Sustainable Agriculture

Sustainable Agriculture

Eric Lichtfouse  Mireille Navarrete Philippe Debaeke  Véronique Souchère Caroline Alberola Editors

Sustainable Agriculture

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Editors Eric Lichtfouse INRA-CMSE-PME 17 rue Sully 21065 Dijon CX France [email protected] Mireille Navarrete INRA-SAD Unite d’Écodéveloppement 84914 Avignon CX 09 France

Véronique Souchère INRA-INA PG UMR SADAPT 78850 Thiverval-Grignon France Caroline Alberola ASEF Europôle de l’Arbois Avenue Louis Philibert 13857 Aix-en-Provence CX 3 France

Philippe Debaeke INRA UMR AGIR 31326 Toulouse CX France

All articles contained in this volume have been previously published in the journal “Agronomy for Sustainable Development” published by EDP Sciences, France. ISBN 978-90-481-2665-1 e-ISBN 978-90-481-2666-8 DOI 10.1007/978-90-481-2666-8 Springer Dordrecht Heidelberg London New York Library of Congress Control Number: 2009926164 c Springer Science+Business Media B.V. 2009 No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission from the Publisher, with the exception of any material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Cover design: Cover legend from top: cotton balls in Uzbekistan, fern in Borneo, tomatoes in France, medlar fruits drying in Uzbekistan, sunflower in France. Cover pictures were kindly provided by Dominique Millot, Dijon, France. Copyright: Dominique Millot 2008. Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com)

Contents

Agronomy for Sustainable Agriculture: A Review : : : : : : : : : : : : : : : : : : : : : : Eric Lichtfouse, Mireille Navarrete, Philippe Debaeke, Véronique Souchère, Caroline Alberola, and Josiane Ménassieu

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Laws of Sustainable Soil Management : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : Rattan Lal

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Part I CLIMATE CHANGE Soils and Sustainable Agriculture: A Review : : : : : : : : : : : : : : : : : : : : : : : : : : 15 Rattan Lal Soils and Food Sufficiency: A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 25 Rattan Lal Denitrification at Sub-Zero Temperatures in Arable Soils: A Review : : : : : : 51 Rebecca L. Phillips Re-Thinking the Conservation of Carbon, Water and Soil: A Different Perspective : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 61 Thomas Francis Shaxson Cropping Systems, Carbon Sequestration and Erosion in Brazil: A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 75 Martial Bernoux, Carlos C. Cerri, Carlos Eduardo P. Cerri, Marcos Siqueira Neto, Aurélie Metay, Anne-Sophie Perrin, Eric Scopel, Razafimbelo Tantely, Didier Blavet, Marisa de C. Piccolo, Mariana Pavei, and Eleanor Milne Influence of Land Use on Carbon Sequestration and Erosion in Mexico: A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 87 J.D. Etchevers, C. Prat, C. Balbontín, M. Bravo, and M. Martínez Rhizodeposition of Organic C by Plant: Mechanisms and Controls : : : : : : : 97 Christophe Nguyen v

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Environmental Costs and Benefits of Transportation Biofuel Production from Food- and Lignocellulose-Based Energy Crops: A Review : : : : : : : : : : 125 Jason Hill Grasslands for Bioenergy Production: A Review : : : : : : : : : : : : : : : : : : : : : : : 141 Enrico Ceotto Plant Drought Stress: Effects, Mechanisms and Management : : : : : : : : : : : : 153 M. Farooq, A. Wahid, N. Kobayashi, D. Fujita, and S.M.A. Basra Part II GENETICALLY MODIFIED ORGANISMS Pharmaceutical Crops in California, Benefits and Risks: A Review : : : : : : : 191 Michelle Marvier Coexistence of Genetically Modified and Non-GM Crops in the European Union: A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 203 Yann Devos, Matty Demont, Koen Dillen, Dirk Reheul, Matthias Kaiser, and Olivier Sanvido Agro-Environmental Effects Due to Altered Cultivation Practices with Genetically Modified Herbicide-Tolerant Oilseed Rape and Implications for Monitoring: A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : 229 F. Graef Bacillus thuringiensis: Applications in Agriculture and Insect Resistance Management – A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 243 Vincent Sanchis and Denis Bourguet Genetically Modified Glyphosate-Tolerant Soybean in the USA: Adoption Factors, Impacts and Prospects – A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : 257 Sylvie Bonny Part III BIODIVERSITY Small Eats Big: Ecology and Diversity of Bdellovibrio and Like Organisms, and their Dynamics in Predator-Prey Interactions : : : : : : : : : : : 275 Shemesh Yair, Davidov Yaacov, Koval Susan, and Edouard Jurkevitch Identification of Traits Implicated in the Rhizosphere Competence of Fluorescent Pseudomonads: Description of a Strategy Based on Population and Model Strain Studies : : : : : : : : : : : : : : : : : : : : : : : : 285 Xavier Latour, Sandrine Delorme, Pascal Mirleau, and Philippe Lemanceau Progress in Mechanisms of Mutual Effect between Plants and the Environment : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 297 Hong-Bo Shao, Li-Ye Chu, and Biao Li Biodiversity: Function and Assessment in Agricultural Areas: A Review : : 309 Boris Clergue, Bernard Amiaud, Frank Pervanchon, Françoise Lasserre-Joulin, and Sylvain Plantureux

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Mixing Plant Species in Cropping Systems: Concepts, Tools and Models: A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 329 E. Malézieux, Y. Crozat, C. Dupraz, M. Laurans, D. Makowski, H. Ozier-Lafontaine, B. Rapidel, S. de Tourdonnet, and M. Valantin-Morison Saffron, An Alternative Crop for Sustainable Agricultural Systems: A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 355 F. Gresta, G.M. Lombardo, L. Siracusa, and G. Ruberto Digital Imaging Information Technology Applied to Seed Germination Testing: A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 377 Antonio Dell’ Aquila Part IV ALTERNATIVE CONTROL Managing Weeds with a Dualistic Approach of Prevention and Control: A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 391 Randy L. Anderson Mechanical Destruction of Weeds: A Review : : : : : : : : : : : : : : : : : : : : : : : : : : 399 D. Chicouene Sustainable Pest Management for Cotton Production: A Review : : : : : : : : : 411 Jean-Philippe Deguine, Pierre Ferron, and Derek Russell Role of Nutrients in Controlling Plant Diseases in Sustainable Agriculture: A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 443 Christos Dordas Crop Protection, Biological Control, Habitat Management and Integrated Farming : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 461 Pierre Ferron and Jean-Philippe Deguine Using Grassed Strips to Limit Pesticide Transfer to Surface Water: A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 471 Jean-Guillaume Lacas, Marc Voltz, Véronique Gouy, Nadia Carluer, and Jean-Joël Gril Part V ALTERNATIVE FERTILISATION Recycling Biosolids and Lake-Dredged Materials to Pasture-based Animal Agriculture: Alternative Nutrient Sources for Forage Productivity and Sustainability: A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 495 Gilbert C. Sigua Symbiotic Nitrogen Fixation in Legume Nodules: Process and Signaling: A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 519 Neera Garg and Geetanjali Factors Responsible for Nitrate Accumulation: A Review : : : : : : : : : : : : : : : 533 Anjana, Shahid Umar, and Muhammad Iqbal

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Role of Phosphate Solubilizing Microorganisms in Sustainable Agriculture – A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 551 Mohammad Saghir Khan, Almas Zaidi, and Parvaze A. Wani Iron and Zinc Biofortification Strategies in Dicot Plants by Intercropping with Gramineous Species: A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 571 Y. Zuo and F. Zhang Soil Exploration and Resource Acquisition by Plant Roots: An Architectural and Modelling Point of View : : : : : : : : : : : : : : : : : : : : : : : : : 583 Claude Doussan, Loïc Pagès, and Alain Pierret Methods for Studying Root Colonization by Introduced Beneficial Bacteria : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 601 Elisa Gamalero, Guido Lingua, Graziella Berta, and Philippe Lemanceau Part VI NEW FARMING SYSTEMS Sustainable Urban Agriculture in Developing Countries: A Review : : : : : : : 619 Hubert de Bon, Laurent Parrot, and Paule Moustier Nitrogen, Sustainable Agriculture and Food Security: A Review : : : : : : : : : 635 J.H.J. Spiertz Conversion to Organic Farming: A Multidimensional Research Object at the Crossroads of Agricultural and Social Sciences – A Review : : : : : : : : 653 Stéphane Bellon and Claire Lamine Triggering Transitions Towards Sustainable Development of the Dutch Agricultural Sector: TransForum’s Approach : : : : : : : : : : : : : : : : : : : : : : : : : 673 A. Veldkamp, A.C. Van Altvorst, R. Eweg, E. Jacobsen, A. Van Kleef, H. Van Latesteijn, S. Mager, H. Mommaas, P.J.A.M. Smeets, L. Spaans, and J.C.M. Van Trijp Spatialising Crop Models : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 687 Robert Faivre, Delphine Leenhardt, Marc Voltz, Marc Benoît, François Papy, Gérard Dedieu, and Daniel Wallach Iterative Design and Evaluation of Rule-Based Cropping Systems: Methodology and Case Studies – A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : : 707 Philippe Debaeke, Nicolas Munier-Jolain, Michel Bertrand, Laurence Guichard, Jean-Marie Nolot, Vincent Faloya, and Patrick Saulas Agri-Environmental Indicators to Assess Cropping and Farming Systems: A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 725 Christian Bockstaller, Laurence Guichard, David Makowski, Anne Aveline, Philippe Girardin, and Sylvain Plantureux

Contents

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Methodological Progress in On-Farm Regional Agronomic Diagnosis: A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 739 Thierry Doré, Cathy Clermont-Dauphin, Yves Crozat, Christophe David, Marie-Hélène Jeuffroy, Chantal Loyce, David Makowski, Eric Malézieux, Jean-Marc Meynard, and Muriel Valantin-Morison Ex ante Assessment of the Sustainability of Alternative Cropping Systems: Implications for Using Multi-criteria Decision-Aid Methods – A Review : : : 753 Walid Sadok, Frédérique Angevin, Jacques-Éric Bergez, Christian Bockstaller, Bruno Colomb, Laurence Guichard, Raymond Reau, and Thierry Doré Comparison of Methods to Assess the Sustainability of Agricultural Systems: A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 769 Christian Bockstaller, Laurence Guichard, Olivier Keichinger, Philippe Girardin, Marie-Béatrice Galan, and Gérard Gaillard Soil-Erosion and Runoff Prevention by Plant Covers: A Review : : : : : : : : : : 785 Víctor Hugo Durán Zuazo and Carmen Rocío Rodríguez Pleguezuelo Integration of Soil Structure Variations with Time and Space into Models for Crop Management: A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 813 J. Roger-Estrade, G. Richard, A.R. Dexter, H. Boizard, S. De Tourdonnet, M. Bertrand, and J. Caneill Management of Grazing Systems: From Decision and Biophysical Models to Principles for Action : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 823 Michel Duru and Bernard Hubert Part VII POLLUTANTS IN AGROSYSTEMS Cadmium in Soils and Cereal Grains After Sewage-Sludge Application on French Soils: A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 845 Denis Baize Mobility, Turnover and Storage of Pollutants in Soils, Sediments and Waters: Achievements and Results of the EU Project AquaTerra – A Review : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 857 J.A.C. Barth, P. Grathwohl, H.J. Fowler, A. Bellin, M.H. Gerzabek, G.J. Lair, D. Barceló, M. Petrovic, A. Navarro, Ph. Négrel, E. Petelet-Giraud, D. Darmendrail, H. Rijnaarts, A. Langenhoff, J. de Weert, A. Slob, B.M. van der Zaan, J. Gerritse, E. Frank, A. Gutierrez, R. Kretzschmar, T. Gocht, D. Steidle, F. Garrido, K.C. Jones, S. Meijer, C. Moeckel, A. Marsman, G. Klaver, T. Vogel, C. Bürger, O. Kolditz, H.P. Broers, N. Baran, J. Joziasse, W. Von Tümpling, P. Van Gaans, C. Merly, A. Chapman, S. Brouyère, J. Batlle Aguilar, Ph. Orban, N. Tas, and H. Smidt Effect of Metal Toxicity on Plant Growth and Metabolism: I. Zinc : : : : : : : : 873 Gyana Ranjan Rout and Premananda Das Phytoremediation of Organic Pollutants Using Mycorrhizal Plants: A New Aspect of Rhizosphere Interactions : : : : : : : : : : : : : : : : : : : : : : : : : : : : 885 Erik Jautris Joner and Corinne Leyval Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 895

Agronomy for Sustainable Agriculture: A Review Eric Lichtfouse, Mireille Navarrete, Philippe Debaeke, Véronique Souchère, Caroline Alberola, and Josiane Ménassieu

Abstract Sustainability rests on the principle that we must meet the needs of the present without compromising the ability of future generations to meet their own needs. Starving people in poor nations, obesity in rich nations, increasing food prices, on-going climate changes, increasing fuel and transportation costs, flaws of the global market, worldwide pesticide pollution, pest adaptation and resistance, loss of soil fertility and organic carbon, soil erosion, decreasing biodiversity, desertification, and so on. Despite unprecedented advances in sciences allowing us to visit planets and disclose subatomic particles, serious terrestrial issues about food show clearly that conventional agriculture is no longer suited to feeding humans and preserving ecosystems. Sustainable agriculture is an alternative for solving fundamental and applied issues related to food production in an ecological way [Lal (2008) Agron. Sustain. Dev. 28, 57–64]. While conventional agriculture is driven almost solely by productivity and profit, sustainable agriculture integrates biological, chemical, physical, ecological, economic and social sciences in a comprehensive way to develop new farming practices that are safe and do not degrade our environment. To address current agronomical issues and to promote worldwide discussions and cooperation we implemented sharp changes at the journal Agronomy for Sustainable Development from 2003 to 2006. Here we report (1) the results of the renovation of the

journal and (2) a short overview of current concepts of agronomical research for sustainable agriculture. Considered for a long time as a soft, side science, agronomy is rising fast as a central science because current issues are about food, and humans eat food. This report is the introductory article of the book Sustainable Agriculture, volume 1, published by EDP Sciences and Springer (Lichtfouse et al., 2009, this book). Keywords Agronomy for sustainable development Biodiversity r Climate change r Farming system Food r Organic farming r Pest control r Pesticide Soil r Sustainable agriculture r Water

r r r

Foreword This article is dedicated to Ms. Josiane Ménassieu. Josiane was the Editorial Secretary of the journal Agronomy for Sustainable Development (ASD) from 2003 and retired in April 2008. The success of the renovation of the journal from 2003 to 2006 is mainly due to her intensive work. Her kindness was greatly appreciated by authors, peer-reviewers and Field Editors. Figure 1 shows a picture of the present that was offered to her by colleagues for her retirement.

1 The Journal Agronomy for Sustainable Development E. Lichtfouse () INRA, Editor-in-Chief, Journal Agronomy for Sustainable Development, INRA-CMSE-PME, 17 rue Sully, 21000 Dijon, France e-mail: [email protected]

Agronomy for Sustainable Development (ASD, agronomy-journal.org) is one of the seven journals of the French Institute of Agronomical Research (INRA,

E. Lichtfouse et al. (eds.), Sustainable Agriculture, DOI 10.1007/978-90-481-2666-8_1, c Springer Science+Business Media B.V. - EDP Sciences 2009. Reprinted with permission of EDP  Sciences from Lichtfouse et al., Agron. Sustain. Dev. 29 (2009) 1–6. DOI: 10.1051/agro:2008054

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E. Lichtfouse et al. Impact factor 1

0 1985

1995

2005

Fig. 2 Impact factor of the journal Agronomy for Sustainable Development (ASD). Note the increase in the impact factor from 0.566 in 2003 to 1.000 in 2007 (+77%). In 2006, the journal had two impact factors due to title change in 2005: 0.863 for the old title, Agronomie, and 0.306 for the new title, ASD. Those factors are not shown in the graph above because their calculation is different from that of normal impact factors. Specifically, the value for the old title is higher because it takes into account only older articles (2004) that thus have higher chances of getting cited (see scientific.thomsonreuters.com/isi for details) Fig. 1 Coquelicots painting by artist Agathe Rémy, who lives near Aix-en-Provence, France. Coquelicots is the French word for poppies

international.inra.fr). The journal is managed by a collaboration of two INRA departments, the Department of Environment and Agronomy (www.inra.fr/ea) and the Department of Sciences for Action and Development (www.inra.fr/sad). Journal issues are produced by EDP Sciences (edpsciences.org). Our Editorial board collaborates with three Associate Editors and 32 Field Editors for manuscript peer-review. ASD publishes research and review articles. Submitted articles are first evaluated by a Pre-Selection Committee that declines about 50% of incoming manuscripts. Selected submissions are then sent to Field Editors for in-depth evaluation. The global rejection rate was 77% in 2006. The current impact factor (2007) is 1.000, ranking the journal 25/49 in the category Agronomy (Fig. 2). ASD Journal was greatly reformed from 2003 to 2006. We changed topics from classical, productionoriented agronomy to sustainable and ecological agriculture (Lichtfouse et al. 2004; Alberola et al. 2008). We integrated social and economic sciences by setting up a collaboration between the INRA Department of Environment and Agronomy and the INRA Department of Sciences for Action and Development. Major journal topics currently include: – Agriculture and global changes – Agricultural production of renewable energies – Ecological pest control and biopesticides

– Organic farming – Genetically modified organisms in cropping systems – Environmental impact on soil, water, air and biodiversity – Risk assessment for food, ecotoxicology – Decision support systems and companion modelling – Social and economic issues of agricultural changes – Innovation in farming systems – Pollutants in agrosystem Major journal changes implemented during the journal renovation are shown in Table 1. They include: the setting up of a pre-selection committee at the submission stage; an increase in the number of Field Editors from 14 to 32; novel topics focusing on sustainable agriculture (Lichtfouse et al. 2004); novel format instructions for more concise articles; a novel title (formerly Agronomie); a switch from hardcopy to fully electronic managing; 100% of articles in English and a novel journal cover; and seven review reports produced per manuscript: three reports from Associate Editors at the pre-selection step, one report from the Field Editor, two reports from peer-reviewers and one report from the Editor-in-Chief; active commissioning of review articles by the Editor-in-Chief. As a consequence, we have observed several positive trends during the last few years (Table 1). The impact factor increased from 0.566 in 2003 to 1.000 in 2007 (+77%, Fig. 2). The rejection rate increased from 44% in 2003 to 77%

Agronomy for Sustainable Agriculture: A Review

Table 1 Major changes in the journal agronomy for sustainable development (ASD) from 2003 to 2006

Title Field Editors Pre-selection Committee Topics Research article format Language Submissions per year Pre-selection rejection (%) Global rejection (%) Research articles per year Review articles per year Impact factor Acceptance delay Article management Article pdf downloads (/yr) E-mail alert subscribers

in 2006. The number of submissions increased from 108 in 2003 to 211 in 2007 (+95%). The number of pdf article downloads at the ASD website increased from 89,158 in 2004 to 231,504 in 2007 (+160%). The number of subscribers to the free e-mail alert increased from 417 in 2004 to 1,307 in 2007 (+213%). The number of published review articles increased from 3 in 2005 to 13 in 2008 (+333%). From 2005, review articles are published both in ASD and in the book series Sustainable Agriculture (Lichtfouse et al., 2009, this book). The first volume is issued in 2009. A call for review articles for the next volumes is posted on the ASD website (agronomyjournal.org). To conclude, the trends observed are very promising to encourage scientists to publish their best results in ASD. In the next section we discuss views on sustainable agriculture.

2 Sustainable Development and Sustainable Agriculture The term “sustainable development” was first defined in 1987 by the Brundtland Commission, formally the World Commission on Environment and Development, solicited by the United Nations: Sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs.

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Before renovation

Actual

Agronomie 14 None Conventional agronomy No size limit 93% English 108 0 44 79 3 0.566 10.3 months Hardcopy, post 89,158 417

Agronomy for sustainable development 32 3 Associate editors Sustainable agronomy Short articles 100% English 211 50 77 44 18 1,000 3.8 months 100% electronic, pdfs 231,504 1,307

This concept was then enhanced by the United Nations Conference on Environment and Development at the Earth Summit, Rio de Janeiro, in 1992. From that time, sustainable development became a key issue in political and scientific bodies, e.g. the Intergovernmental Panel on Climate Change (IPCC, ipcc.ch), the Millennium Ecosystem Assessment (millenniumassessment.org) and more recently, the Grenelle de l’Environnement in France (legrenelleenvironnement.fr). The concept of sustainable development is well accepted by a large public because it has defined global stakes, but is very vague about the practical ways of reaching those stakes. Sustainable agriculture does not escape this weakness, as shown by many reports (Hansen 1996; Hansen and Jones 1996; Gliessman 1998; Gold 1999; Tilman et al. 2002; Boiffin et al. 2004; Dupraz 2005). Many authors and organisations worldwide give their own definition of sustainable agriculture. Some authors consider sustainable agriculture as a set of management strategies addressing the main societal concerns about food quality or environment protection (Francis et al. 1987). Other authors focus on the ability of agricultural systems to maintain crop productivity over the long term (Ikerd 1993). Other authors focus on one main factor of sustainability; for instance, flexibility, which is the adaptive capacity of agriculture to adapt to future changes (Gafsi et al. 2006). Overall, all authors agree on the occurrence of three approaches to the concept of sustainable agriculture: environmental, economic and social approaches. In other words,

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agricultural systems are considered to be sustainable if they sustain themselves over a long period of time, that is, if they are economically viable, environmentally safe and socially fair. Beyond this ideological definition, the practical issue is to build operational solutions to reach global goals. This is a challenging task because the stakeholders do not agree on the criteria to measure the sustainability of a farming system, and on how to balance those criteria. Many indicators have indeed already been produced to evaluate sustainability. The link between sustainable agriculture and sustainable development is not obvious (Legrand et al. 2002). Sustainable agriculture could involve two approaches (Boiffin et al. 2004). The first approach is that agriculture should sustain itself over a long period of time by protecting its productive resources, e.g. maintaining soil fertility, protecting groundwater, developing renewable energies and finding solutions to adapt farming systems to climate change. This first approach considers the farming system as a closed area. The second approach is to consider that agriculture also has to contribute to the sustainability of large territories and social communities. Accordingly, agriculture should help urban areas to manage wastes, e.g. by recycling urban sewage sludge, developing rural employment, and offering a rural landscape for urban people. This second approach has wider goals and does not separate rural and urban areas. To conclude, the vagueness of the concept of sustainable development and sustainable agriculture is a strength because it does not restrict the research field too much, and, in turn gives freedom to scientists to explore wide, unknown domains.

3 Future Sustainable Farming Systems After the Second World War, the development of conventional farming, or “industrial farming,” was promoted in order to increase sharply food production worldwide. This social aim led to extensive use of pesticides, fertilisers and water, and to fast crop rotations and monoculture. Positive effects on yield were rapidly counterbalanced by negative environmental impacts such as soil erosion, groundwater pollution, river eutrophication, excessive water use, and the development of weeds and diseases resistant to chemical control. Industrial farming and other industrial activities have indeed led to the presence of pesticides and persistent organic pollutants in soils,

E. Lichtfouse et al.

water, air and food (Lichtfouse and Eglinton 1995; Lichtfouse 1997a; Lichtfouse et al. 1997, 2005b). Today, to reach economic profitability, environmental safety and social fairness, farming systems should use fewer inputs and resources without drastically reducing yields. As the population is forecasted to increase to 9 billion in the next 50 years, it is necessary to maintain a high level of food production. Nonetheless, farming systems should also meet food quality policies enforced by national and international policies. This issue is particularly relevant given the occurrence of pesticide residues in food products because consumers and environmental associations are concerned about a possible new sanitary crisis. Pesticide use by farmers is thus widely criticised. On the other hand, decreasing pesticide use may lead to negative effects such as toxin risk in food (Le Bail et al. 2005). To reach more sustainable practices, several strategies are described in the literature. Those strategies involve various changes, from simple adjustment of the crop management sequence to fundamental changes at the farming system level. For example, MacRae et al. (1989) proposed the following framework based on efficiency – substitution – redesign (ESR). In the following section we describe three stategies to reach sustainable agriculture: the substitution strategy, the agroecological strategy and the global strategy.

3.1 Level 1: The Substitution Strategy This level refers to the substitution logic, meaning that existing farming systems are only slightly adapted, but not fundamentally altered (Altieri and Rosset 1996). For instance, toxic chemicals and mineral fertilisers (NPK) should be replaced by compounds that are less pollutant, less persistent in soil and less energyconsuming. Applying biopesticides and growing genetically modified plants should decrease both pest development and the use of toxic pesticides. Growing symbiotic N legumes instead of applying costly, energy-consuming N fertilisers should also improve sustainability. Here, research is usually done at the plot level, which is the most common level for agronomists. Collaborations with scientists studying elementary processes, such as geochemists, pathologists and biologists, should be fostered. The substitution logic should be effective for a short time period because it allows a substantial reduction of chemical treatments. However,

Agronomy for Sustainable Agriculture: A Review

it may be not be efficient in the long run due to the appearance of pest resistance following the use of biopesticides, for instance.

3.2 Level 2: The Agroecological Strategy The principle of the agroecological strategy is to build innovative technical scenarios relying on biological regulations in an integrated crop production scheme. This strategy involves applying ecological concepts and principles to the design, development and management of sustainable agricultural systems. Promoting biodiversity in agrosystems provides ecological services such as nutrient cycling, soil structuration and disease control. Biodiversity can be enhanced by cultural practices such as intercropping, rotation, agroforestry, composting and green manuring. Recent studies also address new issues in integrated pest management by combining the use of biological, physical, cultural and genetic control measures (Gurr et al. 2004). Increasing biodiversity by crop rotations (combination in time), intercropping (combination in space) and varietal mixtures has been suggested as an alternative to chemicals (Vandermeer et al. 1998). At this level, agronomy should interact with landscape ecology, because spatial variations in the landscape may be used for pest management. The productivity of farming systems should be increased by developing ecological principles and adapting them to farming systems. The agroecological strategy thus requires the enlargement of the experimental scale. Experimental scenarios should not be designed at plot level, but at the scale of larger territories. Therefore, investigations need a much better understanding of interactions of living organisms at plot and larger levels. They also require input from other disciplines such as ecology and geography.

3.3 Level 3: The Global Strategy The principle of the global strategy is to solve agricultural issues at the global scale, by rethinking its relation to society. Indeed, most failures of intensive agriculture are closely linked to its economic model. There are fundamental contradictions among several

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aims assigned to agriculture. For instance, producing more and cheaper food products without polluting soils; and producing fruits and vegetables without pesticide residues and without visual pest damage appear to be unrealistic aims. Therefore, the global strategy relies on rethinking the role of agriculture in our society, as shown by new trends in agroecology (Gliessman 2006). This approach considers that sustainability cannot be solely reached by farming systems, but should also involve the food system, the relations between farms and food consumption, and the marketing networks. For example, authors studying the relationships between production and marketing highlighted the interest in alternative food networks focused on local production (Lamine and Bellon 2008). The global strategy thus requires interdisciplinary work with scientists from various sciences such as agronomy, ecology, sociology, economics and politics.

4 Agronomical Research for Sustainable Agriculture Agronomy was first defined as the science of crop production. It was mainly focused on the study of relationships between climate, soil, cultural practices and crop yield and quality. Agronomy therefore integrates sciences such as biology, chemistry, soil science, ecology and genetics. Agronomists then enlarged their studies to the individuals performing the cultural practices, namely farmers. This approach raised new issues on the modelling of farmers’ practices, and on the consequences of farmers’ choices on crop production. Agronomists further analysed environmental impacts of farmers’ practices. More recently, they have also studied how agriculture could benefit from the environment and ecosystems to improve crop production, thus leading to the concept of “ecological services.” To study crop production, agronomists have to integrate highly complex sciences that rule farming systems at very different spatial and temporal scales. Agronomists also have to cope with a high environmental variability. As a consequence, results obtained in an experimental field may not be reproducible in another field due to slight – possibly unknown – variations in soil and climate factors. Therefore, a key point of agronomical investigations is to define the validity domain of each finding. Concerning the

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integration of agricultural practices, a key point is to enlarge the classical scales of crop production studies, “plant and plot,” to scales that are meaningful for the farmers, such as combination of plots and farm territory, and even larger scales. In a way, agronomy is a science of complexity aimed at integrating knowledge at various spatial levels from the molecule to the living organism, the farming system and the global scale. Thus, agronomy appears more and more to be the science relevant for global issues because it integrates knowledge from various sciences at various spatial scales. Considered for a long time as a soft, side science, agronomy is rising fast as a central science because current issues are about food, and humans eat food. The systemic dimension is essential because in the next few decades most improvements of farming systems will rely on enhancing positive interactions among various parts of farming systems. To build sustainable farming systems, agronomists will not only have to assess the direct effects of techniques on a crop, but also the indirect effects on the whole ecosystem such as biodiversity changes, water pollution and soil erosion. The economic and social consequences of the new farming systems should also be evaluated with a pluridisciplinary approach with economists or social scientists. Therefore, sustainable agriculture fosters the development of multidisciplinary studies that associate agronomy with ecology, economics, sociology and geography (Lichtfouse et al. 2004). Meynard et al. (2006) identified four different ways to design innovative agricultural systems for sustainable development: – Inventing new farming systems, breaking off with the current ones; – Identifying and improving farming systems built by the local stakeholders; – Giving tools and methods to stakeholders to improve their own systems or evaluate those proposed by scientists; – Identifying the economic, social and organisation conditions that may help the actors to adopt alternative farming systems. These approaches raise several new issues for agronomical scientists. For instance, it is not clear whether solutions will be found either by only a slight adaptation of research practices or by a sharp change in experimentation and modelling. Studying new spatial scales that show heterogeneous areas such as field mar-

E. Lichtfouse et al.

gins will be a challenge. The integration of long-term changes in farming systems, such as soil organic matter turnover (Lichtfouse 1997b) and sewage sludge pollution (Lichtfouse et al. 2005a), should also be investigated and modelled because concepts of resilience and flexibility are relevant. It should also be noted that some farmers already have an accurate expertise in sustainable practices. Some are even ahead of research and are experimenting with new systems for, e.g. organic farming. Here, the issue for the agronomist is to build effective methods to gather local knowledge, to check findings and eventually to redesign experiments. Innovative agricultural systems will benefit from a close collaboration between scientists and farmers.

References Alberola C., Lichtfouse E., Navarrete M., Debaeke P., Souchère V. (2008) Agronomy for Sustainable Development. Ital. J. Agron. 3, 77–78. Altieri M., Rosset P. (1996) Agroecology and the conversion of large-scale conventional systems to sustainable management. Int. J. Environ. Stud. 50, 165–185. Boiffin J., Hubert N., Durand N. (2004) (Eds.) Agriculture et développement durable, Enjeux et questions de recherche, INRA, mission communication. 92 p. ISSN1156-1653.

www.inra.fr/les_partenariats/programmes_anr/agriculture_et_devedeveloppement_durable/documentation. Dupraz (2005) Entre agronomie et écologie : vers la gestion d’écosystèmes cultivés, Revue DEMETER. 16 p., http:// www.montpellier.inra.fr/safe/publications/papers/Dupraz% 20article%20pour%20la%20revue%20Demeter.pdf. Francis C.A., Sander D., Martin A. (1987) Search for a sustainable agriculture: reduced inputs and increased profits. Crops Soils Mag. 39, 12–14. Gafsi M., Legagneux B., Nguyen G., Robin P. (2006) Toward sustainable farming systems: effectiveness and deficiency of the French procedure of sustainable agriculture. Agr. Sys. 90, 226–242. Gliessman S. (1998) Agroecology: ecological processes in agriculture. CRC Press, Michigan, 357 p. Gliessman S. (2006) Agroecology: the ecology of sustainable food systems. CRC Press, Michigan, 2nd ed., 384 p. Gold M. (1999) Sustainable agriculture: definitions and terms. Special reference briefs 99-02, USDA National Agricultural Library (NAL), ISSN 1052-5368, http://www.nal.usda. gov/afsi/AFSI_pubs/srb9902.htm. Gurr G.M., Wratten S.D., Altieri M.A. (2004) Ecological engineering for pest management. CSIRO Publishing, 244 p. Hansen (1996) Is agricultural sustainability a useful concept? Agr. Syst. 50, 117–143. Hansen J.W., Jones J.W. (1996) A systems framework for characterizing farm sustainability. Agr. Syst. 51, 185–201.

Agronomy for Sustainable Agriculture: A Review Ikerd (1993) The need for a system approach to sustainable agriculture. Agr. Ecosyst. Environ. 46, 147–160. Lal R. (2008) Soils and sustainable agriculture. A review. Agron. Sustain. Dev. 28, 57–64. Le Bail M., Verger P., Dore T., Fourbet J.F., Champeil A., Ioos R., Leblanc J.C. (2005) Simulation of consumer exposure to deoxynivalenol according to wheat crop management and grain segregation: Case studies and methodological considerations. Regul. Toxicol. Pharm. 42, 253–259. Legrand P., Fraval A., Laurent C. (2002) INRA faced with Sustainable Development: Landmarks for the Johannesburg Conference (english version), Dossiers de l’Environnement de l’INRA nı 22. Paris, 212 p., INRA Éditions, ISBN: 27380-1049-0, http://www.inra.fr/dpenv/do22-e.htm. Lamine C., Bellon S. (2008) Conversion to organic farming: a multidimensional research object at the crossroads of agricultural and social sciences. A review. Agron. Sustain. Dev., DOI: 10.1051/agro:2008007. Lichtfouse E. (1997a) (Ed.) Soil Pollutants. Analusis Magazine 25, M16–M72. Lichtfouse E. (1997b) Heterogeneous turnover of molecular organic substances from crop soils as revealed by 13 C labeling at natural abundance with Zea mays. Naturwissenschaften 84, 22–23. Lichtfouse E., Budzinski H., Garrigues P., Eglinton T. I. (1997) Ancient polycyclic aromatic hydrocarbons in modern soils: 13 C, 14 C and biomarker evidence. Org. Geochem. 26, 353–359. Lichtfouse E., Eglinton T.I. (1995) 13 C and 14 C evidence of pollution of a soil by fossil fuel and reconstruction of the composition of the polluant. Org. Geochem. 23, 969–973.

7 Lichtfouse E., Habib R., Meynard J.M., Papy F. (2004) Agronomy for sustainable development. Agronomie 24, 445. Lichtfouse E., Sappin-Didier V., Denaix L., Caria G., Metzger L., Amellal-Nassr N., Schmidt J. (2005a) A 25-year record of polycyclic aromatic hydrocarbons in soils amended with sewage sludges. Environ. Chem. Lett. 3, 140–144. Lichtfouse E., Schwarzbauer J., Robert D. (2005b) (Eds.) Environmental Chemistry, Green Chemistry and Pollutants in Ecosystems. 1. Analytical Chemistry. 2. Toxic Metals. 3. Organic Pollutants. 4. Polycyclic Aromatic Compounds. 5. Pesticides. 6. Green Chemistry. 7. Ecotoxicology. Springer, 780 p., ISBN 3540228608, http://www.springerlink.com/ content/n8078j/. Lichtfouse E., Navarrete M., Debaeke P., Souchère V., Alberola C. (2009) (Eds.) Sustainable Agriculture, Vol. 1. Springer, EDP Sciences. MacRae R.J., Hill S.B., Henning J., Mehuys G.R. (1989) Agricultural science and sustainable agriculture: a review of the existing scientific barriers to sustainable food production and potential solutions. Biol. Agric. Hortic. 6, 173–219. Meynard J.M., Aggerri F., Coulon J.B., Habib R., Tillon J.P. (2006) Recherches sur la conception de systèmes agricoles innovants. Rapport du groupe de travail, septembre 2006, 72 p. Tilman D., Cassman K.G., Matson P.A., Naylor R., Polasky S. (2002) Agricultural sustainability and intensive production practices. Nature 418, 671–677. Vandermeer J., van Noordwijk M., Anderson J., Ong C., Perfecto I. (1998) Global change and multispecies agroecosystems: concepts and issues. Agr. Ecosyst. Environ. 67, 1–22.

Soils and Sustainable Agriculture: A Review Rattan Lal

Abstract Enhancing food production and supporting civil/engineering structures have been the principal foci of soil science research during most of the nineteenth and the first seven or eight decades of the twentieth century. Demands on soil resources during the twenty first century and beyond include: (1) increasing agronomic production to meet the food needs of additional 3.5 billion people that will reside in developing countries along with likely shift in food habits from plant-based to animal-based diet, (2) producing ligno-cellulosic biomass through establishment of energy plantations on agriculturally surplus/marginal soils or other specifically identified lands, (3) converting degraded/desertified soils to restorative land use for enhancing biodiversity and improving the environment, (4) sequestering carbon in terrestrial (soil and trees) and aquatic ecosystems to off-set industrial emissions and stabilize the atmospheric abundance of CO2 and other greenhouse gases, (5) developing farming/cropping systems which improve water use efficiency and minimize risks of water pollution, contamination and eutrophication, and (6) creating reserves for species preservation, recreation and enhancing aesthetic value of soil resources. Realization of these multifarious soil functions necessitate establishment of inter-disciplinary approach with close linkages between soil scientists and chemists, physicists, geologists, hydrologists, climatologists, biologists, system engineers (nano technologists), computer scientists and information technologists, economists, social scientists and

R. Lal () Carbon Management and Sequestration Center, The Ohio State University, Columbus, OH 43210, USA e-mail: [email protected]

molecular geneticists dealing with human, animal and microbial processes. While advancing the study of basic principles and processes, soil scientists must also reach out to other disciplines to address the global issues of the twenty first century and beyond. Keywords Biofuels r Climate change r Food security r Soil functions r Sustainable agriculture r Waste management r Water resources

1 Introduction Goals of soil management during the nineteenth century and the first half of the twentieth century, when world population was merely 38% of the 2006 level, was to maintain agronomic productivity to meet the food demands of two to three billion inhabitants. Demands on soil resources are different of a densely populated and rapidly industrializing world of the twenty first century. In addition to food supply, modern societies have insatiable demands for energy, water, wood products, and land area for urbanization, infrastructure, and disposal of urban and industrial wastes. There is also a need to alleviate rural poverty and raise the standard of living of masses dependent on subsistence farming. In addition, there are several environmental issues which need to be addressed such as the climate change, eutrophication and contamination of natural waters, land degradation and desertification, and loss of biodiversity. To a great extent, solutions to these issues lie in sustainable management of world’s soil resources (Fig. 1), through adoption of agronomic techniques which are at the cutting edge of science.

E. Lichtfouse et al. (eds.), Sustainable Agriculture, DOI 10.1007/978-90-481-2666-8_3, c Springer Science+Business Media B.V. - EDP Sciences 2009. Reprinted with permission of EDP  Sciences from Lal, Agron. Sustain. Dev. 28 (2008) 57–64. DOI: 10.1051/agro:2007025

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Fig. 1 World soils and global issues of twenty first century

2 Advancing Food Security The world population of merely 0.2 billion during the biblical era increased by only 0.11 billion (to 0.31 billion) during the next 1,000 years by 1000 AD. However, the population increased 20 times to 6 billion during the next 1,000 years by 2000 AD. The world population is projected to reach 9.4 billion by 2050 and 10 billion by 2100 (Fischer and Heilig 1997; Cohen 2003). The most remarkable aspect of the future population dynamics is the fact that all of the projected increase by about 3.5 billion will occur in developing countries of Asia (mostly South Asia) and Africa (mostly sub-Saharan Africa). These are also the regions where soil resources are limited in extent (per capita), fragile to natural and anthropogenic perturbations, and prone to degradation by the projected climate change and the increase in demographic pressure. Thus, any future increase in agronomic/food production will have to occur through vertical increase in production per unit area, time and input (e.g., nutrients, water, energy) of the resources already committed to agriculture. It is in this context that developing and identification of some innovative methods of soil management are crucial to feeding the world population of

10 billion. These methods must minimize losses by delivering nutrients and water directly to the plant roots during the most critical stages of crop growth. Degraded and desertified soils must be reclaimed through enhancement of the soil organic matter (SOM) pool, creation of a positive elemental budget with balanced supply of all essential nutrients, effective control of soil erosion by water and wind, restoration of soil structure and tilth through bioturbation, and enhancement of activity and species diversity of soil fauna and flora. Soil management techniques must be chosen to ensure: (1) liberal use of crop residues, animal dung and other biosolids, (2) minimal disturbance of soil surface to provide a continuous cover of a plant canopy or residue mulch, (3) judicious use of sub-soil fertigation techniques to maintain adequate level of nutrient and water supply required for optimal growth, (4) an adequate level of microbial activity in the rhizosphere for organic matter turnover and elemental cycling, and (5) use of complex cropping/farming systems which strengthen nutrient cycling and enhance use efficiency of input. Identification, development and validation of such innovations must be based on modern technologies such as GIS, remote sensing, genetic manipulations of crops and rhizospheric organisms, soil-specific management, and slow/time release formulations of

Soils and Sustainable Agriculture: A Review

fertilizers. Increase in crop yields must occur in rainfed/dry farming systems which account for more than 80% of world’s croplands. Breaking the agrarian stagnation/deceleration in sub-Saharan Africa must be given the highest priority by soil scientists and agronomists from around the world. While expanding irrigated agriculture is important, crop yields have to be improved on rainfed agriculture in Asia and Africa, by conserving or recycling every drop of rain, and by not taking soils for granted.

3 Biofuels In comparison with the stone age or bronze age, the industrial era (1750–2050) will be referred to the carbon (C) age or carbon civilization by future generations from 2100 AD and beyond (Roston 2008). The use of fossil fuel, since the onset of industrial revolution 1750, has drastically disturbed the global C cycle with the attendant impact on the climate change and the increase in earth’s temperature along with change in rainfall amount and distribution. The present civilization is hooked on C, and is in need of a big time rehabilitation. Breaking the C-habit will require development of C-neutral or non-carbon fuel sources, and both soil science and agronomy have a major role to play in this endeavor. Not only the recommended agricultural input (fertilizers, pesticides, tillage methods, irrigation) must be efficiently used, the future energy demands will eventually be met by non-carbon fuel, most likely hydrogen. The latter can be generated from biomass produced through appropriate land uses and judicious cropping/farming systems. In the meanwhile, modern biofuels (ethanol, biodiesel) can play an important role in minimizing emissions of greenhouse gases and reducing the rate of increase of atmospheric concentration of CO2 (Brown 1999; Cassman et al. 2006). Converting grains (e.g., corn) to ethanol is rather an inefficient method of energy production, and grains are and will remain in high demands as food staple for humans and feed for livestock and poultry. Crop residues are also being considered as a source of energy (Somerville 2006; Service, 2007). Indeed, 1 Mg (1t) of lignocellulosic residues is equivalent to 250–300 L of ethanol, 15–18 GJ of energy, 16  106 kcal or 2 barrels of diesel (Lal 2005; Weisz 2004). The energy return on investment (EROI) for grain-based ethanol is low. Furthermore, crop

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residues (of corn, wheat, barley, millet, rice) must be used as soil amendment/mulch to control erosion, conserve water and replenish the depleted SOM pool through soil C sequestration, and restore degraded soils and ecosystems (Wilhelm et al. 2004). Crop residues must not be considered a waste, because they have multifarious but competing uses including conservation of soil and water, cycling of nutrients, enhancement of the use efficiency of fertilizers and irrigation water, and above all, as a food of soil organisms which are essential to making soil a living entity. Using crop residues for production of biofuels is “robbing Peter to pay Paul” and all that glitters is not gold, not even the green gold. The price of harvesting crop residues (such as from the US Corn Belt) will be severe soil and environmental degradation (dust bowl), because there is no such thing as a free lunch. It is, thus, important to identify dedicated crops which can be grown

Table 1 Species for establishing biofuel plantations English name Botanical name 1. Warm season grasses  Switch grass  Big blue stem  Indian grass  Blue giant grass  Guines grass  Elephant grass  Kallar/Karnal grass  Molasses grass  Reed canary grass  Cord grass 2. Short rotation woody crops  Popalar spp  Willow spp  Mesquite (Velayti Babul)  Miscanthus  Black locust  Birch 3. Halophytes  Pickle weed  Salt grass  Salt brushes  Algae  Cyanobacteria 4. Drought tolerant trees  Gum tree  Leucaena (Subabul)  Casurinas  Acacia  Teak  Cassia

Panicum virgutum L. Andropogan gerardi, Vitnam Sorghastrum nuttans L. Nas Calanagrostis canadensis Michx Bean L. Panicum maximum L. Pennisetum perpereum schm. Leptochloa frscha L. Melinis minutiflona Phalaris arundinacae L. Spartina pectinata Link. Populus spp. Salix ssp. Prosopis juliflora Miscanthus spp. Robinia pseudoacacia L. Onopordum nervosum Salicornia bigelovii Distichlis palmeri Atriplex spp. Spirulina geitleri Cyanobacteria spp. Eucalyptus spp. Leucaena leucocephala Casurina equisetifolia Acacia spp. Tectona grandis Casia siamea

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Soil and Water Conservation • • • •

Improving the Environment • Sequestering soil carbon • Improving air quality • Increasing biodiversity

Minimizing runoff Controlling erosion Decreasing evaporation Reducing non-point source pollution

Recycling Nutrients Crop Residues As Soil Amendments

• Enhancing soil fertility • Reducing losses out of the ecosystem

Energy Source for Soil Biota • Providing food for soil macro-and micro-fauna • Enhancing bioturbation and improving soil structure

Fig. 2 Site and eco-system specific effects of crop residue management on soil and environment quality must be assessed in relation to improvement in soil quality and sustainable use of natural resources

to establish biofuel plantations (Table 1). Furthermore, new lands (agriculturally marginal/surplus soils; and degraded, disturbed and polluted soils) must be identified to establish appropriate biofuel plantations. In addition to providing the lignocellulosic biomass for conversion to ethanol, establishment of biofuel plantations on degraded soils would also lead to soil C sequestration and enhance soil quality and the ecosystem services that it would provide. The EORI of biofuel production system must be carefully assessed through a comprehensive life cycle analysis. In addition to establishing managed biofuel plantations, lignocellulosic biomass can also be harvested from natural vegetation growth on abandoned/set aside or fallowed land (Tilman et al. 2006). The issue of using crop residues for cellulosic ethanol production must not be resolved on the basis of short-term economic gains. The rational decision must be based on the long-term sustainable use of natural resources (Figs. 2 and 3). Indeed, the immediate needs for fuel must not override the urgency to achieve global food security, especially for almost 1

billion food-insecure people in Africa and Asia. If the crop residues harvested for celunol production are not returned as compost (with enhanced plant nutrients such as N, P, K), the long-term adverse impacts on soil quality (such as has been the case in severely degraded soils of sub-Saharan Africa and South Asia due to perpetual removal of crop residues) will jeopardize global food security and set-in-motion the soil degradation spiral with the attendant impact on social unrest and political instability (Fig. 4).

4 Waste Disposal The importance of soil for the safe disposal of ever-increasing industrial and urban wastes cannot be over-emphasized. The municipal solid wastes generated in the US doubled between 1970 and 2003 (USEPA 2006), as is also the case in western Europe and developing economies, in addition, there are wastes of animal and poultry industry, and food

Soils and Sustainable Agriculture: A Review

19 Competing Uses Of Crop Residues

Short –Term Economic Gains

Long-Term Management of Natural Resources

Traditional/ Modern Biofuel

Erosion Control

Fodder / Feed

Soil Water Management

Construction Material

Soil Quality Enhancement

Industrial raw Material

Ecosystem Services

Fig. 3 An objective assessment of short-term economic gains vs. long-term and sustainable use of natural resources important to the decision-making process for competing uses of crop residues

processing plants and restaurants. These wastes containing biosolids which can be used as energy source (either for direct combustion or conversion to methane or ethanol), and as soil amendment, or both. The by products of biosolids used for production of methane gas or ethanol must be composted and used as soil amendment. Soils of appropriate characteristics (e.g., good drainage, absence of impermeable layer in the vadoze zone, high activity and species diversity of macro and micro-organisms in the surface layer, highly aggregated and stable structure) is also a natural biomembrane which must be used to filter and denature/biodegrade industrial pollutants. Carefully chosen soils and the underlying parent material/geologic strata are being used as a repository for nuclear wastes (e.g., Yucatan mountain range in the southwestern US). Although questionable in terms of effectiveness and economic cost, geologic strata are also being used/considered for storage of industrial CO2 emitted from point sources (Schrag 2007). The importance of soil as a medium for waste disposal is bound

to increase with increase in population and industrialization, and soil scientists must be pro-active in this emerging field of great significance. Similarly, agronomists must be very actively involved in phytoremediation of polluted soils by using plants to denature industrial pollutants.

5 Farming Carbon Carbon sequestration in terrestrial ecosystems (e.g., soils, trees, wetlands), and improving soil quality so that soils can be a net sink for CH4 and release less N2 O, is an important issue which must be addressed by soil scientists, crop scientists, agronomists, foresters, and wetland ecologists. While understanding the processes which impact the ecosystem carbon pool and fluxes is important, soil scientists and agronomists must liaise with economists and policy experts to develop a methodology for trading of carbon credits so that it can be traded like any farm commodity (e.g.,

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R. Lal • Political instability • Civil unrest

Soil Degradation & Desertification

Decline in Soil Quality

Decline in Soil Fertility

Accelerated Erosion

Decline in Soil Structure

• Poverty • Hunger • Malnutrition

• Low NPP • Food deficit

• Negative nutrient balance • Depletion of soil organic matter

• Runoff losses • Soil erosion by water & wind

• Crusting • Compaction

Residue removal

Fig. 4 The classic collapse

corn, millet, poultry). Similar to C sequestered in trees, methodology must be developed to trade C in soils (Breslau 2006; Brahic, 2006). In addition, emissions of CH4 and N2 O can be converted to CO2 equivalent, and also traded. Trading C credits can provide another income stream for farmers, and provide the much needed incentives to invest in soil improvements (e.g., erosion control, fertility management, irrigation). Restoration of degraded soils and ecosystems is an important facet intimately linked to soil C sequestration. Soil degradation and desertification, biophysical processes driven by socio-economic and political factors, are severe problems in developing countries of South Asia and sub-Saharan Africa (Oldeman 1994). Restoration of eroded/degraded soils, through land use conversion via afforestation and conversion of degraded croplands to improved and well managed pastures, will lead to terrestrial C sequestration (in soils and trees) as an ancillary benefit. Soil degradation through land misuse, soil mismanagement, and excessive consumption of water through flood irrigation that leads to salinization and inundation are luxuries that

the land-starved and the water-scarce world cannot afford, not anymore. There is a strong link between soil restoration, carbon sequestration, food security (Lal 2006) and biodiversity (Fig. 5). Improvement of soil quality, gradual and a slow process as it may be, is caused by an increase in the terrestrial C pool. The latter is also linked with biodiversity, water quality and micro and mesoclimate, and emission of greenhouse gases into the atmosphere. Understanding interactive mechanisms, especially those which link processes in soil with those in atmosphere and hydrosphere through biosphere, are of a high priority for soil scientists and agronomists. In addition to quantifying these processes, soil scientists and agronomists must also communicate their findings to policy makers such as the US Congress, European Parliament, and the UN organizations. Through their interactive research outlined above, soil scientists must provide the basic information which is needed to bring together three UN Conventions (i.e., UNFCCC, UNFCBD, and UNFCDC). While providing crucial information on biodiversity, desertification control and climate change to strengthen cross linkages among three UN Conventions, soil scientists can also build bridges to link these organizations with the noble UN Millenium Development Goals of cutting poverty and hunger in half. In agricultural economy, which involves two-thirds to three-fourths of the rural population, increasing agronomic productivity and providing another income stream for farmers through trading of C credits are important strategies to advance food security while alleviating poverty and improving the environment. Generating income through trading of soil/terrestrial carbon credits may be the entry point or the handle to break the vicious cycle of soil degradation-low yields-poverty – hunger-severe soil degradation. It is a truly win-win-win strategy that deserves a serious attention of the world community.

6 Water Resources In addition to fertility and nutrient supply, agricultural productivity will be constrained by lack of water resources, whose severity will be exacerbated by frequent and severe drought stress due to the projected climate change. Whereas agriculture is the largest consumer of water, the competition from industrial and

Soils and Sustainable Agriculture: A Review

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Terrestrial Carbon Sequestration to Mitigate and Adapt to Climate Change

Alleviation of Rural Poverty Through Increase in Agronomic Productivity, Trading of Carbon Credits and Payments for Ecosystem Services

Global Food Security Through Increase in Soil Quality

Desertification Control to Restore Degraded Soils and Ecosystems

Enhancing Biodiversity Through Improvement in Quality of Soil & Biotic Factors

Fig. 5 A positive and synergistic interaction between desertification control, biodiversity improvement, climate change mitigation, and food security. The latter is improved through improvement in soil quality, increase in availability of water

resources, strengthening of elemental cycling, and enhancement of bioturbation in the rhizosphere. Soil scientists and agronomists must be actively pursuing quantification of these synergistic effects

urban uses is increasing with increase in demographic pressure and rapid industrialization (Moldan 2007; Gleick 2003; Kondratyev 2003; Johnson et al. 2001). The scarcity of fresh water is exacerbated by non-point and point source pollutions (Tilman et al. 2006), and will be further aggravated by likely shift in diet in developing economies (e.g., China, India) from plantbased to animal-based products (Clay 2004). In this regard, improved understanding of soils and agronomic processes which enhance water use efficiencies is highly relevant and extremely critical. Soil scientists and agronomists need to work closely with plant breeders to develop genetically engineered plants which have high productivity per unit consumption of water, with irrigation engineers to reduce losses of water during conveyance and delivery, with micrometeorologists to reduce losses from soil evaporation,

with hydrologists to economically and effectively recycle water drained into the sub-soil or ground water, and with municipalities of large urban centers to develop techniques of recycling waste water for irrigation and aquifer recharge. Replacing flood irrigation with subirrigation or drip irrigation techniques is a high priority.

7 Reaching Out The traditional functions of soil have been: (1) the medium for plant growth, (2) foundation for civil structures, and (3) source of raw materials for industry. During the twenty first century and beyond, functions of soil must be expanded to include the following: (1) mitigation of climate change through C sequestration in terrestrial and aquatic ecosystems, (2) purification

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of water through filtration and denaturing of pollutants, (3) disposal of urban and industrial wastes in a way that these do not contaminate water or pollute air, (4) store germ plasm including that of microbes which can be used to combat diseases, (5) archive human and planetary history, (6) support being a reactor of chemical and physical processes, and (7) provide a strategic entity in national and international affairs to give peace a chance. The concept of “sustainable agriculture” needs to be revisited in the context of the need for increasing productivity in developing countries which will entirely inherit the future increase in population of 3.5 billion by the end of the twenty first century. With reference to the densely populated countries of Asia and Africa, sustainable agricultural practices are those which: (1) maximize productivity per unit area, time and input of fertilizers, water and energy, (2) optimize the use of off-farm input, (3) increase household income through increase in production, trading of carbon credits, off-farm employment, and value addition of farm produce, (4) improve quality and quantity of fresh water resources at the farm level, (5) provide education opportunities especially for women, (6) create clean household cooking fuel for the rural population to improve health of women and children and spare animal dung and crop residues for use as soil amendments, and (7) address concerns of the farm family especially food security until the next harvest. It is a fact that indiscriminate use of chemicals, excessive tillage and luxury irrigation have degraded soils, polluted waters and contaminated air. The problem is not with the technology. It has been the over-fertilization, overuse of pesticides, excessive application of irrigation because of free water, unnecessary plowing, complete removal of crop residues along with uncontrolled grazing, and the use of animal dung for household fuel rather than soil amendment that have caused the problems. Access to adequate and balanced food and clean drinking water are the most basic human rights which must be respected. Political stability and ethnic conflicts are caused by hunger and the desperateness created by it. Thus, the concept of sustainable agriculture must be based on the simple fact that agricultural ecosystems are only sustainable in the long-term if the outputs of all components produced balance the inputs into the system. Whether the required amount of input (nutrients) to obtain the desired yield is supplied in organic rather than inorganic form is a matter

R. Lal

of availability and logistics. Plants cannot differentiate the nutrients supplied through the organic or synthetic sources. The important question is of supplying nutrients in adequate quantity and when needed to produce enough food to meet the needs of 6.5 billion people now and 10 billion by the end of the century. In some cases, in vicinity of large livestock or poultry farms, organic manures may be available. In other cases, massive intervention through fertilizer use has no practical alternatives in a world of growing population. In some cases traditional breeding is acceptable, in others the natural process of gene manipulation may have to be accelerated through techniques of genetic engineering. The strategy is to use the technology prudently and with utmost objectivity and rationality. Transgenic plants can be grown on degraded and saltaffected soils to produce biomass for biofuels, and to alleviate biotic and abiotic stresses on dryland agriculture. If effective, why not? While those holding the neo-Malthusian views will again be proven wrong through adoption of already proven and emerging technologies for sustainable management of soil resources, soil scientists and agronomists cannot undertake these serious issues all by themselves. These are far reaching and complex functions that soil scientists may take lead in but must develop close cooperation with other disciplines. While advancing and improving the knowledge of basic processes, soil scientists must also work with geologists, hydrologists, climatologists, biologists, chemists, physicists, computer scientists, nano technologists, system engineers, economists and political scientists to address these emerging issues of the twenty first century. The key strategy is to reach out to other disciplines while strengthening and advancing the science of soil and its dynamics in an ever changing physical, social, economic and political climate. Agriculture, implemented properly, is an important solution to the issue of achieving global food security but also of improving the environment. The agricultural history of 10–13 millenia has taught us that the motto of modern civilization must be “In Soil We Trust”.

References Brahic C (2006) Price cash rattles Europe’s CO2 reduction scheme, Science 312, 1123 Breslau K (2006) It can pay to be green, clean air means profits at the climate exchange, Newsweek, 22 May 2006, 45

Soils and Sustainable Agriculture: A Review Brown KS (1999) Bright future – or brief flare – for renewable energy, Science 285, 678–680 Cassman K, Eidman V, Simpson E (2006) Convergence of agriculture and energy, CAST Commentary QTA 2006-3, CAST, DeMoines, IA, p 12 Clay J (2004) World agriculture and the environment: a commodity by commodity guide to impacts and practices, Island Press, Washington DC, p 570 Cohen JE (2003) The human population: next half century, Science 320, 1172–1175 Fischer G, Heilig GK (1997) Population momentum and the demand on land and water resources, Philos T Roy Soc B 352, 869–889 Gleick PH (2003) Global fresh water resources, Science 302, 1524–1528 Johnson N, Revenga C, Echeverria J (2001) Managing water for people and nature, Science 292, 1071–1074 Kondratyev KY, Krapivin VF, Varotsos CA (2003) Global carbon cycle and climate change, Springer, Berlin, p 388 Lal R (2005) World crop residues production and implications of this use as a biofuel, Environ Int 31, 575–584 Lal R (2006) Enhancing crop yields in developing countries through restoration of soil organic carbon pool in agricultural lands, Land Degrad Dev 17, 197–206 Moldan D (Ed) (2007) Water for food, water for life. Earthscan, USA, p 645

23 Oldeman LR (1994) The global extent of soil degradation, In: Greenland DJ, Szabolcs I (Eds), Soil Resilience and Sustainable Land Use, CAB International, Wallingford, UK, pp 99–118 Roston E (2008) The carbon age: how life’s core element has became civilization’s greatest threat. Walker & Co., New York, p 309 Schrag D (2007) Preparing to capture carbon, Science 315, 812–813 Service RF (2007) Cellulosic ethanol: biofuel researchers prepare to reap new harvest, Science 315, 1488–1491 Somerville C (2006) The billion ton biofuel vision, Science 312, 1277 Tilman D, Hill J, Lehman C (2006) Carbon-negative biofuels from low-input high-diversity grassland biomass, Science 314, 1598–1660 USEPA (2006) Municipal solid waste (online). Available at www.epa.gov/ epaoswer/non-hw/muncpl/facts.htm. USEPA, Washington DC, USA Weisz PB (2004) Bad choices and constraints on long-term energy supplies, Physics Today, July 2004, pp 47–52 Wilhelm WV, Johnson JMF, Hatfield JL, Vorhees WB, Linden DR (2004) Crop and soil productivity response to crop residue management: a literature review, Agron J 96, 1–17

Soils and Food Sufficiency: A Review Rattan Lal

Abstract Soil degradation, caused by land misuse and soil mismanagement, has plagued humanity since the dawn of settled agriculture. Many once thriving civilizations collapsed due to erosion, salinization, nutrient depletion and other soil degradation processes. The Green Revolution of the 1960s and 1970s, that saved hundreds of millions from starvation in Asia and elsewhere, by-passed Sub-Saharan Africa. This remains the only region in the world where the number of hungry and food-insecure populations will still be on the increase even by 2020. The serious technological and political challenges are being exacerbated by the rising energy costs. Resource-poor and small-size land-holders can neither afford the expensive input nor are they sure of their effectiveness because of degraded soils and the harsh, changing climate. Consequently, crop yields are adversely impacted by accelerated erosion, and depletion of soil organic matter (SOM) and nutrients because of the extractive farming practices. Low crop yields, despite growing improved varieties, are due to the severe soil degradation, especially the low SOM reserves and poor soil structure that aggravate drought stress. Components of recommended technology include: no-till; residue mulch and cover crops; integrated nutrient management; and biochar used in conjunction with improved crops (genetically modified, biotechnology) and cropping systems, and energy plantation for biofuel production. However, its low acceptance, e.g., for no-till farming, is due to a range of

R. Lal () The Carbon Management and Sequestration Center, The Ohio State University, Columbus, OH 43210, USA e-mail: [email protected]

biophysical, social and economic factors. Competing uses of crop residues for other needs is among numerous factors limiting the adoption of no-till farming. Creating another income stream for resource-poor farmers, through payments for ecosystem services, e.g., C sequestration in terrestrial ecosystems, is an important strategy for promoting the adoption of recommended technologies. Adoption of improved soil management practices is essential to adapt to the changing climate, and meeting the needs of growing populations for food, fodder, fuel and fabrics. Soil restoration and sustainable management are essential to achieving food security, and global peace and stability. Keywords Biochar r Conservation agriculture r Food security r Integrated nutrient management r No-till farming r Soil degradation r Soil restoration r Sustainable development

1 Introduction Global estimates of food-insecure populations of 825 million (Lobell et al. 2008) to 850 million (Borlaug 2007) have increased to >1 billion in 2009. Regional estimates of the food insecure population include 263 million in South Asia (SA), 268 million in China and Southeast Asia, 212 million in sub-Saharan Africa (SSA), 60 million in South and Central America and the Caribbean, and 50 million in other regions of the world. Contrary to the United Nations’ (UN) Millennium Development Goals of cutting hunger by half by 2015, the number of food-insecure populations in the world will increase. The stock of food grains in

E. Lichtfouse et al. (eds.), Sustainable Agriculture, DOI 10.1007/978-90-481-2666-8_4, c Springer Science+Business Media B.V. - EDP Sciences 2009. Reprinted with permission of EDP  Sciences from Lal, Agron. Sustain. Dev. 29 (2009) 113–133. DOI: 10.1051/agro:2008044

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the world in 2007–2008, the lowest in decades, was only 75 million tons for milled rice and 105 million tons for corn in early 2008 (USDA-FAS 2008). An estimated 75% of the world’s poor (<$2 per day income) live in rural areas and depend directly or indirectly on agriculture (FAO 2006). Food prices are rising (Normile 2008), leading to riots in 30 countries around the globe (Brown 2008; Hoyos and Blas 2008). Share of family income spent on food is estimated at 10% in the USA, 20% in Brazil, 30% in China, 50% in Kenya and 65% in Bangladesh (Hoyos and Blas 2008). Thus, the world’s poor are under great stress, and increase in food prices is a threat to global peace and stability. There is a shortage of good quality soil to bring about the desired increase in food production. Soil fertility decline is an important factor (Sanchez 2002; Sanchez and Swaminathan 2005; IFDC 2006) that cannot be ignored. Indeed, agriculture requires more land, water and human labor than any other industry (FAO 2007; Kiers et al. 2008). Several studies have documented that the potential of genetically modified (GM) crops is appropriate in some contexts, unpromising in others, and unproven in many more (Kiers et al. 2008). The potential of GM crops remains unfulfilled, especially for the subsistence farmers of SSA and SA, where crop yields are strongly constrained by the severe problems of soil degradation and desertification. Furthermore, policy-makers are torn between allocating resources to food security and biofuel production. The objective of this article is to deliberate the need for a soil-based approach to enhance and sustain agronomic production and eliminate world hunger and malnutrition.

2 Prehistoric Farming Techniques and Soil Degradation The literature is replete with prehistoric agriculture and its impact on soils and the environment of the Middle East, and in the valleys of the Nile, Yangtze, Indus and others. However, little is known about prehistoric farming and soil quality in other sites of ancient cultures. Yet, maintaining soil fertility and agronomic productivity have been serious issues facing humanity ever since the dawn of settled agriculture. For example, it has long been recognized that catenal processes result in large fertility contrasts between summit/shoulder slopes and footslope landscape positions (Scholes 1990), and that these contrasts were

R. Lal

accentuated by the activities of prehistoric cultures such as addition of charcoal to create the “terre preta de Indio” (Mann 2002). Two important soil-related constraints faced by prehistoric farmers, drought stress and nutrient depletion, were alleviated by use of the innovative technologies of supplemental irrigation and nutrient recycling, respectively. Irrigation and water management played a major role in the so-called “hydric civilizations”. In addition to the well-known use of irrigation in Asia (Knozer 2000; Premathilake 2006; Groenfeldt 1991; Andrianov 1989; Knauss 2000) there are examples of using irrigation in the prehistoric Americas (Park 1983; Lange 1992). Central America has been the cradle of agriculture in the new world (Clement and Horn, 2001; Goman and Byrne 1998). In the Lake Titicaca Basin, covering the Andean Highlands of Peru and Bolivia, archaeological investigations have shown a massive landscape modification throughout prehistory to intensively cultivate marginal lands. These modifications, over 82,000 ha of land area, included raised beds and large earthen platforms to improve drainage, increase soil fertility by recycling nutrients and improve the micro-climate (Erickson 1992). In the Pampa de Chaparri hyper-arid region of Peru, prehistoric irrigation canals and furrowed fields have been preserved for 500 years (Nordt et al. 2004). On Easter Island (Rapa Nui), Chile, cultivation of yams (Dioscorea spp.) and taro (Colocasia esculenta) began in the twelfth century and continued through the fifteenth century (Stevenson et al. 2006). Archaeological evidence of early irrigation systems has also been discovered in southeastern Arizona. Palacios-Fest et al. (2001) identified several stages of water management in the Santa Cruz River Valley of Southeastern Arizona in the prehistoric era. These researchers reported that prehistoric people first operated their irrigation system in a simple mode involving diversion of ephemeral flows following storms, and later in a complex mode involving diversion of perennial flows. Berger (2004) reported the use of canal irrigation from 1,600 years to 800 years ago (400 AD to 1200 AD) in the flood plains of Salt River in Phoenix, Arizona. Masse (1981) used aerial photographs to reveal complex and extensive remains of ancient Hohokam irrigation systems in the Salt River Valley near Tempe, Maricopa County, Arizona. These systems were probably constructed and used between 850 AD and 1450 AD, and consist of over 2,100 km of canals in the north and south of the Salt River.

Soils and Food Sufficiency: A Review

Nutrient depletion was also a major problem in settled agriculture, except in the flood plains where soil fertility was renewed by alluvial deposition. Prehistoric depletion of soil nutrients was documented after centuries of indigenous agriculture in Hawaii. Hartshorn et al. (2006) observed that farmers began growing dryland taro and sweet potato (Ipomea batatas) using a digging stick on the leeward slopes of East Maui beginning approximately 500 years ago. Centuries of this extractive farming lowered concentrations of Ca2C (49%), Mg2C (28%), NaC (75%), KC (37%) and P (32%) in cultivated compared with uncultivated soils. Similar to the problem of accelerated water erosion faced by the Mayan culture in Central America, wind erosion was a serious constraint faced by the prehistoric farmers of the Kalaupapa field system, Molakai Island, Hawaii (McCoy and Hartshorn 2007). In New Zealand, there is strong evidence of prehistoric cultivation of four introduced Polynesian plants: bottle gourd (Lagenaria siceraria), paper mulberry (Broussonetia papyrifera), sweet potato and taro (Horrocks et al. 2004). These data identify combinations of early Polynesian crops, including both field – and tree – cropping systems, and provide evidence of prehistoric taro cultivation in the South Island. Lepofsky (1995) observed the prehistoric agricultural production and human-induced environmental changes in the Society Islands dating back to the seventh to tenth century AD. Valleys with the greatest arable potential were cultivated earlier than less preferred sites. Similar to the prehistoric civilizations, the problem of nutrient depletion has aggravated the food insecurity in SSA even during the twenty-first century (Smaling et al. 1993; Henao and Baanante 2006; IFDC 2006). Increase in population, because of the transformation of the hunter–gatherer system into settled agriculture, necessitated development of an “ard” or a prehistoric plow (Highman et al. 1981; Lal et al. 2007). Introduction of the plow exacerbated the problem of soil erosion and depletion of soil organic matter (SOM). The plow-induced soil degradation plagued mankind even during the prehistoric era. Accelerated soil erosion had a devastating effect (Bunney 1990) throughout the Middle East and the Central American Mayan culture (Diamond 2004). With the world’s population of 6.7 billion in 2008 and projected to be 9.5 billion by 2050, the issue of producing adequate and nutritious food, the basic human right, is more important now than the challenges faced by sparsely-populated prehistoric farmers.

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3 Constraints to Transforming Traditional Agriculture in Sub-Saharan Africa The SSA remains the only region in the world where the number of hungry and malnourished populations will still be on the increase even by 2020 (Rukuni 2002). While other regions have improved per capita food availability since the 1970s, food production and availability have perpetually declined in SSA. It is both a technological and a political/economic challenge, and cannot be ignored any longer. Agrarian stagnation in SSA has defied numerous attempts at transforming subsistence agriculture, even with due consideration to issues related to biophysical constraints and the human dimensions challenges (Otsuka 2006; Vermeer 1983; Nieuwoudt and Vink 1989). Traditional agricultural practices in SSA have been addressed in relation to soil degradation (Chokor and Odemerho 1994), soil nutrients and SOM depletion (Stromgaard 1991; Dakora and Keya 1997; Sanchez and Leakey 1997; Nye and Greenland 1958), soil pests (Hillocks et al. 1996a,b), pest management (Abate et al. 2000), and plant defense mechanisms (van der Westhuizen 2004). Among commonly promoted strategies for achieving food security in SSA are: cooperative regionalism (Ugwuanyi and Obinne 1998), drought management (Hubbard et al. 1992), improvement of roots and tuberous crops such as cassava (Manihoc esculenta) (Prudencio and Alhassan 1994), use of indigenous knowledge (Oniang’o et al. 2004), integrated food systems (Hulse 2004), macroeconomic and public policy (Rukuni 2002), structural adjustment programs (Amalu 2002), multiple livelihood strategies of women farmers using micro-enterprises (Gladwin et al. 2001), political economy of urban population (Sutherland et al. 1999), income-generating employment (Duncan 1998; McCalla 1999) and policy framework (Van Rooyen and Sigwele 1998). Innovative technologies have been successful in improving agriculture in Asia, especially in China and India. Important among these innovations to intensify traditional agricultural systems are ecological agricultural techniques involving more input of skills, knowledge and labor (Ellis and Wang 1997; Xu 2004; Shi 2002; Battershill and Gilg 1998), integrated farming systems (Li and Min 1999), improved germplasm/transgenic plants grown with efficient systems (Xu and Jeffrey 1995; Soleri et al. 2005)

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With Input Relative Crop Yield

and diversification of farming systems (Short 1997). Yet, repeated attempts at adoption of improved technologies have been met with modest success in SSA, where food insecurity remains a major issue. The agrarian stagnation in SSA has defied all approaches and strategies. The stubborn problem will be solved only when Africans (scientists, farmers, policy-makers and the public at large) collectively resolve to solve it in a manner pertinent to the site-specific situations. SSA must enhance, restore and prudently manage soil and water resources to improve and sustain soil quality. Soil-related constraints to be addressed in SSA and other developing countries are accelerated erosion, SOM depletion, drought stress and soil nutrient management.

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Without Input

Top Soil Depth (cm)

Fig. 1 Relative effects of differences in topsoil depth due to differences in severity of soil erosion on relative crop growth and yield with and without off-farm input. Adverse effects of erosion on crop growth are marked by the use of off-farm inputs. Inputs include fertilizers, manure and mulch

4 Crop Yield and Soil Erosion Accelerated erosion is an important factor adversely affecting sustainability of cropping and farming systems. Erosion is more serious in the tropics than in temperate climates because of the prevalence of fragile soils of high erodibility, harsh climates of high erosivity, and predominately resource-poor farmers who cannot afford to adopt conservation-effective measures. Thus, adverse effects of erosion on agronomic productivity are more severe in Africa, Asia and the Caribbean than in the USA, Europe and Australia. Erosion affects crop yields and agronomic productivity both directly and indirectly. Directly, it reduces the effective rooting depth and available water and nutrient retention capacities. Indirectly, it decreases use efficiency of inputs and increases the amount of fertilizers, water and energy needed to produce the same yield. Erosion has both on-site and off-site impacts. On-site, it reduces seed germination, stand establishment, and plant growth and vigor. Off-site, through runon and deposition, it increases risks of inundation, pesticide damage, and seedling burial. Experimental data on erosional impacts on yield of crops in soils of the tropics are scanty. Synthesis of available data (Lal 1987, 1998; den Biggelaar et al. 2003a,b) indicates stronger adverse effects on farms that do not use off-farm inputs than on those under intensive management. In general, application of fertilizers and soil amendments masks the adverse impacts of accelerated soil erosion (Fig. 1). In Nigeria,

Salako et al. (2007) reported 65–75% reduction in crop yield with 25-cm removal of topsoil when no fertilizers or manure were applied. However, productivity of eroded soils was restored more effectively by the application of manure than by use of chemical fertilizers. Similar experiments by Oyedele and Aina (2006), also conducted on Alfisols in Western Nigeria, indicated that grain yield of maize (Zea mays) decreased from 3.2 Mg ha1 under control to 0.1 Mg ha1 where 20 cm of topsoil was removed. Maize yield decreased exponentially with decrease in the depth/thickness of the remaining topsoil. Drastic reduction in maize grain yield on eroded soil was attributed to the low physical and chemical quality of the exposed sub-soil. Field experiments conducted in the West African Sahel indicated that the grain yields of pearl millet (Pennisetum glaucum) were severely reduced on eroded and unmulched compared with uneroded and mulched soils (Michels and Bielders 2006). Furthermore, millet dry matter yield tripled with P application, and increased by a factor of 13.5 when additional N was applied. These researchers observed that the high P availability was the key factor to reversing decline in crop yields on erosion-affected soils, and manuring was more effective than mulching with straw. Conversely, some studies have documented the beneficial effects of adopting conservation-effective measures on crop yields. For example, installation of stone bunds in large areas of the Tigray Highlands in Northern Ethiopia have shown that average sediment

Soils and Food Sufficiency: A Review

29

accumulation behind 3- to 21-year-old stone bunds is 58 Mg ha1 per year (Nyssen et al. 2007). Consequently, there was an average increase in grain yield of 53% on the lower parts of the landscape in the vicinity of the stone bunds compared with the central and upper parts. Similar positive effects on crop growth and yields have been reported with regards to the use of conservation tillage and mulching. Long-term effects of no-till (NT) farming in conjunction with crop residue mulch are due to improvements in SOM and decrease in soil erosion (Scopel et al. 2005). Effects of accelerated erosion on trends in crop growth and yield in intensively managed soils under commercial farming practices are similar to those of the tropics, but the magnitude of the adverse effects on crop yield is small (den Biggelaar et al. 2003a,b). Experiments conducted on an eroded prairie landscape in the USA indicated that wheat (Triticum aestivum) yields were lowest on soils subject to tillage erosion, and were about 50% of those on uneroded or depositional sites (Papiernik et al. 2005). The data from field experiments, used to validate the results of modeling studies, are in accord with those of experimental studies with regards to the adverse effects of accelerated erosion on soil quality (Izaurralde et al. 2007). The fact that increase in risks of soil erosion hazard due to climate change would have similar adverse effects on crop growth and yield (Zhang 2005) necessitate planning for the use of adaptive measures, espe-

cially in the tropics and sub-tropics where the climate change impacts would be more drastic (Cline 2007), and resource-poor small land-holders do not have the capacity to adapt to the abrupt climate change. The fate of erosion-displaced soil carbon is also a debatable issue that needs to be addressed through long-term research conducted on a watershed scale. Some researchers argue that soil erosion is a source of atmospheric CO2 (Lal 2003). In contrast, others hypothesize that C transported into aquatic ecosystems is a major sink (Van Oost et al. 2004, 2007), and may account for the so-called missing or fugitive CO2 . Resolving this issue is important to enhance the scientific understanding of the complex global C cycle and its impact on the projected climate change.

5 Soil Organic Matter and Crop Yield Regardless of the farming system, e.g., traditional, commercial or modern and innovative, maintenance of soil organic matter (SOM) above the critical level (Aune and Lal 1998) is essential to sustaining productivity and minimizing the risks of soil and environmental degradation (Lal 2006a,b). There are numerous benefits of increasing the SOM concentration and pool on enhancing ecosystem services and improving the environment (Fig. 2). The key factor is

Increase in Production Improvement in Soil Structure

Increase in Use Efficiency

Soil Physical Quality

Soil Chemical Quality Soil C Sequestration Soil Biological Quality

Decrease in Input

Strengthening of Nutrient Cycling

Increase in Water Holding Capacity Decrease in Risks of Soil Degradation

Increase in Farm Income Increase in Soil Biodiversity Improvement in Ecosystem Services

Investing in Soil Improvement Achieving Yield Stability Advancing Food Security

Improving Standard of Living

Erosion Control Desertification Control

Fig. 2 The importance of soil carbon sequestration in ecosystems and environmental sustainability

Ecologic & Environment Sustainability

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R. Lal Relative Crop Yield

With Input

Without Input

Soil Organic Matter (g/kg)

Fig. 3 Relative effects of differences in soil organic matter concentration in the root zone on crop growth and yield. Similar to the effects of accelerated soil erosion, decline in crop yield due to reduction in soil organic matter is more without than with application of fertilizers and other inputs

the improvement in soil quality with the attendant positive impact on soil processes and properties (Lal 2004). Similar to the effect of erosion, adverse effects of decline in SOM concentration on crop yields are also more severe without than with application of fertilizers and soil amendments (Fig. 3). These generalized trends are supported by the data from experiments conducted on representative soils and cropping systems in diverse agroecoregions of sub-Saharan Africa (Pieri 1991). The data from a field experiment conducted in Burkina Faso, West Africa, indicated that optimum SOM concentration and crop performance results from a judicious combination of the use of organic/biosolids and inorganic fertilizers (Ouedraogo et al. 2007). In the Sudano-Sahelian zone of Burkina Faso, Mando et al. (2005a,b) observed that application of manure enhanced SOM concentration and increased sorghum (Sorghum bicolor) grain yield by 56–70%. Furthermore, grain yield was positively correlated with the particulate organic matter (POM) fraction. A long-term field experiment in semi-arid Kenya, designed to assess the sustainability of cereal/legume intercropping, monitored the relationship between SOM concentration and crop yield. Grain yield of sorghum improved with application of manure, whose residual effect lasted for several years (Kihanda et al. 2006). In Morocco, Bessam and Mrabet (2003) reported that conversion of plow tillage (PT) to no tillage (NT) farming sustained crop yields by enhancing the SOM pool. The

mean rate of soil C sequestration over the 11-year period was 0.66 Mg ha1 per year upon conversion from PT to NT farming. Positive effects of enhancing the SOM pool on crop yield and agronomic sustainability have also been reported from experiments conducted in Asia. Shibu et al. (2006) synthesized the available literature on modeling SOM dynamics in the rice (Oryza sativa)-wheat (Triticum aestivum) system for different management scenarios, with impact on crop yield. Swarup et al. (2000) documented the positive effects of SOM on crop yields in India. In the Philippines, Manguiat and Rocamora (2004) reported that application of bio-organics significantly increased the average yield of six crops by 98–153% over the 3year period. Some studies in South America have also documented the beneficial effects of improving SOM on crop yield (Pupiro et al. 2004). In Colombia, Basamba et al. (2006), observed that increase in maize (Zea mays) grain yield with adoption of conservation tillage was due to increase in SOM concentration. In the Pampas of Argentina, Quiroga et al. (2006) observed that the grain yield of barley (Hordeum vulgare) was strongly influenced by the improvements in soil quality caused by increase in SOM concentration. In Sao Paulo, Brazil, Silva et al. (2006) reported the beneficial effects of increase in SOM on growth and yield of radish (Raphanus rativus). In Cuba, biomass yield of leucaena (Leucaena leucocephala) increased with increase in application of worm humus or compost. Similar to the tropics, there is also strong evidence about the positive effects of increasing SOM concentration on crop yields in North America and Europe (Lal 2004, 2006a,b). A strong and positive impact of applying sphagnum peat on the yield of potato (Solanum tuberosum L.) grown on a sandy soil was reported by Li et al. (2004). In Canada, Malhi et al. (2006) observed that practices which improve SOM concentration also enhance crop yields. The data from a study with long-term application of manure on tomato (Solanum lycopersicum) indicated similar yields and soil fertility status to applying inorganic fertilizers (Moccia et al. 2006). Martin-Rueda et al. (2007) documented the positive impacts of reduced tillage and crop rotations on SOM concentration and agronomic yield in some soils of Madrid, Spain. In Denmark, Thomsen and Sorensen (2006) observed that both grain yield and N uptake were highest on soil with

Soils and Food Sufficiency: A Review

the highest SOM level. Long-term field experiments in Estonia showed that rotation and tillage systems with positive impact on SOM concentration also increased and sustained crop yields (Viil and Võsa 2005).

6 Irrigation and Fertilization Management The Green Revolution of the 1970s, which saved hundreds of millions from starvation in Asia and South/Central America, by-passed sub-Saharan Africa (SSA). In Asia, the Green Revolution was ushered in by adoption of input-responsive varieties, grown on irrigated soils with liberal use of fertilizer, especially the heavily subsidized nitrogen. It is probable that agricultural land area could double in SSA (and West Asia) by 2050, and may also increase by 20–25% in the AsiaPacific region and the zone of tropical rainforest in South America and Southeast Asia. Yet there is no substitute for intensification of agriculture on the existing cropland. Intensification implies cultivating the best soils with the best management practices (BMPs) to produce the optimum sustainable yield so that marginal lands in fragile ecosystems, e.g., steep lands, tropical rainforest, can be saved for nature conservancy. It is in this context that the importance of expanding cropland area under irrigation by using small-scale projects, judicious and prudent use of chemical fertilizers, and adoption of genetically improved (GM) crops cannot be over-emphasized. Adoption of these options of agricultural intensification can be easily quadrupled and sustained at the high level of production. Ecosystems utilized by humans can only be sustained if the outputs are balanced by inputs. The latter may be of organic or inorganic origin, because plants cannot differentiate the nutrients supplied through the organic or inorganic sources. It is a question of logistics and availability rather than of the natural or synthetic origin. In addition to these inputs, conversion of traditional hoe-based and plow-based methods of seedbed preparation to NT farming is also essential. The promises of NT farming, used in conjunction with crop residue mulch and cover cropping as integral components of complex cropping systems, can be realized by addressing the challenges (biophysical and socio-economic) that it faces.

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7 Promise and Challenge of No-Till Farming Agronomic techniques to improve soil quality (Table 1) include NT farming and crop residue management, nutrient management and use of biochar. Plowing and other mechanical methods of seedbed preparation are redundant if weeds can be controlled chemically or biologically, seeds can be sown through the crop residue mulch, and fertilizers can be applied without incorporation into the soil. In this regard, NT farming, used in conjunction with crop residue mulch and cover crops, has numerous advantages including conservation of soil and water, saving in time and energy, improvement in SOM concentration, increase in soil biotic activity and decrease in weather-related impacts on crop yields. It is in this regard that the importance of the judicious management of crop residues cannot be over-emphasized. Therefore, planning for SOM management requires the data on the amount of crop residues produced under different cropping systems (Johnson et al. 2006). Application of NT farming for erosion control and moisture conservation under row crop production in soils of West Africa was documented by long-term experiments conducted in Nigeria (Lal 1976, 1989). The potential of judiciously using crop residues as a basis

Table 1 Components of sustainable soil management system for advancing food security in the tropics Farming Operation / Objectives

Recommended Management Practices

1. Seed bed preparation 2. Rotations

No-till farming, crop residue mulch Legume-based crop rotations, agroforestry Mulch farming, water harvesting and recycling, efficient irrigation methods Manuring, BNF, biochar, slow release formulations of fertilizers, nano-enhanced No-till, agroforestry, cover cropping, conditioners No-till, manuring, mulch farming Complex crop rotations, no-till, agroforestry, biochar application, manuring and biosolids

3. Water management

4. Fertility management

5. Erosion control 6. Soil biotic activity 7. Enhancing SOM pool

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of SOM management was also reported by Shittu and Fasina (2006), who observed that residue mulching improved crop yields in Western Nigeria. Incorporation of a fallow (no cropping) period in the rotation cycle also improved SOM concentration and enhanced soil quality in Burkina Faso (Bostick et al. 2007). In the Mediterranean climate of north Africa, Masri and Ryan (2006) observed that incorporation of medic (Medicago sativa) and vetch (Vicia faba) increased the SOM pool compared with continuous wheat. In India, Mandal et al. (2007) concluded that the SOM level can be sustained with annual application of 2.9 Mg of biomass-carbon per hectare through manure and other biosolids. Ghosh et al. (2006) also observed that the groundnut (Arachis hypogea) – wheat systems contributed more carbon through root biomass than groundnut – chickpea (Cicer arietinum) systems, and recommended that additional crop residues along with fertilizers are essential to maintaining SOM levels on Vertisols in Central India. In northeastern China, Xu et al. (2006) reported that SOM concentration increased for Ustepts with application of crop residues and fertilizers. In El Batan, Mexico, Govaerts et al. (2006, 2007) observed that NT farming with crop residue mulch was essential to enhancing SOM and improving soil quality. In western Mexico, Scopel et al. (2005) also observed significant yield benefits of growing NT maize under the semi-arid conditions. In southern Brazil, De Bona et al. (2006) and Bayer (2006) observed that NT farming along with high input systems are needed to counter-balance the higher SOM decomposition rates in a sub-tropical Acrisol. In a clayey Oxisol of Brazil, Razafimbelo et al. (2006) reported the management of sugarcane (Sacchrum rotundum) residue (burning vs. no burning) had a strong impact on SOM dynamics. A large body of literature available on the impact of crop residue management and NT farming in the USA and Brazil (West and Post, 2006) is not reviewed in this report. Thus, a few recent examples are discussed herein. Venterea et al. (2006) assessed the effects of rotational (biennial) tillage on SOM dynamics under corn-soybean (Glycine max L.) rotation, and concluded that biennial chisel plowing in the upper mid-west USA can enhance C storage in soil, reduce fuel costs and maintain yields compared with intensive annual tillage. Long-term effects of tillage and crop residue management in the sub-Arctic region of Alaska were assessed by Sparrow et al. (2006).

R. Lal

They observed that adoption of reduced tillage can improve soil quality and conserve SOM, but longterm NT may not be feasible because of the weed problem and progressive buildup of crop residues on the soil surface in the cold regions. In the northern Great Plains region, Sainju et al. (2006a–c) assessed tillage and crop rotation effects and concluded that reduced tillage and increased cropping intensity enhance the SOM pool. Furthermore, use of hairy vetch (Vicia villosa Roth) and rye (Secale cereole) biculture was effective in sequestering more C than monocultures or no cover crop. A long-term study in central Texas, USA, indicated that NT associated with enhanced cropping intensity and N fertilization increased SOM and N pools (Dou et al. 2006). Also in the hot climate of Texas, Zibilske and Bradford (2007) observed that SOM accumulation may be stimulated by growing cover crops with higher polyphenolic contents and restricting soil – O2 availability with NT farming. In Michigan, USA, Kravchenko and Thelen (2007) observed that use of winter wheat residue decreased the amount of plant-available N and increased grain moisture and test weight of corn grains at harvest. In California’s Mediterranean climate, Veenstra et al. (2007) concluded that conservation tillage alone does not accumulate or stabilize more C than conventional tillage. Therefore, addition of cover crop biomass is essential to increasing total soil C accumulation. Incorporating cover crop in the rotation cycle is even more significant for the cotton (Gossipium hirsutum)-based systems in Alabama to enhance SOM and improve soil quality (Paudel et al. 2006). In addition to the amount of chemical composition of crop residues, accumulation of SOM in soil using higher residue-producing conservation systems and manure is also scale-dependent. On the basis of their data from a 9-ha field in Alabama, Terra et al. (2005) observed that the potential to sequester C in degraded soils in the southeastern USA may be higher than previously expected. In the Colombia Basin of Washington state, Cochran et al. (2007) studied the SOM dynamics in a semiarid shrub-steppe ecosystem recently converted to irrigated agriculture. They concluded that cultivation, crop residue incorporation and dairy manure compost amendments contributed to increase in the total soil C pool. Johnson et al. (2007) tested the hypothesis that SOM decomposition is a function of biochemical composition when all other variables are constant, e.g., particle size, temperature and moisture. Variation

Soils and Food Sufficiency: A Review

in biochemical composition was created by selecting residues of five species including alfalfa (Medicago sativa L.), corn, cuphea (Cuphea viscosissi-ma Jacq), soybean and switchgrass (Panicum virgatum L.). A stepwise multivariate regression indicated that chemical recalcitrance slows root decomposition and explained why roots contribute more C to the SOM pool than surface residues. Thus, root activity is also an important factor in total CO2 production (Chen et al. 2005). In the mid-South USA, Brye et al. (2006) studied SOM dynamics in a wheat-soybean double crop system for a range of wheat residue management scenarios. Their data showed that SOM, along with total N and C concentrations, increased with NT at one of the two locations in east-central Arkansas but not in the other. Brye and colleagues concluded that in a wheat-soybean double-crop production system in a relatively warm and wet environment, numerous soil properties can be improved with NT when crop residues are left unburnt. Measurements, made on peat soils in Ohio (Elder and Lal 2008a,b) and Florida (Gesch et al. 2007), indicated that respiration-induced subsidence can be reduced by conversion to NT farming. In Ohio, the rate of SOM buildup in NT soil increased with application of manure (Wang et al. 2006; Hao et al. 2002), and with retention of crop residue mulch (Blanco-Canqui and Lal 2007, 2007b, 2008). Similar to the data from the USA, the positive impact of residue mulch and NT farming on the SOM pool have been reported by several studies in Canada (Malhi et al. 2006; Campbell et al. 2007; Janzen 2006; Wang et al. 2006; Liang et al. 2005; Singh and Malhi 2006; Gregorich et al. 2006). These conclusions of improvements in soil quality are also supported by the data from Europe and Australia. Positive impacts of improving cropping intensity and eliminating tillage on increasing the SOM pool were also observed in Brazilian Cerrado by In addition to cropping intensity, the impact of the climate gradient (temperature and moisture regimes) is also important to SOM and N dynamics (Ortega et al. 2005). Similar to the environmental problems of burning sugarcane residues in Brazil and Colombia, there is a strong interest in a system of “green can trash blanketing” (GTCB). In addition to improving SOM, there may also be advantages in saving of fertilizers over a long time (Robertson and Thorbun 2007). The data from a long-term experiment in northeastern Italy showed that residue incorporation

33

enhanced the SOM pool at the mean rate of 0.17 Mg Cha1 ya1 (Lugato et al. 2006). Morari et al. (2006) also document the SOM dynamics under NT farming in north-eastern Italy. In semi-arid northern Spain, Bescansa et al. (2006) observed that conservation tillage improved the plant-available water capacity and increased crop yield during the dry years. A critical analysis of the studies reviewed above, and others reported elsewhere, show that the effects of NT systems and crop residue management on the net gain of the soil C pool depend on a range of complex and often interacting factors. It is, therefore, highly challenging to generalize the results. The information available to date can be summarized as follows: (a) Ecosystem services: Conversion of PT and other reduced tillage or conservation tillage systems, along with use of crop residue mulch and cover crops, provides numerous ecosystem services regardless of the biophysical and socio-economic environment. Important among these are: erosion control, water conservation, reduction in non-point source pollution, savings in time and energy and other inputs, stabilization of crop yields against drought, and decrease in the C footprint of the agricultural systems. (b) Crop yields: Effects of conservation tillage and residue mulching on crop yields are variable and depend on many factors. Important among these are soil use history, soil quality at the time of conversion to NT farming, soil properties and profile characteristics, crop species (e.g., cereal, legume, root and tubers), climate, amount and composition (C to N ratio) of crop residues, the rate of N application, effectiveness of the seed drill in cutting through the crop residues and establishing seed-soil contact, drainage conditions, soil temperature at the time of seeding and during seed germination and seedling establishment, and the duration since conversion to NT farming. Some reduction in crop yields may occur in soils that have been recently converted to NT, and also in soils with persistent weeds and incidence of other pests (stem borer, slugs, etc.). Despite reduction in crop yields, the net benefit may still be higher with NT farming because of lower costs (e.g., diesel, machinery, fertilizers). (c) Soil C pool: It is widely recognized that conversion of PT to NT leads to C sequestration in soils (Campbell et al. 2007; Sa et al. 2001; West and Post 2002; Vagen et al. 2005). These are some concerns about the hidden C costs (Schlesinger 2006), and the net C gains. Despite the low C footprint, it is also

34

R. Lal

possible that the total soil C pool in NT is either equal to, or in some cases even lesser than, that in the PT soil. In most cases, there is an increase in the soil C concentration in the surface 0–20 cm depth in NT compared with the PT soil. In others, however, the PT soil may have more C in the sub-soil than in the surface layer (Fig. 4) (Blanco-Canqui and Lal 2008; Baker et al. 2007). Location of crop residue in the soil (surface vs. deep incorporation) can strongly affect its decomposition (Coppens et al. 2006). Soil texture, initial soil C pool and internal drainage also play an important

Soil organic carbon Concentration No-till 20 cm

Plow till

Soil Depth

Fig. 4 Schematics of carbon profile in some soils managed by no-till and plow till

role in determining the net C gains with regards to the tillage system (Puget and Lal 2005). There exists a strong interaction between availability of N and the soil C pool. Addition of N may increase the soil C pool in N-deficient soils (Jaycinthe et al. 2002; Jagadamma et al. 2008), or decrease it while enhancing mineralization in others (Khan et al. 2007). Soil aggregation plays an important role in retention and turnover of rootderived C (De Gryze et al. 2006). The overall goal of managing the soil C pool is to create a positive C budget in the soil by increasing input more than the output (through moderation of erosion, mineralization and leaching), and increase its mean residence time (MRT). Despite the promise of NT farming in revolutionizing agriculture, its adoption rate is disappointingly low, especially so in developing countries of the tropics where it is needed the most. Total cropland area under NT farming is less than 100 Mha or merely 6% of the world’s cultivated area (Derpsch 2007). Most of the NT farming is practiced by large-scale commercial farmers in the USA, Brazil, Argentina, Canada, Australia, Chile, Paraguay, etc. The lack of adoption of this important innovation by small land-holders is attributed to a range of biophysical and socio-economic factors (Table 2). The adoption of this innovation can be facilitated only if these constraints are effectively addressed. The issue is not that NT farming does not work; the goal is to make it work by alleviating constraints through site-specific and adaptive research conducted under on-farm conditions and with a participatory approach.

Table 2 Biophysical, socio-economic, political and cultural factors affecting the adoption of no-till farming in developing countries Biophysical

Economic

Social

Political

1. Soil type (texture, clay, minerals) 2. Climate (drought), rain amount and distribution, temperature 3. Terrain (slope gradient) 4. Weeds 5. Insects and pathogens

1. Competing uses of crop residues 2. Availability of inputs (herbicides, seed drill, fertilizers) 3. Yield reduction 4. Small size farm 5. Lack of credit

1. Land tenure

1. Policy interventions

2. Mind set

2. Infrastructures

3. Community participation 4. Gender and social equity 5. Inappropriate demonstration techniques

3. Institutional support 4. Subsidies or lack thereof 5. Lack of political leadership, willpower and vision

6. Nitrogen immobilization

6. Price control

6. Lack of innovative platform

7. Susceptibility to erosion 8. Soil health at the time of conversion to NT

Soils and Food Sufficiency: A Review

8 Integrated Nutrient Management Similar to the C budget, the nutrients removed from the soil must also be replaced. Nutrient depletion, with the attendant adverse impacts on crop yield, occurs when the nutrient removal (harvest, erosion, leaching and volatilization) exceeds the nutrient input: recycling, biological nitrogen fixation (BNF), animal manure, fertilizers, runon and aerial deposition. Nutrient depletion by indiscriminate mining through extractive farming has adversely impacted crop yields in SSA, SA and elsewhere in developing countries (IFDC 2006; Tan et al. 2005). There is a need to adopt management strategies which create and sustain positive nutrient and C budgets in managed ecosystems.

9 Soil Fertility Management Judicious management of soil, water and plant nutrients is one of the strategies to adapt to climate change. Nutrient depletion and imbalance in soil adversely affect crop growth and yield, and are serious issues in soils of SSA (IFDC 2006; Smaling and Dixon 2006) and elsewhere in Asia, Central America and the Caribbean. Tan et al. (2005) estimated that globally nutrient depletion occurs at the rate of 18.7 kg ha1 yr1 of N, 5.1 kg ha1 yr1 of P and 38.8 kg ha1 yr1 covering 59%, 85% and 90% of harvested area in 2000. Tan and colleagues estimated the global annual nutrient deficit at 5.5 Tg of N, 2.3 Tg P, and 12.2 Tg K, causing a total production loss of 1,136 million tons of food grains. Soil nutrient depletion is attributed to lack of or insufficient use of fertilizers, unbalanced fertilization, and losses caused by erosion, leaching, volatilization and weeds. Increasing the input of plant nutrients into the ecosystem is crucial to creating a positive nutrient budget. Nutrients may be applied from inorganic or organic sources (Goulding et al. 2007). Nitrogen is the most limiting factor in crop production, and its use efficiency remains low because of the severe losses caused by volatilization and leaching (Eickhout et al. 2006). Nitrogen management is closely related to the soil C pool and its dynamics, and soils of the tropics are highly depleted of their soil C pool because of extractive farming practices used for centuries and millennia. Using INM techniques is important for enhancing and sustaining soil fertility (INM: integrated nutrient

35

management). INM involves combined use of mineral and organic fertilizer sources along with the adoption of legume-based, tree-based and animal-based farming systems. Several studies conducted in subSaharan Africa (SSA) and South Asia (SA) have documented the long-term and positive impacts of using INM techniques for improving soil fertility (Alemu and Bayu 2005; Smaling and Dixon 2006). The use of fire must be minimized because of numerous adverse impacts on ecosystem processes (Shriar 2007). The direct link between anthropogenic emissions and atmospheric abundance of CO2 (Houghton 2002, Broecker 2007), necessitates adoption of mitigation strategies (Gleason et al. 2005), along with afforestation (Clawson 1979) and restoring prairie wetlands. There are numerous strategies of mitigating climate change (Fig. 5). Improving soil quality through C sequestration (King et al. 2004) is one of these options.

10 Biochar Application of biochar (charcoal or black C) to soil can improve soil fertility. Ever since the discovery of “terra preta do Indio” (Indian Black Earth) in the Amazon (Sombroek et al. 2003; Marris 2006; Lima et al. 2002; Rumpel et al. 2006), there is a growing interest in using biochar as a soil amendment and for sequestering C and improving soil quality. There are several mechanisms by which application of biochar can enhance soil quality: (1) increase in soil’s cation exchange capacity, (2) decrease in losses of nutrients by leaching, runoff and volatilization, (3) increase in soil’s microbial activity that accentuates soil’s resilience, (4) increase in soil structure and water retention capacity, (5) increase in buffering against soil acidification, and (6) reduction in emission of CH4 and N2 O (Fowles 2007). A pot experiment with three rates (10, 50 and 100 Mg ha1 ) of biochar showed that application of biochar without application of N fertilizer had no impact on the yield of radish (Raphanus sativus) grown in a degraded Alfisol. However, higher yield increases were observed with increasing rates of biochar in the presence of N fertilizer, indicating an increase in N fertilizer use efficiency (Chan et al. 2007). A field experiment in Manans, Brazil, showed that charcoal (11 Mg ha1 ) significantly improved plant growth and doubled grain production only if applied in conjunction with NPK fertilizers. The highest crop yield of

36

R. Lal

12.4 Mg ha1 was obtained with application of poultry manure. Similarly, there are a range of soil processes that increase C sequestration through application of biochar: (1) it is a relatively stable/recalcitrant C with long residence time, and (2) it is translocated into the sub-soil away from the zone of natural and anthropogenic perturbations. Furthermore, black C is also translocated into the ocean (Dickens et al. 2004; Schmidt 2004). Therefore, it is argued that charcoal can be used in a climate-neutral manner whereby one mole of CO2 emitted can be balanced by one mole of CO2 sequestered (Seifritz 1993). Air-dried wood can be used to produce charcoal as per equation (1):

C in the wood converted into charcoal (Seifritz 1993). Charcoal conversion efficiency of wood is about 50%. A coal-fired power plant of 1 MW h power plant produces 6,800 Mg of CO2 or 1,855 Mg of C. The CO2 thus released can be sequestered in 1,130 ha (11.3 km2 / plantation of poplar. Therefore, biochar sequestration in terrestrial ecosystems is considered a viable option of enhancing soil fertility while mitigating climate change (Lehmann et al. 2006; Laird 2008). Despite its potential, three issues which remain to be addressed are (1) the logistics of producing a large quantity of biomass for making biochar to be used at high rates of application on soils at 10–20 Mg ha1 or more, (2) loss of humus or the soil organic carCH1:44 O0:66 .dry wood/ C 0:43O2 .from air/ bon (SOC) pool because of the removal of biomass soot 0:6C.charcoal/ C 0:4CO2 .g/ C 0:72H2 O: (1) for charcoal (Wardle et al. 2008), and (3) possible increase in mineralization of the SOC pool due to The process shown in equation (1) can produce 750 kg soil application of biochar (Shneour 1966, Wardle of charcoal from 3 Mg of air-dried wood, with 60% of et al. 2008).

Mitigative Options

Adaptive Options

Terrestrial Biosphere

Aquatic Ecosystems

Reducing Emissions

Sequestering Emissions

Forests

Wetlands

Energy Efficiency

Biotic sequestration

Agriculture

Coastal Ecosystems

Low-carbon fuel sources

Geologic Injection

Deserts

Glaciers

No-carbon fuel sources

Chemical Processes

Alpine Ecosystems

Fresh Water Supply

Life style changes

Oceanic Technologies

Policy Options • Cap and Trade • CO2 Tax

Fig. 5 Technological options for adapting to and mitigating atmospheric abundance of CO2

Innovative Emerging Technologies

Adapting Ecosystems to Climate Perturbations

Strategies For Mitigating CO2 Enrichment

Soils and Food Sufficiency: A Review

37

11 Crop Yields and Agronomic Input Strategies for improving soil quality may be built upon traditional knowledge, but must strongly rely on proven scientific innovations (Lal 2007). With the world average crop yield of milled rice at 4 Mg ha1 , the yield is about 3 Mg ha1 in India compared with 8.5 Mg ha1 for the USA. Similarly, for the world average yield of 5 Mg ha1 for corn, the grain yield is 2 Mg ha1 in India compared with 9.5 Mg ha1 in the USA. Crop yields in developing countries are strongly related to input, especially of fertilizer and irrigation. The data in Fig. 6 show that fertilizer use has drastically increased in all regions, except in SSA. Yields of crops in India have increased with increase in fertilizer use (Fig. 7), especially in irrigated wheat and rice. Because of low fertilizer use, and degraded/depleted soils, crop yields in countries of SSA are extremely low, as is shown by the data in Fig. 8 for Nigeria, Fig. 9 for Senegal, Fig. 10 for Uganda, Fig. 11 for Ghana and Fig. 12 for Kenya. Fertilizer response also depends on the cropland area under irrigation. Similar to trends in fertilizer use, cropland area under irrigation is also low in Africa (Fig. 13). Out of the total world irrigated area of about 275million hectare (Fig. 14),

that in Africa is about 5 million hectare, or <2% of the world total. In the long run, therefore, it is important to make these inputs available to minimize the adverse impacts of weather and soil degradation, and advance food security. There is no reliable substitute for the judicious use of inputs. Improved germplasm cannot extract water and nutrients from degraded/depleted soils where these do not exist. Therefore, making water and nutrients available at the critical time and in appropriate forms is essential to obtaining high yields. Growing improved varieties can help, but these are not substitutes for the essential inputs. The scientific challenge lies in (1) understanding soil processes, (2) characterizing and mapping soil resources, and (3) predicting soil behavior under a variety of land uses and management scenarios (Miller and Wali 1995). The strategy is to make economic-agricultural development congruent with ecological, social and political realities, use and conserve indigenous genetic resources, and restore degraded soils and ecosystems (Miller and Wali 1995). Using the ecological footprint, Kitzes et al. (2007) and Hazell and Wood (2007) proposed two scenarios to balance human demands and ecosystem supply: (1) managing the consumption of food, fiber and energy,

6

Fertilizer Consumption (10 Mg/yr) 25

Western Europe

20

15

10

South Asia South America

5 Sub- Saharan Africa

0 1961

1971

1981 Year

Fig. 6 Trends in regional and national fertilizer use (reported from IFDC 2004)

1991

2001

38

R. Lal

Fertilizer Use Rice Fertilizer Use (million t)

Crop Yield (kg/ha)

Wheat

Corn

Sorghum Millet

Year Fig. 7 Trends in fertilizer use and crop yields in India (data compiled from FAO Production Yearbooks, and IFDC 2004) 14000

450 400

12000

350

Yam

8000

300 250

Fertilizer Use

6000

150

Sweet Potato

4000

200

100

Fertilizer Use (1000 MT)

Crop Yield (kg/ha)

Cassava

10000

Rice

2000

50 Corn

0

0 1970

1980

1990

2000

2002

Year Fig. 8 Trends in fertilizer use and crop yields in Nigeria (data compiled from FAO Production Yearbooks)

and (2) maintaining and increasing the productivity of agricultural ecosystems. It is important to understand the linkages between human needs, agriculture and the environment. The strategy is to develop agricultural systems which balance the positives and negatives of farming and to protect the production capacity and

wellbeing of the land (Pollock et al. 2007). Several technological options relevant to achieving these goals include: agrobiodiversity (Thrupp 2000), conservation agriculture (Hobbs et al. 2007) and social/ political factors which determine farmers’ interest in adopting recommended practices (Shriar 2007).

Soils and Food Sufficiency: A Review

39 40

12000

35 10000

Crop Yield (kg/ha)

8000 25 20

6000

15 4000

Fertilizer Use (1000 MT)

30

10 2000 5 0

0 1970

1980

1990

2000

2002

Year Fig. 9 Trends in fertilizer use and crop yields in Uganda (data compiled from FAO Production Yearbooks) Fertilizer Use (1000 MT)

Crop Yield (kg/ha)

10

16000

9

14000

8 12000 7 10000

6

Fertilizer Use

8000

5 Cassava 4

6000

3 4000

Sweet Potato 2

2000

Corn

1

Rice 0

0 1970

1980

1990

2000

2002

Year

Fig. 10 Trends in fertilizer use and crop yields in Senegal (data compiled from FAO Production Yearbooks)

Recommended practices are those that enhance eco-efficiency or the sustainable use of resources in farm production and land management (Wilkins 2007; Pretty 2007). In addition to advancing food security, soil C management is also important for controlling the

abundance of CO2 in the atmosphere (Dyson 1977). While the precious natural resource (SOM pool) competes with biofuel for alternate uses of crop residues (Jenny 1980) and for improving nutrient flow to plants (Janzen 2006), its judicious management is important for sustainable use of soil resources.

40

R. Lal

Fig. 11 Trends in fertilizer use and crop yields in Ghana (data compiled from FAO Production Yearbooks) 160

12000 Fertilizer Use

140

Crop Yield (kg/ha)

Cassava

120

Sweet Potato

8000

100

6000

80

Yam

60 4000

Fertilizer Use (1000 MT)

10000

40

Rice

2000

20

Corn

0

0 1970

1980

1990

2000

2002

Year Fig. 12 Trends in fertilizer use and crop yields in Kenya (data compiled from FAO Production Yearbooks)

12 Climate Change and Food Security Three biophysical factors which need to be addressed are soil quality, water availability or drought stress, and climate change. There is a strong interaction among these factors. For example, adverse effects of soil degradation and drought stress are exacer-

bated by differences in the amount and distribution of rainfall, and increase in temperature, especially during the flowering stage of crop growth. Adverse effects of soil erosion may also be exacerbated by climate change (Meadows 2003). Over and above the effect of extreme events, equally important is the impact of more gradual changes as well as

Soils and Food Sufficiency: A Review

41

6

Irrigation (10 ha) 250

200

150

Asia

100 China

50 India

Africa South America

0 1950

1960

1970

1980 Year

1990

2000

2010

Fig. 13 Regional and national trends in irrigated land area (data compiled from FAO Production Yearbooks) 6

6

Fertilizer (10 Mg)

Irrigation (10 ha)

160

300

140 250 Irrigation

120

200 100 150

80 Fertilizer

60 100 40 50 20 0 1950

0 1960

1970

1980

1990

2000

2010

Year

Fig. 14 Trends in global fertilizer use and irrigated agricultural land area (data compiled from FAO Production Yearbooks, and IFDC 2004)

interaction with social and economic factors (Vogel 2005). The effect of governance and economic development cannot be ignored. In some cases, however, there may be a positive effect of increase in atmospheric concentration of CO2 on grain yield.

Walker and Schulze (2006) observed that doubling of CO2 and increase in rainfall by 10% may increase maize grain yield by 200–1,000 kg ha1 depending on the use of manure and chemical fertilizers, respectively.

42

Land-use conversion and soil cultivation can be both a source and sink of atmospheric CO2 (IPCC 2007). Adoption of sustainable practices can make the world’s soils a major C sink. Soil C sequestration can off-set fossil fuel emissions and mitigate climate change. Soil C sink capacity is about 78 ˙ 12 Pg, equal to the historic C loss from the world’s soils (Lal 1999, 2006a,b). The land-based C sink may be decreasing (Canadell et al. 2007), probably due to soil/ecosystem degradation. The rate of C sequestration depends on soil and crop management, and soil type and climate (Lal 2004; IPCC 2000), and can be as much as 1 Pg C per year for the next several decades (Pacala and Socolow 2004). Increase in the soil C pool is important for improving soil quality, and enhancing agronomic production (Lal 2003, 2006a,b). It is in this context that C sequestration in soils is deemed a win-win situation. While advancing food security through increase in use efficiency of input in soil of improved quality, it also accentuates numerous ancillary benefits (e.g., water quality, biodiversity). Most productivity models, however, have indicated adverse effects of climate change on agronomic production in Africa. Parry et al. (2005) predicted that the region of greatest risk to decline in food production is SSA. Furthermore, the impact of climate change on risk of hunger depends on the degree of economic development, with the most adverse effect on low-income and poorly developed communities. Kurukulasuriya et al. (2006) conducted a survey of 9,000 farmers across 11 countries in SSA. They observed that farm revenues are likely to fall with warming for dryland crops and livestock. In contrast, revenues may rise for irrigated crops. In general, irrigated farmers are less sensitive to climate change than dryland/rainfed farmers. Verdin et al. (2005) reported that Ethiopia will need special attention because of persistent dryness and the positive trends in the Indian Ocean sea surface temperature. Gbetibouo and Hassan (2005) observed a distinct shift in farming practices and patterns due to climate change. These include a shift in the crop calendar and growing seasons including disappearance of some crops from climate-sensitive regions. Jones and Thornton (2003) estimated an overall reduction of only 10% in maize production by 2055, equivalent to losses of $2 billion per year. However, the impact will be highly variable among regions.

R. Lal

13 Biofuel and Food Security Conundrum Increasing energy demand is a global issue. Modern biofuels, bioethanol and biodiesel produced from grains (corn, soybean) or biomass (crop residues and biosolids from energy plantations such as switch grass, poplar, willow, etc.) are important for the future of sustainable energy (Goldemberg 2007; Goldemberg et al. 2004), but are also intimately linked with food security. One of the major issues is the source of feedstock for cellulosic ethanol (Somerville 2006; Kennedy 2007). Maintenance of SOM being closely linked to the soil application of crop residues, it has been widely recognized that it is either humus or ethanol (Jenny 1980) but not both. Soil application of crop residues is extremely important for soil quality (Wilhelm et al. 2004; Blanco-Canqui and Lal 2007, 2008). It is in this context that Tilman et al. (2006) proposed low-input high-density grassland biomass as a potential source for biofuel feedstock. Despite its importance and the urgency to replace fossil fuel by alternative energy sources, the impact of using grains or crop residue on food security must be objectively and critically addressed. In addition to the direct impact, food security is also impacted by competition for land, water and nutrients, to establish biofuel plantations. It is the question of a sustainable use of limited resources in a world of a growing population of 6.7 billion in 2008, projected to be 7 billion by 2012 and increasing at the rate of 1.3% per year.

14 Conclusion The food crisis of 2008 is attributed to numerous interacting factors. Important among these are drought and soil degradation, both of which are especially severe in SSA and South Asia (SA). Then, adverse effects on crop yields are aggravated by high energy costs and poor support services. With predominantly smallsized land-holders, who do not have the resources to purchase the much-needed input, crop yields are low and highly dependent on weather. Weather-dependence of crop yields can only be reduced by improving soil quality, its waterholding capacity and overall soil fertility.

Soils and Food Sufficiency: A Review

The resource-poor small land-holders in economically less developed regions of the tropics may be more vulnerable to climate change than large-scale commercial farms in developed economies of temperate regions. Therefore, making agriculture less susceptible to climate change implies development of irrigation, establishment of conservation-effective techniques, making fertilizers and soil amendments available to farmers, and development of farming/cropping systems which are less vulnerable to declining effective rains and warming temperatures. Several promising technologies for restoration of degraded ecosystems and sustainable management of soil resources have existed since the 1960s and 1970s (e.g., NT farming, mulching, growing cover crops and integrated nutrient management). However, these proven technologies have not been adopted, especially in the developing countries of SSA and SA. Creating another income stream for farmers, through payments for ecosystem services (e.g., C sequestration, water quality, biodiversity) may be an important strategy to promote adoption of improved technology (Flugge and Abadi 2006). Involving farmers in the decision-making process, in choosing and implementing technological packages, is another important consideration. Soils must not be taken for granted. While adoption of improved varieties is important, agronomic production can neither be improved nor sustained unless soil quality is restored and maintained. Maintaining soil quality and water resources at optimal level is essential to realizing the potential of improved varieties. Degraded soils do not respond to other inputs unless their physical, chemical and biological quality is restored. Even the GM crop varieties cannot extract water and nutrients from soil where they do not exist. Agronomic management of soil and water must go hand-in-hand with improved germplasm. Seeds of the second Green Revolution will be sown in improved and restored soils which have a favorable soil moisture regime, optimal soil structure, and positive carbon and nutrient budgets.

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Denitrification at Sub-Zero Temperatures in Arable Soils: A Review Rebecca L. Phillips

Abstract Nitrogen (N) in agricultural fertilizers is denitrified by soil bacteria when oxygen is limited, which effectively removes plant-available N from the soil to the atmosphere. Reported denitrification rates range from 0 to 239 kg N ha1 yr1 , and, depending upon environmental conditions and management, may reduce the amount of N available for crop growth by 27%. Denitrification in soils also results in emissions of nitrous oxide (N2 O), which is a recognized pollutant that contributes to stratospheric ozone destruction and radiative forcing in the troposphere. Practitioners of sustainable agronomy aim to improve plant N-use efficiency and reduce emissions of the greenhouse gases by synchronizing N application and plant nutritional requirements. However, it is difficult to predict denitrification rates during and after the growing season based on current knowledge. High rates are consistently reported in irrigated cropping systems following heavy applications of fertilizer-N, but few studies report denitrification during the dormant season. Denitrification in winter may represent a significant sink for fertilizer-N in cropping systems, but further research at sub-zero soil temperatures is needed. Here, the three factors required for microbial denitrification: limited O2 availability, electron donors and electron acceptors, are reviewed based on soil research performed both above and below 0ı C. Gaps in the knowledge of denitrification rates in cropping systems, particularly when soils are frozen, are identified. Sustainable management of N in cropping systems such as greater

R.L. Phillips () USDA-ARS, Box 459, Mandan, ND 58554, USA e-mail: [email protected]

N-use efficiency and lower greenhouse gas emissions could be advanced by greater understanding of denitrification in winter. Keywords Fertilizer r Nitrogen Sub-zero temperatures

r

Nitrous oxide

r

1 Introduction The greatest agronomic uncertainty in balancing the nitrogen (N) budget of agricultural landscapes is the rate of denitrification, which converts plant-available N into gaseous N (Galloway et al. 2004). Specifically, it is not known when denitrification in the rooting zone reduces the availability of N to crops or the magnitude of N losses via denitrification. Current average N-use efficiency in cropping systems (% recovery of applied N) is reported to range from 30 to 50% (Cassman et al. 2002). A major reason for low N-use efficiency is the loss of gaseous-N from agricultural soils worldwide (Davidson and Seitzinger 2006). Denitrification may transfer up to 27% of agricultural N back to the atmosphere (Bouwman et al. 2005). However, spatial and temporal heterogeneity in denitrification rates, lack of quantitative data and inconsistencies between laboratory vs. field measurements contribute to uncertainties in the rate of denitrification, despite decades of research (Davidson and Seitzinger 2006). Achieving synchrony between N supply and crop demand without sacrificing yield or protection of the environment requires greater knowledge of denitrification rates, yet knowledge of denitrification during the dormant season is limited. In many cases, gaseous-N

E. Lichtfouse et al. (eds.), Sustainable Agriculture, DOI 10.1007/978-90-481-2666-8_5, c Springer Science+Business Media B.V. - EDP Sciences 2009. Reprinted with permission of EDP  Sciences from Phillips, Agron. Sustain. Dev. 28 (2008) 87–93. DOI: 10.1051/agro:2007045

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losses at sub-zero soil temperatures are unknown or considered negligible. Consequently, wintertime N losses are rarely considered in crop fertilization recommendations. The body of evidence suggests microbes are physiologically active when soils are frozen (Clein and Schimel 1995; Mikan et al. 2002; Price and Sowers 2004; Rivkina et al. 2000), calling for agronomists to question what is known about the physicochemical and biological properties of soil below 0ı C. Microbial emissions of N2 O and N2 occur at sub-zero soil temperatures (Phillips 2007; Röver et al. 1998), but processes controlling denitrification rates in frozen soils are currently unknown. Gaps in the knowledge below 0ı C need to be filled because there may be unforeseen opportunities for conservation of fertilizer-N and for reductions in greenhouse gas emissions during winter. Denitrification is a ubiquitous process, occurring globally in both terrestrial and aquatic ecosystems (Davidson and Seitzinger 2006). This review focuses on denitrification through the process of anaerobic microbial respiration known to occur in sub-oxic soil microsites (Myrold and Tiedje 1985; Parkin 1987) specifically in cropped soils. Other, non-respiratory pathways observed in aquatic systems (e.g., chemodenitrification, aerobic ammonium oxidation) are outside the scope of this review (Hulth et al. 1999; Kuypers et al. 2005). Three fundamental factors are required for anaerobic microbial denitrification: (1) suboxic or anoxic conditions (herein referred to anoxic), (2) electron donors (herein referred to as organic C), and (3) electron acceptors nitrite (NO 2 ) or nitrate  ), (herein referred to as NO ). Each factor is re(NO 3 3 viewed separately with respect to denitrification when soil temperatures are greater or less than 0ı C. Research that is necessary to unravel how denitrification might occur at sub-zero soil temperatures is proposed.

2 Denitrification Overview Denitrification is classically defined as the microbial oxidation of organic matter, where NO 3 is the terminal electron acceptor. It is a heterotrophic process of anaerobic respiration conducted by facultative bacteria using oxidized forms of N to accept electrons when O2 is limited (Firestone et al. 1980). The end product is N2 , but some intermediate compounds (such as N2 O

R.L. Phillips

and NO) may also be produced, depending upon environmental conditions (Firestone and Davidson 1989). The primary factor controlling the rate of denitrification is O2 availability because in sub-oxic conditions (<0.2 mg O2 L1 /, some facultative microbes that normally use O2 as an electron acceptor will use NO 3 (Firestone et al. 1980; Seitzinger et al. 2006). Sub-oxic, as defined here, is three orders of magnitude lower than the density of O2 in moist air (290 mg O2 L1 /. Numerous laboratory incubation studies indicate that, for similar soils incubated at a constant temperature, denitrification rates can be manipulated by varying percent water-filled pore space (%WFPS), electron donors, and electron acceptors (Firestone and Davidson 1989; Myrold and Tiedje 1985; Sexstone et al. 1988). Numerous field studies indirectly point to these factors by reporting how differences in drainage (O2 status), soil organic matter form and quantity (electron donors), and fertilization form and application (electron acceptors) alter rates of denitrification (Aulakh et al. 1984, 2001; Hofstra and Bouwman 2005). Accordingly, for a given soil and temperature, the kinetics of denitrification can largely be explained by these three factors. An intermediate gaseous product of denitrification, N2 O, has received a great deal of attention (Bouwman et al. 1995; Davidson et al. 2000; Dobbie et al. 1999; Dobbie and Smith 2003; Jungkunst et al. 2006) because of its importance in the processes of ozone destruction and radiative forcing (Prather et al. 2001). The global warming potential of N2 O is nearly 300 times greater than CO2 by mass (Forster et al. 2007). Two soil microbial processes, nitrification and denitrification typically produce this gas, so N2 O fluxes measured at the soil surface are not necessarily products of denitrification. Losses resulting from complete reduction of NO 3 to N2 are rarely measured because the large atmospheric background of N2 makes it analytically difficult to detect small increases in N2 from denitrification (Davidson and Seitzinger 2006). While knowledge is growing with respect to cropping systems and surface fluxes of N2 O in winter (Kaiser et al. 1998; Maggiotto and WagnerRiddle 2001; Wagner-Riddle et al. 1997), measurements of total gaseous losses of N via denitrification in frozen soils are lacking. Observed fluxes of N2 O at the surface of frozen soils suggest microbial denitrification may occur at sub-zero soil temperatures (Röver et al. 1998) but specific mechanisms are unknown. One question is: how much total N (N2 O + N2 / is from

Denitrification at Sub-Zero Temperatures in Arable Soils: A Review

denitrification below 0ı C and how much is degassed from accumulated products of denitrification prior to freezing? Current knowledge of how management influences denitrification rates is largely garnered from experiments conducted during the growing season and at laboratory temperatures. Many field and laboratory experiments are conducted by manipulating the three factors required for denitrification (O2 status, organic C, NO 3 ). For example, greater denitrification rates are found in fertilized cropped soils at high %WFPS (Barton et al. 1999; Hofstra and Bouwman 2005) because O2 diffusion is restricted under saturated conditions and the proportion of soil volume that is anoxic increases (Sexstone et al. 1985; Smith 1980). In some cases, organic C and N additions will increase denitrification rates (Burford and Bremner 1975; Myrold and Tiedje 1985; Paul et al. 1993). Fertilizer-N and residue inputs might fuel denitrification in frozen soils at high moisture levels during the winter, but empirical studies are needed to estimate N transformation rates and controls in frozen soil. Filling these knowledge gaps during the winter will improve agronomic recommendations, with potentially positive economic and environmental benefits.

3 Water-Filled Pore Space in Frozen Soil Application of soil %WFPS (1) as a proxy for soil O2 status is fundamental to current models of denitrification (Firestone and Davidson 1989). %WFPS D .™v =¥/  100

(1)

where ™v D percent volumetric water content, ™m  ¡b ¥ D percent total porosity D .1  ¡b =¡p /  100 ™m D percent gravimetric water content ¡b D soil bulk density (mg m3 ) ¡p D soil particle density (2.65 mg m3 /. It is a well accepted approximation that at soil %WFPS >70 (where water is liquid), gaseous N emissions are the result of microbial denitrification (Bateman and Baggs 2005; Davidson 1991), although exact %WFPS values vary with soil mineralogy. It is less clear how %WFPS influences microbial denitrification when soil water is transformed to ice. Effects of

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freezing on the soil physical environment may influence rates of microbial denitrification at sub-zero soil temperatures. Calculation of soil %WFPS becomes less tractable at sub-zero soil temperatures because the majority of liquid water becomes ice, rendering changes in bulk density (Kay et al. 1985), hydraulic conductivity (Pikul and Allmaras 1985), pore space volume (Loch and Kay 1978), and water content (Pikul et al. 1989). Liquid water in frozen soil is mobile (Pikul and Allmaras 1985) and flows along unfrozen liquid water channels (Edwards and Cresser 1992), which change in thickness as temperature decreases (Anderson and Hoeckstra 1965). Formation of ice pushes soil particles apart to increase soil pore size (Loch and Miller 1975), and ice lenses forms to create additional pores (Kay et al. 1985). The percent of water occupying soil pores is not constant because liquid water content and soil pore space are not constant at sub-zero soil temperatures. The presence of both ice and unfrozen water in soil could enhance denitrification at oxic/sub-oxic interfaces controlled by the thermal gradient. Oxic/sub-oxic interfaces facilitate transport of oxidized forms of N from oxic to sub-oxic zones (Seitzinger et al. 2006). If these interfaces are present in frozen soil, then the amount of ice vs. unfrozen water could influence denitrification rates. However, the presence of oxic/suboxic interfaces is not likely to remain static in frozen soil because the amount of unfrozen water, the thickness of the water films, the size of transport channels, and hydraulic conductivity are controlled by soil temperature (Hoeckstra 1966; Pikul and Allmaras 1985). The temperature gradient continuously transforms ice to films of water (Kay et al. 1985), potentially creating sites for denitrification at oxic/sub-oxic interfaces. Freezing also induces changes in soil structural stability (Bullock et al. 1988; Lehrsch et al. 1990), which interacts with water and temperature to affect soil pore space. Liquid water is replaced by ice lenses that weaken soil aggregates (Bullock et al. 1988; Edwards and Cresser 1992; Lehrsch et al. 1990). As the frost front moves into the soil and the majority of soil water is transformed to ice, soil cohesion is lost to shearing forces (Bullock et al. 1988). Slightly soluble chemicals precipitate at the surface of soil particles. As the thermal gradient vacillates diurnally and seasonally, ice crystals collapse and return to unfrozen water. As freezing progresses deeper into the soil, water also

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migrates upward towards the freezing front to increase water content near the surface (Hoeckstra 1966). Soil aggregates frozen at high water contents (>15% v/v) will be more strongly affected by freezing than soil aggregates frozen at lower water contents (Lehrsch et al. 1990), with greater loss of aggregate stability and cohesion across a wide range of soil series (Bullock et al. 1988). Disruption to the soil matrix as a result of freezing, as described above, is complex and dynamic. Freezing affects soil volume, migration of water, aggregate stability, precipitation of solutes, ice crystal formation, and ice crystal collapse – all of which could alter soil O2 status. Microbes can remain physiologically active when films of unfrozen soil water are present (Mikan et al. 2002; Priemé and Christensen 2001; Rivkina et al. 2000), and denitrification has been measured in soils at –2 ı C (Dorland and Beauchamp 1991; Phillips 2007). Initial %WFPS at freezing may be fundamental to understanding interactions between soil physics and soil microbial activity, but diffusion of O2 may be limited by other factors. Current %WFPS thresholds for denitrification (Firestone and Davidson 1989) in frozen soil may need adjustment and/or other potential factors (e.g., thermal gradient, ice-filled pore space) considered.

4 Soil Organic Carbon in Frozen Soil Organic C often limits denitrification in cropped soils at soil temperatures above 0ı C (Beauchamp et al. 1989; Burford and Bremner 1975; McCarty and Bremner 1993; Sainz Rozas et al. 2001), but it is uncertain how organic C influences denitrification below 0ı C. Alternatively, studies of aerobic microbial respiration below 0ı C suggest soil organic C can limit microbial activity (Feng et al. 2007; Michaelson and Ping 2003; Priemé and Christensen 2001; Schimel and Clein 1996) and that aggregate disruption from freezing releases potentially mineralizable C to microbes when soils thaw (Christensen and Tiedje 1990; Priemé and Christensen 2001). Greater microbial respiration observed following freeze-thaw cycles (Christensen and Tiedje 1990; Mikan 2002; Teepe et al. 2004) may be linked to soil organic C (Breitenbeck and Bremmer 1986; Mikan 2002; Schimel and Clein 1996;

R.L. Phillips

Skogland et al. 1988), since freeze-thaw events positively influence amounts of small, hydrophilic compounds (Michaelson and Ping 2003) and phospholipid fatty acids (Feng et al. 2007). Suggested mechanisms for C availability following freezing and thawing include rupture of cellular membranes in microbial biomass (Skogland et al. 1988), the release of organic matter previously bound in aggregates (Christensen and Christensen 1991), and exposure of fresh reactive surfaces (Edwards and Cresser 1992). In addition, thawing may enhance the availability of C required for anaerobic respiration through the collapse of ice crystals and diffusion of substrate to anoxic microsites. Rarely are effects of freezing on microbial activity separated into components of C availability vs. physical soil disturbance. Reported pulses of aerobic microbial respiration in frozen soils may be due to disturbance from aggregate disruption (Edwards and Cresser 1992), release of organic C (Christensen and Christensen 1991; Myrold and Tiedje 1985; Schimel and Clein 1996), changes in the form of organic C (Feng et al. 2007), or to changes in the diffusion of gases or solutes in frozen soil. The causative factor of respiratory activities (physical disturbance, substrate amount/form, diffusion) likely contributes to the magnitude and duration of observed pulses. This may be critical to denitrification questions in agronomy because physical disturbances from freezing can be modified, to some extent, with tillage and residue management. Diurnal and seasonal soil temperature extremes are modulated and depth to frost is reduced with standing stubble (Pikul et al. 1986), with mulch application (Kohnke and Werkhoven 1963), and with reduced tillage (Kay et al. 1985). On the other hand, carbon can limit denitrification (McCarty and Bremner 1993), and addition of plant residues can promote denitrification activity (Aulakh et al. 1984; McCarty and Bremner 1993). Laboratory studies using soils from no-till cropping systems point to greater soil C as the reason for higher denitrification rates, compared to conventional tillage (Aulakh et al. 1984; Liu et al. 2007; van Bochove et al. 2000). Understanding and quantifying effects of management on denitrification in frozen soil require separation of physical disturbance (loss of aggregate cohesion and stability) from release and transport of organic C potentially bound in soil aggregates. From here, if organic C is

Denitrification at Sub-Zero Temperatures in Arable Soils: A Review

55

limiting denitrification, both tillage and residue management recommendations could be balanced to potentially reduce denitrification rates.

5 Nitrogen in Frozen Soils Effects of fertilizer-N on denitrification have been studied extensively (see review by Hofstra and Bouwman 2005). Fertilized soil at high %WFPS will promote facultative anaerobic bacteria to reduce N oxides and consume organic matter (Firestone and Davidson 1989; Mulvaney et al. 1997). The magnitude and duration of NO 3 additions varies with soil texture, pH, climate, crop, management, etc (Jungkunst et al. 2006; Kaiser et al. 1998; Nieder et al. 1989; Sainz Rozas et al. 2001). It is generally accepted that cropped soils with high rates of fertilizer-N inputs generally exhibit higher denitrification rates than soils not receiving fertilizer-N additions (Aulakh et al. 2001; Barton et al. 1999; Jarvis et al. 1991; Kaiser et al. 1998). Manures and animal slurries amendments also enhance denitrification rates (Calderón et al. 2004; Ginting et al. 2003; Lessard et al. 1996; Mogge et al. 1999; Paul et al. 1993; Petersen 1999). The proportion of fertilizer-N denitrified in crop fields varies widely across soil series and climates (see review by Nieder et al. 1989), with 2.5% reported in Colorado, USA (Mosier et al. 1986) and 60% reported in Denworth, UK (Colbourn et al. 1984). The effect of fertilizer-N application on denitrification rates in cropped soils during the winter is less known, particularly when soils are frozen. Field studies instead have reported significant emissions of N2 O (Goossens et al. 2001; Maggiotto and Wagner-Riddle 2001; Ruser et al. 2001; WagnerRiddle et al. 1997). Nitrous oxide studies are labor intensive, and microbial activity in frozen soil is often assumed to be negligible; consequently N2 O flux data collected at the surface of frozen soil are rare (Phillips 2007; Röver et al. 1998). Evidence of greater microbial N2 O production in fertilized soil during winter suggests that denitrification may occur in anoxic soil microsites at low soil temperatures. Manure amendments were found to increase N2 O flux and denitrification in the field and in soil cores (30 cm depth) incubated at sub-zero soil temperatures (Phillips 2007), but further research is needed to

Fig. 1 Conceptual illustration of a soil column where ice and unfrozen liquid water exist in surface soil (frozen soil depth varies with frost depth) and liquid water exists in subsurface soil. Brown represents soil, blue represents water, and white with blue represents ice

determine the geographic prevalence of denitrification in frozen soils, and how form and timing of fertilizer-N application might alter gaseous N losses in winter. Determining the location of denitrification in the soil profile is also paramount to understanding N management in winter (Fig. 1). Denitrification in the soil profile could occur (1) below the freezing front in subsurface soil, (2) in the frozen surface soil, (3) at the interface between frozen/unfrozen soils, or (4) throughout the profile in both frozen and unfrozen soil. In each case, the distance denitrification products would need to travel upward to the surface would significantly affect emissions at the surface because there may be opportunities for complete reduction of NO or N2 O to N2 as they migrate from below the freezing front to the surface. Further, as suggested earlier, denitrification controls in the frozen surface soil are likely different from denitrification controls in the unfrozen subsurface. Snow cover, residue cover, and tillage will reduce the frozen soil depth; consequently, understanding the vertical distribution of denitrification activity within the soil profile will point to how management might influence N emissions at the soil surface by manipulating soil temperature.

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R.L. Phillips

6 Conclusion

for denitrification (limited O2 , organic C, NO 3 /, a few of which are summarized below.

What is not known about denitrification in cropped soils at sub-zero soil temperatures far exceeds what is known. Physical differences induced by soil freezing suggest use of %WFPS as a proxy for soil aerobic status is insufficient for predicting heterotrophic anaerobic respiration below 0ı C. Organic C may limit aerobic microbial respiration below 0ı C, calling into question if organic C might also limit anaerobic respiration. If so, management practices implemented in autumn (e.g., residue incorporation, compost or fertilizer-N application) could promote N and C losses via denitrification, with important agronomic implications (Fig. 2). Moreover, observed fluxes of N2 O in fertilized cropped soils during the dormant season point to potential losses of fertilizer-N inputs via denitrification, but total annual N losses need quantification. The agronomic importance of timing and form of fertilizerN during the growing season is well known. Less known is how post-season N application influences the N-budget and plant-available N the following spring. Denitrified N losses from cropping systems at sub-zero soil temperatures may or may not amount to a significant portion of the N budget. However, the preponderance of the evidence suggests denitrification should not be considered negligible without further investigation. A number of agronomic research questions have been raised with respect to the three factors required

– How much N is denitrified during the off-season, particularly when soils are frozen? – How does water content below 0ı C influence denitrification compared to above 0ı C? – What is the O2 status of frozen soil and how does this change with the advancement of the freezing front? – How does migration of water below 0ı C influence denitrification? – Is denitrification in frozen soil limited by organic C? How available is organic C to microbes below 0ı C? – How is substrate transport affected by soil comprised of ice and unfrozen soil water? How available are solutes to microbes in frozen soil? – How is fertilizer-N transformed in frozen soil? At what point in the N-cycle is N transformation inhibited by freezing temperatures? – Does the type of fertilizer-N applied to crop fields (e.g., compost, urea, anhydrous ammonium) influence denitrification in winter? – Do plant residue fermentation products enhance denitrification below 0ı C? – Does denitrification occur in both the frozen soil near the surface and in the unfrozen subsurface soil? The evidence indicates microbial denitrification occurs during the winter in previously-cropped soils at sub-zero soil temperatures, and the potential exists for some mediation with management. However, management studies should be preceded by basic knowledge of how frozen soil conditions alter soil O2 status and anaerobic transformation of NO 3 to gaseous N2 O and N2 . From there, questions of soil pH, texture, residue quantity and quality, fertilizer-N form and timing, etc., can be more parsimoniously addressed. Potential economic and climate change implications warrant continued, mechanistic research. Acknowledgements Special thanks to Drs. Joe Pikul, Steve Merrill, Bill Schlesinger, Steve Whalen, and Mark Liebig for their valuable comments.

Fig. 2 Summary of potential crop management effects on denitrification in frozen soil. Positive effects are indicated with .C/ and negative effects by ./. Some of the indirect effects of management on denitrification in soil, designated by (?), are unknown

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58 Jarvis S., Barraclough D., Williams J., Rook A. (1991) Patterns of denitrification loss from grazed grassland: Effects of N fertilizer inputs at different sites, Plant Soil 131, 77–88 Jungkunst H.F., Freibauer A., Neufeldt H., Bareth G. (2006) Nitrous oxide emissions from agricultural land use in Germanya synthesis of available annual field data, J. Plant Nutr. Soil Sci. 169, 341–351 Kaiser E.A., Kohrs K., Kücke M., Schnug E., Heinemeyer O., Munch J.C. (1998) Nitrous oxide release from arable soil: Importance of N-fertilization, crops and temporal variation, Soil Biol. Biochem. 30, 1553–1563 Kay B.D., Grant C.D., Groenevelt P.H. (1985) Significance of ground freezing on soil bulk density under zero tillage, Soil Sci. Soc. Am. J. 49, 973–978 Kohnke H., Werkhoven C.H. (1963) Soil temperature and soil freezing as affected by an organic mulch, Soil Sci. Soc. Proc. 27, 13–17 Kuypers M.M.M., Lavik G., Woebken D., Schmid M., Fuchs B.M., Amann R., Jørgensen B.B., Jetten M.S.M. (2005) Massive nitrogen loss from the Benguela upwelling system through anaerobic ammonium oxidation, Proc. Natl. Acad. Sci. (USA) 102, 6478–6483 Lehrsch G.G., Sojka R.E., Carter D.L., Jolley P.M. (1990) Freezing effects on aggregate stability affected by texture, mineralogy, and organic matter, Soil Sci. Soc. Am. J. 55, 1401–1406 Lessard R., Rochette P., Gregorich E.G., Pattey E., Desjardins R.L. (1996) Nitrous oxide fluxes from manure-amended soul under maize, J. Environ. Qual. 25, 1371–1377 Liu X.J., Mosier A.R., Halvorson A.D., Reule C.A., Zhang F.S. (2007) Dinitrogen and N2 O emissions in arable soils: Effect of tillage, N source and soil moisture, Soil Biol. Biochem. 39, 2362–2370 Loch J.P.G., Kay B.D. (1978) Water redistribution in partially frozen, saturated silt under several temperature gradients and overburden loads, Soil Sci. Soc. Am. J. 42, 400–406 Loch J.P.G., Miller R.D. (1975) Tests of the concept of secondary frost heaving, Soil Sci. Soc. Proc. 39, 1036–1041 Maggiotto S.R., Wagner-Riddle C. (2001) Winter and spring thaw measurements of N2 O, NO and NOx fluxes using a micrometeorological method, Water Air Soil Poll. 1, 89–98 McCarty G.W., Bremner J.M. (1993) Factors affecting the availability of organic carbon for denitrification of nitrate in subsoils, Biol. Fert. Soils 15, 132–136 Michaelson G.J., Ping C.L. (2003) Soil organic carbon and CO2 respiration at subzero temperature in soils of Arctic Alaska, J. Geophys. Res. 108, DOI: 10.1029/2001JD000920 Mikan C.J., Schimel J.P., Doyle A.P. (2002) Temperature controls of microbial respiration in arctic tundra soils above and below freezing, Soil Biol. Biochem. 34, 1785–1795 Mogge B., Kaiser E., Munch J. (1999) Nitrous oxide emissions and denitrification N-losses from agricultural soils in the Bornhöved Lake region: Influence of organic fertilizers and land-use, Soil Biol. Biochem. 31, 1245–1252 Mosier A., Guenzi W.D., Schweizer E.E. (1986) Soil losses of dinitrogen and nitrous oxide from irrigated crops in northeastern Colorado, Soil Sci. Soc. Am. J. 50, 344–348 Mulvaney R.L., Khan S.A., Mulvaney C.S. (1997) Nitrogen fertilizers promote denitrification, Biol. Fert. Soils 24, 211–220 Myrold D.D., Tiedje T. (1985) Diffusional constraints on denitrification in soil, Soil Sci. Soc. Am. J. 49, 651–657

R.L. Phillips Nieder R., Schollmayer G., Richter J. (1989) Denitrification in the rooting zone of cropped soils with regard to methodology and climate: A review, Biol. Fert. Soils 8, 219–226 Parkin T.B. (1987) Soil microsites as a source of denitrification variability, Soil Sci. Soc. Am. J. 51, 1194–1199 Paul J.W., Beauchamp E.G., Zhang X. (1993) Nitrous oxide and nitric oxide emissions during nitrification and denitrification from manure-amended soil in the laboratory, Can. J. Soil Sci. 73, 539–553 Petersen S.O. (1999) Nitrous oxide emissions from manure and inorganic fertilizers applied to spring barley, J. Environ. Qual. 28, 1610–1618 Phillips R.L. (2007) Organic agriculture and nitrous oxide emissions at sub-zero soil temperatures, J. Environ. Qual. 36, 23–30 Pikul J.L., Allmaras R.R. (1985) Hydraulic potential in unfrozen soil in response to diurnal freezing and thawing of the soil surface, Trans. ASAE 28, 164–168 Pikul J.L., Zuzel J.F., Greenwalt R.N. (1986) Formation of soil frost as influenced by tillage and residue management, J. Soil Water Cons. 41, 196–199 Pikul J.L., Boersma L., Rickman R.W. (1989) Temperature and water profiles during diurnal soil freezing and thawing: Field measurements and simulation, Soil Sci. Soc. Am. J. 53, 3–10 Prather M., Ehhalt D., Dentener F., Derwent J., Dlugokencky E., Holland E.A., Isaksen I., Katima J., Kirchhoff P., Matson P.A., Midgley P., Wang M. (2001). Atmospheric chemistry and greenhouse gases, in: Houghton J.T., Ding Y., Griggs D.J., Noguer M., van der Linden P.J., Dai X., Maskell K., Johnson C.A. (Eds.), Climate change 2001: The scientific basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, UK, pp. 240–287 Price P.B., Sowers T. (2004) Temperature dependence of metabolic rates of microbial growth, maintenance, and survival, Proc. Natl Acad. Sci. 101, 4631–4636 Priemé A., Christensen S. (2001) Natural perturbations, dryingwetting and freezing-thawing cycles, and the emission of nitrous oxide, carbon dioxide and methane from farmed organic soils, Soil Biol. Biochem. 33, 2083–2091 Rivkina E., Friedmann E., McKay C., Gilichinsky D. (2000) Metabolic activity of permafrost bacteria below the freezing point, Appl. Environ. Microbiol. 66, 3230–3233 Röver M., Heinemeyer O., Kaiser E.A. (1998) Microbial induced nitrous oxide emissions from an arable soil during winter, Soil Biol. Biochem. 30, 1859–1865 Ruser R., Flessa H., Schilling R., Beese F., Munch J.C. (2001) Effect of crop-specific field management and N fertilization on N2 O emissions from a fine-loamy soil, Nutr. Cycl. Agroecosys. 59, 177–191 Sainz Rozas H.R., Echeverría H.E., Picone L.I. (2001) Denitrification in maize under no-tillage: Effect of nitrogen rate and application time, Soil Sci. Soc. Am. J. 65, 1314–1323 Schimel D.S., Clein J.S. (1996) Microbial response to freezethaw cycles in tundra and taiga soils, Soil Biol. Biochem. 28, 1061–1066 Seitzinger S., Harrison J.A., Bohlke J.K., bouwman A.F., Lowrance R., Peterson B., Tobias C., Van Drecht G. (2006) Denitrification across landscapes and waterscapes: A synthesis, Ecol. Appl. 16, 2064–2090

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Re-Thinking the Conservation of Carbon, Water and Soil: A Different Perspective Thomas Francis Shaxson

Abstract Sustaining soil productivity requires continuing actions of soil organisms on organic materials for optimizing of soil porosity and of movements of roots, water and gases in the root-zone. Soil is more quickly formed and self-renewed from the top downwards than only by slow additions from the bottom upwards. Loss of porosity diminishes soil’s infiltration capacity and water-holding potential. Factors that provide insufficient organic substrates for soil organisms and that unduly accelerate oxidation of soil organic matter hinder the self-recuperation of soil and facilitate ‘Stage-1’ loss of carbon from within soil aggregates. They predispose the soil to lose rapidly even more carbon, in particulate form, through ‘Stage-2’ losses during consequent processes of runoff and erosion. Forms of land use and management are advocated that favour the functioning of soil-inhabiting organisms, including plants, such that carbon’s capture in photosynthesis is increased, its usefulness in the soil as a rooting medium is prolonged, and its subsequent immobilization in the process of sequestration ameliorates the rate of increase of carbon dioxide concentration in the global atmosphere. Keywords Carbon sequestration r Porosity r Self-recuperation r Soil-organisms r Sustainability

T.F. Shaxson () Greensbridge, Winterborne Kingston, Dorset, England e-mail: [email protected]

1 Introduction After seven decades of conservation programs that have consumed millions of dollars of economic resources and vast quantities of human resources, soil erosion and subsequent degradation of water resources remain serious environmental issues within the United States. (Napier, 2001)

The same may be said of other countries, in particular those covered by the Intertropical Convergence Zone where high temperatures and unstable masses of moist air lead to heavy storms and high intensities of erosive rainfall (Pereira, 1989). In places where farmers’ capacities to manage the soil are insufficient or inappropriate to maintaining it, soil productivity is in decline, or its maintenance is increasingly costly, and the land’s use less profitable. However, if even the wealthy USA, which has invested so much over so many years, has not solved this problem, then it is not surprising that other countries which adopted a similar paradigm have also not managed to prevent the degradation of their soil and water resources. Superimposed on this broad concern is a worry, being addressed at this Colloquium, that carbon selectively removed during erosion processes may be contributing to carbon dioxide in the atmosphere and thus to global warming, adding urgency to the need to find solutions to ongoing degradation of soil and water resources. The deliberately provocative question is therefore implied: “Is ‘erosion control’ (however defined) the best approach for solving these problems?”

E. Lichtfouse et al. (eds.), Sustainable Agriculture, DOI 10.1007/978-90-481-2666-8_6, c Springer Science+Business Media B.V. - EDP Sciences 2009. Reprinted with permission of EDP  Sciences from Shaxson, Agron. Sustain. Dev. 26 (2006) 9–19. DOI: 10.1051/agro:2005054

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2 Some Assumptions and Their Consequences Many of the relevant facts about land degradation and soil erosion had been identified, and their implications assessed, by the 1940s and 1950s (Bennett, 1939; Stallings, 1957). However, in many countries, Governments’ subsequent selective interpretations for purposes of developing policies and strategies to minimise land degradation and maintain soil productivity appear to have missed key points and misplaced important emphases. Pronouncements about accelerated erosion’s effects on soil productivity appear to have rested in part on some unquestioned but doubtful assumptions, which seem to have led also to some more or less inadequate approaches to controlling erosion. These have been typified by widespread primary reliance, in many Governments’ programmes in past years, on physical conservation works such as terraces, bunds, and silt traps. The evidence of this is plain to see on huge areas of cultivated land across many countries of the world. Such works have failed to solve the problem on their own and the problem of carbon loss through soil erosion by water was probably unnecessarily severe. The difficulty appears to arise from a too-narrow an emphasis on soil erosion control and too limited an application of insights from other disciplines in the development of better means for improving and sustaining the resilience and productivity of soils in the face of severely erosive climatic events. Through much of the earlier literature and policies on soil erosion and its control, common assumptions appear to have included: (a) That productivity decline is commonly caused by soil erosion, and the relation between the two ought to be definable by knowing the quantities of soil lost: therefore measure soil loss and attempt to predict productivity changes under different scenarios (b) That soil erosion is some sort of a force in its own right, capable of destroying land: therefore it has to be combated (c) That runoff is the prime factor in erosion: therefore runoff must be controlled (d) That soil productivity depends largely on its chemical constituents, and these – as well as clay and organic materials – are selectively eroded: therefore

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fertilizers must be added to counterbalance such losses in order to maintain/raise yields (Bridges et al., 2001; Doolette and Magrath, 1990) This line of thinking became to an extent hallowed over time not only by respect for those who first paid serious attention to trying to solve the problems of land degradation on a large scale, but also by repetition of only selected aspects of what these pioneers had propounded. The approach, as applied in practice, has led to much dedicated research and many kilograms of reports; but it also tended to ‘tramline’ thinking and action towards ever-more detailed investigation of the same parameters, and the application of more refined forms of the same erosion-control technologies. It has not fostered much lateral thinking about the subject as a whole. So, the common response has been to ‘fight erosion’ with ‘erosion control’, which has often translated, primarily through government policies, programmes and projects, into the construction of physical works to control and divert runoff and contain the soil it carries. For many, ‘soil conservation’ and ‘erosion control’ appeared almost synonymous. Even with financial inducements, farmers have not responded to this approach very enthusiastically, and these approaches have not often been widely adopted on the one hand nor, on the other, is there much evidence that these physical works, on their own, have improved the quality of the soil (even though they may have slowed the rate of gross soil loss). The justifications for recommending such an approach are further diminished by the difficulty that quantitative erosion-productivity relationships for most soils are not known (Eswaran et al., 2001; Sonneveld., 2002), although such information for a number of soils in tropical and subtropical regions has been clarified in recent years (Stocking et al., 2002; Stocking and Tengberg 1999; Tengberg and Stocking 2001). A result has been that many conservation enthusiasts tended in the past to assume they were correct in their analyses of land degradation and that farmers were resistant in not sharing the faith or favouring the recommendations made. The only way to improve this unsatisfactory situation appeared to be for governments to pressure farmers more strongly, and/or reward them more generously, until they readily complied. However, the reason for non-adoption by farmers is more likely to have lain in the frustration they felt at not being able to get advice that is relevant to their

Re-Thinking the Conservation of Carbon, Water and Soil: A Different Perspective

problem of falling productivity: this implies a need for advice that can be seen to be effective and that is both feasible and of net benefit – or at least of no net disbenefit – to their farming enterprises. The danger has been that, without adequate proof that such an approach can solve problems, governments might become disillusioned and reduce support for seemingly unproductive investments in ‘SWC’ (soil and water conservation), as occurred in, for example, the case of Lesotho in 1990 where continuing support for an aid programme for soil conservation was withdrawn (author’s experience). While there has been developing a growing awareness, over the last three decades in particular (Hudson, 1981) of a wider range of ways of addressing the problem, it has still been necessary to point out recently that “there is a need to re-think what we are doing, rather than re-fashioning dated concepts” (Hannam, 2001). This paper contributes to this necessary re-thinking.

3 Views from a Different Vantage-Point 3.1 Some Anomalous Results There exists a small number of ‘inconvenient’ results of erosion’s effects on productivity that open a mental door to another perception. Some researchers have recorded certain instances, in research plots and in farmers’ fields, where yields have risen rather than fallen after significant erosion has occurred (e.g. in Malaysia [Morgan, 1995], Papua/New Guinea [Kerr, personal communication], and Australia [Sanders, personal communication]. An example of such a condition of the soil is illustrated for example in Lesotho [Shaxson and Barber, 2003]). Though the number of such recorded instances is very small it would be unwise to discard such, apparently anomalous observations as mistakes or as special ‘exceptions which prove the rule’ because, in fact, they are important pointers to an unacknowledged but significant reality. These odd observations indicate that the condition of a previously subsurface layer was better for rooting than that of the former surface layer; it was exposed at the surface (before the next season’s crop was sown) when the topmost layer had been stripped off through erosion (Shaxson, 1997). This is in direct

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contrast with the more-common situation, where the subsurface layer that has been newly exposed by erosion is poorer for root growth than that which formerly covered it].

3.2 Different Considerations Reassessment of some already-known facts allows some alternative interpretations of the assumptions a, b, c, d noted above: (a) Difference between yields before and after soil erosion is more clearly related to differences between in-situ characteristics of the soil as a rooting environment before and after erosion than to the quantity of soil removed (Shaxson and Barber, 2003). The quantity and chemical quality of eroded soil provides an inadequate explanation of soil-productivity decline. Three-dimensional pore spaces and interlinkages of soil particles, organic matter and soil micro-organisms are important for good root-growth and function in the surface layers of soil, as also at sub-surface levels (FAO., 2001; Wild, 1988). While this is acknowledged in the recent study of erosion:productivtiy relations (Stocking and Tengberg, 1999) their damage and loss is more attributed to the erosion process itself, rather than damage to them being perceived as a precondition for accelerated erosion and runoff. A more-effective strategy to maintain soil productivity should therefore emphasise protecting the soil surface and favouring the improvement of the complex integrity of the soil which remains in place into a good condition for the growth and functioning of roots. In this instance, this is a matter of changed emphasis rather than changed approach: emphasise the accelerated protection and improvement of what remains in situ (positive view) more than regret that which has been lost (negative view). (b) Undue erosion is a foreseeable ecological consequence of changes (often management-induced) in relations between the components of the natural environment both above and below ground – geology, topography, vegetation, hydrology, soil, fauna and flora, all under the influence of climate, gravity, and the effects of people’s actions.

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From soil not adequately protected from erosive raindrop impact the active movement of eroded soil during a rainstorm is visible manifestation that this adjustment is happening. It may be occurring as a result of a detrimental change in condition of the surface soil, resulting in the ecosystem at that place changing from one level of metastable equilibrium to another, often (but not necessarily always) of lower productivity (Downes 1982). If this is true, then soil erosion itself is not the first cause of destabilization and soil loss. The predisposing cause is a disturbance to the balance among environmental factors, which is often provoked by people’s damage to the soil’s cover and its three-dimensional architecture through inappropriate management. (c) Runoff control measures offer barriers/diversions in the lateral dimension, after water has begun to flow down-slope. They have no effect on the prior impact of erosive raindrops falling in the vertical dimension, which cause splashing of soil particles, hammering of the surface, and its interstitial sealing by filtered-out fine particles, with loss of infiltration capacity through the first few millimeters of the soil surface and rapid saturation, thereby provoking runoff at the air/soil interface. If the porous condition of the surface can be maintained and improved by the interpolation of a permeable organic cover between rain and the soil surface, infiltration rates can remain surprisingly high, with little or no partition of the rainfall into runoff. In this context physical cross-slope works against runoff are ‘blunt instruments’ for keeping the root-zone intact and in place, and not-very-effective substitutes for (though useful complements to) adequate amounts of cover, and good water-stability of the soil aggregates, against damaging effects of rainfall. (d) Soil productivity, as expressed through plant growth, inheres in the dynamic interactions between its physical  chemical  biologic  hydric constituents that define the soil as a rooting environment, and not merely in a reservoir of useful chemicals (Squire, 1990; Wild, 1988). Both plant roots and soil moisture need to be explicitly

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identified as constituents, otherwise they tend to get overlooked when hidden within the ‘biologic’ and ‘physical’ segments. Soil moisture at plant-available water potentials is a key control of soil productivity. The effect of its inadequacy on plant growth-functions is almost immediate – within hours or days rather than weeks – not only after the transpiration rate is diminished by slowed water transfer from soil to roots, but also as stressed plants are revived when rainwater re-enters the desiccated root-zone. A range of pore-sizes enables water to be held in the soil at a range of water-potentials (in kPa) between Field Capacity and Wilting Point which is available to the transpiration stream of plants under the influence of evaporative demand (Allan. and Greenwood, 1999). Pores which are large enough to allow free drainage to below the root-zone allow excess rainwater to pass downwards towards the groundwater. Components of any soil productivity-enhancing strategy should therefore include (a) whatever is necessary to ensure that rainwater can enter the soil without avoidable hindrance, (b) that the soil is maintained in an appropriately-porous physical condition to retain high proportions, or all, of its soil moisture at low tensions, so that plants can retrieve it readily, and (c) that unproductive water loss by direct evaporation from the soil surface is minimized.

3.3 Soil Porosity and Biological Activity Soil porosity based on water-stable aggregates is of primary importance in the joint consideration of both soil productivity and soil erosion, because it moderates the movement of water, gases and roots within the soil. In undisturbed conditions, both the physical processes of wetting, drying and weathering, and – under favourable conditions such as in the forest floor, prairie grasslands, well-managed pastures, and other managed situations rich in organic materials – the effects of biological activity contribute significantly to the build-up and maintenance of soil porosity. This is a consequence of (a) the activities of micro-organisms such as bacteria and fungi in transforming organic materials into humic gums, which cause soil particles

Re-Thinking the Conservation of Carbon, Water and Soil: A Different Perspective

to clump-together into irregularly-shaped aggregates, within and between which are the voids which form useful soil pores; (b) the expansion and subsequent decay of roots which leave tubes of various diameters in which organic materials have been disintegrated and transformed; (c) burrowing activities of mesofauna such as termites, worms, and other soil-inhabiting fauna. Water acceptance is very high, a situation less often found under conventional tillage systems than under residue-based no-till systems. The literature abounds in references to soil organic matter, but the same seldom refer to the parallel necessity for the presence of active soil organisms which can effect its transformations. If the soil is inimical to their activity – too low in organic matter, too hot, too dry, too acid etc. – soil-benefitting transformations do not take place. In the author’s experience in Lesotho, for instance, maize-stalks and leaves that had been ploughed-under 3 years previously re-appeared unaltered (and thus of no benefit to soil structure) when the field was again ploughed because there was no biological activity in the soil. The combination of sufficient organisms, organic materials, water, and nutrients in soil provides for the continuing resurgence of biological activity from year to year. Maintaining a good soil-architecture – a soil with the pore spaces and structural stability that is most conducive to biological productivity – depends on (a) not losing it in the first place, and (b) if it has become damaged, its rapid recuperation. This latter can only be achieved by the microbially-induced transformations of organic matter to form the aggregates that both contain and bound the spaces in the soil, where life goes on, where waters and air move. On the one hand there must be self-perpetuating populations of living organisms to effect such transformations, and on the other there must be permanent or recurrent supplies of organic materials as a sufficient substrate for their activities. These may be variously provided by roots themselves, by the retention of residues from previous crops, and by transporting-in of organic materials from elsewhere as raw or composted additions. In the majority of agricultural situations (in cropland, pasture-land, forest-land, range-land), the key factors for avoiding (rather than controlling) runoff and erosion are surface cover and soil porosity. Both depend on living organisms, plants and other soil inhabitants, and their proper husbandry, which thus

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contributes to extending the useful life of carbon in complexes within the ecosystem and to preventing its premature return back to the atmosphere.

3.4 Tillage and the Loss of Soil Pores Ploughs and discs and even hand hoes (Shaxson, 1999) through their capacity to shatter aggregates, excessively aerate the soil leading to high rates of organicmatter oxidation, and can cause severe subsurface compaction. Tillage provokes not only gross physical alteration to soil-architecture but also results in accelerated oxidation of dead organic matter by soil organisms, with untimely release of respired carbon dioxide back to the atmosphere (Mrabet et al., 2001; Reicosky, 2001; Stewart, personal communication). Exposure of unprotected soil to direct solar radiation also results in breakdown of the complex organic materials which give coherence and stability to soil aggregates, resulting in collapse of soil architecture, loss of pore-spaces and densification of the soil. This tillage-induced oxidation can quickly negate the carbon-accumulating effects of photosynthesis, thereby short-circuiting and hindering the possibility of any subsequent processes of carbon sequestration by more-profound immobilization. Repeated oxidation induced by tillage has often also resulted in net depletion of earlier-accumulated soil organic matter (as in chernozems in C. Asia, and other ‘over-worked’ soils across the world. The process occurs more rapidly in warmer climatic zones of the world. In different situations some or all the carbon fixed by plants may therefore not persist beyond the first year in the soil – next year’s tillage can result in loss of some/most/all the carbon fixed by photosynthesis in the previous year. The benefits of rotational covercrops and/or rotational pastures in improving soil conditions can be more or less completely undone again by any tillage which subsequently actively incorporates them back into the soil (Hudson, 1981). Loss of pore spaces from the soil, whether due to compaction, collapse, pulverization, oxidation of organic matter, or interstitial sealing, represents loss of useful voids in the soil matrix, hindering or even

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preventing exploration by root-hairs and root-tips, expansion of maturing roots, movements of water and gases and, from the moist boundaries of these spaces, the absorption of nutrients (McGarry, 2002). These spaces are comparable with rooms in a building: all the important activities take place within the voids, not within the structural materials themselves. Their loss is comparable to the effects of demolishing a building: the mass of the rubble, glass, steel beams and other construction-materials is the same, but the value of those materials has vanished because the useful spaces of the architecture have disappeared. Loss of pore-space in the soil diminishes the value of the physical component of productivity. This damage to soil architecture, at the surface and below, results in quicker saturation of affected soil horizons and an increased likelihood of early onset of runoff. The improvement of soil by getting organic matter back into the profile is best achieved by soil-inhabiting organisms. Their energy comes free of charge and their actions tend, directly or indirectly, to improve the porosity of the soil. Heavy farm equipment’s physical effects cannot emulate, nor even simulate, organisms’ biological effects in improving soil condition.

3.5 Towards Sustainability – Prolonging the Usefulness of Resources Throughout the history of agriculture, soil conditions have been modified by people’s activities, and native vegetation widely substituted by other plants capable of producing higher yields and/or different plant products of greater use to people. Ideally, the substituted systems of use and management should be at least as stable and biologically sustainable in the face of the range of anticipated recurrent weather conditions as the native ecosystems they have supplanted. The GAMMA Project of the universities in Montreal defined ‘conservation’ as ‘prolonging the usefulness of resources’ (Downes, 1978). In the context of this paper, resources of carbon, water and of life itself (expressed in the forms of e.g. soil-inhabiting plants and organisms) can properly be included in the development of this capacity. Schrödinger indicated that, “metaphorically, the most amazing property and capacity of life is its ability

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to move upstream against the flow of time” (Lovelock, 1988) with the capacity to assemble complex energyrich materials against the otherwise opposite entropic tendency of breakdown to simpler units, which is accompanied by dissipation of energy as time progresses. This capacity of life provides a common thread which interconnects both concepts and dynamic aspects of ‘ecosystems’, ‘soil health’, ‘resilience’ (of both soils and plants), ‘sequestration and combination of carbon’, ‘self-recuperation capacity’ (of ecosystems and their living components), and ‘sustainability’. Improvement of the soil as a rooting environment leads to more carbon-capture by plants, increasing the cycle of mutual benefit. It also favours retention of this carbon within the soil, and further prolongs its usefulness by increasing the net amounts which are subsequently sequestered in relatively-immobile forms. The concomitant sustaining of soil quality and of waterholding and water-transmitting capacities can greatly diminish the volumes of surface flow of excess water, and hence most or all losses of organic-matter particulate fragments in erosional runoff. Better management of landscapes therefore contributes significantly not only to increased stability and productivity (of water and of plants) in agricultural and other ecosystems (Haigh and Gentcheva-Kostadinova, 2002; Shaxson and Douglas, 2004) but also, and simultaneously, to lessening the quantities and detrimental effects of carbon dioxide gas in the global atmosphere. Managing an organic-rich agriculture ensures the regular addition of organic materials to the soil surface, by crop residues and cover-crops, manures etc. These protect the soil surface, provide food for the soil organisms and raw materials for transformation by them, and keep humic materials already within the soil in conditions that are shielded from ultra-violet solar radiation, which is capable of breaking chemical bonds in organic molecules. Results from unirrigated residue-based No-till (NT) systems for a range of crops in Brazil (Landers, 1998), from mulching experiments in many situations – (e.g. on young tea in Malawi [Tea Research Stations, 1963] and from NT wheat in central Italy [Pisante, personal communications] and Morocco [Mrabet, 2002] in areas of annual rainfalls ranging from about 2,000 mm to 300 mm), show that improved surface-cover conditions – by diminishing direct insolation of the soil surface – prolong the usefulness of both rainwater and carbon in the soil. This enabled longer duration of early

Re-Thinking the Conservation of Carbon, Water and Soil: A Different Perspective

growth of young plants of tea (a perennial crop), and of duration of grain-filling of durum wheat (an annual crop), by delaying the onset of growth-inhibiting moisture stress when rainless conditions set in during and at the end of a rainy season (Shaxson and Douglas, 2004). To obtain the full carbon-capturing effects of ‘notill’ agricultural systems, it is essential, as a first task, to bring the soil to good condition for water-acceptance (possibly involving subsoiling etc. to break any earlier induced compacted layers at or below the surface). Then the retention of crop residues in the field becomes the means of ensuring net additions of organic matter, which are gradually and progressively incorporated by the soil biota into the soil matrix of the root zone and below. Such systems have the characteristics of (a) continually adding to the soil surface the raw materials for biological transformation into soil organic matter and (b) reducing rates of oxidation of the same organic matter by precluding unnecessary mechanical disturbance of the soil and thereby avoiding its excess oxygenation. These aspects of the continuity of biological activity and of self-recuperation over time are fundamental to the sustainability of chosen land uses. These results were formerly achieved by ‘resting’ the soil (allowing/enabling biological self-recuperation of the soil) between phases of intense production. In traditional tropical agricultural systems, ‘bush fallow’ periods of maybe as long as 50 years were used in long rotational cycles with crops and pastures (Wrigley, 1961; Critchley et al., 1992). Given today’s increasing pressures of population, many small-holders’ declining farm size and persistent poverty, a major challenge is to achieve the same degree of sustainability by simulating fallows’ restorative effects very much more quickly. Even in rotational copping systems which involve tillage, three options (preferably used together, for synergy) can be outlined:  Increase the soil’s biological capability for recuperation – assist more organic activity.  Reduce the time during which the soil suffers damage – rotate crops at shorter rather than longer intervals (Hudson, 1981).  During the period of suffering damage, reduce the severity of its impact: use equipment, pasturemanagement etc., which is least-damaging to soil in optimum root-favourable condition, in preference to practices capable of causing adverse mechanical disturbance (Shaxson, 1993).

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3.6 Soil as a Renewable and Self-Renewing Resource Many consider soil to be, in practical terms, a nonrenewable resource. This perception relates to the slowness with which a soil’s deep parent materials are weathered into root-usable materials. However, in situations where organic matter and organisms accumulate on and within the upper horizons of the soil, the rooting-zone is enriched and increased (Shaxson, 1981). Further, it is possible that organic acids moving down from the surface may raise the rate of ‘weathering’ of mineral particles and so liberate nutrient ions within the root-zone. Soil can be thus be said to be formed top-down as a recurrent process, almost independent of deep weathering (Wild, 1988) (Fig. 1). Based on this understanding, soil restoration and development can be actively implemented in agricultural systems, more rapidly than with unimproved fallow periods, by using residue-based zero-tillage systems, as in Brazil (Derpsch, 2001; Landers, 2001a). By the same token, former thinning of soil-depth by

Fig. 1 Progressive top-down formation of the rooting environment through plants and other soil-inhabiting organisms contributing and transforming organic matter, combining with the effects of chemical weathering in the development of a soil. (Kasupe, Malawi, T.F. Shaxson)

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erosion may also be reversed by improved forms of soil management that encourage the accumulation of organic matter in the soil surface layer. In different farming situations in a number of countries in the tropics and sub-tropics this is dramatically achieved also with the occasional/rotational use of e.g. Mucuna spp. as a dense clambering smother-crop across field-surface (Shaxson, 1999; Stocking, 2003). This provides excellent control of weeds, thick and complete cover to the soil, and large quantities of nitrogen-rich biomass whose fallen leaves produce excellent litter beneath, as a food-source for the soil biota, leading to increase in the soil’s content of organic matter. In other situations, various legumes such as Tephrosia vogelii and Vetches (Vicia spp.) are used to achieve similar improvements in soil quality (Shaxson and Barber, 2003). This accumulation of carbon, with associated improvements in the physical and chemical qualities of a soil, is generally accompanied by a rising potential for increased productivity.

3.7 A Biological Definition of Soil Many scientists include soil biology as a defining characteristic of a soil. A weathered layer that has no soil is called regolith, or sediment if it has been transported across the environment. A true soil is a biologically constructed zone at the interface between rock and atmosphere. It is reasonable, therefore to argue that soil should be valued more for the dynamics and diversity of its living components, which benefit plantproduction, than only for non-living pedological characteristics such as its arrangement of horizons, range of mineral-particle sizes, and/or parent materials. Arguably, society might may take better care of soil if it were considered less as an inorganic physical unit of mineral particles, air, water and nutrient ions that happens to contain life, but more descriptively as a living system, a complex and dynamic subsurface ecosystem of diverse living organisms (including plant roots), non-living organic matter, and biologicallytransformed organic/humic products, which inhabits, modifies and interpenetrates an inorganic mix of mineral particles, air, water and nutrient ions, and which changes dynamically over the fourth dimension of time.

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4 Land Husbandry Influences 4.1 Effects of Good Land Husbandry Principles of good land husbandry (Shaxson, 1993; Downes et al., 1997) are shown in Table 1a,b. In the field they are well illustrated by an increasingly large number of Brazilian farmers, for instance, who have developed integrated residue-based farming systems with minimal soil disturbance whose total area has grown from around 1,000 to more than 14 million ha in 30 years (Pieri et al., 2002). These systems specifically pay attention to improving the protection and sustainability of soil productivity (Fig. 2). They combine commercial crops, legumes and cover-crops with no-tillage and direct drilling in rotational systems of farming. The principles of soil protection and improvement may be extended to managed pasture-based systems also. Benefits include, among others: greater yields and their stability in the face of unpredictable vagaries of weather/rainfall; improved conditions of soil architecture accompanied by greater water-holding capacity; lowered costs of production; release of space and time for diversification of crops and of people’s activities; much-reduced soil erosion and surface runoff; more-reliable and increased stream flow; reduced floods and related infrastructure damage; reduced costs of maintenance of rural roads and of watertreatment for urban consumption (Landers, 1998, 2001; Mrabet et al., 2001). Increased attention to prolonging the usefulness of carbon (in organisms and organic matter) on and in the soil is being shown, by e.g. the Brazilian experiences with residue-based NT systems, also to have positive hydrological effects – via benefits to soil porosity as well as to water storage, not only as soil moisture but also to groundwater. The results are (a) to diminish the duration of, and damage to plant production by, infrequent but significant periods of drought; (b) to make streamflow more regular and prolonged, whose corollary is to lessen the frequency, duration and severity of floods, following uncommon amounts of rainfall (Landers, 1998). Conventional soil bulk-density measurements alone may not adequately explain observed positive improvements in water function because soils under residuebased NT systems are also penetrated by scattered but significant large diameter wormholes and root tubes which may not be adequately sampled.

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Table 1a Better land husbandry – altering some technical perceptions Newer view Older view 1. Chief causes for concern are (a) decline of land’s in-situ productive potentials, and (b) insufficiency of soil moisture. 2. Improving and managing soil to ensure optimum rainwater absorption and retention will have more sure and widespread effects on plant production than only constructing physical cross-slope works to catch or direct runoff water and soil already on the move. 3. Accelerated runoff and erosion are foreseeable ecological processes, and consequences of other aspects of land degradation. 4. Post-erosion yields at any site after erosion are closely related to the quality of soil which still remains in situ.

5. Rainfall’s erosivity can be minimised by breaking the force of large raindrops by ensuring some form of cover over the soil surface. 6. Soil’s erodibility is increased or diminished over time by effects of management of the soil. 7. More intensive use of land at a particular site – such that it (a) improves soil architectural conditions by favouring soil-organic transformations and minimising tillage-damage, and (b) increases density, duration and frequency of cover over the soil – can improve rather than diminish conservation-effectiveness of the particular use. 8. Increased production of plant parts – with improvements in soil architectural conditions, and in the amounts of cover over the soil – is an effective way of achieving conservation of water and soil as a consequence of better husbandry within the farm production system. 9. Because the land system is dynamic, maintaining its capacity to continue producing what we want requires its active and conservation-effective management over time, at the same time as any re-allocations of land uses and imposition of any necessary physical works. 10. Solving problems of low productivity and of erosion and runoff requires an inter-disciplinary approach to match the inter-relatedness of the problems’ causes. etc.

1a. The primary cause for concern was with quantities lost of soil particles and water. 2a. It was commonly assumed that cross-slope physical conservation works would result in significant increases in yield, by holding back soil, water and nutrients in narrow bands across the slope. 3a. Accelerated runoff and erosion were visualised as primary active causes of land degradation. 4a. It was generally assumed that decline in yields post-erosion could be related closely to quantities of water, soil particles and plant nutrients lost in the erosion process. 5a. Erosivity of rainfall was usually implicitly assumed to be an unalterable feature of each rain event. 6a. Erodibility of a soil series was assumed to be an inherent characteristic of that series. 7a. If at a particular site the land use was ‘too intensive’ for the Land Use Capability classification of that site, it was recommended to reduce the land use intensity until it matched that permitted for that Class.

8a. It was usually insisted that soil conservation be done/implemented before yields could rise.

9a. It was implied that land would be least subject to erosion when its uses are allocated across the land in accordance with maps of ‘Land Use Capability Classification’, and treated with types and layouts of physical and biological conservation measures. 10a. It was assumed that soil conservation required a mono-disciplinary specialist approach, independent of other specialisations, and needing separate institutional arrangements. etc.

Table 1b Better land husbandry – altering some socio-economic perceptions Newer view Older view 11. Farm families have their own observations and perceptions about land degradation, and other views of the reality than those of non-farm agriculturists/specialists: they should be allowed to judge what is best in their situation. 12. The rural community, and the development of its abilities to manage its own environment, is the most appropriate focus of development assistance. 13. Resource-poor small farmers have considerable knowledge about their environments, and make rational decisions about allocation of their resources within the ‘envelopes’ of constraints within which they make those decisions; the challenge is to lessen constraints and improve the shape of the ‘envelope’.

11a. Specialists’ perceptions of the land degradation problems and solutions were presumed to be the correct ones: outsiders should judge what is best.

12a. Land conservation, production and economic efficiency have usually been proposed as the primary foci for development assistance. 13a. It was implicitly, or even explicitly, assumed that small resource-poor farmers are by nature conservative, irrational and ignorant of good land use; the task was to change farmers’ rationality.

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Table 1b (continued) Newer view 14. Rural families ultimately decide what will be done on the land, and whether it would be in their interests to change according to recommendations; resource-poor small farmers are more vitally concerned than any outsider to maintain their lands’ productivity in both the short and long term. 15. To get conservation-effective agriculture improved, it is important to start from where people are now, assist them to do better what they are already trying to do, and remove constraints that inhibit their doing better. 16. A community, and the land it occupies and uses, is the optimum focus for village planning, and for integrating inputs of various ‘disciplines’. 17. ‘Participation’ signifies technical advisers participating with farm families in helping people to identify and rank their most important problems, to decide what do about them, to implement decided actions, and to monitor the outcomes. 18. Advisory workers should be promoters of dialogue and of two-way information-transfer, catalysts of interactions, and facilitators of interchange and of farmers’ well-informed actions. 19. Until they have proved themselves to the satisfaction of individual farmers, technical advisers have very low credibility at the outset of their interactions with farm families. etc.

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Older view 14a. Governments assumed that they decide what would be done on the land, as they assumed they had a greater long-term concern to maintain productivity and halt land degradation than do small farmers with (supposedly) short-term time-horizons. 15a. Adoption of recommended changes and innovations were promoted as being essential for getting agriculture moving. 16a. The topographic catchment/watershed, with the people it contains, was stated to be the logically optimum unit for planning, and for demonstrating the effects of technical recommendations. 17a. ‘Participation’ was commonly taken to mean ‘the people participating in implementing plans’, devised by outsiders, which are considered good for them.

18a. Extension workers were trained as demonstrators and one-way transmitters of information to farm families, in a process of transfer of technology’. 19a. Technical advisers armed with scientific knowledge assumed themselves to be 100% credible from the outset. etc. (Downes et al., 1997).

4.2 Effects of Poor Land Husbandry

Fig. 2 From this farmer’s own comparative field trial, the clod of soil on the right represents the farm’s soil conditions after many years of conventional tillage with disc equipment and removal of crop residues. After less than 5 years of no tillage plus direct-drilling through the retained crop residues, the clod on the left shows the major improvements in organic-matter content and in soil porosity that have been achieved as a result. (Ponta Grossa, Brazil, T.F. Shaxson)

By contrast, poorly managed systems – which by overgrazing, fire or excessive tillage, for example, allow or encourage breakdown of the complex compounds of carbon (‘Stage-1 carbon-loss’) in porous soil aggregates – thus pre-dispose the land to lose yet more carbon, as particles of litter and other organic remains, in subsequent processes of erosion and runoff (‘Stage-2 carbon-loss’). Concentrating attention only on the Stage-2 carbonloss (as in ‘erosion-control’ work) fails to take sufficient account of the effects the preceding Stage-1 loss of carbon from the ecosystem. This sidelines the very serious and far-reaching consequences of poor land husbandry, which include the increasing exposure of the soil surface, decline in soil-structural stability, diminution of soil porosity, lowering of productivity, and consequent increases in occurrence and severity of runoff, erosion, and water stress in plants (Fig. 3).

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5 Conclusions The points discussed above appear to accord more closely with agro-ecological realities in the field than do some of those provided by the earlier common paradigm which had been widely accepted for so long. There are, therefore, a number of implications which should alter the balance of emphases in research, training and advisory work, as well as in the policy framework within which they would more-effectively foster better land husbandry leading to biologicallysustainable land uses.

5.1 Implications for Research

Fig. 3 Stage-2 carbon loss has occurred due to erosion, following Stage-1 carbon loss via oxidation of organic matter and associated degradation of soil architecture, compounded by compaction due to conventional disk-tillage, over only a few years. (São João de Aliança, Brazil, A.W. Bell)

4.3 The Need for Better Land Husbandry All farmers who depend on the land try to look after it (‘husband’ it) in one way or another – some better, some worse. Land degradation and loss of productivity (of both plants and of water) is a usual and widespread result of poor land husbandry. Writings about land husbandry in English (e.g. Shaxson et al., 1989; Chinene et al., 1996) are paralleled by those in French (Roose E., 1996). Improvements in land husbandry are necessary to move from the ‘poor’ condition (still all-too-common) to the ‘good’ condition, and then to sustain it. It should aim to assist the plants and other organisms of the chosen agro-ecosystem to optimize between themselves the dynamic relations between the physical  biologic  hydric  chemical components of the soil’s productivity, aided by farmers’ decisions and actions.

Through reading potentially relevant technical literature and reinterpreting the basic research data which is reported there (as well as re-examining one’s own understanding of field experiences), it may be found that much of the detail needed to fill in the picture of sustainable organic-rich agriculture, as sketched above, already exists. However, additional experimentation may be needed to disentangle the real effects of improved soilmoisture conditions in the three dimensions of space and the fourth dimension of time from those of erosion control itself, with respect to their comparative effects on plant growth. There are indications that some of the plant-growth benefits attributed to ‘erosion control’ are in fact attributable to benefits of additional soil moisture due to the measures used, such as crossslope trash lines ‘fanya juu’ terraces, and conservation banks, where runoff has accumulated locally along upslope sides of the banks, and thus had more time to soak in than where runoff had been diverted along a cross-slope shallow gradient (Hudson, 1992; Hellin and Haigh, 2002; Shaxson, 1999). Research is needed to determine, in specific situations of cropland, pasture, rangeland, forest land, what proportion of declining production of biomass (at a constant, not rising, input-cost) is due to (a) insufficiency of plant nutrients (as commonly supposed) and/or to (b) root-impedance and soil-moisture deficiency following loss of soil porosity by whatever cause.

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The contention that the difference between soil conditions before and after erosion provide a better explanation for yield-difference than the quantity of soil eroded’ needs investigation, with a view to resolving the uncertainty about the ‘grey box’ of soil loss/yield loss relationship (Stocking and Tengberg, 1999; Stocking et al., 2002). In view of the great need to extend plant growth into rainless periods and dry seasons, emphasis in plantbreeding may be directed to selection for root systems better capable of exploring soils for stored moisture that can freely enter the transpiration stream.

5.2 Implications for Training and Advisory Work While the difficulties of implementing such an approach and strategy may be considerable in various socio-cultural and agro-climatic situations, notably in small-farmer and subhumid and dryland areas, the ecological principles remain valid in all situations. The challenge is to assist farmers to devise appropriate means of putting them into harmonious practice – using the resources of rainwater, soil, organisms, organic materials, and the energy available to themselves as farmers – to their better advantage and in ways that are simultaneously productive, sustainable and conservation-effective. This implies the need for training advisory staff in the principles and practice of better land husbandry, in both its agro-ecological and socio-economic aspects, building on, expanding and, where necessary, remoulding knowledge they already have so as better to fit those realities. It is important to appreciate and show the two-way linkages between components at microscale (root hairs, soil pores, bacteria, ecosystems etc.) and those at macro-scale (weather, landscape, land use systems, institutions, etc.). A key need in such training is to match the ecology of agro-environmental situations being considered with the ecology of disciplines that teaching staff need to deploy for their training activities. Moving to organic-rich systems of agriculture with much-improved soil-water relations greatly reduces the hazard of soil erosion at a given place, because the soil is better protected against raindrop damage and

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is more porous and absorptive. Therefore the technical ‘erosion-hazard’ class of a particular land unit – commonly assigned VIII-I from ‘least safe’ to ‘most safe’ (Shaxson et al., 1977) can be up-graded (e.g. from hazard-class IV to III, etc.), indicating greater flexibility of safe use and a wider range of suitable land-use types which could safely be allocated. By this means the ‘marginality’ of lands which are increasingly being brought under tillage by small resource-poor farmers can be modified by improving their organic quality and reducing their hazards of being eroded out of production. This would help to resolve the dilemmas encountered when attempting to classify land as ‘non-arable’ which is already covered with people already making arable use of it, as is often the case on steeplands in the tropical regions (Shaxson, 1999).

5.3 Implications for Policy Within governments, relatively independent departmental policies which at present are variously aimed at ‘soil conservation’, ‘the environment’, ‘agriculture’ etc. need an overarching agro-ecological policy framework that interconnects the concerns they have in common. These include soil conditions, biomass production, erosion, flooding, sedimentation, irrigation, and related matters, which are all linked through their common features of sustainability of organic potentials, soil porosity and water use efficiency. The basis for such a framework should be a concern to encourage, develop and support systems of land use and soil management that are actively ‘pro-biotic’ with respect to life in the soil, at the same time discouraging those approaches of the past, which have allowed soils to degrade by default and inappropriate management, and which, in this sense, have turned out to be somewhat ‘anti-biotic’.

5.4 A Valid Perspective The perspective outlined in this paper appears valid for two main reasons: (a) It suggests some credible alternatives to some commonly-held but doubtful assumptions.

Re-Thinking the Conservation of Carbon, Water and Soil: A Different Perspective

(b) It offers a positive approach to enhancement of resources’ value, agricultural sustainability, environmental improvement, and carbon sequestration,