jose, and R Monies, editors. La capacidad bioprodutiva de sabanas. IVIC, Caracas, Venezuela. Eiten, G. 1972. The cerrado vegetation of central Brazil. Bot. Rev. 38:201-341. EMBRAPA, 1978. Levantamento de reconhecimento dos solos do Distrito Federal. Boletim Tecnico no. 53. Service Nacional de Levantamento e Conservacao de Solos. Rio de Janeiro, Brazil. Emmerich, K. H. 1990. Influence of landform, landscape development and soil moisture balance on forest and savanna ecosystem patterns in Brazil. Pedologie. XL(1):5-17. Ernst, W. H. O., and D. J. Tolsma. 1989. Mineral nutrients in some Botswana savanna types. Pages 97-119. in J. Proctor. Mineral nutrients in tropical and savannah ecosystems. Blackwell, Oxford. Fassbender, H. W. 1985. Ciclos da materia organica e dos nutrientes em ecossistemas florestais dos tropicos. Pages 202-230. in P. C. Rosand, editor. Reciclagem de nutrientes e agricultura de baixos insumos nos tropicos. CEPLAC/SBCS, Ilheus, Bahia, Brazil. Fassbender, H. W., L. Alpizar, J. Heuveldop, G. Enriquez, and H. Folster. 1985. Ciclos da materia organica e dos nutrientes em agrossistemas com cacaueiros. Pages 230257. in P. C. Rosand, editor. Reciclagem de nutrientes e agricultura de baixos insumos nos tropicos. CEPLAC/SBCS, Ilheus, Bahia, Brazil. Fearnside, P. M. 1992. Forest biomass in Brazilian Amazonia: comments on the estimate by Brown and Lugo. Interciencia 17 Qan-Feb):19-27. Furley, P. A. 1992. Edaphic changes at forest-savanna

82

boundary with particular reference to the neotropics. Pages 91-117. in P. A. Furley, J Proctor, and J. A. Ratter, editors. Nature and Dynamics of Forest-Savanna Boundaries. Chapman and Hall, London. Furley, P. A., and J. A. Ratter. 1988. Soil resources and plant communities of the central Brazilian cerrado and their development. Journal of Biogeography 15:97-108. Furley, P. A., J. Proctor, and J. A. Ratter. 1992. Nature and dynamics of forest-savanna boundaries. Chapman and Hall, London. Haridasan, M. 1987. Distribution and mineral nutrition of aluminium-accumulating species in different plant communities of the cerrado region of central Brazil. Pages 309-348. in J. J. San Jose, and R Montes. editors. La capacidad bioprodutiva de sabanas, IVIC. Caracas, Venezuela. Haridasan, M. 1992. Observations on soils, foliar nutrient concentrations and floristic composition of cerrado sensu stricto and cerradao communities in central Brazil. Pages 171-184. in P. A. Furley, J Proctor, and J. A. Ratter, editors. Nature and Dynamics of Forest-Savanna Boundaries. Chapman and Hall, London. Haridasan, M. 1993 Solos. Pages 309-328. in M. NovaesPinto, editor. Cerrado: caracterizacao, ocupacao e perspectivas. Second Edition. Editora Universidade de Brasilia, Brasilia. Haridasan, M., and Araujo, G. M. 1988. Aluminium-accumulating species in two forest communities in the cerrado region of central Brazil. Forest Ecology and Management 24: 15-26. Haridasan, M., J. M. Felfili, M. C. Silva Jr., A. V. Rezende, and P. E. N. Silva 1997. Gradient analysis of soil properties and phytosociological parameters of some gallery forests on the Chapada dos Veadeiros in the cerrado region of central Brazil. Pages 168-180. in Proceedings of the International Symposium on Assessment and Monitoring of Forests in Tropical Dry Regions with Special Reference to Gallery Forests. Editora da Universidade de Brasilia, Brasilia, Brazil. Haridasan, M., A. A. M. C. Pinheiro, and F. R. R. Torres. 1997. Resposta de algumas especies do estrato rasteiro de um cerrado a calagem e a adubacao. Pages 87-91. in Leite, L. L., and C. H. Saito, editors. Contribuicao ao conhecimento ecologico do cerrado. Universidade de Brasilia, Brasilia, Brazil. Hoeflich, E. R., J. C. Cruz, J. Pereira, F. F. Duque, and H. Tollini. 1977. Sistema de producao agricola no cerrado. Pages 37-58. in M. G. Ferri, editor. IV Simposio sobre o Cerrado: Bases para Utilizacao Agropecuaria. Editora Itatiaia and Editora da Universidade de Sao Paulo, Sao Paulo, Brazil. Jordan, C. F. 1987. Amazonian rainforests: Ecosystem disturbance and recovery. Springer, New York. King, L. C. 1956. Geomorfologia do Brasil central. R. Bras. Geog. 18(2):l47-265. Klinge, H., J. Adis, and M. Worbes. 1995. The vegetation of a seasonal varzea forest in the lower Solimoes river, Brazilian Amazonia. Acta Amazonica. 25:201-220. Kauffman, J. B., D. L. Cummings, and D. E. Ward. 1994. Relationships of fire, biomass and nutrient dynamics along a vegetation gradient in the Brazilian cerrado. Journal of Ecology. 82:519-531. Leite, L. L., C. R. Martins, and M. Haridasan. 1994. Efeito da

M. Haridasan descompactacao e adubacao do solo na revegetacao espontanea de uma cascalheira no Parque Nacional de Brasilia. Pages 527-534. in Anais do I Simposio Sul-americano e II Simposio Nacional sobre Recuperacao de Areas Degradadas. Foz de Iguacu, Parana, Brazil. Lopes, A. J., and F. R. Cox. 1977. A survey of the fertility status of surface soils under cerrado vegetation of Brazil. Soil. Sci. Soc. Am. Jour. 41:752-757. Macedo, J., and R. B. Bryant. 1987. Morphology, mineralogy, and genesis of a hydrosequence of oxisols in Brazil. Soil Sci. Soc. Am. J. 51:690-698. Medina, E., and E. Cuevas. 1996. Biomass production and accumulation in nutrient-limited rain forests: implications for responses to global change. Pages 221-239. in J. H. C. Gash, C. A. Nobre, J. M. Roberts, and R. L. Victoria, editors. Amazonian deforestation and climate. Institute of Hydrology, London. Meirelles, M. L, and R. P. B. Henriques. 1992. Producao primaria liquida em area queimada e nao queimada de campo sujo de cerrado, Planaltina, DF. Acta bot. bras 6:3-14. Meir, P., J. Grace, A. Miranda, and J. Lloyd. 1996. Soil respiration in a rainforest in Amazonia and in cerrado in central Brazil. Pages 319-329. in J. H. C. Gash, C. A. Nobre, J. M. Roberts, and R. L. Victoria, editors. Amazonian deforestation and climate. Institute of Hydrology, London. Miranda, A. C., H. S. Miranda, J. Lloyd, J. Grace, J. A. Mclntyre, P. Meir, P. Riggan, R. Lockwood, and J. Brass. 1996a. Carbon dioxide fluxes over a cerrado sensu stricto in central Brazil. Pages 353-363. in J. H. C. Gash, C. A. Nobre, J. M. Roberts, and R. L. Victoria, editors. Amazonian deforestation and climate. Institute of Hydrology, London. Miranda, A. C., H. S. Miranda, J. Lloyd, J. Grace, R. J. Francey, J. A. Mclntyre, P. Riggan, R. Lockwood, and J. Brass. 1996b. Fluxes of carbon, water and energy over Brazilian cerrado: An analysis of eddy covariation and stable isotope. Plant Cell and Environment. Miranda, H. S., C. H. Saito, and B. F. S. Dias. 1996c. Impactos de queimadas em areas de cerrado e restinga. Departamento de Ecologia, Universidade de Brasilia, Brasilia, Brazil. Nascimento, M. T, and C. N. Cunha. 1989. Estrutura e composicao floristica de um cambarazal no Pantanal de Pocone - MT. Act. Bot. Bras. 3:3-23Oliveira-Filho, A. T., G. J. Shepherd, F. R. Martins, and W. H. Stubblebine. 1989. Environmental factors affecting physiognomic and floristic variation in an area of cerrado in central Brazil. Journal of Tropical Ecology 5:413:431. Oliveira Jr., R. 1985. Efeito dos microartropodos sobre a decomposicao do folhedo em um cerrado. Master's thesis. Departamento de Biologia Vegetal, Universidade de Brasilia, Brasilia, Brazil. Parada, J. M., and S. M. Andrade. 1976. Cerrados - Recursos Minerais. Pages 195-209 in M. G. Ferri, editor. IV Simposio sobre o Cerrado: Bases para Utilizacao Agropecuaria. Editora Itatiaia and Editora da Universidade de Sao Paulo, Sao Paulo, Brazil. Peres, J. R. R., R. Suhet, M. A. T. Vargas, and A. Drosdowicz. 1983. Litter production in two cerrado vegetations in Brazil. Pesquisa Agropecuaria Brasileira 18:1037-43.

Nutrient Cycling as a Function of Landscape and Biotic Characteristics in the Cerrados of Central Brazil Pivello-Pompeia, V. R., and L. M. Coutinho 1992. Transfer of macro-nutrients to the atmosphere during experimental burnings in an open cerrado (Brazilian savanna). Journal of Tropical Ecology. 8:487-497. Pompeia, S. L. 1989. Aspectos da dinamica dos nutrientes minerals em solo sob vegetacao de campo cerrado (Mogi-Guafu, SP). Master's thesis, Universidade de Sao Paulo, Sao Paulo, Brazil. Ponomareva, V. V., and V. V. Dokuchayev.1984. Watermineral nutrition of plants as a major factor in phylogenesis and soil formation. Pochvovedeniye. 1984(4):29-38. Proctor, J. 1989- Mineral nutrients in tropical and savannah ecosystems. Blackwell, Oxford. Ratter, J. A. 1971. Some notes on two types of cerradao occurring in North Eastern Mato Grosso. Pages 100-102. in M. G. Ferri, editor. Ill Simposio sobre o cerrado. Editora da Universidade de Sao Paulo, Sao Paulo. Ratter J. A., P. W. Richards, G. Argent, and D. R. Gifford. 1977. Observances adicionais sobre o cerradao de solos mesotroficos no Brasil central. Pages 306-316. in M. G. Ferri, editor. IV Simposio sobre o cerrado. Editora da Universidade de Sao Paulo, Sao Paulo. Ratter J. A., P. W. Richards, G. Argent, and D. R. Gifford. 1978. Observations on the forests of some mesotrophic soils in central Brazil. Revta brasil Bot 1:47-58 Ratter, J. A., and T. C. D. Dargie. 1992. An analysis of the floristic composition of 26 cerrado areas in Brazil. Edinburgh Journal of Botany, 49:235-250. Ratter, J. A. 1992. Transitions between cerrado and forest vegetation in Brazil. Pages 417-419- in P. A. Furley, J Proctor, and J. A. Ratter, editors. Nature and Dynamics of Forest-Savanna Boundaries. Chapman and Hall, London. Ratter, J. A., S. Bridgewater, R. Atkinson, and J. F. Ribeiro. 1996. Analysis of the floristic composition of the Brazilian cerrado vegetation. II: Comparison of the wood) ".gelation of 98 areas. Edinburgh Journal of Botany. 53:153-180. Ribeiro, J. F. 1983. Comparacao da concentracao de nutrientes na vegetacao arborea e nos solos de um cerrado e um cerradao no Distrito Federal. Master's thesis.

83

Departamento de Ecologia, Universidade de Brasilia, Brasilia, Brazil. Richter, D. D., and L. I. Babbar. 1991. Soil diversity in the tropics. Advances in Ecological Research 21:315-389. Rodrigues, T. E., and E. Klamt. 1978. Mineralogia e genese de uma seqiiencia de solos no Distrito Federal. R. Bras. Cienc. Solo. 2:132-143. Ross, S. M. 1992. Soil and litter nutrient losses in forest clearings close to a forest-savanna boundary on Maraca island, Roraima, Brazil. Pages 119-143. in P. A. Furley, J Proctor, and J. A. Ratter, editors. Nature and Dynamics of Forest-Savanna Boundaries. Chapman and Hall, London. Sarmiento, G. 1992. A conceptual model relating environmental factors and vegetation formations in the lowlands of tropical south America. Pages 583-601. in P. A. Furley, J Proctor, and J. A. Ratter, editors. Nature and Dynamics of Forest-Savanna Boundaries. Chapman and Hall, London. Schiavini, I. 1983. Alguns aspectos da ciclagem de nutrientes em uma area de cerrado (Brasilia, DF): chuva, producao e decomposicao de liter. Master's thesis. Departamento de Ecologia, Universidade de Brasilia, Brasilia, Brazil. Silva, F. C. 1990. Compartilhamento de nutrientes em diferentes componentes da biomassa aerea em especies arboreas de um cerrado. Master's thesis. Departamento de Ecologia, Universidade de Brasilia, Brasilia, Brazil. Silva Junior, M. C. 1995. Tree communities of the gallery forests of IBGE Ecological Reserve, Federal District, Brazil. Ph. D. Thesis. University of Edinburgh, Edinburgh, UK. Solbrig, O. T., E. Medina, and J. F. Silva. 1996. Biodiversity and savanna ecosystem processes - a global perspective. Ecological Studies, Vol. 121. Springer-Verlag, Berlin, Heidelberg. Villela, D. M. V., and M. Haridasan. 1994. Response of the ground layer community of a cerrado vegetation in central Brazil to liming and irrigation. Plant and Soil 163:25-31. Vitousek, P. M., and R. L. Sanford. 1986. Nutrient cycling in moist tropical forest. Annual Review of Ecology and Systematics 17:137-167

6 Linking Biogeochemical Cycles to Cattle Pasture Management and Sustainability in the Amazon Basin Moacyr B. Dias-Filho, Eric A. Davidson, Claudio J. Reis de Carvalho

Pasture development has become the observed (reviewed by Serrao and Toledo largest anthropogenic disturbance of forest 1990). If left uncontrolled, these invader land in the Amazon basin (Skole et al. 1994, species slowly become dominant and lead to Serrao and Toledo 1990). The area of forests "pasture degradation," a condition characterconverted to cattle pasture in Amazonia is ized by a complete dominance of the weedy currently estimated at approximately 20 community. If left to secondary succession, million hectares. In the Brazilian Amazon forest vegetation usually becomes reestabbasin, most of the conversion of forest land lished on these degraded pasture lands in the to pasture began during the early 1960s to Amazon, although the species composition is the late 1980s, as a consequence of the usually different than that of the primary opening of Amazon highways and govern- forest (Nepstad 1989). The nutrient status of ment policies aimed at regional develop- the degraded pasture soils is among the ment (Hecht 1982, Nepstad et al. 1991, factors that affect the rate of regrowth of the secondary forests. Serrao et al. 1979). One of the first attempts to study soil Pasture productivity and longevity in the Amazon basin seem to be closely related nutrient dynamics under cultivated pastures to soil fertility and nutrient cycling (e.g., Dias in the Amazon basin was conducted in the Filho and Serrao 1987, Serrao et al. 1979). early 1970s by Falesi (1976). The results of Thus, understanding the major biogeo- that chronosequence study in different soil chemical cycles that influence soil fertility types suggested that soil nutrient cycling in under pasture is vital for predicting the pastures differed from that of the traditional consequences of continued conversion of slash-and-burn agriculture. The decline in the tropical forests to cattle pastures. This under- levels of some nutrients in the soil was found standing is also important for devising to be more gradual, and the decline in management technologies that enhance the productivity over time in planted pastures sustainability of these areas and thus slow could, at least in part, be associated with the behavior of available phosphorus in the soil. further deforestation. Although during the first three to five years Although the sampling methodology of after establishment, the productivity of pas- Falesi's chronosequence study has been tures is often good, after that period a rapid criticized (see Hecht 1982), other studies decline in productivity of the planted grasses considering the dynamics of soil fertility in associated with an increased presence of pasture areas in the Amazon basin found herbaceous and woody invaders is generally similar results (Diez et al. 1991, Hecht 1982,

Linking Biogeochemical Cycles to Cattle Pasture Management and Sustainability in the Amazon Basin

Buschbacher 1984, Moraes et al. 1996, Teixeira 1987). The connection between soil fertility (mainly available phosphorus) and pasture productivity and longevity in the Amazon basin 'was further established in many studies conducted from the late 1970s to the early 1990s (Azevedo et al. 1982a, 1982b; Dias Filho and Serrao 1980, 1981, 1982, 1987, Dias Filho et al. 1989, Dias Filho and Simao Neto 1992, Embrapa 1980, Italiano et al. 1982, Serrao et al. 1979). The promotion of efficient nutrient recycling has been acknowledged as a means of enhancing the stability of pasture systems in tropical America (e.g., Dias Filho 1986, Serrao and Toledo 1990, Spain and Salinas 1985). In the Amazon basin, efficient nutrient recycling is even more important, as most of the soils where pasture development is concentrated are nutrient-poor Oxisols and Ultisols. However, there has been very little progress in understanding major biogeochemical cycles in active and abandoned pastures in the Amazon basin. An attempt to directly address the role of grazing animals in nutrient recycling of an active pasture in Amazonia was made by Buschbacher (1984, 1987). More detailed studies have followed this, however, without directly addressing the role of cattle and grazing, and considering specifically only nitrogen and carbon dynamics in pasture soils (Dejardins et al. 1994, Feigl et al. 1995a, 1995b, Neill et al. 1995, 1997, Nepstad et al. 1994, Trumbore et. al. 1995). We review these studies here, and we attempt to identify the most important issues for our understanding of pasture biochemistry: How does nutrient availability in pastures change over time? What nutrients are most critical? What is the role of cattle and grazing? What are the effects of altered hydrology? How does weed invasion affect pasture biogeochemistry? Also, we attempt to identify what management practices are important to promote pasture longevity and sustainability, and to reclaim degraded pasture areas.

85

Land Clearing Methods and Pasture Establishment The method of land clearing can have important and long-lasting consequences for pasture productivity and longevity (Dias Filho 1986, Dias and Northcliff 1985, Seubert et al. 1977). Land clearing for pasture establishment on forested areas in the Amazon basin normally involves cutting the forest and then burning of the vegetation (Dias Filho 1986, Serrao and Toledo 1990, Toledo and Morales 1979). Clearing is normally done with chain saws, generally after the valuable wood species have been removed. Mechanical clearing for pasture establishment using heavy machinery is seldom practiced in the Amazon basin, mainly because of high operational costs, but also due to the detrimental effects mechanical clearing is known to have on soil properties, reducing pasture productivity (reviewed by Dias Filho 1986). Perhaps one of the earliest and most influential studies that compared the agricultural impact of manual (slash-and-burn) and mechanical (bulldozer) clearing for pasture establishment in the Amazon basin was that of Seubert et al. (1977) in Peru. In that study, pasture (Panicum maximum) biomass production, over a 2-year period, was 68 % higher in the area cleared manually. Other studies have followed, with the general conclusion that pastures manually cleared from forest were more productive and that mechanical clearing adversely affected soil physical and chemical properties (e.g., Alegre 1985, Toledo and Morales 1979, Dias and Northcliff 1985). The basic difference between the two land clearing methods is that in the "manual" process, fire is the major agent employed to remove vegetation cover and prepare the land for pasture establishment. From an agricultural perspective, burning can be regarded as a fast and economical process to

86

easily prepare the area for cultivation. From an ecological viewpoint, however, it causes immediate disruption of major biogeochemical cycles through the production and release into the atmosphere of particulates and environmentally significant gases, and the release into the soil of relatively large amounts of nutrients previously immobilized in the aboveground biomass. Forest biomass burning also affects the hydrological cycle by changing rates of land evaporation and water runoff. Losses from the system due to combustion of biomass can be quite high for carbon, nitrogen, and sulfur, and less for major nutrients like phosphorus, potassium, and calcium (Ewel et al. 1981, Frost and Robertson 1985, Kauffman et al. 1995, Sanchez 1976, Wright and Bailey 1982). However, immediate improvement in the soil chemical characteristics - higher pH, reduced aluminum toxicity, and higher nutrient content—is usually observed after forest cutting and burning in the Amazon basin (Buschbacher 1984, Dantas 1989, Dantas and Matos 1980, Diez et al. 1991, Falesi 1976, Seubert et al. 1977, Smyth and Bastos 1984). The general improvement in soil chemical properties after burning is mainly due to constituents in the ash that provide substantial inputs of bases and other elements to the soil. Also, heating caused by burning may contribute to the improvement of soil fertility by enhancing the mineralization of elements (N, Ca, Mg, and P) formerly linked and complexed with organic matter (Giovannini et al. 1990). During pasture establishment, available soil nutrients are generally at their highest levels due to ash deposition (after forest clearing) or chemical fertilization (in reclaimed pastures). In addition, the soil is left unprotected against high temperatures, which accelerates the decomposition rates of fresh organic matter, and against wind and raindrop impact, which contribute to erosion

Moacyr B. Dias-Filho et al.

and soil compaction. For these reasons, a fast and efficient soil cover by the forage plant is key in determining the future of pasture productive longevity. Any failure in pasture establishment will expose the area to an increased nutrient loss and weed invasion that ultimately may lead to premature pasture degradation. According to Dias Filho (1986), forage seed quality, seeding rates, time of planting and of beginning grazing, and the agronomic characteristics of the forage species are the most crucial aspects affecting successful pasture establishment.

Carbon Cycling The most important effect of forest-topasture conversion on the carbon cycle is the release of 100-200 tons C ha'1 from aboveground forest biomass to the atmosphere. However, soil C stocks are often as large or larger than aboveground biomass-C, and the changes in soil C stocks, although much less than changes in aboveground C, can also be significant (Nepstad et al. 1994). Inventories of soil C stocks following tropical pasture formation have shown increases (Bushbacher 1984, Cerri et al. 1992, Chone et al. 1991, Feigl et al. 1995, Fisher et al. 1994, Neill et al. 1996), decreases (Bushbacher et al. 1988, Desjardin et al. 1994, Detweiler 1986, Eden et al. 1991, Garcia-Oliva et al. 1994, Luizao et al. 1992, Street 1982, Veldkamp 1994), and mixed results (Brown and Lugo 1990, Trumbore et al. 1995). The key to understanding these conflicting responses is variation in the productivity of pasture grasses. A comparison of C budgets for forests and pastures in the eastern Amazon was made by Trumbore et al. (1995). In a reformed and fertilized pasture of Brachiaria brizantha, they estimated gains, relative to forest soil C stocks, of over 20 tons soil C ha~ 1 in the top 1 m of soil and a loss of about 0.5 tons C ha'1 in the 1-8 m soil depth interval during the first 5 years following pasture

Linking Biogeochemical Cycles to Cattle Pasture Management and Sustainability in the Amazon Basin

87

reformation. In contrast, they calculated a 10-100 cm depth interval and to 80-90% loss of about 9 tons soil C ha'1 in the top below 1 m depth. Hence, large changes in 1 m of an abandoned, degraded pasture soil soil C can be observed near the surface in a and no change below 1 m. As shown in matter of years, although modest changes in Table 6.1, total C inputs were greatest in the modern soil C at depth are also significant forest, due to high rates of litterfall, but when summed over the entire profile of 8 or 20-30% of the litter C probably decomposes more meters. to CO2 within the litter layer. Root inputs Losses of soil C were also found in pastures were greatest in the top 1 m of soil in the of the eastern Amazon by Desjardin reformed productive pasture, which resulted et al. (1994), whereas increases of soil C in in gains of soil C stocks there. However, the Amazonian cattle pastures have been reportdeeply rooted woody vegetation had been ed in the "western Brazilian Amazon (Cerri et removed from the reformed pasture, result- al. 1992, Chone et al. 1991, Feigl et al. 1995, ing in very low rates of root inputs to deep Neill et al. 1996). The studies in the western soils, causing losses of deep soil C in the Brazilian Amazon were of pasture soils that productive pasture. Low rates of litter and had not been fertilized. It is tempting to root inputs in the degraded pasture caused interpret these results as an indication that loss of soil C in the topsoil. Hence, both total the soils in the west tend to have greater productivity and the distribution of C inputs native soil fertility than those in the east, thus to the soil among litter and roots of varying characteristically supporting more productive depth are the key to determining whether C pastures without fertilization, but this broad will be lost or gained in topsoil and subsoil. generalization may be too simplistic. It is true These changes in soil C stocks are relative- that the probability of finding eutrophic soils ly rapid, due to rapid turnover of soil organ- increases in a westerly direction toward the ic matter (SOM) in these tropical soils. Using Andes, but both eastern and western regions radiocarbon derived from atmospheric test- have a wide variety of soil types. The studies ing of nuclear weapons in the 1960s as a in the western Brazilian Amazon included tracer, Trumbore et al. (1995) estimated that Oxisols, Ultisols, and Alfisols, but these the mean residence time of C in the top 10 taxonomic classifications are too broad for cm of soil is about 3 years for 30% of the reliable regional extrapolation. Both eutrophSOM and 10-30 years for 60% of the SOM. ic and dystrophic soils occur within the Only about 10% of the C in the top 10 cm of orders of Oxisols and Ultisols. soil cycles on a millennial time scale. This In addition to soil fertility, there are differvery old C fraction increases to 40-80% in the ences in climate and there may be differences

Table 6.1 Comparison of C inputs to soils at Fazenda Vitoria, Paragominas, Brazil. C Inputs

Forest (tons C ha-1)

Degraded Pasture (tons C ha-1)

Productive Pasture (tons C ha-1)

Litter Roots (0-1 m depth) Roots (1-5 m depth) Total

4.5 1.9 0.9 7.3

1.4 1.2 1.3 3.9

2.0 3.4 0.2 5.6

Source: Trumbore et al. 1995

Moacyr B. Dias-Filho et al.

88

in management practices between the eastern and western regions of the Brazilian Amazon basin. Most of the eastern Amazon, including the Paragominas study site of Trumbore et al. (1995), has a 3-5 month dry season that limits pasture productivity due to severe water limitation and that may increase the frequency of fire. Frequent fire would reduce nitrogen stocks that might, in turn, decrease grass productivity. Regional differences in pasture management are partly historical. Forest clearing along the Belem-Brasilia highway began in the 1960s and 1970s (Nepstad et al. 199D, when the use of less productive grass species was common and when less was known about optimal grazing intensity, which resulted in severe overgrazing in many areas. Examples of overgrazing probably exist in all regions, but they may be more common in the eastern Amazon. In summary, we know that pasture productivity is the key to predicting whether soil C stocks will increase or decrease following forest conversion to pasture, and we know that several factors, including native soil fertility, fertilization, climate, fire frequency, and grazing intensity, influence pasture productivity. We do not, however, know which of these factors has had the greatest influence in the past and which

will be most important in the future for determining productivity in Amazonian pastures. Nor do we know the areal extent of degraded v. productive pastures. Quantitative estimates of areas of productive and nonproductive pasture lands would be necessary to calculate basin-wide effects of forest-to-pasture conversion on the regional C budget. Finally, we wish to emphasize that sustainability of pasture management practices and maintenance of soil C stocks are practically synonymous.

Phosphorus Cycling Of all the essential plant nutrients, P is most commonly considered a limiting factor to primary productivity in tropical ecosystems on highly weathered soils. Due to its low mobility in the soil and high stability (no biologically important form of P is gaseous), and because inputs (e.g., wet and dry deposition) are roughly equivalent or even higher than outputs (e.g., erosion and runoff), P exists in relatively constant amounts or even increases over time scales of years to centuries in some natural terrestrial ecosystems. In managed ecosystems, however, net losses of P may be high because of harvest of products like grain,

Table 6.2 Forms of P in the top 10 cm of mineral soil at Fazenda Vitoria, Paragominas, Para. NaOH Extractable Site

Primary forest Degraded pasture Reformed pasture 20-yr Secondary forest

Total P (kgPha-i)

194 250 298 209

Organic-P (kg P ha-1)

Inorganic-P (kg P ha-1)

19 30 36 26

26 30 38 28

Mehlich 3 (kgPha- 1 )

1.6 1.2 1.1 0.8

Source: Unpublished data from D. Markewitz and E. A. Davidson. ' Higher values in the pastures are due more to higher bulk density in the compacted pasture soils than due to higher P concentrations.

Linking Biogeochemical Cycles to Cattle Pasture Management and Sustainability in the Amazon Basin

fiber, and roots. The generally higher vulnerability of these ecosystems to surface runoff and erosion can also contribute to P losses. This condition results from incomplete soil coverage by the crop plant and lower infiltration capacity of disturbed soils. Highly weathered tropical soils of oxic and kaolinitic mineralogies often contain large stocks of total P, but the vast majority of this is bound to iron and aluminum oxides in forms that are generally thought to be un-available to plants. Table 6.2 demonstrates that the stocks of total P in the top 10 cm of the mineral soil of a kaolinitic Oxisol at Paragominas, Para, are about 200 kg ha'1, but that less than 1% of the total P is extracted by the Mehlich 3 extraction method. Agronomists often use the Mehlich extraction as an index of P available to plants, but, clearly, this pool is insufficient to provide plant needs of P and must be replenished frequently from other pools (Mengel and Kirkby 1987). It is unclear whether the P that replenishes the plant-available Mehlich pool is derived from enzymatic decomposition of organic P or from gradual desorption of inorganic P, or both. Below 10 cm depth, the concentrations of total P remain similar to those in the top 10 cm, indicating that P is abundant in these soils. However, the concentrations of NaOH-extractable and

Mehlich-extractable P decline to barely detectable levels below 50 cm depth. Phosphorus is most likely a limiting nutrient for plant growth in those ecosystems. For the great majority of Amazon basin soils, available P in the topsoil is known to be naturally low (Leon and Hammond 1985). After forest clearing and burning, available P in the topsoil usually increases due to ash deposition and slash decomposition (e.g., Falesi 1976, Seubert et al. 1977). However, unlike other nutrients which are often maintained at more or less stable levels under pasture, available P in top soil often falls sharply with time after pasture establishment (Falesi 1976), suggesting that a mining effect might be occurring. This decrease in available soil P is normally associated with a decrease in forage grass biomass production, generally leading to pasture degradation (i.e., an increase in weed biomass) and abandonment. In these degraded pasture areas, forage grasses often respond to P fertilization (Table 6.3) but fail to respond to fertilization with other nutrients (Table 6.4), implying that P availability is the major impediment to pasture productivity. In view of its importance to pasture primary production, maintaining relatively high levels of available P is a major challenge to pasture managers in the Amazon basin. However, insufficient knowledge of the biogeochemistry of P in active Amazonian pastures is a

Table 6.3 Response of Andropogon gayanus to P and N fertilization. Fertilizer (kg ha'1)*

Dry Matter (kg ha-1)**

0N +0P

828 1456 405 2875

0 N + 50 P 75 N + 0 P 75 N + 50 P

89

Source: Dias Filho (1986) * Area near Paragominas, Brazil, on yellow latosol (Haplustox) originally under degraded pasture, reclaimed by burning, tilling, planting, and fertilizing in May 1980 (end of wet season). ** Dry matter production evaluated at 20 cm from the soil surface in February 1981.

Moacyr B. Dias-Filho et al.

90

Table 6.4 Dry matter production of a 12-year-old degraded Panicum maximum pasture after weeding and fertilizer application at Paragominas on a yellow latosol (Haplustox). Treatment Treatment

Dry Matter Matter (kg ha' ha-1)*

Control Legumes interplanted Complete** minus N Complete minus P Complete minus K Complete minus S Complete minus lime Complete minus micronutrients Complete

7428 6127 12508 4654 11120 15667 12183 11630 12249

Source: Bias Filho and Serrao (1987). * Tukey's HSD = 6230. ** Complete = 150 N +100 P + 100 K + 50 S + 1000 Lime + 30 micronutrients (in kg ha'1).

major limitation to achieving this goal. The biogeochemical P cycling of active pastures differs from forest and, to some extent, abandoned pasture and agricultural ecosystems in its complexity and unpredictability. Cattle grazing affects the rate of P movement and distribution within the system and increases its potential for loss. In active pasture systems for beef production, the harvested product is the animal, with a potential to export relatively low amounts of P. Considering an average carrying capacity of 0.8 animal units per hectare per year (1 animal unit = 450 kg of live weight) for a typical Brachiaria brizantha pasture with medium productivity, the amount of P that could potentially be exported from the system in one year, due to meat production, would be around 2.5 kg per ha (based on a body composition of 0.7 % P for beef cattle). Under the management conditions typical of most beef cattle pasture systems in the Amazon basin, animals would get their dietary P from two major sources: mineral supplementation and pasture plants. Involuntary soil intake could also be a potential, but difficult to quantify, input source of P.

Because of the low P content (usually < 0.2 %) of forage plants, P supplementation is commonly practiced in most ranches throughout the Amazon basin, particularly in Brazilian Amazonia. Assuming a stocking rate of 0.8 animal units per hectare per year, the annual input of P to the system through animal consumption of mineral supplementation would be around 2.0 kg, which is close to the amount expected to be exported annually by animal products (2.5 kg). The amount thus needed to balance the cycle would be only 0.5 kg. In the absence of phosphate fertilizer inputs, this P must come from the soil pool reserves, through forage consumption. Under the assumption of a forage dry matter P content of 0.12 %, and a stocking rate of 0.8 animal units per hectare, we can estimate that 3.9 kg of P would be ingested by the animal in one year through forage consumption. This gives a total amount of available P for recycling of 3.4 kg ha'1 yr1 (3-9-0.5). Therefore, most of the P taken from the soil by plant uptake and consumed in the forage by the grazing animal on a daily basis, would be returned to the system through animal excreta.

Linking Biogeochemical Cycles to Cattle Pasture Management and Sustainability in the Amazon Basin

Although it may seem straightforward that there is an efficient P recycling through the return of ingested P in animal excreta, in practical terms this efficiency is highly questionable, because feces and urine are not distributed uniformly over the pasture but deposited on specific areas. Usually, areas close to drinking troughs, mineral feeders, or shade receive greater amounts of feces and urine. For example, in a Brachiaria decumbens pasture in the Venezuelan Amazonia, it was found that more than one-half of the dung excreted by Zebu cattle was concentrated in only 30 percent of the pasture area (Buschbacher 1987). The proximity of cattle gathering areas to streams and ponds produces the potential for runoff of P to significantly alter nutrient cycling in aquatic ecosystems, but this topic has not been studied to our knowledge. In addition to greatly altering P distribution within the pasture area due to the patchy distribution of excretion, cattle can also indirectly enhance the potential of P movement in the landscape. This is because cattle transit within the pasture area can create trails which are channels for runoff, and overgrazing and trampling may result in open areas that facilitate losses through runoff of paniculate and dissolved P and erosion of surface soil and plant material. Since most of the excreted P is in the feces

91

and most of the feces are deposited in areas where cattle transit and concentration are more intense, the potential for P loss from the system into surface waters through erosion and runoff can be greatly enhanced during the heavy rains typical of most areas in the Amazon basin. Comparison of the water infiltration capacity of adjacent forest and reclaimed pasture soils in the Paragominas region, eastern Amazonia, has shown that water can infiltrate up to twenty times slower in the soil under pasture (M. B. Dias Filho, unpublished data). Moreover, P transferred in feces to bare trails and campsites will have little or no effect on forage production and P recycling. Cattle also have a great impact on the internal P (nutrient) cycling of forage species. Because grazing is selective and often the plant parts with the greatest amount of nutrients (i.e., young leaves) are preferentially consumed, there is a profound impact on the resorption efficiency of P, as well as other nutrients. This, in turn, stimulates soil P uptake, and if recycling through animal excretion is not efficient (which is often the case, mainly under low stocking rates) and there is no P fertilizer input, a slow depletion of soil P will usually occur. The amount of P removed from the soil by forage grass in a typical Amazonian pasture can be substantial (Table 6.5). Ongoing

Table 6.5 Annual dry matter production (mean of two years) and P uptake (kg of P in plant tops) of Brachiaria brizantha cv. Marandu. Fertilizer (kg P ha'1)*

Dry Matter (kg ha-1 year1)**

P Uptake (kg ha-1 year1)**

0

13900 (696)

13.7 (1.07)

22

23334 (834)

29.5 (1.75)

44

28159 (1651)

39.7 (2.26)

Source: Adapted from Dias Filho and Simao Neto (1992). * Area at Paragominas, Brazilian Amazonia on yellow latosol (Haplustox) originally under degraded pasture, reclaimed by burning, tilling, planting and fertilizing with simple superphosphate. ** Data are mean (± s.e.) n= 4.

92

Moacyr B. Dias-Filho et al.

Table 6.6 P and nitrogen concentration* (% of dry weight) of forage grass (Hyparrhenia rufa), herbaceous weedy vegetation and litter** of a 10-year-old degraded pasture in eastern Brazilian Amazonia. Source

P

N

Grass Weeds Litter

0.10 0.20 0.06

0.63 1.70 0.78

Source: Dantas (1989). * Mean of three sample periods during 2 consecutive years. ** Dead and dry material from many species (mainly weeds).

research (C. J. R. de Carvalho and M. B. Dias Filho, unpublished data) has shown that forage grasses like Brachiaria humidicola and B. brizantha have the ability to respond to low available P in the soil by secreting acid phosphatase at the rhizosphere. Acid phosphatase can solubilize organic phosphate in the soil, increasing the availability of P for plant uptake. If not grazed, or if the pasture is not burned, most of the P absorbed in plant tops will be immobilized by plant tissue and, thus, will be unavailable in the soil. Transfer of P and N by nutrient resorption in forage plants and in the weedy vegetation prior to leaf fall induces low nutrient concentration in litter (Table 6.6) and may in part explain the decreasing amount of available soil P often associated with an increase in the weed biomass of a degrading pasture or with pasture age. Without this biological immobilization, however, these nutrients would be more vulnerable to loss through runoff and erosion. Immobilization of P in the soil in the short or long term by geochemical fixation processes might, in some situations, play an important role in the cycling of P in pasture ecosystems. However, unlike in the acid savanna soils, where high P fixation is prevalent, only a relatively small proportion of Amazon soils (mainly Oxisols and Ultisols of

clayey topsoil texture) are known to have the capacity to fix large quantities of P into relatively insoluble forms (Sanchez 1987). For this reason, P fixation has received far less attention than the role of the biologically mediated organic P transformations in this ecosystem. Geo-chemically fixed P is probably not available for plant uptake and subsequent animal consumption, but could be vulnerable to loss from the system via soil erosion. In summary, we know that available P is often found in low amounts in the great majority of Amazon basin soils. After forest clearing and burning, available P levels increase in topsoil, but during pasture utilization these levels often fall sharply. This decrease in P availability is usually associated with a decline in pasture productivity and an increase in weed biomass. The accumulation of weed biomass sequesters soil P and, thus, temporarily decreases its availability in the soil. In these degraded pastures, forage grasses often respond to P fertilization but fail to respond to fertilization with other nutrients, strongly suggesting that P availability is the major limitation to pasture productivity. Cattle probably play a key role in the P dynamics of active pastures areas by affecting P distribution patterns within the pasture area and facilitating P losses through runoff and erosion.

Linking Biogeochemical Cycles to Cattle Pasture Management and Sustainability in the Amazon Basin

93

Calopogonium spp., Desmodium spp.) can be found in most active pasture areas, and their Land use changes, such as conversion contribution to the N budget is unknown. of tropical forests to cattle pastures, affect When degraded pastures are reformed and biological N fixation, mineralization, nitrifi- fertilized with P, grass biomass production cation, and denitrification (Davidson et al. can be substantial (Tables 6.4 and 6.5), and 1993, Keller and Reiners 1994, Matson et al. it is difficult to believe that all of the N in 1987, Montagnini and Buschbacher 1989, this new plant biomass ( > 100 kg N ha*1) Neill et al. 1995). Matson et al. (1987) esti- could be derived from nutrient-poor degradmated that N mobilized annually from ed pasture soil. Perhaps symbiotic N fixation deforestation was equivalent to more than by the spontaneous herbaceous legumes half of the industrial N fixed globally and or associative N fixation in the grass rhizosgreater than the total amount of N delivered phere may be necessary to supply sufficient to oceans by rivers. N to meet plant needs. If N fixation does Tropical ecosystems on highly weathered occur in active pastures, that may explain soils present an enigma to our current the absence of a response in grass growth to understanding of the N cycle. Based on N fertilization. ratios of elements found in foliage and litAssociative N fixation, the process in terfall of both leguminous and nonlegumi- which free-living N fixing bacteria utilize nous trees, N appears to be an abundant root exudates as energy sources and can nutrient relative to P in forest vegetation maintain active nitrogenase enzymes in growing on many soils dominated by the low O2 environment of the rhizosphere, kaolinitic and oxic mineralogies (Vitousek has been inferred for tropical grasses such and Sanford 1986). And yet, despite the as sugar cane (Dobereiner et al. 1972) apparent relative abundance of N in these and several species of pasture grasses ecosystems, leguminous trees are abundant (Boddey and Victoria 1986). Conditions in in these forests, sometimes being nodulated the grass rhizosphere may be conducive to and sometimes not nodulated (Salati et al free-living N fixing bacteria, including 1982). Do these legumes allocate energy to abundant root exudates, respiration that N fixation despite high availability of N in consumes O2, soil pH usually in the range these ecosystems? Is N abundant in these of 6 to 7, and sufficient P from fertilization. forest ecosystems because symbiotic N fixa- Given the high productivity of these tion exceeds plant demands? (See Cuevas, tropical grasses, it seems plausible that their this volume, for a thorough discussion of rhizosphere N fixation could be significant, nutrient cycling in Amazon forests.) but conclusive evidence for associative Biological N fixation may also be impor- N fixation in pastures is still lacking. tant in active and abandoned pastures, Chronosequence studies in Rondonia although the evidence for it is mostly showed similar or higher N stocks in the inferential. In abandoned pastures, nodulat- surface soil of pastures compared to ed leguminous species are common early forests, although spatial heterogeneity made successional plants, suggesting that symbiot- it difficult to conclude that significant ic N fixation is advantageous for growth in increases in N stocks due to associative N these degraded soils. The use of planted fixation in the pastures had occurred grass-legume pastures is not common in (Piccolo et al. 1994). the Amazon, but spontaneous growth of Comparisons of total N stocks may belie herbaceous legumes (e.g., Centrosema spp., more important differences in "plant-avail-

Nitrogen Cycling

Moacyr B. Dias-Filho et al.

94

able N" among forest and pasture soil. Total ed pasture (Table 6.7). These results provide N is a poor indicator of plant-available, further inferential evidence that biological N actively cycling N pools. Preliminary results fixation or some other N inputs result in from studies at Paragominas (L. V. Verchot partial recovery of the N cycle in secondary and E. A. Davidson, unpublished data) forests and improved pastures. Nutrients mineralized from soil organic show that net N mineralization and net nitrification rates are low in the pastures matter during forest clearing usually support compared to mature forest (Table 6.7). The shifting agriculture or productive pastures NO and N2O flux data follow the same trend for only a few years in many tropical as the net N mineralization data: primary regions, resulting in nutrient-poor, degraded forest > secondary forest > improved pas- soils in abandoned farms and pastures ture > degraded pasture (Table 6.7). (Keller and Reiners 1994, Tiessen et al. 1994, Because N gas emissions are strongly affect- Serrao and Toledo 1990). In the case of ed by N cycling rates and N availability in N, there are several mechanisms by which the soil (Firestone and Davidson 1989), N can be rendered unavailable, redistribthese data also imply decreased N availabil- uted, or lost from pasture soils. Root systems of tropical pasture grasses ity in the pastures. In Rondonia, a similar pattern has been observed, in which cattle and decomposing litter are known to pastures ranging from 3 to 80 years old had immobilize significant amounts of N lower rates of net N mineralization and net (Bushby et al. 1992, Robbins et al. 1989), nitrification than did adjacent forest soils perhaps into organic-N forms that are not readily remineralized. This immobiliza(Neill et al. 1995). The secondary forest at Paragominas has tion may induce N deficiency by reducing recovered much of the net N mineralization both pasture and animal production in some capacity that is characteristic of primary tropical pasture ecosystems (Robbins et al. forest, and the improved (reformed) pasture 1989). It is expected that an increase in stockhas higher net nitrification than the degrad- ing rate or pasture burning, by reducing the

Table 6.7 Mineralization indices and N trace gas emissions for soils from primary forest, secondary forest, reformed pasture, and degraded pasture at Fazenda Vitoria.* Land Use

NetN Mineralization* * (mg N g-i 7d-J)

Primary forest

2.57 (0.62) a

2.25 (0.22) a

2.5 (0.7) a

1.2 (0.3) a

Secondary forest

1.55 (0.39) ab

0.30 (0.29) be

0.9 (0.2) b

0.3 (0.1) b

Net Nitrification** (mg N g-1 Td-1)

N2O Emissions*** (kg N ha-1 yr1)

NO Emissions*** (mg N g-i 7d-i)

Improved pasture

0.97 (0.09) be

0.46 (0.09) b

0.3 (0.3) b

0.8 (0.3) b

Degraded pasture

0.33 (0.11) c

-0.22 (0.05) c

0.0 (0.1) b

0.7 (0.3) b

Source: L. V. Verchot and E. A. Davidson, unpublished data. * Means and (s.e.) within a column followed by the same letter are not statistically different at = 0.05 by Duncan's Multiple Range test. ** Net N mineralization and net nitrification values were obtained from laboratory incubations (7 days) at room temperature. *" Annual trace gas emissions are extrapolated from measurements on 9 dates during the wet season and 6 dates during the dry season.

Linking Biogeochemical Cycles to Cattle Pasture Management and Sustainability in the Amazon Basin

amount of litter, can reverse this trend, but this mobilization of N would also make the N in the system more prone to loss from the ecosystem. Nitrogen losses due to the direct effect of the grazing ruminant have usually not been accounted for in studies of N dynamics in active Amazonian pastures (but see Buschbacher 1987). Cattle interfere in the internal N cycle of pasture plants, alter nitrogen distribution in the pasture area, and change botanical composition, and these effects are important for the N cycle of active pastures. Much of the N loss that occurs in active pastures is accounted for by the heavy deposition of excreta around campsites, drinking and mineralization troughs, and cattle trails. The greatest potential for N loss are in urine patches which may receive very high amounts of N (> 300 kg N ha^1). Nitrogen returned in urine is rapidly hydrolyzed to ammonia, becoming available for plant uptake. However, because of the very high amount of N in urine patches, plant uptake and utilization is usually not efficient, and large amounts of N may be lost through ammonia volatilization, which is further enhanced by the high temperatures at ground level characteristic of tropical pastures. As pasture utilization increases it is expected that N losses from the system would also increase as a result of more N passing through the animal and being excreted. For a typical active B. brizantha beef cattle pasture in Brazilian Amazonia with an average carrying capacity of 0.8 animal units per hectare per, the amount of N that would be converted in live weight gain and potentially exported from the system in one year as animal product would be about 7 kg N ha*1 (based on a body composition of 2% N for beef cattle). Assuming an annual herbage consumption of about 3300 kg (9 kgd'1 x 365d) of B. brizantha dry matter per hectare, we can estimate that 33 kg of N are ingested by the animal in one year

95

(based on an average N content of 1% in B. brizantha dry matter). Considering that only about 10% of the N ingested by the animal is retained, 30 kg of the ingested N is excreted in urine and dung. If a 30% recovery of excreta N by pasture plants is assumed (Ball et al. 1979, Ledgard and Sanders 1982, Ledgard et al. 1982), then about 20 kg of N returned to each hectare of pasture via excreta could potentially be lost during the period of one year through volatilization of ammonia, leaching, erosion, and denitrification. Since N fertilization and protein or urea supplementation are not common management practices in beef cattle pasture systems in the Amazon basin, N inputs are mostly restricted to biological N fixation and additions in precipitation. Loss of N from the system must be balanced by atmospheric deposition, N fixation, or depletion of soil N through mineralization of soil organic matter. Increases in emissions of NO and N2O have been observed in young tropical cattle pastures relative to forests (Keller et al. 1993, Luizao et al. 1992), which is due to an initial pulse of net N mineralization following forest clearing that provides mineral N for nitrifying and denitrifying bacteria (Firestone and Davidson, 1989)- However, Keller and Reiners (1994) found that N gas emissions declined below levels of the primary forest in pastures that were older than 10 years in the Atlantic coastal plain of Costa Rica. There are indications that the period of enhanced N gas emissions in young pastures of the Amazon may be more brief (i.e., only 1—3 years) than "was observed in Costa Rica (personal communication of preliminary results from Rondonia by Paul Steudler and from Para by Louis Verchot). The reason for this difference is unknown, but the volcanic soils of Costa Rica may be more fertile with respect to N availability. If it is true that N2O emissions from soils of most Amazonian pastures

96

Moacyr B. Dias-Filho et al.

that are > 3 years old are lower than emissions from the primary forests (Table 6.7), then one effect of deforestation at the basin scale may be a net reduction in N2O emissions. Seasonality of precipitation is also an important factor controlling N trace gas emissions from Amazonian soils. At Paragominas in the highly seasonal eastern Amazon, N2O is the most important N gas during the wet season when diffusivity is restricted, thus favoring the more reduced N gas form (Table 6.8). During the dry season, the more oxidized NO form is the dominant N gas emitted. Differences in total N gas emissions among land uses is controlled by N availability as indicated by net N mineralization and net nitrification (Table 6.7), while the seasonal variation in soil moisture content provides the controls over the relative proportions of NO and N2O emissions (Table 6.8). In addition to affecting trace gas emissions, lower N availability in old pastures relative to primary forests may also have implications for stream water chemistry. Preliminary results from studies in Rondonia and Para indicate very low concentrations of NO3 in first-order streams draining pastures (C. Neill and D. Markewitz, personal communication). In summary, N cycling processes clearly change when forests are converted to cattle pastures. In Amazonia, soil N availability

generally declines (except perhaps during a brief period of high N availability immediately after clearing), which results in lower emissions of N trace gases and probably lower hydrologic export of NO3. Direct export of N via the cattle is estimated at about 7 kg N ha"1. Cattle are also important in redistributing N within the pasture by concentrating N in excreta around campsites, drinking troughs, and cattle trails, where the potential for further N loss may be as high as 20 kg N ha"1. It is not clear whether the N needs of pasture grasses are provided by associative N fixation or if the most successful introduced grass species are simply well adapted to obtaining N from nutrient-poor soils. Similarly, it is unclear whether reduced N availability in old pasture soils limits rates of regrowth of secondary forests upon pasture abandonment, or if N fixation rates in young secondary forests keep up with demand for N while some other factors limit forest regrowth. For much of the Amazon basin, N fertilization does not appear necessary, but nevertheless, the N nutrition of pasture grasses needs to be better understood for developing sustainable management practices. Moreover, altered N cycling processes following land use change clearly affect the exchange of N between the atmosphere and the biosphere and between terrestrial and aquatic ecosystems.

Table 6.8 Preliminary results for seasonal variation in N gas emissions at Fazenda Vitoria, Paragominas, Brazil, 1995. March(Wet Season) N2O NO (ng N cm-2 hr1) (ng N cnr2 hr1) Primary forest Secondary forest Active pasture Degraded pasture

2.6

1.6 1.0 0.7

1.0 0.4 0.9 0.5

Source: L. V. Verchot and E. A. Davidson, unpublished data.

September(DrySeason) N20 NO (ng N cnr2 hr1) (ng N cm'2 hr-1)

0.6 0.3 0.1 0.1

1.5 0.3 0.2 0.4

Linking Biogeochemical Cycles to Cattle Pasture Management and Sustainability in the Amazon Basin

Acidity and Base Cations The effective cation exchange capacity (ECEC) of the variable-charge kaolinitic and oxic soils of the Amazon is typically low, and what little ECEC exists is pH dependent. At pH 4 to 5 (in water), which is characteristic of most of the primary forest soils, the ECEC is often in the range of 2-3 cmol charge per 100 g topsoil. Depending on the soil type, 20-80% of the ECEC may be base saturated (with the lower end of this range being more common), and the remainder is exchangeable acidity (mostly aluminum). When the forest is cleared and the site is burned, the pH of the new pasture soil often increases to 6 or 7. The ECEC often doubles and the base saturation increases. In effect, a significant fraction of the base cations (Ca, Mg, K) that were present in the above ground biomass of the forest is retained in the soil as a result of the increased ECEC caused by the change in pH of these variable charge soils. While observations of increased base cation content in the soil following fire are common (Andreux and Cerri 1989, Ewel et al. 1981, Kaufman et al. 1995, Moraes et al. 1996, Sanchez et al. 1983) cation budgets that trace masses of nutrients from the primary forest biomass to the soil and then to pasture or secondary forest vegetation are lacking. Kaufman et al. (1995) calculate that < 10% of the base cations in the aboveground forest biomass are lost during forest clearing and burning in Para and Rondonia, but they caution that most of the cations were found in the ash immediately after the fire, which could be prone to subsequent loss by erosion. On the other hand, if the ash becomes incorporated into the soil, the increased pH-dependent ECEC of the soil may be sufficient to retain those nutrients. Studying chronosequences of pastures in Para and Mato Grosso, Falesi (1976) found that the pH, ECEC, and base saturation of the topsoil remained elevated in all pastures. The

97

oldest pasture he studied was 11 years. These results indicate that, provided serious erosion is avoided by good pasture management, base cations can be retained. A similar result was observed by Moraes et al. (1996) in two pasture chronosequences in Rondonia. The pH and the sum of base cations remained elevated in pasture soils relative to the forest in all cases except an 81-year-old pasture, where the pH was still slightly elevated and the sum of base cations was about the same as the primary forest. They also show a trend of gradual decreasing pH and sum of base cations in pastures older than 5 years. Whether the gradual loss of base cations is due to leaching, erosion, repeated fires, or harvest exports is not known. Hence, base cations are not retained by the soil indefinitely, but it appears that losses often do not become significant for a decade or more. Fertilization trials conducted throughout the Amazon basin have shown no significant responses of unproductive (i.e., degraded) pasture areas, ranging from 8 to 13 years old, to liming or potassium amendments (Dias Filho and Serrao 1987, Serrao et al. 1979), reflecting the adequate pH and base saturation status of these areas. As in the case for P and N, cations are obtained by foraging over a large area and are then deposited as excreta on a small area, thereby becoming concentrated, principally near waterers, mineral feeders, and campsites. Potassium is mostly excreted in the urine, while the major pathway of excretion for calcium and magnesium is in the feces (reviewed by Barrow 1987). About 90% (circa 70 kg ha"1 yr1 for a B. brizantha pasture •with a medium productivity) of the ingested potassium is returned to the soil via excreta in an immediately available form (K). Because it is excreted in a soluble form, highly concentrated in urine patches, small amounts of K can be lost from soil through leaching if it is not efficiently absorbed by roots or retained in the soil. Pasture over-

98

Moacyr B. Dias-Filho et al.

grazing and the premature grazing of recent- ary forest growth is limited by the availability of ly burned areas can greatly contribute to these nutrients. Intuitively, it seems that growth increase these losses by directly interfering in of pasture and forest vegetation on highly the uptake efficiency by pasture species and weathered, acidic soils with low cation by increasing the potential of erosion and exchange capacity should be limited by the leaching. Periodic pasture burning could aid availability of base cations, but Amazonian pasin the cycling of base cations, particularly the ture soils appear surprisingly capable of retainless mobile Ca that is immobilized in slash ing base cations, and the same may be true for (woody residue from forest), litter, and soils of secondary forests. standing weedy vegetation. On the other hand, repeated fires or fires too late in the Hydrological Cycle dry season, when pasture recovery would be slow, could make base cations (and other The modification of natural ecosystems, as when all the plant cover is substituted nutrients) more prone to loss by erosion. For beef cattle pasture systems in the during conversion of forests to cattle pasAmazon basin, only calcium, among the base tures, affects the transport of water within cations, is normally added to the system via these systems. These changes result from pasture management practices. This is modifications of turbulence, radiation balbecause animal supplementation with bone ances, temperature, and humidity. Pasture meal or bicalcium phosphate is commonly canopies are more uniform than forests, have practiced in most beef cattle systems a more smooth surface, and, therefore, pasthroughout Amazonia. For a typical active tures have a different microclimate (Grace pasture with a medium carrying capacity, the 1994). The difference in the conditions amount of calcium entering the system in between pastures and forests are greatest in one year through mineral supplementation the dry season. Measurements made above would be around 4.0 kg per hectare which is the canopy in active pastures of B. brizantha close to the amount expected to be exported in Ji-Parana, Rondonia, during the dry season in animal products. show that the specific humidity deficit (D) We are not aware of published results on the above the surface in an area of pasture is changes in soil acidity and base cation reten- higher (19.0 g kg"1), than that of a forest area tion in secondary forests of the Amazon. (12.5 g kg-1) (McWilliam et al. 1996). Close to Preliminary results of a study of a 20-year-old the soil surface, D is even higher. In a forest at Paragominas indicate that soil pH, degraded pasture in the region of ECEC, and base saturation remain elevated in Paragominas, mean maximum values of D 10 the top 20 cm of mineral soil (D. Markewitz cm from the soil surface were in the order of and E. A. Davidson, unpublished data). 24.7 g kg"1 in the dry season, while below However, this forest is one of the sites the canopy of an adjacent primary forest classified by Uhl et al. (1988) has having had these values were not above 6.0 g kg'1. The "light use" while in the pasture phase. It is pos- higher values of D in areas of degraded sible that more intensive use of pastures that pastures were a result of the high temperamight result in greater harvest export, greater tures and low air humidity typical of these erosion, or a longer period for leaching may areas during the dry season (Nepstad et al. result in depleted base cation stocks upon pas- 1996). During the rainy season, the ture abandonment. Woody forest vegetation maximum temperatures and D are similar requires large quantities of cations, particularly between forest areas and abandoned Ca, but we do not know if the rate of second- pastures (Nepstad 1989). Studies conducted

Linking Biogeochemical Cycles to Cattle Pasture Management and Sustainability in the Amazon Basin

during many years in Amazonia show that the substitution of forest by pastures has caused a reduction of 11% in net radiation, due mainly to differences of albedo and long-wave net radiation (Wright et al. 1992, Gulf et al. 1996). Mobilization of "water from the soil is closely related to root depth and root density in each layer of soil. Fine roots of active B. brizantha pastures, established in deeply weathered clayey soils in eastern Amazonia, reach depths of 8 m or more (Nepstad et al. 1994). In abandoned pastures (50% B. humidicola and P. maximum cover and 50% invading shrubs and small trees), fine roots ( < 1 mm in diameter) were found at depths of 12 m (Nepstad et al. 1994). Fine-root biomass in the superficial soil layers of an active pasture in Paragominas, eastern Amazonia, was 3 times higher than that found in an adjacent primary forest area. Fine root biomass in the active pasture decreased by a factor of 100 between the surface and 6 m depth. In an abandoned pasture area, the distribution pattern of fine-root biomass was similar to that observed in the deeper soil layers of the forest ecosystem. This pattern is associated with the fine roots of the existing dicotyledonous invading species. The monitoring of the soil plant available water (PAW) between 0 and 8 m depth in the soil has shown that during the dry season, in active B. brizantha pastures areas, a considerable use of water reserves occurs primarily in the top 2 m of soil. This is similar to observations made in primary and secondary forest areas. However, below 2 m, the depletion of the soil water reserves was greater in the forest ecosystem (Jipp et al. 1998). In general, active pasture ecosystems have a greater proportion of fine roots in soil layers between 0—2 meters, and the water in these layers is depleted more quickly, while a major part of the water reserve in the soil is stored in deeper layers. As the pasture

99

is invaded by dicotyledonous species, which often have deeper root systems, nutrient cycling may be intensified as water reserves (and nutrients) in deep soil layers are accessed. For example, the surface soil below Cordia multispicata plants in degraded pastures is richer in Ca, Mg, and K relative to surface soil under other species as a result of inputs from the nutrient rich Cordia litter (Vieira et al. 1994). It has also been shown that in Cecropia palmata, a pioneer species that maintains high stomatal conductance, P levels in leaf tissue are always high, even in dry periods (Denich 1989). These results suggest that the ability of these species to access soil water and maintain a transpirational flux may also be influencing their ability to absorb soil nutrients. The progression of plant succession through the formation of "vegetation islands" and later secondary forest greatly changes the hydrological cycle at a local scale. This can be demonstrated by the similarity between the water depletion profiles obtained in secondary and primary forests (Jipp et al. 1998). If the reduction in plant-available water below 4 m depth in pastures is small and the losses of rainwater through grass canopy interception are also low, it is possible that there is a greater infiltration to the water table from pasture soils, as well as nutrient losses through erosion and runoff during the rainy season (Dunin 1987, Nepstad et al. 1994). Most of the water lost to the atmosphere in the pasture system is subjected to stomatal regulation, which is influenced by environmental variables such as available water in the soil, and D. In Manaus, central Amazonia, at the beginning of the dry season, very high values for stomatal conductance (gs) at mid day were observed in the forage grass B. decumbens. These reached extremes of 1.0 mol nr2 s"1, without a correlation between gs and D. However, these values fell drastically in response to

Moacyr B. Dias-Filho et al.

100

reductions of soil available water (Roberts et al. 1996). On the other hand, the values for maximum gs obtained in pastures of P. maximum during the rainy season in Maraba, eastern Amazonia, rarely exceed 0.40 mol nr2 s"1, in the period between 10:00 am and 2:00 pm. The most frequently observed values were around 0.25 mol rrr2 s"1. Stomatal behavior of P. maximum showed a negative correlation with an increase of D in the air in the dry season. The lowest values of gs (0.10 mol rrr2 s'1) were observed in periods when D was greater than 15 g kg"1- In B. brizantha pastures, no clear relationship between gs and D was observed. However, the effect of the reduction of soil available water was drastic, causing the gs close to midday to be reduced from values of 0.62 mol rrr2 s"1 to only 0.17 mol rrr2 s'1 at the beginning of the dry season (MeWilliam et al. 1996). Under normal conditions of water availability, values for LAI in pastures of B. brizantha have been measured above 4.0. However with the establishment of a water deficit in the soil, these values decrease to below two or even lower in pastures of P. maximum (Roberts et al. 1996). A similar situation is found in abandoned pastures in eastern Amazonia, where a reduction of approximately 68% of green tissue has been observed in the dry season, while in an adjacent area of primary forest this reduction was only 16% (Nepstad et al. 1994). Primary forests, which have deep root systems and little seasonal variation in LAI, maintain stable subcanopy microclimatic conditions and transpirational flux, even during the dry season. Because of an evergreen forest canopy, the return of the rainy season has less impact on the microclimate near the soil in the forest than in the pastures, and the deep soil water stores are also more efficiently recharged in the forest. In the active pastures, the response to environmental water deficits can be influ-

enced by the grass species. In general, however, gs values are normally high when water supply is adequate, but decreases abruptly in situations when both high evapotranspiration demand and low rainfall occur. Despite stomatal control of water vapor loss, the drying of most aboveground pasture biomass is usually observed in areas with an intense dry season. This leads to a dramatic reduction in the flux to the atmosphere of water stored in soil layers below 4 m. This desiccation and reduction in LAI exposes pasture areas to more severe conditions of temperature, wind, and humidity, which consequently cause greater disposition to fire. After the beginning of the rainy season, the upper layers of the pasture soil become saturated quickly, creating a risk of surface runoff, erosion of sediments and nutrients to rivers, and increased water loss to groundwater seepage. In abandoned pastures invaded by deep-rooted dicotyledonous shrubs, some of the plant-available water below the rooting depth of the grasses can be transpired. As secondary forests grow, the hydrologic characteristics of the forest become more like those of the primary forest.

Conclusions The biogeochemistry of active pastures is perhaps one of the least studied processes within managed systems of the Amazon basin. Although the dynamics of the major nutrients in this ecosystem are fairly well known, mostly through agronomic trials conducted during the last two decades, the biogeochemical processes governing these dynamics are still poorly understood. One of the greatest gaps in understanding the biogeochemistry of active pasture systems concerns the role of the grazing cattle in affecting the nutrient cycling processes. Recent studies conducted in active Amazon basin pastures have emphasized the dynam-

Linking Biogeochemical Cycles to Cattle Pasture Management and Sustainability in the Amazon Basin

ics of N and C without paying sufficient attention to important pasture management issues like stocking rate or the frequency (and methods) of weed control. Phosphorus, probably the most important element for pasture primary production in the Amazon basin, has received far less attention than N and C. Most of what we know about phosphorus cycling in active Amazonian pastures is derived from agronomic trials conducted in the Brazilian Amazon and extrapolation of research results obtained in other regions. The increased vulnerability of bioactive elements to loss from the pasture ecosystem is often believed to be one of the greatest differences between this and the antecedent primary forest ecosystem. While this is true during the pasture establishment phase, when the soil is left unprotected from rain, wind, and excessive solar heat, it has been observed that well-managed pastures are usually capable of maintaining soil nutrients at rather constant levels through time, the exception being available phosphorus. In many circumstances, low-productivity pastures can be reclaimed with relatively small additions of phosphate fertilizer, strongly suggesting that all other major plant nutrients (and soil pH) are present in adequate levels in the soil. While phosphorus nutrition can be met relatively easily with fertilization, another challenge for developing sustainable management practices in pastures of the

101

Amazon basin is to balance the need for and means of weed control in order to avoid land degradation. Weed control for pasture reclamation often includes the use of herbicides and/or the removal of tree root biomass by bulldozers, which prevents resprouting of treelets. This practice, along with frequent use of fire, may very likely delay the rate of regrowth of secondary forests, if the pasture is eventually abandoned. Hence, while some management practices, like maintaining good ground cover and minimizing erosion, are salutary for both agroecosystems and successional forest ecosystems, other practices, like bulldozing and herbiciding, may slow ecosystem recovery and commit the site to long-term dependency on intensive inputs to maintain pasture productivity. Intensification of pasture management, if done soundly, may lead to sustainable and permanent pasture usage for some areas. If done poorly, it may lead to impoverished and degraded lands. As abandoned pasture areas become increasingly invaded by shrubs and small trees, which in time lead to secondary forest formation, the biogeochemical cycles of plant nutrients and the hydrological cycle are expected, eventually, to resemble the cycles originally found in the primary forest. The rate of recuperation of biogeochemical cycles in secondary forests and the factors that influence those rates, however, deserve further attention.

Literature Cited Alegre, J. C. 1985. Effects of land clearing and preparation methods on soil physical and chemical properties and crop performance on an Ultisol in the Amazon Basin. Ph.D. thesis, Raleigh, North Carolina State University. Andreux, R, and C. C. Cerri. 1989. "Current trends in the research on soil changes due to deforestation, burning and cultivation in the Brazilian tropics." Toxicology and Environmental Chemistry 20/21: 275-283. Azevedo, G. P. C. de, A. P Camarao, J. B. da Veiga, and E.A.S. Serrao. 1982a. "Introdu^ao e avaliagao de forrageiras no

municipio de Maraba-PA." Belem, EMBRAPA-CPATU. p. 12. (EMBRAPA-CPATU. Boletim de Pesquisa, 46). Azevedo, G. P. C. de, A. P Camarao, J. B. da Veiga, and E.A.S. Serrao. 1982a. 1982b. "Introducao e avaliacao de forrageiras no municipio de Sao Joao do Araguaia, Estado do Para." Belem, EMBRAPA-CPATU. p. 23. (EMBRAPA-CPATU. Boletim de Pesquisa, 47). Ball, P. R., D. R. Keeney, P. W. Theobald, and P. Nes. 1979. "Nitrogen balance in urine-affected areas of a N.Z. pasture." Journal of Agronomy 71: 309—314.

102

Boddey, R. M., and R. L. Victoria. 1986. "Estimation of biological nitrogen fixation associated with Brachiaria and Paspalum grasses using 15N labeled organic matter and fertilizer." Plant and Soil 90: 265-292. Borrow, N. J. 1987. "Return of nutrients by animals." In: Managed Grasslands: analytical studies, Ecosystems of the world; 17B, ed. R. W. Snaydon (Elsevier, Amesterdam), pp. 181-186. Brown, S., and A. E. Lugo. 1990. "Effects of forest clearing and succession on the carbon and nitrogen content of soils in Puerto Rico and U.S. Virgin Islands." Plant and Soil 124: 53-64. Buschbacher, R. 1984. Changes in productivity and nutrient cycling following conversion of Amazon rainforest to pasture. Ph.D. Dissertation, University of Georgia, Athens. Buschbacher, R. 1986. "Tropical deforestation and pasture development." Bioscience 36: 22-28. Buschbacher, R. 1987. "Cattle productivity and nutrient fluxes on an Amazon pasture." Biotropica 19: 200-207. Buschbacher, R., C. Uhl, and E. A. S. Serrao. 1988. "Abandoned pastures in eastern Amazonia II. Nutrient stocks in the soil and vegetation." Journal of Ecology 76: 682-699. Bushby, H. V. A., I. Vallis, and R. J. K. Myers. 1992. "Dynamic of C in a pasture grass (Panicum maximum var. trichoglume)-soi\ system." Soil Biology and Biochemistry 24: 381-387. Cerri, C. C., B. Volkoff, and F. Andreux. 1991. "Nature and behavior of organic matter in soils under natural forest, and after deforestation, burning and cultivation, near Manaus." Forest Ecology and Management 38: 247-257. Cerri, C. C., J. F. L. Moraes and B. Volkoff. 1992. "Dinamica do carbono organico em solos vinculados a pastagens da Amazonia Brasileira." Rev. INITA Inv. Agr. No. 1, Tomo I, pp. 95-102. Charley, J. L., and B. N. Richards. 1983. "Nutrient allocation in plant communities: mineral cycling in terrestrial ecosystems." In: Physiological plant ecology IV: ecosystem processes, mineral cycling, productivity and man's influence, ed. Vol. 12D, eds. O. L. Lange, P. S. Nobel, C. B. Osmond, and H Ziegler, (Encyclopedia of plant physiology; new ser.), pp. 5-45. Chone, T., F. Andreux, J. C. Correa, B. Volkoff, and C. C. Cerri. 1991. "Changes in organic matter in an oxisol from the central Amazonian forest during eight years as pasture, determined by 13C composition." In: Diversity of environmental biogeochemistry, ed. J. Berthelin (Elsevier, New York), pp. 307-^05. Culf, A. D., J. L. Esteves, A. de O. Marques Filho, and H. R. da Rocha. 1996. "Radiation, temperature and humidity over forest and pasture in Amazonia." In: Amazonian deforestation and climate, eds. J. H. C. Gash, C. A. Nobre, J. M. Roberts, and R. L. Victoria (John Wiley & Sons, London), pp. 175-191. Dantas, M. 1989. Studies on succession in cleared areas of the Amazon rain forest. Ph.D. dissertation. Oxford University, Oxford. Dantas, M., and A. de O. Matos. 1980. "Estudos fitoecologicos do tropico umido Brasileiro: III. Conteudo de nutrientes em cinzas de floresta e capoeira, Capitao Poco, PA." Belem, EMBRAPA-CPATU. p. 23. (EMBRAPACPATU. Boletim de Pesquisa, 24).

Moacyr B. Dias-Filho et al. Davidson, E. A., P. A. Matson, P. M. Vitousek, and R. Riley. 1993. "Processes regulating soil emissions of NO and N2O in a seasonally dry tropical forest." Ecology 74: 130-139. Denich, M. 1989. "Estudo da importancia de uma vegetacao secundaria nova para o incremento da produtividade do sistema de producao na Amazonia oriental brasileira." EMBRAPA/CPATU—GTZ, Belem, Brasil. Desjardins, T., F. Andreux, B. Volkoff, and C. C. Cerri. 1994. "Organic carbon and 13C contents in soils and soil size-fractions, and their changes due to deforestation and pasture installation in eastern Amazonia." Geoderma 61: 103-118. Detweiler, R. P. 1986. "Land use change and the global carbon cycle: the role of tropical soils." Biogeochemistry 2: 67-93Dias, A. C. C. P., and S. Nortcliff. 1985. "Effects of two land clearing methods on the physical properties of an Oxisol in the Brazilian Amazon." Tropical Agriculture. 62: 207-212. Dias Filho, M. B., 1986. "Especies forrageiras e estabelecimento de pastagens na Amazonia." In: Pastagens na Amazonia, eds. A. M. Peixoto, J. C. de Moura, and V. P. de Faria (Congresso Brasileiro de Pastagens. FEALQ, Piracicaba, Brazil), pp. 27-54. Dias Filho, M. B., and E. A. S. Serrao. 1980. "Recuperacao de pastagens de capim coloniao {Panicum maximum) atraves de niveis de fosforo e introducao de leguminosas em Paragominas, Para." Belem: EMBRAPA-CPATU. p. 2. (EMBRAPA-CPATU. Pesquisa em Andamento, 34). Dias Filho, M. B., and E. A. S. Serrao. 1981. "Introducao e avaliacao de gramineas forrageiras na regiao de Paragominas, Estado do Para." Belem, EMBRAPA CPATU. p. 14. (EMBRAPA-CPATU. Circular Tecnica, 17). Dias Filho, M. B., and E. A. S. Serrao. 1982. "Introducao e avaliacao de leguminosas forrageiras na regiao de Paragominas, Pa." Belem, EMBRAPA-CPATU. p. 18. (EMBRAPA-CPATU. Circular Tecnica, 29). Dias Filho, M. B., and E. A. S. Serrao. 1987. "Limitacoes de fertilidade do solo na recuperacao de pastagem degradada de capim coloniao (.Panicum maximum Jacq.) em Paragominas, na Amazonia oriental." Belem, EMBRAPA/CPATU, p. 19. (EMBRAPA-CPATU. Boletim de Pesquisa, 87). Dias Filho, M. B., M. Simao Neto, and E. A .S. Serrao. 1989- "Utilizacion de roca fosforica parcialmente acidulada y superfosfato simple en el estabelecimiento de Brachiaria brizantha cv. Marandu." Pasturas Tropicales 11: 25-8. Dias Filho, M. B., and M. Simao Neto. 1992. "Eficiencias agronomica e economica de um fosfato parcialmente acidulado em Brachiaria brizantha cv. Marandu em solo de floresta na Amazonia." Pesquisa Agropecuaria Brasileira 27: 395-401. Diez, J. A., A. Polo, C. C. Cerri, and F. Andreux. 1991. "Influencia do pousio e da pastagem sobre a dinamica de nutrientes em oxissolos recentemente desflorestados na Amazonia oriental." Pesquisa Agropecuaria Brasileira 26: 77-83. Dobereiner, J., J. M. Day, and P. J. Dart. 1972. "Nitrogen activity in the rhizosphere of sugarcane and some other tropical grasses." Plant Soil 37: 191-196.

Linking Biogeochemical Cycles to Cattle Pasture Management and Sustainability in the Amazon Basin Dunnin, F. X. 1987. "Run-off and drainage from grassland catchments." In: Managed grasslands. Analytical studies. Ecosystems of the world series 17B, ed. R. W. Snaydon (Elsevier, Amsterdam), pp. 205—213. Eden, M. J., P. A. Furley, D. F. McGregor, W. Milliken, and J. A. Ratter. 1991. "Effect of forest clearance and burning on soil properties in northern Roraima, Brazil." Forest Ecology and Management 38: 283-290. Embrapa. 1980. "Centro de Pesquisa Agropecuaria do Tropico Umido, Belem. Projeto de Melhoramento de Pastagens da Amazonia Legal—PROPASTO;" Relatorio Tecnico 1976/1979. Belem, Brazil. Ewel, J., C. Berish, B. Brown, and N. Price. 1981. "Slash and burn impacts on a Costa Rica wet forest site." Ecology 62: 816-829. Falesi, I. C. 1976. "Ecossistema de pastagem cultivada na Amazonia brasileira." Belem, EMBRAPA-CPATU. p. 193. (EMBRAPA-CPATU. Boletim Tecnico, 1) Feigl, B. J., J. Melillo, and C. C. Cerri. 1995. "Changes in the origin and quality of soil organic matter after pasture introduction in Rondonia (Brazil)." Plant and Soil 175: 21-29. Feigl, B. J., P. A. Steudler, and C. .C. Cerri. 1995. "Effects of pasture introduction on soil CO2 emissions during the dry season in the state of Rondonia, Brazil." Biogeochemistry 31: 1-14. Firestone, M. K., and E. A. Davidson. 1989- "Microbial basis of NO and N2O production and consumption in soil." In: Exchange of Trace Gases Between Terrestrial Ecosystems and the Atmosphere, eds. M. O. Andreae, and D. S. Schimel (John Wiley and Sons, New York), pp. 7-21. Fisher, M. J., I. M. Rao, M. A. Ayarza, C. E. Lascano, J. I. Sanz, R. J. Thomas, and R. R. Vera. 1994. "Deep-rooted grasses store carbon in South American soils." Nature 371: 236-238. Frost, P. G. H., and F. Robertson. 1985. "The ecological effects of fire in savannas." In: Determinants of Tropical Savannas, ed. B. H. Walker (CSIRO, Zimbabwe), pp. 93-140. Grace, J. 1994. "Responses of trees to stress." Scottish Forestry 48 (2): 90-95. Garcia-Oliva, F., I. Casar, P. Morales, andj. M. Maass. 1994. "Forest-to-pasture conversion influences on soil organic carbon dynamics in a tropical deciduous forest." Oecologia 99: 392-396. Giovannini, G., S. Lucchesi, and M. Giachetti. 1990. "Effects of heating on some chemical parameters related to soil fertility and plant growth." Soil Science 149: 344-50. Hecht, S. B. 1982. "Deforestation in the Amazon Basin: Magnitude, dynamics and soil resource effects." Studies in Third World Societies 13: 61-101. Italiano, E. C., E. de Moraes, and A. do C. Canto. 1982. "Fertilizacao de pastagens de capim coloniao em degradacao." Manaus, EMBRAPA-UEPAE de Manaus. p. 3. (EMBRAPA-UEPAE de Manaus. Comunicado Tecnico, 31) Jipp, P. H., D. C. Nepstad, D. K. Cassel, and C. R. de Carvalho. 1998. "Deep soil moisture storage and transpiration in forests and pastures of seasonally-dry Amazonia." Climatic Change 39: 395^412. Kauffman, J. B., D. L. Cummings, D. E. Ward, and R.

103

Babbitt. 1995. "Fire in the Brazilian Amazon: 1. Biomass, nutrient pools, and losses in slashed primary forests." Oecologia 104: 397-408. Keller, M., and W. A. Reiners. 1994. "Soil-atmosphere exchange of nitrous oxide, nitric oxide, and methane under secondary succession of pasture to forest in the Atlantic lowlands of Costa Rica." Global Biogeochemical Cycles 8: 399-409. Keller, M., E. Veldkamp, A. M. Weitz, and W. A. Reiners. 1993. "Effect of pasture age on soil trace-gas emissions from a deforested area of Costa Rica." Nature 365: 244-246. Ledgard, S. F, and W. M. H. Sanders. 1982. "Effects of nitrogen fertiliser and urine on pasture performance and the influence of soil phosphorus and potassium status." New Zealand Journal of Agricultural Research Agric. Res. 25: 541-547. Ledgard, S. F., K. W. Steele, and W. M. H. Sanders. 1982. "Effect of cow urine and its major constituents on pasture properties." New Zealand Journal of Agricultural Research 25: 61-68. Leon, L. A., and L. L. Hammond. 1985. "Phosphorus limitations and management considerations." In: Land in Tropical America, eds. T. T. Cochrane, L. G. Sanchez, L. G. de Azevedo, J. A. Porras, and C. L. Garver (Centro Internacional de Agricultura Tropical (CIAT)-Centro de Pesquisa Agropecuaria dos Cerrados (CPAC), Planaltina, D. F., Brazil), pp. 105-110. Ludlow, M. M., M. J. Fisher and J. R Wilson. 1985. "Stomatal adjustment to water deficits in three tropical grasses and a tropical legume grown in controlled conditions and in the field." Australian Jounal of Plant Physiology 12: 131-149. Luizao, R. C. C., T. A. Bonde, and T. Rosswall. 1992. "Seasonal variation of soil microbial biomass-the effects of clearfelling a tropical rainforest and establishment of pasture in the Centaral Amazon." Soil Biology and Biochemistry. 24: 805-813. Luizao, F., P. Matson, R. Livingston, and P. M. Vitousek, 1989. "Nitrous oxide flux following tropical land clearing." Global Biogeochemical Cycles 3: 281-285. Matson, P. A., P. M. Vitousek, J. J. Ewel, M. J. Mazzarino, and G. P. Robertson. 1987. "Nitrogen transformations following tropical forest feeling and burning on a volcanic soil." Ecology 68: 491-502. McWilliam, A.-L. C., O. M. R. Cabral, B. M. Gomes, J. L Esteves, and J. M. Roberts. 1996. "Forest and pasture leaf-gas exchange in south-west Amazonia." In: Amazonian deforestation and climate, eds. J. H. C. Gash, C. A. Nobre, J. M. Roberts, and R. L. Victoria (John Wiley & Sons, London), pp. 266-285. Mengel, K., and E. A. Kirkby. 1987. Principles of plant nutrition. International Potash Institute, Bern, Switzerland. Montagnini, F., and, R. Buschbacher. 1989. "Nitrification rates in two undisturbed tropical rain forests and three slash-and-burn sites of the Venezuelan Amazon." Biotropica 21: 9-14. Moraes, J. E, B. Volkoff, C. C. Cerri, and M. Bernoux. 1996. "Soil properties under Amazon forest and changes due to pasture installation in Rondonia, Brazil." Geoderma 70: 63-81. Neill C, M. C. Piccolo, P. A. Steudler, J. M. Melillo,

104

B. J. Feigl, and C. C. Cerri. 1995. "Nitrogen dynamics in soils of forests and active pastures in the western Brazilian Amazon basin." Soil Biology and Biochemistry. 27: 1167-1175. Neill, C., B. Fry, J. M. Melillo, P. A. Steadier, J. F. Moraes, and C. C. Cerri. 1996. "Forest and pasture-derived carbon contributions to carbon stocks and microbial respiration of tropical pasture soils." Oecologia 107: 113-119Neill, C., J. M. Melillo, P. A. Steudler, C. C. Cerri, J. F. L. de Moreaes, M. C. Piccolo, and M. Brito. 1997. "Soil carbon and nitrogen stocks following forest clearing for pasture in the southwestern Brazilian Amazon. Ecological Applications 7: 1216-1225. Nepstad, D. C. 1989. Forest regrowth in abandoned pastures of eastern Amazonia: limitations to tree seedlings survival and growth. Ph.D. Thesis, Yale University, New Haven. Nepstad, D. C., C. Uhl, and E. A. S. Serrao. 1991. "Recuperation of a degraded Amazonian landscape: forest recovery and agricultural restoration." Ambio 20: 248-255. Nepstad, D. C., C. J. R. de Carvalho, E. A. Davidson, P. Jipp, P. Lefebvre, G. H. Negreiros, E. D. da Silva, T. A. Stone, S. Trumbore, and S. Vieira. 1994. "The role of deep roots in the hydrological and carbon cycles of Amazonia forests and pastures." Nature 372: 666-669. Nguyen, M. L., and K. M. Goh. 1992. "Nutrient cycling and losses based on a mass-balance model in grazed pastures receiving long-term superphosphate applications in New Zealand. 1. Phosphorus." Journal of Agricultural Science, Cambridge 119: 89-106. Piccolo, M. C., C. Neill, and C. C. Cerri. 1994. "Natural abundance of 15N in soils along forest-to-pasture chronosequences in the water Brazilian Amazon basin." Oecologia 99: 112-117. Rosswall, T. 1982. "Microbiological regulation of the biogeochemical nitrogen cycle." Plant and Soil 67: 15-34. Roberts, J. M., O. M. R Cabral, J. P. Costa, A.-L. C McWilliam, and T. D. de A. Sa. 1996. "An overview of the leaf area index and physiological measurements during ABRACOS." In: Amazonian deforestation and climate, eds. J. H. C. Gash, C. A. Nobre, J. M. Roberts, and R. L. Victoria (John Wiley and Sons, London), pp. 287-306. Robbins, G. B., J. J. Bushell, and G. M. McKeon. 1989. "Nitrogen immobilization in decomposing litter contributes to productivity decline in ageing pastures of green panic (Panicum maximum var. trichoglume)." Journal of Agricultural Science, Cambridge 113: 401-406. Salati, E., R. Sylvester-Bradley, and L. Victoria. 1982. "Regional gains and losses of nitrogen in the Amazon basin." Plant and Soil 67: 367-376. Sanchez, P. A. 1976. "Soil management in shifting cultivation areas." In: Properties and Management of soils in the tropic, ed. P. A. Sanchez (John Wiley & Sons, New York), pp. 347-412. Sanchez, P. A., 1987. "Management of acid soils in the humid tropics of Latin America." In: Management of acid

Moacyr B. Dias-Filho et al. tropical soils for sustainable agriculture: proceedings of an IBSRAM inaugural workshop, eds. P. A. Sanchez, E. R. Stoner, and Pushparajah (Bankok, Thailand), pp. 63-107. Sanchez, P. A., J. H. Villachica, and D. E. Bandy. 1983. "Soil fertility dynamics after clearing a tropical rainforest in Peru." Soil Science and Social American Journal 47: 1171-1178. Serrao, E. A. S., I. C., Falesi, J. B. da Veiga, andj. F. Teixeira Neto. 1979- "Productivity of cultivated pastures in low fertility soils of the Amazon of Brazil." In: Pasture production in acid soils of the tropics, eds. P. A. Sanchez, and L. E. Tergas (Centro Internacional de Agricultura Tropical, Cali, Colombia), pp. 195-225. Serrao, E. A. S., and J. M. Toledo. 1990. "The search for sustainability in Amazonian pastures." In: Alternatives to Deforestation: Steps Toward Sustainable Utilization of Amazon Forests, ed. A. B. Anderson (Columbia University Press, New York), pp. 195-214. Seubert, C. E., P. A. Sanchez, and C. Valverde. 1977. "Effects of land clearing methods on soil properties of an ultilsol and crop performance in the Amazon jungle of Peru." Tropical Agriculture. 54: 307-321. Skole, D. S., W. Chomentowski, W. A. Salas, and A. D. Nobre. 1994. "Physical and human dimensions of deforestation in Amazonia." Bioscience 44: 314-322. Smyth, J., andj. B. Bastos. 1984. "Alteracoes na fertilidade de um Latossolo Amarelo Alico pela queima da vegetacao." Revista Brasileira de Ciencia do Solo. 8: 127-132. Spain, J. M., and J. G. Salinas. 1985. "A reciclagem de nutrientes nas pastagens tropicais." In: Reciclagem de nutrientes em agricultura de baixos insumos, ed. P. Cabala-Rosand (CEPLAC/SBCS, Ilheus, Brazil), pp. 259-299. Street, J. 1982. "Changes of carbon inventories in live biomass and detritus as a result of the practice of shifting agriculture and the conversion of forest to pasture: case studies in Peru, New Guinea and Hawaii." International seminar on geography and the third world. Department of Geography, University Kebangsaan Malaysia Bangi, Selangor, Malaysia. Teixeira, L. B., 1987. "Dinamica do ecossistema de pastagem cultivada em area de floresta na Amazonia Central." Tese de doutorado. INPA/FUA, Manaus, Brazil. Tiessen, H., E. Cuevas, and P. Chacon. 1994. "The role of soil organic matter in sustaining soil fertility." Nature 371: 783-785. Toledo, J. M., and V. A. Morales. 1979. "Establishment and management of improved pastures in the Peruvian Amazon." In: Pasture production in acid soils of the tropics, eds. P. A. Sanchez, and L. E. Tergas (Centro Internacional de Agricultura Tropical, Cali, Colombia), pp. 177-194. Trumbore, S. E., E. A. Davidson, P. B. de Camargo, D. C. Nepstad, and L. A. Martinelli. 1995. "Below-ground cycling of carbon in forests and pastures of eastern Amazonia." Global Biogeochemical Cycles 9: 515-528. Uhl, C., R. Buschbacher, and E. A. S. Serrao. 1988. "Abandoned pastures in Eastern Amazonia, I: Patterns of plant succession." Journal of Ecology 76: 663-681. Veldkamp, E. 1994. "Organic carbon turnover in three trop-

Linking Biogeochemical Cycles to Cattle Pasture Management and Sustainability in the Amazon Basin ical soils under pasture after deforestation." Soil Science and Social American Journal 58: 175—180. Vieira, I. C. G, C. Uhl, and D. C. Nepstad. 1994. "The role of the shrub Cordia multispicata Cham, as a 'succession facilitator' in an abandoned pasture, Paragominas, Amazonia." Vegetatio 115: 91—94. Vitousek, P. M., and R. L. Sanford. 1986. "Nutrient cycling in moist tropical forests." Annual Review of Ecology and Systematics 17: 137-167.

105

Wright, H. A., and A. W. Bailey. 1982. "Soil and water properties." In: Fire ecology. United States and Southern Canada, eds. H. A. Wright, and A. W. Bailey Qohn Wiley and Sons, New York), pp. 24-48. Wright, I. R., J. H. C. Gash, H. R. da Rocha, W. J. Shuttleworth, C. A. Nobre, G. T. Maitelli, C. A. G. P. Zamparoni, and P. R. A. Carvalho. 1992. "Dry season micrometeorology of central Amazonian ranchland." Q. J. Roy. Met. Soc. 118: 1083-1099.

7 Nutrient Considerations in the Use of Silviculture for Land Development and Rehabilitation in the Amazon Horencia Montagnini

Tropical plantations serve diverse economic, social, political, and ecological functions. With considerably higher yields than managed native forests, tropical and subtropical plantations make substantial contributions to world timber and pulp production (Wadsworth 1983, Evans 1992). Tree plantations can also be a source of cash, savings, and insurance for individual farmers. Plantations may help stabilize rural populations in regions where shifting agriculture is the predominant land use. In combination with subsistence and commercial crops (agroforestry) or cattle (agrosilvopastoral systems), plantations have been used as tools in rural development projects worldwide. Plantations are often seen as alternatives to deforestation as they can provide products that otherwise would be taken from natural forests (Fearnside 1990, McNabb et al. 1994). Nutrient cycling characteristics of tropical plantations differ from those of natural forests in a number of ways. Natural forests are adapted to ecological niches by intricate and effective physiological adaptations of growth in the environment (see Cuevas, this volume). Instead, tropical plantations are simplified, generally monospecific ecosystems that occupy the site for a limited period of time that can range from 4-12 years (for biomass, pulpwood, or fuelwood) up to 2040 years (timber). In many instances plantations are composed of species that are exot-

ic to the region, or even when indigenous, are new to the particular plantation site. Since plantation tree species have been generally selected for production of timber or other aboveground tree parts, they tend to maintain a smaller fraction of total tree biomass nutrients in roots than natural forests (Vogt et al. 1997). In rain forests growing on poor soils, high tree productivity is in part due to the existence of important nutrient conserving mechanisms mediated by the root system (Cuevas, this volume). The smaller biomass of plantation root systems may thus make them more susceptible to nutrient and water stress. Smaller root systems may also make plantation forests more susceptible to disturbances from strong winds and pathogens that attack aerial parts (Vogt et al. 1997). Nutrient demands by plantation trees vary from season to season and with the developmental age of the stand (Drechsel and Zech 1993). During the life of the plantation, large quantities of nutrients are returned to the soil by above- and belowground litter, harvest residues, stem flow, and throughfall. These elements are mineralized in the soil and may be used by the trees or by associated crops. Deposition of certain mineral elements from the atmosphere can also be important, sometimes even compensating from losses in stemwood removal at harvest (Bruijnzeel 1989)- Retranslocation of absorbed nutrients

Nutrient Considerations in the Use of Silviculture for Land Development and Rehabilitation in the Amazon

can be also of considerable magnitude, often supporting sustained growth in the short and long term (Nambiar 1984). Plantations are frequently managed intensively, with silvicultural inputs, for maximum wood or biomass production in the shortest time. Plantation management (e.g., thinning, pruning, coppicing, fertilizing) profoundly affects stand structure, crown characteristics, and plantation micro environment, all of which in turn can affect nutrient cycling processes. Other forestry operations, especially during harvest and those associated with the establishment of the following crop (biomass removal, soil disturbance, residue disposal by burning, intense site preparation such as ploughing, discing or blading, weed control, and fertilization) have profound influences on the productivity of a site (Lundgren 1980, Sanchez et al. 1985). The effects of perturbation on nutrient losses are greatly accelerated in short rotations of intensively managed plantations, and these considerations have often led to the concept of economic v. ecological rotations (Nambiar 1984, Drechsel and Zech 1993). Fast growing tropical tree plantations can incorporate considerable amounts of nutrients in their biomass over a relatively short period of time. Site fertility declines can limit sustained plantation forestry in tropical regions: soil fertility can be decreased through excessive removal of nutrients in living biomass, particularly if nutrients in tree crowns are lost through harvest or site preparation (Jorgensen and Wells 1986, Perry and Maghembe 1989)- Site fertility declines can be more serious if the trees are harvested in short rotation schemes, and if whole-tree harvesting is applied. This can be particularly serious when plantations are established on soils that are inherently poor, as occurs over vast areas of the Amazon basin. If plantation species are chosen with knowledge of their nutrient-use efficiencies

107

and recycling capacities, they can be highly productive and even serve a function in ecosystem restoration projects. In particular, mixed-species designs can offer product diversification and can complement nutrient resource use with positive impacts on soil nutrients (Alvim 1989, Silva and Uhl 1992, Montagnini et al. 1995). In addition, rotation length and harvest techniques can be adjusted to minimize nutrient losses and maintain site productivity in the long term. This chapter considers large-scale industrial tree plantations, as well as small- and medium-scale plantations for timber and land rehabilitation. Aspects that influence plantation sustainability are emphasized, and suggestions are given regarding plantation design and management.

Plantations in the Amazon Tree plantations in the Amazon are used at small, medium, and large scales, each of which serves different functions, such as rural development, land rehabilitation, or industrial purposes. The first major entrepreneurial effort in Amazonian tree plantations was the establishment of native rubber (Hevea brasiliensis) in 1928 at the Fordlandia Estate owned by Henry Ford on the lower Tapajos river in the state of Para, Brazil. These plantations were abandoned in the late 1940s due to low latex productivity that resulted from a high incidence of the South American leaf blight (Rankin 1985). After this failure, large-scale tree plantations were not undertaken again in the region until 1968 when Jari Forestal e Agropecuaria was established, also in Para. Other early attempts at plantation establishment in the Brazilian Amazon include small plantations of Brazil nut (Bertholletia excelsa) near Manaus in 1931, which had generally low nut yield but good tree growth, and scattered efforts to cultivate the valuable Meliaceae. Of the Meliaceae, Swietenia macrophylla (mahogany)

108

and Cedrela odoratd), were planted most often, but neither these nor other species were successful, as all were stricken by the shoot borer Hypsipyla grandella (Palmer 1977). Plantations have been sometimes recommended as a potentially viable land use for the Amazon region (Alvim 1989, Hoyos et al. 1992, McNabb et al. 1994). Silvicultural plantations have also been promoted as an alternative means of supplying wood and paper needs while also reducing pressure for additional rain forest clearing (Fearnside 1990). However, as in other tropical humid regions of the world, many authors have raised concerns about the ecological and economic sustainability of large-scale plantations in the Amazon basin: fast-growing exotics planted in monospecific stands deplete soil nutrients in a few rotations and often suffer from pest and disease problems (Johnson 1976, Fearnside and Rankin 1980, Rankin 1985, Palmer 1977, Russell 1987). In the Brazilian Amazon, hardwood species have been restricted to experimental settings, but fast-growing species for pulp, plywood, and sawlogs have been planted in an increasing number of commercial plantations that are partially or fully owned by foreign investors. Examples include 20,000 ha of Caribbean pine (Pinus caribaea) in the Amapa territory, 500,000 ha of Caribbean pine near Portel in the state of Para, and the highly publicized Jari project, also in Para.

Nutrient Cycling in Industrial Plantations of Fast- Growing Exotics: the Jari Project Located along the Jari River (a tributary 300 km from the mouth of the Amazon), the 1.6 million ha Jari estate was originally purchased by the American multimillionaire Daniel Ludwig in 1967. In the first fifteen years of the project, more than 100,000 ha of forest were cleared and planted with exotic

Florencia Montagnini

species, mostly Gmelina arborea, Pinus caribaea var. hondurensis, and Eucalyptus spp. (McNabb et al. 1994). In 1982 Ludwig sold the forestry operation to a consortium of Brazilian businesses, taking a substantial loss on the original investment (Jordan, this volume). Now called the Monte Dourado Forestry Company, this estate has become a modern forestry enterprise similar to those established in southeastern Brazil. Current forestry, agricultural, and mining uses encompass about 28% of the property, while the remaining 72% is still relatively undisturbed rain forest (McNabb et al. 1994). Controversy has surrounded the capacity of the soils to support long-term plantation productivity at Jari. For example, Irion (1981) suggested that the soil would be completely exhausted after the second generation (14-20 years) even though the Gmelina arborea plantations had reached a height of 20 m in 7 years in the first generation, and the other species (Pinus caribaea and Eucalyptus deglupta) had shown few signs of nutrient deficiency at the time. Other authors have suggested that soil fertility could be improving under the forest plantations at Jari. For example, company researchers showed that in the plantations there was no economic response to the application of mineral fertilizers in the early years (Palmer 1977). Likewise, Greaves (1979) stated that the fertility and moisture retention of the soils was apparently improving under the forest plantations, making reference to the thick organic layer developed under both pine and Gmelina species. Russell (1987) examined whole ecosystem nutrients within a native forest, a 6-month pine plantation, a 9-5-year pine plantation, an 8.5-year Gmelina plantation, and a 1.5-year second generation pine plantation that was established on a previous 8.5-year Gmelina plantation. A synthesis of the changes in nutrient stocks during the course of plantation establishment and growth is presented

Nutrient Considerations in the Use of Silviculture for Land Development and Rehabilitation in the Amazon

in Figure 7.1. At the end of the first rotation of Gmelina (8.5 years) or Pinus caribaea (9.5 years), total plant biomass was about 40-60% of that in the virgin forest. Most of the losses

Fig. 7.1 Total nutrient stocks and in plant biomass plus soil in rain forest, newly planted Pinus caribaea (6 months old), P. caribaea, and Gmelina arborea plantations at the end of the first rotation (9.5 and 8.5 years old, respectively), and second rotation P. caribaea (1.5 years old) at Jari (Sanchez et al. 1985). H = harvest loss from trees taken when clearing the rain forest for the plantations; L = leaching. Total nutrient stock is defined as the sum of all the nutrients in plant biomass (aboveground, litter, detritus, roots) plus total N, available P (extracted by the Mehlich method), and exchangeable K, Ca, and Mg in the top meter of the soil.

109

were accounted for by the dry matter extracted by harvest, as well as by the disturbance caused by site preparation. The plantations of all ages contained about 60% of the total N stock of the rain forest. Most losses occurred shortly after rain forest clearing, but thereafter the plantations maintained a relatively constant level of N. The ecosystem, therefore, lost 40% of its N, and then reached a new equilibrium level. Stocks of P ranged from 76% to 116% of the rain forest values, with the decrease in P at the start of the rotation largely accounted for by the P removed in the first rotation harvest. Significant losses of K occurred after rain forest clearing, with a decrease to about 32% of that in the forest before clearing. Most of the losses were accounted for by the removal at harvest, as well as by rapid leaching losses recorded during this period, an expected result since K is a highly mobile ion and it is readily lost through leaching. Afterwards, there were slight increases, to about 40% of the rain forest values. Ca stocks decreased to about 56% of the rain forest values upon planting of the first pine rotation. Losses were again accounted for by Ca removal at forest harvest and small amounts that were leached. This reduced level of Ca remained relatively stable with Pinus, but increased to above preclearing levels with Gmelina. Magnesium stocks decreased with age in Pinus, but mature Gmelina plantations maintained a steady level of about 75% of that in the rain forest. Most of the ecosystem nutrient losses occurred during the plantation establishment phase because of the removal of forest debris at clearing and soil disturbance during mechanized operations. Despite the potential improvement in Ca levels and maintenance of Mg levels as mentioned for Gmelina, both the extraction of nutrients during harvesting, and leaching losses prior to canopy closure, lead to a depletion of key nutrients, particularly potassium, that must be replaced by fertilization if yields are to be maintained (Sanchez et al. 1985, Russell 1987).

no Likewise, Spangerberg (1994) reported on nutrient removal by eucalyptus at Jari and stressed the need for fertilization after the first rotation. Studies were made on first rotation, 4.5-year-old plantations of Eucalyptus urograndis (a hybrid of E. urophylla and E. grandis) in order to calculate nutrient losses due to the removal of wood and bark during harvesting for pulpwood. Plantation trees exhibited no nutrient deficiencies, although foliar concentrations of Ca seemed to be rather low. Average nutrient losses due to removal of wood and bark amounted to 64.6% for N, 54.2% for P, 76.3% for Ca, 57.2% for K, and 60.7% for Mg, of the whole tree biomass. If the same nutrient removal was assumed in the next rotation, there would be a deficiency of Ca, and compensations of up to 250 kg Ca/ha would be necessary. Results of experimental research on nutrient dynamics in industrial plantations elsewhere in the Amazon also point to the need for supplemental fertilization to sustain yields in the medium to long term. In experimental plantations at the National Institute for Amazonian Research (INPA) in Manaus, results of studies by Magalhaes et al. (1986) showed how height of several tree species growing in 3-year-old plantations correlated strongly with soil fertility parameters (exchangeable bases, organic matter, exchangeable Al, total Zn and Mn). However, nutrient requirements vary among plantation species and with plantation age. For example, in experiments using 4 doses of lime (calcium carbonate) during establishment of plantations on acid soils at La Selva, Costa Rica, only 1/3 of the species exhibited a significant response in growth (Soto et al. 1996). The species tested were all native to the region, and they were probably adapted to growing on the acid soil conditions of the experimental area. In addition, responses to fertilizer also vary with plantation age. In the early stages of stand development prior to canopy closure, the annual rate of nutrient accumu-

Florencia Montagnini

lation increases rapidly and tree growth is very dependent on current nutrient uptake. Mineral deficiencies are frequently observed and responses to fertilizer application are common during this stage. Once the canopy has closed, the reduction in rate of nutrient uptake is associated with attaining maximum foliage biomass, high internal retranslocation of mobile nutrients, as well as increasing nutrient recycling via litterfall. This decreases the nutrient contribution by the soil, thus fertilizer responses are unlikely during this stage (Drechsel and Zech 1993). The type, amounts, and timing of fertilization used in plantations need to be adjusted according to species requirements, ideally using results of field fertilization experiments. In summary, in high-yield, short-rotation plantation forestry in rain forest sites such as Jari, and other locations with similar ecological conditions, nutrients are likely to become limiting after the second or third rotation; therefore, yields will not be maintained without extra fertilizer inputs. Repeated fertilization can upset soil nutrient balances and alter microbial populations, and it would also pose problems for the economics of producing low-value commodity timbers (see also Jordan, this volume). In spite of the lessons from Jari, other large scale plantation development plans continue in the Brazilian Amazon. If developed, the Grande Carajas Program would consume large areas of tropical forest in the eastern Amazon in order to obtain charcoal for the smelting of pig-iron from the Carajas' mines. The area of Eucalyptus plantation to produce the required amount of charcoal would be over 700,000 ha-more than 10 times the area of E. deglupta already cultivated in the Jari Project (Fearnside 1989).

Small-Scale Plantations for Timber Nutrient cycling in timber plantations in the Amazon has not been studied as much as

Nutrient Considerations in the Use of Silviculture for Land Development and Rehabilitation in the Amazon

industrial plantations, yet these systems have great potential to provide a more sound land use option for the region: they can produce high-quality timber, help to restore degraded lands, and contribute to rural development. Because there are still large areas of native forest available for management and extraction of valuable timber species, plantations of exotic and native hardwoods remain small and mostly experimental. A lack of information on the silvicultural management for some of the most preferred timber species, which are often difficult to grow in open plantations, also limits the current expansion of timber plantations in the Amazon. For example, in recent experiences from the Peruvian Amazon, populations of Hypsipyla grandella (the mahogany shoot borer) grew rapidly in the rainy season as food availability increased from new growth sprouted on the host trees (Cedrela odorata, C. fissilis, and Swietenia macrophylla). C. odorata was more susceptible to the pest than S. macrophylla (Yamazaki et al. 1990). It was suggested that planting trees in small cleared areas and avoiding clean weeding may reduce the ease with which the pest finds its host species. Several other native species have been tried with success in plantations in the Peruvian Amazon. Results of 20 years' experience with plantations of 9 exotic and 104 native species at Jenaro Herrera, near the Ucayali river 200 km upstream from Iquitos, were summarized by Claussi et al. (1992). As a result of the failure of some exotics and attacks by Hypsipyla on the Meliaceae, the plantation "work concentrated on non-Meliaceae native species. Cedrelinga catenaeformis and Simarouba amara [Quassia simarouba] were the two best timber species, with mean annual increments of over 1.5 cm in diameter and 1.5 m in height at 10 years of age. Two species suitable for rural construction work, Guatteria elata and G. hyposericea, had mean annual increments of over 1.5 cm

111

in diameter and 1.5 m in height. Other suitable timber species recommended by these authors include Parkia multijuga, Ormosia spp., and Sclerolobium off. tinctorium. Recommendations on plantation establishment included species mixtures, such as C. catenaeformis with Q. simarouba and Guatteria elata, and agroforestry combinations with agricultural crops. In the Brazilian Amazon, the earliest experiences reported are those from timber plantations at EMBRAPA's Curua-Una station in Santarem, Para (Pedroso 1973a, 1973b; SUDAM 1979). These experiences provided data on species trials established since 1959, as well as information on silvicultural methods and regeneration techniques developed for establishing plantations of indigenous and exotic species in the Amazon region. Fourteen to eighteen years after planting, tree growth of 47 native and 18 exotic species showed that promising native species included Anacardium giganteum, Bagassa guianensis, Bertholletia excelsa, Buchenavia huberi, Didymopanax morototoni, Goupia glabra, Jacaranda copaia, Parkia multijuga, Simarouba amara, Vatairea guianensis, Virola cuspidata, and Vochysia maxima (SUDAM 1979). Performance of Pinus caribaea var. hondurensis and Eucalyptus spp. were also good. Other species recommended for their inclusion in tree improvement programs were the natives Carapa guianensis, Platonia insignis, Schizolobium amazonicum, and Virola surinamensis and exotics such as Gmelina arborea and Terminalia ivorensis (Pitcher 1976). Cordia alliodora has also been recommended as a good prospect for commercial, large-scale plantations (Carpanezzi et al. 1982). Important lessons regarding plantation management strategies have been learned from the results of pioneering studies in the eastern Amazon. From their experiences in Paragominas (in Para), Nepstad et at. (1991), and Uhl et al. (199D suggested that burning

112

Florencia Montagnini

The experiences in Paragominas, Para, Brazil mentioned in the previous section and experiments at EMBRAPA stations constitute isolated, pioneering examples of this type of silvicultural plantation in the Amazon. Keys to the success of these systems are correct species choice and design. Choice of species should be based on the nutrient use efficiency and growth rate of each species. Recent studies on the natural impact of native trees on soil fertility and nutrient cycling in the Atlantic forest region of Bahia, and in the Atlantic humid lowlands of Costa Rica, help to illustrate these points. During trials in Bahia, Brazil, 15 out of 20 tree species planted in monospecific stands had positive effects on at least 5 soil fertility parameters 13-15 years after planting. Several species contributed to increased soil carbon and nitrogen levels in the topsoil: Inga affinis, Parapiptadenia pterosperma, Plathymenia foliolosa (leguminous, N fixing species), Caesalpinia echinata, Copaifera luscens (leguminous, non N fixing), Eschweilera ovata, and Pradosia lactescens (Montagnini et al. 1994). In experiments at La Selva Biological Station in Costa Rica, improved soil conditions were found just after tree canopy closure in pure plantations of timber trees compared to pasture lands. The highest values for soil carbon, total nitrogen, and phosphorus were found under Vochysia ferruginea, a valuable timber species native to Central America. After three years the soils under pure stands of native species had soil fertility values Plantations as a Tool for Land similar to those found in 20-year-old natural Rehabilitation and Rural Development secondary forests originated after abandonment of cattle pastures (Montagnini and Plantations for land rehabilitation are Sancho 1994). generally grown at a small to medium scale In the same region of Costa Rica, the use because they usually require intensive initial of improved fallow systems yielded land valcare, often including external inputs (mycor- ues of $5-12 thousand per hectare at a 5% rhiza inoculation, use of fertilizers or herbi- real interest rate, after inflation (Montagnini cides). In some cases, initial intercropping and Mendelsohn 1996). This system involved with annual plants can help to offset costs. planting native tree species to replenish soil

before planting would decrease competition by grass. In trials with 27 native species, they concluded that the addition of fertilizer or manure was not essential for establishment. They also suggested planting trees in patches of already established shrubs, such as Cordia multispicata, which presumably will offer a more favorable microclimate for tree establishment. Among the best performers in these trials were fruit trees such as Anacardium occidentale (caju), and Bixa orellana, as well as timber trees such as Swietenia macrophylla. As seen, a considerable number of valuable timber species can be grown in plantations with relative success in the Amazon region, and a few silvicultural guidelines are available to aid in their establishment and management. Tree plantations, especially with indigenous species, can contribute to soil restoration and facilitate natural forest regeneration (Guariguata et al. 1995). Plantations of exotics can also be beneficial on many degraded sites and can facilitate natural regeneration in some circumstances, as explained in the next section. Over large areas of the tropics, with pressing human needs for land, the restorative effects of tree plantations can be realized if local people are willing to plant trees for their products such as timber. Overall, in the Amazon basin this is probably the most important role of silvicultural plantations, the most unrealized one, and the one in need of greater research and promotion.

Nutrient Considerations in the Use of Silviculture for Land Development and Rehabilitation in the Amazon

and provide economically valuable timber. Although the experiences are site specific, the species used have broad distribution in Latin America. Therefore, the systems would be transferable in other areas with similar ecological and economic conditions.

Reforestation of mine spoils Under the highly degraded conditions present in mine spoils, serious nutrient deficiencies limit tree growth and require heavy use of external inputs to ensure success of restoration projects. In experiences on reforestation of bauxite mining sites at Porto Trombetas (western Para), low levels of soil organic matter, N, P, K, Ca, and Mg were found to be the principal factors for reduced growth of planted trees (Ferraz 1993). The plantations present great variability in growth and degree of vegetation cover, even in areas of apparently homogeneous soils. The restored areas receive surface soil as an amendment before reforestation. This topsoil

113

is stored after removing it to allow for bauxite mining, thus the length and conditions of soil storage may affect its suitability to serve as bed for the newly planted seedlings. Additions of mycorrhizae and supplemental nutrients are often necessary. This increases the per hectare cost of reforestation although levels remain reasonable for the mining company (Knowles, personal communication). In the past, reforestation relied on the establishment of monospecific plantations, often with exotics (e.g., Eucalyptus, Pinus, Acacia spp.) or a limited number of native tree taxa for which seeds are readily available and silvicultural practices have been developed (Knowles and Parrotta 1995). Also at Porto Trombetas, about 160 native forest species in mixed plantings were evaluated for their suitability for forest restoration on bauxite mine land over a 14-year period. Observations over 600 ha of plantings have yielded information on ecological characteristics of the species and cost-effective propagation methods. The plantations of exotics and natives are expected to catalyze natural forest succession in the understory and thus accelerate the rate at which species-rich native forest stands develop on severely degraded lands.

Nutrient Cycling and Plantation Design and Management The impacts of trees on soil fertility depend on nutrient recycling characteristics such as litter chemistry and decomposition rates. Tree litter can be used as mulch with different outcomes: a fast mulch decomposition rate may accelerate the growth of associated crops on poor soils, while in other cases a more persistent litter may provide a steady source of nutrients and a better soil cover Fig. 7.2 Nitrogen and phosphorus use efficien- year round. In the experiments in Costa cy of 4-year-old stands of 4 indigenous timber Rica described in the previous section, high species grown in pure plantation at La Selva rates of litterfall and slower decomposition Biological Station, Costa Rica. resulted in high litter accumulation and high

114

soil organic matter under V. ferruginea, making this species well suited for protecting soils against erosion. In contrast, litter from another species of Vochysia, V. guatemalensis, may be especially important for Ca and Mg recycling (Montagnini et al. 1993). Although in this experiment the litter of Hieronyma alchorneoides was less abundant than the other 3 species, it had a relatively faster decomposition rate and higher nutrient content. These characteristics promoted fast nutrient recycling, especially of N, Ca, Mg, K, and P. The ability of a species to produce large amounts of biomass with less nutrients may be an important consideration in choosing species for degraded, nutrient-poor sites. When put in context with nutrient recycling characteristics of a species, Nutrient Use Efficiency (NUE) values, calculated as the annual biomass increments per nutrients in annual leaf litterfall, can indicate appropriate system design and management to maintain productivity and recover or conserve nutrients over the long term. Results from experiments in Bahia, Brazil, suggest that Bombax macrophyllum and Plathymenia foliolosa, with overall high NUE values, would grow well on relatively nutrient-poor soils, and thus could be good alternatives for reforestation of degraded sites following the abandonment from agriculture and pasture that is frequent in the region (Montagnini 1994). B. macrophyllum stands tended to accumulate high amounts of litter under its canopy while P. foliolosa stands had relatively high amounts of organic matter and total N in the topsoil in comparison with adjacent areas of secondary forest (Montagnini et al. 1994). These features indicate that these species may be well suited for soil rehabilitation, including increasing soil organic matter content and protecting against soil erosion. Also at Bahia, species such as Buchenavia grandis and Hymenaea aurea, with overall lower NUE values, would be most appropri-

Florencia Montagnini

ate for agroforestry combinations where crops could benefit from nutrients recycled from litter. Species choices and designs should be suited to the kind of soil rehabilitation desired, a goal that should be tightly connected with the future use expected from the rehabilitated land. At La Selva, Vochysia ferruginea showed comparatively low efficiency values for N and P (Fig. 7.2), confirming the beneficial role of this species in recycling organic matter and positively impacting soil fertility as shown above, while the higher NUE values shown for V. guatemalensis illustrate the relatively large allocation of nutrients to stem biomass and comparatively low recycling in leaf litter. The relatively low efficiency (high recycling) of N and P found for Stryph-nodendron microstachyum and Hyeronima alchorneoides (Fig. 7.2) was also shown in experiments where maize grown with mulch of these species grew better and absorbed more N and P than with mulch of other species (Montagnini et al. 1993). Nutrient allocation in trees can impact soil nutrients in plantations when different parts of the trees are harvested and removed. When biomass nutrient allocation was compared among species at La Selva, the forest floor appeared to be an important compartment for long-term recycling of N, with marked differences among species (Fig. 7.3a). Similar results were obtained with respect to Ca and Mg, which also had relatively low use efficiency values (Montagnini 1994). If the forest floor is burned or collected for fuelwood, a substantial loss of organic matter and nutrients may occur, while if the litter is left after harvest, it represents a significant reservoir for the next tree rotation. In contrast, most species had higher use efficiency of P, and this nutrient was found in higher proportion in live tree biomass (Fig. 7.3b); a similar pattern was found for K (Montagnini 1994). Roots were also important for P, in comparison with

Nutrient Considerations in the Use of Silviculture for Land Development and Rehabilitation in the Amazon

115

Fig. 7.3 Nitrogen and phosphorus stocks in above ground biomass, forest-floor litter and roots of 4-year-old stands of 4 indigenous timber species grown in pure plantation at La Selva Biological Station, Costa Rica.

116

forest-floor P. Roots of tropical trees appear to decay at slower rates than leaf tissue, which means that they may function as a longerterm storage mechanism for nutrients (Bloomfield et al. 1993). The biomass of fine roots in tropical plantations is generally 2 to 4 times lower than in adjacent secondary forest of similar age (Lugo 1992, Vogt et al. 1997). Certain indigenous species that tend to maintain a relatively large proportion of living tree biomass nutrients in roots, while still producing high stem biomass increments (such as V. ferruginea and H. alchorneoides), would be preferred in sites where nutrient losses are a major concern. This can be especially critical for P and K, often mentioned as the nutrients which are most likely to be depleted from soils with subsequent rotations (Wadsworth 1983, Bowen and Nambiar 1984, Bruijnzeel 1984). Altering the rate of nutrient removal in products is probably one of the most important design considerations in planning sustainable plantations (Wang et al. 1991). Variation between species in nutrient allocation and the parts of the tree removed from the site will determine the extent to which the nutrients are removed during harvesting. For example, whole tree harvests of V. guatemalensis -would result in substantial nutrient removals, especially of Ca and Mg, while the harvest of H. alchorneoides would result in large removals of P and K (Montagnini and Sancho 1994). This can be assessed through nutrient and biomass sampling and estimation, and harvesting guidelines can be developed that reduce the extent of nutrient losses. The degree to which harvest losses will impact on forest production will depend on the amount of nutrients stored in remaining pools and the rate at which they are mineralized from these pools. Finally, a key issue in nutrient management of tree plantations is to find the best compromise between ecological requirements

Florencia Montagnini

and rotation length so as to ensure adequate nutrient supply in the long term. Due to increasing nutrient use efficiency in the stands with time, lengthening the rotation age of fast growing species will reduce net nutrient uptake from soil reserves (Drechsel and Zech 1993).

Other considerations for management and design The recovery of the productive capacity of soils is frequently expensive, thus the techniques involved must produce financial returns to ensure they are adopted by local farmers. In soil rehabilitation projects, species such as B. macrophyllum and P. foliolosa (Bahia) or V. guatemalensis (Costa Rica) should be grown first as they can generally tolerate more impoverished conditions than other species while producing valuable timber and protecting soils against erosion. Other species, such as B. grandis and H. aurea (Bahia), or H. alchorneoides (Costa Rica), with lower nutrient use efficiency and potentially producing higher nutrient cycling, might be planted at the same time, or underplanted later. The calculation of NUE and its interpretation in context with the recycling capacities of the species could contribute to species selection and system design focusing on the potential role of each species on productivity and soil rehabilitation. Mixed-species designs can be more advantageous than tree monocultures for site nutrient rehabilitation if systems are planned to complement each species' nutrient demands and effects. For the fastest-growing, light-demanding species included in a mixture, tree productivity can be higher than in monospecific stands, while for the slowergrowing, shade-tolerant species of generally more valuable timber, the mixture offers a more adequate growth environment than a single-stratum, open plantation (Montagnini et al. 1995). In experiments with mixed and

Nutrient Considerations in the Use of Silviculture for Land Development and Rehabilitation in the Amazon

117

Fig. 7.4 Aboveground biomass and roots (0-15 cm soil depth) of 3-year-old stands of 4 indigenous tree species: Terminalia amazonia, Hieronyma alchorneoides, Albizia guachapele, and Virola koschnyi, grown in pure plots, and a mixture of the 4 species at La Selva Biological Station, Costa Rica. pure species plantations at La Selva, Costa Rica, mixed stands of four indigenous species ranked first or second in terms of above ground biomass, in comparison with pure stands of each species (Fig. 7.4). The data for roots in Fig. 7.4 were only for the top 15 cm layer of mineral soil; it would be interesting to see if roots explore the soils in a more efficient way in mixed than in pure stands. The advantages to mixed forest designs in terms of longterm effects on ecosystem nutrients have yet to be demonstrated, but mixed systems show promise as they may be preferred to diversify product outputs, decrease risks of failure, and contribute to species and landscape diversity.

Combinations of Plantation Trees with Agricultural Crops or Cattle (Agroforestry) In plantations for timber, with relatively slower growth and longer rotations than pulp or energy species, the first years of plantation establishment can be costly, and combinations with agricultural crops or cattle in agroforestry or agrosilvopastoral systems often can help to defray these costs. Based on available information on the agroecological characteristics of humid tropical regions, Alvim (1989) recommended agroforestry systems that include black

118

pepper, cacao, rubber, and some successful silvopastoral systems similar to those in the cacao region of Bahia and in the Amazon region of Brazil. Likewise, Sombroek (1992) pointed to timber plantations and agroforestry systems as approaches for the reclamation of degraded and abandoned areas. The potential of agroforestry as an ecologically and economically sound land use alternative in the Amazon is also discussed by Jordan (this volume). A variety of experimental settings involving forestry and agroforestry practices have been established by EMBRAPA over the last 10-15 years in uplands (terra firme) near Santarem, on the Tapajos river, a tributary of the Amazon. For example, along the Santarem/Cuiaba road, research has been initiated to help small land holders to adopt more environmentally sustainable farming practices and to obtain additional sources of capital. Fast growing species with high economic value have been introduced into farming areas. Systems include combinations of corn, banana, freijo (Cordia goeldiana), and mahogany (Swietenia macrophylla) (Brienza-Junior and Yared 1991). Other agroforestry models involve combinations of crops, fruit trees, and timber tree species designed to provide immediate returns and longer-term investment for the small holders. Food crops associated with tree establishment reduced tree plantation costs and decreased the frequency of crop weeding. For example, the trees tatajuba (Bagassa guianensis), parapara (Jacaranda copaia), and freijo were planted in a mixture with cowpea (Vigna unguiculatd) with good results in terms of crop yields and tree growth. The use of agrosilvopastoral systems is an alternative to the ranching system operated over large areas of Amazonia, which results in progressive loss of soil fertility. Such agrosilvopastoral systems have been introduced to the Paragominas region of eastern Para using the forestry species

Florencia Montagnini

Eucalyptus tereticornis, B. guianensis and Schizolobium amazonicum, the crop Zea mays, and the forage species Brachiaria brizantha, B. humidicola and Panicum maximum. Performance of the forestry and forage species has been satisfactory and the maize yield was greater than the regional average. The cattle, at a low stocking rate, grazed normally on the site and caused no harm to the planted trees (Brienza-Junior and Jared 1991). The sustained management of tree plant-ations and tree-crop combinations (agroforestry) are potentially biologically and socially feasible alternatives for soils unsuitable for local agriculture because of severe degradation. Tree plantations or agroforestry also represent productive uses of lands that have poor regeneration due to a lack of nearby sources of forest propagules. As the area of degraded land expands, there is an increasing emphasis on the planting of tree species which can grow in such conditions and still yield potentially profitable products (such as timber and fuelwood) as well as environmental benefits (such as soil conservation and watershed protection). The choice of appropriate species is the key to success of such systems. Ideally, species should be indigenous to the region, have fast growth, good economic value, and positive impacts on soil fertility. Additionally, management systems must be compatible with local farming and cultural practices and avoid increasing labor requirements beyond acceptable levels. These conditions should aid the adoption of these systems by local farmers. Because of the importance of nutrients in soils of humid tropical regions, nutrient cycling studies in timber plantations and agroforestry systems, including those using valuable timber species, are needed to determine appropriate designs and management so that systems and practices can be promoted throughout the Amazon basin.

Nutrient Considerations in the Use of Silviculture for Land Development and Rehabilitation in the Amazon

Conclusions Though the total area in the Amazon basin dedicated to plantation silviculture is relatively small compared to other managed systems, plantations have been encouraged as an alternative land use for the region, and their area has been increasing steadily over the last decade. Large scale plantations with fast-growing exotics planted in monospecific stands can deplete soil nutrients in a few rotations and often suffer from pest and disease problems. Most of the ecosystem nutrient losses occur during plantation establishment because of the removal of forest debris at clearing and soil disturbance during mechanized operation. Long-term productivity of short rotation plantations can generally be sustained with the application of fertilizers, in heavily subsidized systems much like those in southern Brazil and in several industrialized countries of the temperate region. In comparison with natural forests, plantations tend to have a smaller standing biomass, and a smaller proportion of their biomass in the root system. Therefore, belowground nutrient cycling in plantations is potentially diminished compared with natural forests. Management guidelines generally focus on the aboveground component of plantations since the ultimate goal is the production of wood and other aboveground tree products. There is an important need to understand the role of roots and root symbionts in acquiring nutrients and water in plantations since they control carbon allocation patterns within plants.

119

Nutrient cycling in small-scale timber plantations in the Amazon has not been the subject of extensive study. However, using longer rotations of native species, or trees in combination with agriculture, these systems are a potentially more sound land use option for the region, as they can produce high quality timber, contribute to the restoration of degraded lands, and serve a rural development purpose. A considerable number of valuable timber species can be grown in plantations in the Amazon region, and a few silvicultural guidelines are available to aid in their establishment and management. Further information on the silvicultural management of preferred timber species would enhance the successful establishment of these plantations. Choice of species for land rehabilitation should be based on their nutrient use efficiencies (NUE) and growth rates. Recent studies show the recycling abilities and ameliorating effects of indigenous species on soils and suggest guidelines for their use in economically and ecologically sound land use systems. Species with high NUE values can produce high biomass on poor sites, while species with low efficiencies would be preferred for nutrient recycling. Mixed-species designs can be more advantageous than tree monocultures for site nutrient rehabilitation if systems are planned to complement each species' nutrient demands and effects. Acknowledgments: Victoria Derr provided useful comments and editorial remarks.

Literature Cited Alvim, P. 1989."Tecnologias appropriadas para a agricultura nos tropicos umidos." Agrotropica 1: 5-26. Bloomfield, J., K. A. Vogt, and D. J.Vogt. 1993. "Decay rate and substrate quality of fine roots and foliage of two

tropical tree species in the Luquillo Experimental Forest, Puerto Rico." Plant and Soil 150: 233-245. Brienza-Junior, S., and J. A. G. Yared. 1991. "Agroforestry systems as an ecological approach in the Brazilian

120

Amazon development." Forest Ecology and Management 45: 319-323. Bowen, G. D., and E. K. S. Nambiar. 1984. Nutrition of plantation forests. Academic Press, New York. pp. 516. Bruijnzeel, L. A. 1984. "Immobilization of nutrients in plantation forests of Pinus merkusii and Agathis dammara growing in volcanic soils in central Java, Indonesia." In: Proceedings of International Conference on Soils and Nutrition of Perennial Crops, eds. A. Tajib, and E. Pushparajah (Malaysian Soil Science Society, Kuala Lumpur, Malaysia), pp. 19-29. Bruijnzeel, L. A. 1989. "Nutrient content of bulk precipitation in south-central Java, Indonesia." Journal of Tropical Ecology 5: 187-202. Carpanezzi, A. A., M. Kanashiro, I. A. Rodrigues, S. BrienzaJunior, and L. C. T. Marques. 1982. "Informacoes sobre Cordia alliodora (R. & P.) Oken na Amazonia Brasileira." Unidade Regional de Pesquisa Florestal Centro Sul, EMBRAPA, Brazil. No. 10: 105-115. Claussi, A., D. Marmillod, and J. Blaser. 1992. Descripcion silvicultural de las plantaciones forestales de Jenaro Herrera. Instituto de Investigaciones de la Amazonia Peruana, Iquitos, Peru. pp. 334. Drechsel, P., and W. Zech. 1993. "Mineral nutrition of tropical trees." In: Tropical Forestry Handbook, Volume 1, ed. L. Pancel (Springer-Verlag, New York), pp. 516-567. Evans, J. 1992. "Plantation forestry in the tropics." In: Tree planting for industrial, social, environmental, and agroforestry purposes. 2nd. ed. Clarendon Press. Oxford. England, pp. 403. Fearnside, P. M., and J. M. Rankin. 1980. "Jari and development in the Brazilian Amazon." Interciencia 5: 146-156. Fearnside, P. M. 1989. "The charcoal of Carajas: a threat to the forests of Brazil's eastern Amazon region." Ambio 18: 141-143. Fearnside, P. M. 1990. "Predominant land uses in Brazilian Amazonia." In: Alternatives to Deforestation: Steps Towards Sustainable Use of the Amazon Rain Forest, ed. A. B. Anderson (Columbia University Press, New York), pp. 233-251. Ferraz, J. B. S. 1993- "Soil factors influencing the reforestation on mining sites in Amazonia." In: Restoration of tropical Forest Ecosystems, eds. H. Lieth, and M. Lohman (Kluwer Academic Publishers, The Netherlands), pp. 47-52. Greaves, A. 1979. "Gmelina-. large scale planting, Jarilandia, Amazon basin." Commonwealth Forestry Review. 58: 267-269. Guariguata, M. R., R.. Rheingans, and F. Montagnini, 1995. "Early woody invasion under tree plantations in Costa Rica: implications for forest restoration." Restoration Ecology 3: 252-260. Hoyos, J. B, F. J. Bruseke, A. K. O. Homma, J. B. Veiga, N. M. Silva, and W. L. Overal. 1992. Desenvolvimento sustentavel: um novo caminho? J. B. Hoyos, editor. Serie Universidade e Meio Ambiente. Universidade Federal do Para, Nucleo de Meio Ambiente, Belem, Brazil, pp. 119. Irion, G. 1981. Holzplantage im Urwald? aturwissnschaften 68: 133-138. Johnson, N. E. 1976. "Biological opportunities and risks associated with fast-growing plantations in the tropics." Journal of Forestry 74: 206-211.

Florencia Montagnini Jorgensen, J. R., and C. G. Wells. 1986. "Tree nutrition and fast-growing plantations in developing countries." International Tree Crops Journal 3: 225-244. Knowles, O. H., and J. A. Parrotta. 1995. "Amazonian forest restoration: an innovative system for native species selection based on phenological data and field performance indices." Commonwealth Forestry Review 74: 230-243. Lugo, A. E. 1992. "Comparison of tropical tree plantations with secondary forests of similiar age." Ecological Monographs 62: 1-41. Lundgren, B. 1980. Plantation forestry in tropical countries-physical and biological potentials and risks. Swedish University of Agricultural Sciences. International Rural Development Centre." Rural Development Studies 8. Uppsala, pp. 134. Magalhaes, L. M. S., W. E. H. Blum, and N. P. Fernandes. 1986. "Crescimento de Eucalyptus deglupta Blume em solos de diferentes texturas. Caracteristicas edaficonutricionais de plantios florestais na regiao de Manaus." 1. Acta Amazonica. 16-17: 509-521. McNabb, K., J. Borges, and J. Welker. 1994. Jari at 25. "An investment in the Amazon." Journal of Forestry 92: 21-26. Montagnini, F., K. Ramstad, and F. Sancho. 1993. "Litterfall, litter decomposition and the use of mulch of four indigenous tree species in the Atlantic lowlands of Costa Rica." Agroforestry Systems 23: 39-61. Montagnini, F. 1994. "Rehabilitating forest ecosystems in the humid tropics: Recent experiences from Latin America." In: Proceedings from IUFRO International Symposium on Growth and Yield of Tropical Forests. Sept. 26^Oct. 1, 1994 (Tokyo University of Agriculture and Technology. Fuchu, Tokyo, Japan), pp. 224-234. Montagnini, F., and F. Sancho. 1994. "Above-ground biomass and nutrients in young plantations of four indigenous tree species: implications for site nutrient conservation." Journal of Sustainable Forestry 1: 115-139. Montagnini, F., A. Fanzeres, and S. G. da Vinha. 1994. "Studies on restoration ecology in the Atlantic Forest region of Bahia, Brazil." Interciencia 19: 323-330. Montagnini, F., and R. Mendelsohn. 1996. "Managing forest fallows: improving the economics of swidden agriculture." Ambio 26(2): 118-123. Montagnini, F., E. Gonzalez, R. Rheingans, and C. Porras. 1995. "Mixed and pure forest plantations in the humid neotropics: a comparison of early growth, pest damage and establishment costs." Commonwealth Forestry Review 74: 306-314. Nambiar, E. K. S. 1984. "Plantation forests: their scope and perspective on plantation nutrition." In: Nutrition of Plantation Forests, eds. G. D. Bowen and E. K. S. Nambiar (Academic Press, New York), pp. 1-15. Nepstad, D., C. Uhl, and E. A. S. Serrao. 1991. "Recuperation of a degraded Amazonian landscape: forest recovery and agricultural restoration." Ambio 20: 248-255. Palmer, J. R. 1977. "Forestry in Brazil-Amazonia." Commonwealth Forestry Review 56: 115-130. Pedroso, L. M. 1973a. Informacoes sobre o atual comportamento de especies exoticas na regiao do Medio Amazonas. SUDAM-Document 5: 21-31.Superintendencia do Desenvolvimento da Amazonia (SUDAM), Belem, Para, Brazil.

Nutrient Considerations in the Use of Silviculture for Land Development and Rehabilitation in the Amazon

Pedroso, L. M. 1973b. Alguns aspectos sobre o florestamento e reflorestamento na Amazonia. SUDAM Document 5:35-49. Superintendent do Desenvolvimento da Amazonia (SUDAM), Belem, Para, Brazil. Perry, D. A., and J. Maghembe. 1989. "Ecosystem concepts and current trends in forest management: time for reappraisal." Forest Ecology and Management 26: 123-140. Pitcher, J. A. 1976. Forestry development and research. Brazil. A tree improvement programme for Amazonia. FAO-Report. 1976, No. FO: DP/BRA/71/545 Technical Report 3. pp. 42. Rankin, J. M. 1985. "Plantations." In: Key Environments, eds. G. T. Prance and T. E. Lovejoy (Amazonia. Pergamon Press, New York), pp. 379-387. Russell, C. E. 1987. "Plantation forestry. Case study No. 9: The Jari project, Para, Brazil." In: Amazonian Rain Forests. Ecosystem Disturbance and Recovery. Ecological Studies 60, ed. C. F. Jordan (Springer-Verlag, New York), pp. 76-89. Sanchez, P. A., C. A. Palm, C. B. Davey, L. T. Szott, and E. C. Russell. 1985- "Tree crops as soil improvers in the humid tropics?" In: Attributes of trees as crop plants, eds. M.G.R. Cannell, and J.E. Jackson (Institute of Terrestrial Ecology, Natural Environmental Research Council, Abbots Ripton, Huntingdon, England), pp. 327-350. Silva, N. M, and Uhl, C. 1992. "Forest management for timber production: A sustainable use for the Brazilian Amazon." Anais da Academia Brasileira de Ciencias 64 (suppl. 1): 89-95. Sombroek, W. G. 1992. "Strategies for protection of soil fertility in the Amazon Region." In: Sustainable land use systems and human living conditions in the Amazon region: proceedings of a meeting held in Bonn, Germany, 1-2 November 1991, eds. H. Jaenicke, and P. Flynn (Commission of the European Communities, Luxembourg), pp. 43-46. Soto, G., Alvarado, A. y Montagnini, F. 1996. "Encalado de especies forestales nativas." In: Suelos: «;Puede la

121

Agricultura Sostenible ser Competitiva? Memorias, X Congreso Nacional Agronomico y de Recursos Naturales/II Congreso de Suelos. Volumen III. Ed., ed. F. Bertsch, W. Badilla, and y E. Bornemisza (Universidad Estatal a Distancia/Ed. de la Universidad Nacional, San Jose, Costa Rica), pp. 144. Spangenberg, A. 1994. Nahrstoffvorrate und exporte von Eucalyptus urograndis, Plantagen in Ostamazonien (Jari), Brasilien. Gottingen Beitrage zur Land und Forstwirtschaft in den Tropen und Subtropen No.93. Gottingen, Germany; Institut fur Pflanzenbau und Tierhygiene in den Tropen und Subtropen. SUDAM. 1979. Caracteristicas silviculturais de especies nativas e exoticas dos plantios do Centro de Technologia Madeireira-Estacao Experimental de Curua-Una. Superintendencia do Desenvolvimento da Amazonia, Belem, Brazil, pp. 351. Uhl, C., D. Nepstad, J. M. Cardoso da Silva, and I. Vieira. 1991. "Restauracao da floresta em pastagems degradadas." Ciencia Hoje 13: 22-31. Vogt, K., H. Asbjornsen, A. Ercelawn, F. Montagnini, and M. Valdes. 1997. "Roots and mycorrhizas in plantation ecosystems." In: Management of Soil, Nutrients and Water in Tropical Plantation Forests, eds. E. K. S. Nambiar, and A. G. Brown.(ACIAR/CSIRO/CIFOR. ACIAR, Canberra, Australia), pp. 247-296. Wadsworth, F. H., 1983. "Production of usable wood from tropical forests." In: Tropical Rain Forest Ecosystems. Structure and Function. Ecosystems of the World 14A, ed. F. B. Golley (Elsevier, New York), pp. 279-288. Wang, D., F. H. Bormann, A. E. Lugo, and R. D. Bowden,. 1991. "Comparison of nutrient-use efficiency and biomass production in five tropical tree taxa." Forest Ecology and Management 46: 1-21. Yamazaki, S., A. Taketani, K. Fujita, C. P. Vasques, and T. Ikeda, 1990. "Ecology of Hypsipyla grandella and its seasonal changes in population density in Peruvian Amazon forest." Japan Agricultural Research Quarterly 24: 2, 149-155.

8 Extractive Reserves and Participatory Research as Factors in the Biogeochemistry of the Amazon Basin I. Foster Brown, Karen A. Kainer, Andrea S. Alechandre, Eufran do Amaral

The word Amazonia conjures up diverse images, ranging from an exotic jungle to resources for development to a vast web of ecosystems that interact with global element cycles-the focus of this book. This chapter examines the biogeochemical role of extractive reserves, a relatively new land use type within Amazonia in which nontimber forest extraction is the defining human activity. The chapter also provides examples of how participatory research with local communities can enhance the quality of the results and improve their transmission to society. Humans have been a part of the Amazon for the past several thousand years. Amerindian activities have affected forest structure in significant manners by selective planting and clearing (Balee 1989) and by increasing fire frequency, particularly during mega-El Nino events (Meggers 1994). During the last few centuries, neo-Europeans have tragically reduced native indigenous populations by several million and made wide-scale transformations in the tropics of the Americas (Crosby 1993, Ribeiro 1996). The booms in rubber extraction in the late 1800s and during World War II brought waves of nonindigenous migrants to Brazilian Amazonia (Dean 1989). More recently, largescale implantation of cattle ranching and colonization projects, and to a lesser degree, mining activity, have accelerated change in Amazonian landscapes (Schmink and

Wood 1992). In addition, the ensuing road network and infrastructure left in the wake of these recent activities increased access to primary forest, precipitating further deforestation. By 1996, about 52 million hectares, nearly the size of France, had been deforested in Brazilian Amazonia (INPE 1998). At the average rate of deforestation from 1992 to 1996 (1.9 million hectares per year), another area equivalent to this figure will be added by the year 2025, a time frame within the career of many reading this book. Continuation of the present trends will result in an increasing savannization of the Amazonian region, with pastures, secondary forests, and crop lands expanding into areas once occupied by closed-canopy forests. This phenomenon may also be called the "Africanization" of Amazonia because most of the pastures are planted with grasses imported from Africa, such as Bracharia brisanthum, which are notably different in their response to rainfall patterns and to fire than the forests that they replace. This human-induced transformation leads to major impacts on biogeochemical cycles, many of which can be considered deleterious and are detailed in the other chapters. The tragedy of this type of forest conversion in Amazonia is that in spite of its costs in environmental terms, little economic, social, or cultural benefit accrues to local populations. The resultant landscape is one

Extractive Reserves and Participatory Research as Factors in the Biogeochemistry of the Amazon Basin

123

that is economically as well as ecologically research that involves local researchers and impoverished (Anderson 1990). This crisis communities in the research process in an has forced critical examination of alterna- attempt to enhance local capabilities for tive land use strategies that encourage improved resource management. simultaneous economic use and biological Extractive Reserves and conservation of existing forests. Extractive reserves are one of the few Biogeochemical Cycling land use alternatives in Brazilian Amazonia that reconciles traditional resource use patFrom a biogeochemical perspective, land terns that maintain forest cover with subsis- uses such as extractive reserves that maintence and cash needs of local populations. tain most forest cover and ecosystem funcFirst established in 1990 in western tion intact are least likely to modify element Brazilian Amazonia, extractive reserves are cycling. Extractive reserves are actually government-owned, protected lands desig- comprised of a mosaic of land uses, includnated for sustainable extraction and conser- ing pasture areas, shifting cultivation plots vation of renewable natural resources by (including various successional stages), and resident populations with a tradition of mature forest. The impact on biogeocheminon-timber extraction (Allegretti 1989, 1990, cal cycles of each of these land uses can be 1994). This land use strategy is unique in evaluated independently as has been done that it institutionalizes collective land use in previous chapters. However, in the case areas for nonindigenous populations, and of extractive reserves, the relative percentassigns management responsibilities to age of land devoted to pasture and shifting local people who have a long-term stake in cultivation plots is small in comparison to maintenance of the resource base. the area covered by mature forests. Extractive reserves have been praised as an Nontimber forest product extraction forms important land use alternative that combines the basis for extractivist livelihoods; hence, sustainable development and conservation of for economic reasons, most forest cover forest cover (Fearnside 1989, Alegretti 1994, is maintained. In addition, shifting cultivaAnderson 1992). They have also been tion and pasture areas within reserves are criticized as having limited chances for also relatively small and within close proxeconomic success (Browder 1992a, 1992b, imity to mature forest. Therefore, once Homma 1989) and for their limitations as abandoned, these agricultural plots convert conservation units (Redford and Stearman rapidly to secondary forest. 1993), particularly for megafaunal diversity Extractive reserves provide a new concept (Almeida 1992). The authors view them as for Amazonia. Most implemented models for evolving social experiments that will take Amazonian development and conservation years, if not decades, to fully evaluate their are based on a preservationist/developmenenvironmental, political, and socioeconomic talist dichotomy (Foresta 1991). In this strengths and weaknesses. dichotomy, forests and their ecosystem The purpose of this chapter is twofold: components are to be preserved in parks first, to analyze the role of extractive and biological reserves. Areas designated reserves as a potential factor in Amazonian for development are exploited at varying biogeochemistry, using changes in forest degrees of intensity that usually involve cover and land use as drivers, and the car- the complete elimination of forest cover bon cycle as the example; and second, to (pasture and annual crops) or partial extracdiscuss a participatory model for scientific tion and degradation (selective logging). This

I. Foster Brown et al.

124

Table 8.1 Distribution of extractive reserves and settlements in Brazilian Amazonia.

Extractive Settlements (INCRA)* Project Name

Decree and Date of Creation

Municipality/State

Area (ha)

Population Capacity

Antimary Canary

Port. 1055/98 de 28.07.88

Boca do Acre/AM Bujari/AC Epitaciolandia/AC

260,227 8054

4340

Port. 011/97 of 10/03/97 Port. 158/88 of 08/03/88

Chico Mendes -(Cachoeira) Limoeiro Porto Dias Porto Rico Remanso Riozinho Santa Quiteria Santo Antonio Mourao Terrua Maraca Praialta/Piranheira

24898

135 340

Port. 11/98 of 19/03/98

Bujari/AC

11,150

Resol. 40/89 of 20/10/89 Resol. 43/91 of 11/07/91 Port. 472/87 of 04/06/87 Resol. 39/89 of 20/10/89 Port. 886/88 of 24/06/88 Resol. 1855/92 of 20/08/92

Acrelandia/AC Epitaciolandia/AC Capixaba/AC Sena Madureira/AC Brasileia/AC Eirunepe/AM

22,145 7530 39,570 35,896

Port. 01/89 of 04/01/89 Port. 017/97 of 28/04/97

Pauini/AM Mazagao/AP

2320 5340

Port. 42/97 of 22/08/97

Nova Efigenia/PA Subtotal for extractive settlements

139,236 363,500 22,000 999,936

17,695

44,205 21,525

185 415 230 790 600 750 1000

1250

Extractive Reserves (IBAM A)** Name

Decree and Date of Creation

Principle Municipalities

Area (ha)

Estimated Population

Alto Jurua

98.862/90- 23/01/90

506,186

6000

Chico Mendes

99.144/90-12/03/90

970,570

7500

Rio Cajari Ciriaco Quilombo do Frexal Mata Grande Rio Ouro Preto Extreme Norte do -Tocantins

99.145/90-12/03/90 534/92-20/05/92 536/92-20/05/92 532/92-20/05/92 99.166/90-12/03/90 535/92-20/05/92

Cruzerio do Sul.Taumaturgo de Azevedo/AC Rio Branco, Xapuri, Brasileia, Assis Brasil/AC Laranjal doJarimasaga/AP Imperatriz/MA Imperatriz/MA Imperatriz-Joao Lisboa/MA Guajara-Mirim/RO Augustinopolis Sampaio/TO8280

481,650 7050 9542 10,450

5000 1150 900 1500 3410

Subtotal for extractive reserves Total of reserves and settlements

2,198,311 3,199,691

204,583 2000

27,460 46,155

Source: Data from INCRA * The National Institute for Agrarian Reform (INCRA) is responsible for establishing extractive settlements. ** The Brazilian Institute for Environment and Natural Renewable Resources (IBAMA) has governmental responsibility for management of extractive reserves. Population estimates for extractive reserves based on an average of 5 persons/ family. Population estimates are of variable accuracy and differences between areas reported in decrees and digitized from maps can vary by several percent. State extractive reserves from Rondonia are not included.

Extractive Reserves and Participatory Research as Factors in the Biogeochemistry of the Amazon Basin

preserved/exploited dichotomy is useful for maintenance of biodiversity of rare species, particularly megafauna such as jaguars and harpy eagles that have large natural ranges and require minimal forest disturbance. But between the end points of the landuse spectrum (national parks and extensive pastures/ intensive agriculture), lies a sizable portion of Brazilian Amazonia where human activity exists, but at relatively low intensities. It is for this region where lessons learned from extractive reserves have implications for how human populations and conservation of ecosystem function can coexist. Extractive reserves are not the panacea for forest conservation of all of Amazonia; the appropriate conditions for such reserves exist at maximum for 25 percent of Brazilian Amazonia (Meneses 1994). Nevertheless, such an area would exceed that of France. At present, extractive reserves administered by IBAMA, the Brazilian Institute for Environment and Natural Renewable Resources, and extractive settlements administered by INCRA, the National Institute for Agrarian Reform, cover about three million hectares, representing about one percent of the closed canopy forest in Brazilian Amazonia (see Table 8.1). From a carbon perspective, these reserves and settlements store about 6 x 108 Mg (tons) C (400 Mg/ha x 0.5 Mg C/Mg biomass x 3 x 106 ha). This is equivalent to about 10 percent of the global annual flux of carbon to the atmosphere from fossil fuel sources. Currently, the state of Acre in western Brazilian Amazonia has nearly half the area of extractive reserves and settlements in Amazonia (Table 1). Of the 1.7 million ha in reserves and settlements within the state, Chico Mendes Extractive Reserve is the largest, covering 970,000 ha. It is located in the Brazil nut-rich southeastern portion of Acre and it is this extractive region that is the focus of our subsequent discussions.

125

Extractivist Maintenance of Forest Cover Approximately 94% of the total area within Chico Mendes Extractive Reserve is retained in mature forest designated for nontimber forest extraction (Brown et al. 1992). This defining activity of extractive reserves provides the principle cash-generating products of the reserves, primarily rubber and Brazil nuts, and a wide variety of raw materials for home consumption such as fuelwood, wild fruits, medicines, game, and construction and artisan materials (Kainer and Duryea 1992). The provision of both cash and subsistence products makes the standing forest valuable to extractivist families, motivating them to maintain a high percentage of their land in mature forest rather than convert it to other forms of land use. The forest is also a key component in shifting cultivation, the mode of crop production in Acre's extractive reserves. Each family clears and burns approximately 0.25 to 2 ha of secondary or primary forest each year in preparation for a 3- to 6-year cropping cycle involving rice, corn, beans and finally manioc. When the forest is burned, some of the Ca, K, and Mg in the biomass is converted to ash and deposited on the soil surface, and perhaps more importantly, burning dramatically raises soil pH, increasing phosphorous availability (Nye and Greenland 1964, Ewel et al. 1981). After the cropping cycle, some of the land may be converted to pasture, or more typically, successional vegetation invades the site, thus beginning a 10- to 15-year fallow period. The fallow helps sustain agriculture by positively affecting two key elements: (1) restoration of soil fertility, and (2) decline in agricultural pest populations (Ewel 1986). Therefore, the mature forest and subsequently secondary forest, or fallow, provides critical biological services to the extractivists. Without these services, sustained crop production would become prohibitively costly in terms of labor

126

I. Foster Brown et al.

The basic equation is: as well as other energy inputs such as fertilizers and pesticides. Carbon content in _ carbon in biomass + The enormous value Acre's extractivists a landscape unit carbon in soil (1) place on the forest is exemplified in the well-documented and highly publicized Most research focuses on the time rate of rubber tapper resistance movement. During change of these stocks, stratified by vegetathe wholesale conversion of forest to tion type. Using the chain rule (see appenpasture during the 1970s and 1980s, many dix), we can derive the following relationship extractivist families in Acre risked their lives where the time rate of change of carbon in when threatened with expulsion from their the landscape unit is a function of four forested lands (Allegretti 1990, Hecht and groups of fluxes. These fluxes are listed as aCockburn 1989). Indeed, it was this d for each of the vegetation types movement that eventually led to the in the landscape unit: creation of the first extractive reserves in Change in carbon content in a landscape Acre. Simply stated, deforestation eliminates unit = the resource base from which the extractivist (a) Carbon in vegetation x migration of livelihood depends. vegetation type +

Carbon cycling and extractive reserves

(b) Carbon in secondary regrowth x area of vegetation type +

To help understand the effect of extrac(c) Erosion x carbon content of soil tive reserves on biogeochemical cycles, we in vegetation type + have selected carbon as the keystone reference element. The cycling of carbon drives (d) In situ changes of soil carbon x area of vegetation type, (2) the cycling of many other elements such as nitrogen and potassium. Carbon composes about half the dry biomass of tropical summed over all the vegetation types in the forests and consequently reflects ecosystem landscape unit. These fluxes are a product of natural and structure. Carbon stocks in biomass can vary more than ten-fold between primary anthropogenic factors, as discussed in the forest ecosystems and the agroecosystems other chapters. Flux (a) includes deforestathat replace them. Soil organic carbon also tion, natural migration of vegetation types, strongly affects the physical and chemical and pasture expansion. In the case of deforproperties of most tropical soils; it modifies estation where pasture substitutes forest, the the ion exchange capacity of soils, nitrogen rates of migration of forest and pasture are stocks, soil structure, and soil water reten- equal in magnitude but opposite in sign. The tion. Because much of the soil carbon is flux from this process equals the difference located close to the soil surface, its stocks in carbon content between forest and can be affected by changing land use. pasture. As the carbon content of tropical We can apply differential calculus and pastures is typically an order of magnitude the law of mass conservation to derive a less than that of closed canopy forests, this conceptual model for carbon dynamics in difference is roughly equal the original forest a forested landscape unit where carbon carbon content, when averaged over several is sequestered in either biomass (alive or burning seasons. dead but identifiable) and soil carbon (see The term "secondary regrowth" in flux (b) appendix for derivation). for forests also includes natural daily,

Extractive Reserves and Participatory Research as Factors in the Biogeochemistry of the Amazon Basin

seasonal, decadal, or secular changes in forest biomass that can affect the carbon content of the forest. For the purpose of this chapter, we will first examine how human activities affect these fluxes, estimating the relative importance of the various anthropogenic fluxes of equation (2) for Brazilian Amazonia; the landscape unit for our estimates is therefore the entire original forested region (Table 8.2). Then, we will demonstrate how extractive reserves and extractivist activities are related to these broader anthropogenically induced fluxes. The relative magnitudes of the fluxes described in equation (2) can be compared graphically in Fig. 8.1. Although the erosion flux may be important on a local basis, it probably represents more a redistribution of carbon between the terrestrial and fluvial systems than a flux to the atmosphere. The dominant gas flux is from deforestation. Consequently, land uses that maintain forest cover and/or reduce the deforestation rate will have a major impact on the anthropogenic fluxes, thereby affecting the total

127

atmospheric carbon flux in the Amazon basin. It is in this context that extractive reserves, and other land uses that maintain forest cover, have their importance for biogeochemical cycling. Extractive reserves can reduce deforestation by two mechanisms. First, local residents limit deforestation in their specific colocacoes or landholdings within the reserve in which they reside. Currently, federal regulations governing extractive reserve activities restrict deforestation to 10 % of the extractivist's landholding (Brasil 1995). As a result, a typical extractivist household that manages a landholding of 350 ha, acts as a guardian for more than 50,000 Mg of carbon of their managed forest (Brown et al. 1992). The fate of that carbon is largely determined at the household level, where decisions are made as to whether the high-biomass closed-canopy forest or a converted low-biomass agroecosystem provides a higher quality of life for that household. A second mechanism for reducing deforestation results from the buffering effect of reserves, limiting the extension of cattle

Fig. 8.1 A graphical representation of carbon fluxes directly associated with anthropogenic activities in Brazilian Amazonia, based on Table 8.2. Note the importance of the deforestation flux, calculated as committed emissions.

I. Foster Brown et al.

128

Table 8.2 Estimates of relative importance of the various anthropogenic fluxes of equation (2) for Brazilian Amazonia. Component

Estimate*

Source

Other estimates*

Flux (a): Deforestation rate

1.9 x loV"1

Original forest mass

400 Mg ha'1

INPE (1998), average for 1992-1996 One significant figure estimate, based on other estimates

464 Mg ha'1 Fearnside (1997), 424 Mg ha"1 Brown et al. (1992)

Carbon content of biomass (primary and secondary forests)

0.5 Mg C (Mg biomass)'1

Idem

0.48 Mg C (Mg biomass)'1 Carvalho et al. (1995)

Committed carbon emissions from biomass burning/decomposition

+400 x 106 Mg C yr1

Result of calculation, to one significant figure

+261 x 106 Mg C yr1 Fearnside (1997)

Regrowth rate of secondary forests

4 Mg ha'1 yr1

Salomao et al. (1996)

4-8 Mg ha'1 yr1 Saldarriaga et al. (1988) 6 Mg ha"1 yr"1 Schroeder and Winjum (1995)

Area of secondary forests

26 x 106 ha

one-half of deforested area in 1996 (INPE 1998)

17.5x 106 ha Schroeder and Winjum (1995)

Sink of carbon by secondary forests

- 50 x 106 Mg C yr1

Result of calculation, to one significant figure

-29 xlO6 Mg C yr1 Fearnside and Guimaraes (1996)

Flux (b):

Flux (c): Erosion loss

???

Flux (d): In situ change in soil carbon per unit area

-1 to 2 Mg C^ha-V calculated from Trumbore over a decade et al. (1996)

Area

52 x 106 ha

deforested area in 1996 (INPE 1998)

Flux

-50 to 100 x 106 Mg C yr1

Result of calculation, to one significant figure

*+ signifies source to atmosphere, - signifies sink from atmosphere.

Extractive Reserves and Participatory Research as Factors in the Biogeochemistry of the Amazon Basin

ranching and colonization projects. Differences between forest cover in Chico Mendes Extractive Reserve and the surrounding countryside are marked (Fig. 8.2), and the eastern boundary of the reserve was drawn precisely to halt the expansion of the ranches. At the scale of this image, extractivist clearings for shifting cultivation plots and pastures do not appear. The buffering role of these reserves only increases in importance as frontier expansion continues. The Chico Mendes Extractive Reserve, for example, borders the Brazilian Highway BR-317 that someday will connect western Brazilian Amazonia with a Peruvian seaport.

The Role of Research in Extractive Reserve Viability Although Acrean extractivists have been utilizing and managing their forests for decades, they are constantly challenged to successfully address ever-changing constraints and opportunities in the local, regional, national, and global socioeconomic and environmental arena. These spatial and temporal factors are multiple (Table 8.3), and their complexity is aggravated by their interactions as can be seen by citing a few examples. From a regional perspective, some parts of the Chico Mendes Extractive Reserve are dominated by extensive (101 to 104 hectares) patches of bamboo (Guadua sp). In these patches, transportation is difficult and Brazil nut trees are nearly absent; not surprisingly, these areas have the lowest population densities in the Reserve. In this case, regional environmental factors affect not only abundance of economic species, but also inhibit transportation of goods and general communication. On a global scale, success of rubber plantations in diverse areas such as Malaysia and southern Brazil have drastically altered the price of natural rubber, making extractive rubber production less lucrative

129

(Homma 1992). At these same scales, Acrean rubber tappers, lead by Chico Mendes in the mid 1980s, forged alliances with the global environmental movement that brought them international recognition and eventually led to the creation of extractive reserves (Hecht and Cockburn 1989, Keck 1995). Technological innovations, road networks, and changing national aspirations have all affected extractivist activities. In addition to environmental and socioeconomic drivers operating at various spatial and time scales, extractivists are faced with changing political and conceptual challenges. For example, the Chico Mendes Extractive Reserve joined over forty seringals into one

Fig. 8.2. Landsat TM image of the central portion of the Chico Mendes Extractive Reserve in eastern Acre, Brazil. The white line outlines the boundary of the reserve. North is to the top of the image, which covers about 90 km x 90 km. Xapuri is the local urban center. The light areas are deforested lands, principally large cattle ranches. The national highway BR-317 threads through the center of the deforested lands along the eastern side of the image. Data is from August 1989, TM bands 3, 4, and 5.

I. Foster Brown et al.

130

Table 8.3 Some factors affecting the viability of extractive reserves as functions of temporal and spatial scales. Socioeconomic Factors*

Environmental Factors*

Local

Household structure, local community organization, access to information

Distribution and abundance of economic species, topography, soil type

Regional

Transport network, education and health services, regional organizations and markets

Large-scale dieoffs of dominant species (e.g. bamboo), faunal distributions, alteration by smoke of Photosynthetically Active Radiation (PAR)

National

Policy decisions (e.g. rubber subsidies, creation of reserves), inflation control, recession

Global

Commodity prices, political pressure from other countries

Climate changes, particularly of rainfall patterns (e.g. El Nino)

Seasonal

Cash inputs and outputs

Excessive rainfall, dry season, burning activity

Annual

Market fluctuations, health of individual household members, fluctuating strength of representative political and economic organizations

Fluxes in productivity of economic species (e.g. Brazil nut), pest and predator populations

Decadal

Changes in political climate, household labor availability by life cycle

Recruitment of economic species, El Nino events

Generational

Education and cultural transmission

Scale Spatial

Temporal

* Note that factors also interact across space and time as well as across environmental and socioeconomic domains.

political entity. Extractivists in this reserve are faced with forging a new identity in order to make such an entity work. The rising importance given to biodiversity conservation has provoked the debate as to whether the conservation role of these extractive reserves is to be a byproduct of their economic activities or should be subsidized

by national and global beneficiaries (Almeida 1994, Carvalho and Brown 1996). These issues are complex and research can help understand that complexity and tease out the more critical points. While extractivists have made heady gains in their struggle for land rights, access to social services, and cultural dignity (Keck 1995),

Extractive Reserves and Participatory Research as Factors in the Biogeochemistry of the Amazon Basin

there still exists a pressing need for sustainable economic options and technical assistance in extractive reserves. The scientific community can play an important role in facilitating the identification, testing, and evaluation of the myriad production and forest conservation strategies open to these rain forest peoples. Moreover, scientists can further enhance research effectiveness by embracing a participatory approach in their research efforts, collaborating with both the extractive community and local professionals who contribute to reserve viability.

Participatory research as one form of scientific inquiry The decision to adopt a participatory approach in our research reflected the premise that the responsibility of environmental scientists is to provide reliable information that will help society resolve its environmental problems. While the premise may be laudatory, it is slippery to apply. For example, the typical means of supplying new information to society takes the form of scientific papers published in English in international journals (e.g. Brown et al. 1992, Kainer et al. 1998). In Brazilian Amazonia, we have found, however, that information in this format does not easily reach the local and regional decisionmakers, few of whom read English or have effective access to scientific literature. Also, those who have the most at stake, the local communities, require a different mode of information transfer. We also considered that involvement of extractivists in our research was essential. Ultimately, the extractivists are the ones who will make the decision to retain the majority of their land under forest cover or convert it to other types of managed land use less favorable for maintenance of natural biogeochemical cycles. In addition, by proactively seeking collaboration with local professionals, we felt that we could use our knowledge,

131

contacts, and financial resources to learn from and train a cadre of local individuals and organizations interested in the fate of extractive reserves. In adopting a participatory approach, we expected that the research process, not simply the findings, would provide a rich, immediate learning experience for all who make forest management decisions affecting extractivist living conditions and conservation of their environment. These considerations led us to adopt a more participatory approach, the principles of which are listed in Table 8.4. The following two case histories illustrate aspects of a more participatory approach to research that we have used in an attempt to enhance economic viability in extractive reserves while minimizing disruption of forest cover. The objectives of our research were both result and process oriented. First, our goal was to provide information to those who make decisions concerning reserve management. Second, we wanted to execute our research in such a way as to build the capacity of those who are most intimate with the day-to-day management of the reserves so that they may use the information. The first case concerns research directed to enriching the forest with Brazil nut trees, and the second addresses mapping as a key activity for local land use control.

Brazil Nut Research Brazil nut (Bertholletia excelsa Humbolt & Bonpland) is arguably the most important species to many extractivists in eastern Acre due to its multiple indigenous uses and the cash income generated from nut sales. Ecologically, the importance of Brazil nut within the local ecosystem is equally impressive (Ortiz 1995), and further, this species exemplifies the functional role of large trees on the biogeochemical cycle in extractive reserves. As the epithet excelsa implies, Brazil nut is a giant in the Amazon forest. It is a

132

I. Foster Brown et al.

Due to the fundamental role of Brazil nut dominant, upper-canopy tree often reaching 50 m in height, found on nonflooded lands in extractivist livelihoods, and ultimately in (terra firme) in the Amazon basin and the the success of extractive reserves, the Guianas (Prance 1990). In eastern Acre, Brazil National Council of Rubber Tappers called nut trees comprised 20% of the total biomass for research on Brazil nut ecology, processof a 5 ha survey of the primary forest ing, and political economy (ECOTEC and (F. Brown, personal observation). Clearly, on CNS 1990). One specific recommendation was mass alone, Brazil nut trees figure prominent- to increase Brazil nut densities through enrichly in the broader biogeochemical cycles of ment plantings (CNS et al. 1991)- In response, participatory research was carried out in 1993 the region. Human interaction with Brazil nut is long- to 1994 in three extractive communities near standing. Throughout its range, Brazil nut Xapuri, Acre, to assist extractivists in deteroften has an erratic distribution, occurring in mining which sites within the reserves are the stands of 50 to 100 individuals with each most appropriate for establishing Brazil nut stand separated from another by distances of seedlings. The specific research objectives of up to 1 km (Prance and Mori 1979). These the doctoral study were to examine and comdistribution patterns have been attributed to pare environmental and socioeconomic conAmerindian interventions (Miiller et al. 1980, straints to seedling establishment in forest Balee 1989). Even the type specimen of the gaps, shifting cultivation plots, and pastures first recorded description of the species made (Kainer et al.1998). Aside from these specific by Humboldt and Bonpland in 1807 came research objectives, the scientists had a from a tree in a Venezuelan churchyard that second participatory agenda that included the may have been cultivated from a Brazilian following modest development and empowseed source (Prance 1990). Brazil nut erment objectives. Carry out the research "on-site" in order to currently provides a host of raw materials used for household consumption in extrac- insure that research treatments, results, and tive reserves from food and medicinal prod- recommendations better respond to the socioecoucts to children's games. Most critically, nut nomic realities of the extractive reserves. sales are currently the greatest source of cash Selection of the specific landholdings for income for the many extractivist households, seedling establishment took place in a generparticularly those living in southeast Acre al community meeting wherein the research proposal was introduced and feedback solicited. (Campbell 1996). Table 8.4 Working guidelines for participatory research in extractive reserves. 1. Seek situations where local organizations are effective, available, and interested enough to incorporate the results of the research (multiplier effect) 2. Carry out research "on-site" if experimental results are to be applied locally 3. Clarify both researcher and participant expectations 4. Develop a base of trust by becoming involved in community functions, even if unrelated to the research 5. Become as fluent as possible in local language and culture 6. Include as many local institutions as possible in the research process 7. Include a training component for building capacity & interest of local professionals 8. Return research results in a timely manner and in forms appropriate for local consumption * Note that not all items are necessary nor desirable in all research situations.

Extractive Reserves and Participatory Research as Factors in the Biogeochemistry of the Amazon Basin

133

Objectives, treatments, and procedures were EMBRAPA (Agronomic Research Institute of discussed in detail to insure that all volunteer Brazil) participated in various phases families clearly understood responsibilities if of the research. In addition, a local they chose to participate. nongovernmental organization, PESACRE The on-site nature of the research and (Agroforestry Systems Research and inclusion of socioeconomic variables great- Extension Group of Acre), was a key ly affected final results and recommenda- collaborator during the entire study. tions. Based on environmental variables Intensively train local professionals in the alone (light, water, and nutrients), shifting research process. Trained researchers in the cultivation plots and pastures appear to be Amazon are few, and a young, local agronoequally suitable for enrichment plantings. my student participated fully in the entire Employing solely socioeconomic criteria field research process. She subsequently (labor considerations and overall compati- received a grant to accompany the laboratobility with the extractivist system), forest ry and analysis phases of the research at the gaps are the best enrichment sites. University of Florida. During this period, an However, upon consideration of both envi- extension booklet of the Brazil nut research ronmental and socioeconomic variables, was also designed. shifting cultivation plots are clearly the best Expose extractivists to the research experience. sites for enrichment plantings, as confirmed Day to day exposure to field studies takes by growth results. the mystery out of research. This gives extracCarrying out research on site also assisted tivists a fuller understanding of how research in our understanding of the practical results and conclusions are reached, and it constraints of extractivist living as well as validates their own local experimentation. For important cultural and language elements. To example, one extractivist who was not even enhance our interactions with extractivists, directly involved in the research has we lived with extractivist families throughout constructed a tree nursery and is outplanting the study period, participating in as many his seedlings into his shifting cultivation plots. community functions as possible. In addition, as extractivists are exposed to Include as many local institutions as possible research, foreigners, and urban Brazilians, in the research process. Despite the fact that they are broadening their understanding of the rural poor are a major target group for where they fit within the global scheme. This many governmental and nongovernmental empowerment experience gives them greater organizations, there is limited exposure and social and technical skills for managing the understanding of rural realities on the part extractive reserves, and helps them refine of most of these local, urban-based their ability to negotiate with the outside research, development, and conservation world, thus increasing control of their own organizations. In addition to facilitating development. linkages between extractivists and the Return results in an appropriate and timely institutions interested in and/or responsible manner. Meetings about research progress for supporting extractive reserve success, were held periodically, and preliminary the personal exposure and involvement by results were returned one year into the study individuals of these organizations creates a at a final community meeting that included a powerful human connection that often band, dance, and banquet with traditional leads to further personal commitment. foods made of Brazil nuts. One and a half Professionals from CAEX (Agroextractive years later, final results and recommendaCooperative of Xapuri), IBAMA, and tions were returned in oral form to the

134

professional research and extension community in Acre, the local nongovernmental organization most responsible for extractivist needs (CAEX), and the extractivists in the reserves. An extension booklet outlining the study in a simple, didactic manner was also distributed at this time to the extractivist community. Clearly, these guidelines and the manner in which this one research project was carried out does not ensure improved forest management in the reserves, but we believe that the participatory approach we adopted contributed greatly to the observed positive outcomes. Of equal importance, we feel that the science itself was fortified by using a participatory approach. We were easily able to tap into local knowledge and experience that improved treatments and provided ecological insights into the biology and cultivation of Brazil nut trees.

Land use mapping using satellite imagery Spatial location forms part of the human experience. Extractivists use mental maps to navigate within their own landholdings, colocacoes, typically covering several hundred hectares. Contiguous landholdings compose a seringal or former rubber estate. These former estates range from a few thousands to tens of thousands of hectares in size. Extractivists normally know their own rubber estate extremely well and possibly neighboring ones; however, the larger extractive reserves cover several hundred thousand hectares, well beyond the mental maps of individual extractivists. The National Council of Rubber Tappers realized that management of these reserves would require geographic information of human populations, resources, and land use, in short, maps. They requested help in mapping the Chico Mendes Extractive Reserve (Brown et al. 1995).

I. Foster Brown et al.

Much of the information available from residents in the reserve is referenced to their landholdings. In order to create detailed thematic maps, our first step has been to map the location of about 700 landholdings living within the reserve. Naming the landholdings, however, can only be done with the active participation of community residents. We have found this participation to be most effective when residents gather for meetings with union representatives or during other similar activities. Once extractivists understand that these maps will enable them to communicate with other extractivists, union representatives, and government officials, we began the mapping exercise using Landsat TM images (Fig. 8.3). Extractivists superimpose their "mental maps" with the image and typically produce a map of clearings over thousands of hectares in a few hours. There have been several positive spinoffs from incorporating extractivists into the research process. These mapping visits have spawned demand in local communities for more training in mathematics in order to calculate areas for agroforestry systems. The use of Global Positioning System (GPS) units for locating landholdings stimulated extractivists to learn about coordinate systems. Frequently, we used inflatable globes that illustrated not only the logic behind latitude and longitude, but also helped extractivists understand the geographical context of their reserve. Two local professionals who engaged in the mapping activities have continued on for their Masters degrees, using their experiences as a basis for their dissertations. The mapping of the Chico Mendes. Extractive Reserve has served two purposes. It has helped generate information necessary for the management of the reserve, the initial goal of the research. More importantly, however, it has stimulated dozens of extractivists to learn more about their reserve and

Extractive Reserves and Participatory Research as Factors in the Biogeochemistry of the Amazon Basin

135

Fig. 83 Example of local professionals building the capacity of local extractivists in the use of satellite images for mapping. Black and white images and tracing paper are used for the exercise. On wall at back is a preliminary map of landholdings of the Chico Mendes Extractive Reserve made in collaboration with The Technological Foundation of the State of Acre (FUNTAC) and the Remote Sensing Center of IBAMA.

encouraged them to develop skills that are not only useful for mapping, but also in deciding their own fate. During the continued mapping of the Chico Mendes Extractive Reserve, we anticipate that extractivists will continue to take more responsibility for the mapping and posterior quality control, as well as to expand this mapping to other areas.

Ecological research with local participation The creation of extractive reserves alone will not resolve the issues of ecosystem impoverishment and alteration of biogeochemical cycles in Amazonia (Nepstad et al. 1992). These complex issues need to be addressed at several scales simultaneously (Carvalho and Brown 1996). Nevertheless, the explicit managerial position of extractivists as stewards of the reserves, and therefore the forests they inhabit, suggests that a more collegiate relationship between researcher and local resident is warranted. Extractive reserves therefore provide rich opportunities for participatory research in which scientists and local communities may act together for improved conservation and development.

There is no fixed manner in which participatory research should be conducted. The examples above are simply two ways in which ecological research "was carried out with a certain degree of participation by the extractivist community and local professionals. To our knowledge, little ecological or biogeochemical research has been carried out in a participatory manner, but there exists a great body of experiences with local participation in agronomic research (Hildebrand 1986, Chambers et al. 1989, Quiros et al. 1991). Documentation of participation in development work (Cohen and Uphoff 1977) and discussion on how to carry it out in the field (Bunch 1982) has an even longer history. Participatory conservation is a relatively new offshoot of these efforts, and Little (1994) provides a review of some issues and experiences relevant to the incorporation of local participation in conservation work. Specific methods and tools, as well as thoughtful discussion on local participation and empowerment, are becoming increasingly available for those working with rural peoples (FAO 1990, Slocum et al. 1995, Pretty et al. 1995), and can be adapted for use in scientific research. There is an opportunity to incorporate such participatory research in the program for

136

economic and ecological zoning of Brazilian Amazonia, under the ageis of federal and state agencies. As the scale of this zoning becomes more detailed, local communities can make significant contributions to thematic mapping and, through such activity, learn to participate more effectively in zoning decisions that will affect their lives.

Appendix. Derivation of the Model for carbon fluxes We can apply the rules of differentiation to derive a model of the change in carbon content of a landscape unit covered by vegetation types of differing carbon contents. Equation (A-l) expresses the carbon content of the unit as the sum of carbon in the biomass and in the soil of the vegetation types:

I. Foster Brown et al.

(A-l) with respect to time results in the following first-order differential equation that can be simplified by integrating the biomass component over the soil depth z, so that Bv is in more conventional values of mass/area, and assuming that fv is not a function of time:

Equation (A-2) summarizes several processes. The first term includes

, the rate of

migration of vegetation types for which the carbon flux is proportional to the biomass Bv. The second term,

, is often

considered secondary regrowth, but also can include daily, seasonal, decadal, or secular changes in forest biomass (Keller et al. 1996). The third term, -

where C is the mass of carbon in a threedimensional landscape unit, Av is the area covered by vegetation type v, Zv is the depth of soil to which the vegetation type affects carbon storage, Bv is the biomass of the vegetation type (above-and belowground) in mass per soil volume, fv is the fraction of the biomass that is carbon in vegetation type v, Pv is the soil density, and Sv is the fraction of the soil that is carbon (mass/mass). All the values are averaged for the vegetation type. The total carbon is then the sum (D of the carbon in vegetation and soil of t