Biological nitrogen fixation in trees in agro-ecosystems
Biological Nitrogen Fixation for Sustainable Agriculture
Introduction Measurement of BNF in trees The total nitrogen difference (TND) method The acetylene reduction (ARA) assay The ~SN isotope methodology The natural ~SN abundance or 6 ~SN method Nitrogen-15 fertilizer enrichment methods Selection of reference plant Rate, formulation, time and frequency of ~SN fertilizer addition Sampling of plants Effect of remobilized N Effect of litter turnover Ureides as a measure of BNF Nitrogen fixation potential of trees The occurrence of nodulation Genotypic
... ariation in BNF Plant and Rhizobium factors The age factor The Rhizobium component Specificity in nodulation Effectiveness Management practices and environmental effects on BNF Effects of management Environmental factors Season Soil moisture Temperature Soil nitrogen Soil toxicity 178 Danso et al. Nutrition Mycorrhizas Conclusion and future research needs References Abstract acetylene especially methods. promising NFTs). Various The integration of trees, especially nitrogen fixing trees (NFTs), into agroforestry and silvo-pastoral systems can make a major contribution to sustainable agriculture by restoring and maintaining soil fertility, and in combating erosion and desertification as well as providing fuelwood. The particular advantage of NFTs is their biological nitrogen fixation (BNF), their ability to establish in nitrogendeficient soils and the benefits of the nitrogen fixed (and extra organic matter) to succeeding or associated crops. The importance of NFTs leads to the question of how we can maximise or optimize their effects and how we can manage BNF and the transfer of nitrogen to associated or succeeding plantings. To be able to achieve these goals, suitable methods of measuring BNF in trees are necessary. The total nitrogen difference (TND) method is simple, but is better suited for low than high soil N conditions. The reduction assay (ARA), although sensitive and simple, has many technical limitations for NFTs, and the estimates of BNF have generally been very low, compared to other For NFTs, the ~SN techniques are still under development, but have already given some results (e.g., has been used to measure large genetic variability in BNF within different factors affect BNF in trees. They include the age of trees, the microbial component, soil moisture, temperature, salinity, pH, soil N level and plant nutrient deficiencies. Some of the factors, e.g. temperature, affect the symbiosis more than plant growth, and differences in the effects of these factors on BNF in different NFT genotypes have been reported. These factors and research needs for improving BNF in trees are discussed.