Introduction Contribution of BNF to N balance Beneficial effects of legumes Nitrogen effect Non-N rotational effects Ways to improve BNF in the SAT Host-related aspects Host-variability for nodulation and nitrogen fixation Breeding for increased BNF and nitrate tolerance Management practices Nitrogen Intercropping Tillage Land form Deep sowing Other nutrients Insects Use of inoculants Need for inoculation Competitive and effective strains Factors affecting performance of inoculant strains Yield responses to inoculation. Conclusion and future areas of research Acknowledgements References 30 AbstractSustainable agriculture relies greatly on renewable resources like biologically fixed nitrogen. Biological nitrogen fixation plays an important role in maintaining soil fertility. However, as BNF is dependent upon physical, environmental, nutritional and biological factors, mere inclusion of any NE-fixing plant system does not guarantee increased contributions to the soil N pool. In the SAT where plant stover is also removed to feed animals, most legumes might be expected to deplete soil N. Yet beneficial legume effects in terms of increased yields in succeeding cereal crops have been reported. Such benefits are partly due to N contribution from legumes through BNF and soil N saving effect. In addition, other non-N rotational benefits, for example, improved nutrient availability, improved soil structure, reduced pests and diseases, hormonal effects are also responsible. In this paper we have reviewed the research on the contribution of grain legumes in cropping systems and the factors affecting BNF. Based on the information available, we have suggested ways for exploiting BNF for developing sustainable agriculture in the semi-arid tropics (SAT). A holistic approach involving host-plant, bacteria, environment and proper management practices including need based inoculation for enhancing BNF in the cropping systems in the SAT is suggested. CropNitrogen fixed (kg ha -

Sustainable agriculture relies greatly on renewable resources like biologically fixed nitrogen. Biological nitrogen fixation plays an important role in maintaining soil fertility. However, as BNF is dependent upon physical, environmental, nutritional and biological factors, mere inclusion of any Nrfixing plant system does not guarantee increased contributions to the soil N pool. In the SAT where plant stover is also removed to feed animals, most legumes might be expected to deplete soil N. Yet beneficial legume effects in terms ofincreased yields in succeeding cereal crops have been reported. Such benefits are partly due to N contribution from legumes through BNF and soil N saving effect. In addition, other non-N rotational benefits, for example. improved nutrient availability. improved soil structure. reduced pests and diseases. hormonal effects are also responsible. In this paper we have reviewed the research on the contribution of grain legumes in cropping systems and the factors affecting BNF. Based on the information available, we have suggested ways for exploiting BNF for developing sustainable agriculture in the semi-arid tropics (SAT). A holistic approach involving host-plant, bacteria. environment and proper management practices including need based inoculation for enhancing BNF in the cropping systems in the SAT is suggested.

A b s t r a c tThe quantity and patterns of net mineralization of soil nitrogen (N) were studied in Vertisols under different cropping systems in the semi-arid tropical areas. Eight cropping systems were selected; three contained pigeonpea (PP), one contained PP and cowpea (COP), and two contained chickpea (CP) as legume component crops, one included sequence cropping with nonlegumes during the rainy and postrainy seasons, and one system was kept fallow (F) during the rainy season and sown to sorghum (S) during the postrainy season. Cropping systems with PP as a component crop increased mineralizable N(No) content two-fold in the soil compared with fallow + sorghum (F+S)-F+S system. The N mineralization rate constant (k) was not significantly affected by previous cropping history of the soil; however, a numerically higher rate constant was observed in the COP/PP intercrop, followed by sequential S+safflower (SF) system as compared to the other soils. Mineral N accumulation curves for six soils were more accurately described by the exponential model than the linear model. The active N fraction (No/Ntot %) varied between 8 and 16% for different systems and a direct relationship was observed between No/Ntot and total N for the soils under diverse cropping systems.

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Responses to inoculation with Nl-fixing bacteria were studied" in relation to genotypic differences in pea~1 millet, effect of nitrogen levels, and FYM additions in India. In some experiments, inoculation increased mean grain yield up to 33% over the uninoculated control, whereas in the remaining 11 experiments'there was no significant increase. Increased grain yields~ > 10% over the uninoculated controls were observed in' 46% of the experiments with AzospiriJ/um lipojerum (18.7% average increase) and with Azotobacter chroococcum (13.6% average increase). Yield increases were nil or reduced in three experiments with Azos. /ipojerum and four experiments with A'ztb. chroococcum. I,n two experiments continued inoculation fot two