Biological nitrogen fixation and residual effects of winter grain legumes in rice and wheat cropping systems of the Indo-Gangetic Plain

Legumes have long been recognized by farmers and scientists alike as builders and restorers of soil fertility. Chickpea, the dominant winter legume in India and Pakistan can potentially fix at least 80% of its nitrogen (N) needs from air (like other legumes) and can acquire up to about 140 kg N ha"1 from air. Lentil, a major winter legume in Nepal and Bangladesh can potentially acquire about 190 kg N ha"1 from air. But the quantities of N2-fixation by these legumes in farmers' fields in these countries are normally much less than half the potential fixation levels, according to experiments using 15N methods. An increase in irrigation facilities, ready and cheap availability of chemical fertilizers, relatively less stable yields of legumes, and government policies favoring cereal production have driven away the legumes from the intensive cropping systems of the Indo-Gangetic Plain. Any significant increase in area under winter legumes in the intensive cereal-cereal cropping system will require a change in this scenario..