IAEA-TECDOC-1053 XA9848493_:P?2, The use of nuclear techniques in the management of nitrogen fixation by trees to enhance fertility of fragile tropical soils Results of a co-ordinated research project organized by the Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture INTERNATIONAL ATOMIC ENERGY AGENCY The originating Section of this publication in the IAEA was: Soil and Water Management and Crop Nutrition Section International Atomic Energy Agency " Wagramer Strasse 5 P.O. Box 100 A-1400 Vienna, Austria THE USE OF NUCLEAR TECHNIQUES IN THE MANAGEMENT OF NITROGEN FIXATION BY TREES TO ENHANCE FERTILITY OF FRAGILE TROPICAL SOILS IAEA, VIENNA, 1998 1AEA-TECDOC-1053 ISSN 1011^289 ©IAEA, 1998 Printed by the IAEA in Austria November 1998 The IAEA does not normally maintain stocks of reports in this series. However, microfiche copies of these reports can be obtained from IN IS Clearinghouse International Atomic Energy Agency Wagramerstrasse 5 P.O. Box 100 A-1400 Vienna, Austria Orders should be accompanied by prepayment of Austrian Schillings 100,- in the form of a cheque or in the form of IAEA microfiche service coupons which may be ordered separately from the INIS Clearinghouse. FOREWORD Increasing population pressure on arable land in the tropics is leading to replacement of traditional shifting cultivation with unsustainable systems such as those with shorter-duration fallow periods, sedentary agriculture on small-scale land holdings and expansion onto marginal areas, causing soil degradation and deterioration of the environment. Some 65% of tropical soils are fragile and lose fertility rapidly under cultivation; within as little as two years, yields can fall to a small fraction of those obtained in the first season after clearing native vegetation, a' trend that is particularly marked in marginal soils. Loss of soil organic matter accompanies cultivation, with concomitant decrease in desirable soil physical properties and increased erosion. Nutrient depletion through leaching and run-off exacerbates the problem. Depletion of soil nutrients can be arrested by addition of chemical fertilizers, but financial considerations preclude this as a solution for the majority of farmers in developing countries. Therefore, it is imperative to explore alternative integrated soil and nutrient-management approaches with minimum or zero risk of environmental degradation. In this context, the use of trees for rehabilitating soils and maintaining fertility is particularly attractive. In dry areas, trees are often able to exploit soil water not available to shallow-rooted plants, and their perennial nature obviates the need for annual planting, thus reducing erosion. In the humid tropics leguminous and actinorhizal trees have been incorporated into agroforestry systems, thereby enhancing their contributions due to fixation of atmospheric nitrogen. Depending on the type of agroforestry system and management practices, a substantial portion of this fixed nitrogen can be transferred to soil and to arable crops. Trees also provide fodder and fuel-wood, components of great importance to the subsistence farmer. Despite broad recognition of the value of nitrogen-fixing trees in agroforestry, there are few reliable data on the magnitude of fixed nitrogen contributions or on the potential to manage and maximize nitrogen fixation by trees. There has been little effort to identify species and provenances of superior nitrogen-fixing potential. Even for the well studied Leucaena leucocephala there has been no critical determination of the amounts of nitrogen fixed compared with nitrogen taken up from soil, of its nutritional constraints to nitrogen fixation or, in particular, of the effects of management practices, e.g. frequency and severity of pruning, on fixation and transfer of nitrogen to soil or to associated crops via prunings. Changes in soil characteristics due to the presence of trees have also received scant attention. The Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture initiated a Co- ordinated Research Project in 1990 on The Use of Nuclear and Related Techniques in Management of Nitrogen Fixation by Trees for Enhancing Soil Fertility and Soil Conservation in Fragile Tropical Soils. This project was underpinned by extensive experience in the use of 15N-labelled fertilizer in quantifying nitrogen fixation by food and pasture legumes; the isotope-dilution technique, recognized as the most accurate mode of quantifying fixation, was developed at the IAEA and has been used profitably in Co-ordinated Research Projects that have focused on aspects of sustainability in agriculture in developing countries in which food security is most under threat. The effort to improve understanding of the potential contributions of trees to soil fertility and to increased production of food for human consumption, which are timely in terms of current global needs, grew out of a realization at the IAEA that a comprehensive multi-disciplinary approach was needed to optimize results from Co-ordinated Research Projects. S.K.A. Danso and G. Keerthisinghe were the Project Officers during 1990-1995 and 1997, respectively. The IAEA thanks the OPEC Fund for International Development for its generous financial support. The TECDOC was edited for publication by A.R. J. Eaglesham. EDITORIAL NOTE In preparing this publication for press, staff of the IAEA have made up the pages from the original manuscripts as submitted by the authors. The views expressed do not necessarily reflect those of the IAEA, the governments of the nominating Member States or the nominating organizations. Throughout the text names of Member States are retained as they were when the text was compiled. The use of particular designations of countries or territories does not imply any judgement by the publisher, the IAEA, as to the legal status of such countries or territories, of their authorities and institutions or of the delimitation of their boundaries. The mention of names of specific companies or products (whether or not indicated as registered) does not imply any intention to infringe proprietary rights, nor should it be construed as an endorsement or recommendation on the part of the IAEA. The authors are responsible for having obtained the necessary permission for the IAEA to reproduce, translate or use material from sources already protected by copyrights. CONTENTS Summary 1 Management and quantification of nitrogen fixation in Leucaena leucocephala 5 E.Y.Safo Nitrogen fixation improvement in Faidherbia albida 15 O. Toure, M. C. Dasilva, S. Badji, M. Dianda, I. Ndoye, M. Gueye Nitrogen fixation in four dryland tree species in central Chile 23 C. Ovalle, S. Arredondo, J. Aronson, L. Longeri, J. Avendano Nitrogen fixation in Leucaena leucocephala and effects of prunings on cereal yields 33 M. Bekunda Nitrogen fixation and effects of pruning on Gliricidia sepium and Leucaena leucocephala 41 M. de S. Liyanage Evaluation of Frankia and rhizobial strains as inocula for nitrogen-fixing trees in saline conditions 61 F. Y. Hafeez, S. Hameed, K.A. Malik Nitrogen fixation in Acacia auriculiformis and Albizia lebbeck and their contributions to crop-productivity improvement 79 N. Mbaya, K. N'K. Mwange, N. Luyindula Nitrogen fixation by Gliricidia sepium: Decomposition of its leaves in soil and effects on sweet-corn yields 97 A.R. Zaharah, H.A.H. Sharifuddin, R. Anuar, A.R. Bah, K.N'K. Mwange, P. Kathuli, P. Juma Quantitative estimates of uptake and internal cycling of 15N-depleted fertilizer in mature walnut trees 127 S. Weinbaum, C. van Kessel List of Participants 139 Recent IAEA Publications on Soil and Water Management and Crop Nutrition 141 SUMMARY The global population is expected to double within the next four decades, and much of that increase will occur in developing countries in which hunger is already a reality. The need to make arable land satisfy the present demand for food is resulting in briefer fallow periods, with the commissioning of bush-fallow land that has not fully regenerated since its previous use for crop production - thus a vicious cycle is initiated that results in ever decreasing productivity per unit area. When soil fertility rapidly declines, erosion often increases with far-reaching deleterious environmental consequences that may be impossible to reverse. In such circumstances, crop productivity may not be significantly improved by applications of fertilizers, even if available to the subsistence farmer, because levels of organic matter that sustain fertility have reached critically low values. The pressing need to reverse these trends demands that land use systems be developed to achieve sustainable agricultural production with minimum environmental degradation. Soil fertility should not be sacrificed for the short-term production of crops. The recycling of renewable resources is an important factor in the maintenance of soil organic matter and hence soil fertility. Furthermore, the nutrients that are removed from the system in the harvested crop must be replaced. Where organic sources of nutrients are available, new systems must be designed and developed to exploit them. In this context, leguminous or actinorhizal trees have a potential role in the rehabilitation of nutrient-depleted soils for food production because of their capacity to fix atmospheric nitrogen. Depending on the type of cropping system involved and management practices employed, a substantial portion of this fixed nitrogen can be transferred to companion crops and to soil.