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NITROGEN UPTAKE AND RECOVERY FROM N FERTILIZER AND LEGUME CROPS IN WETLAND RICE MEASURED BY 15N AND NON-ISOTOPE TECHNIQUES

Intensive rice-based cropping systems rely on nitrogenous fertilizer for optimum grain production and legume crops could be used as an alternative nitrogen (N) source for rice. We investigated the fate of N applied to dual cropping wetland rice in the form of legume residue and 15N labeled fertilizer. In 2001–2002, hairy vetch and broad bean accumulated 131 and 352 kg N ha−1 of which 41 and 78% was derived from N2 fixation. In 2002–2003, hairy vetch accumulated 64 kg N ha−1 and broad bean accumulated 320 kg N ha−1 of which 21 to 24% was derived from hairy vetch and 31 to 82% N was derived from broad bean by N difference and 15N-natural abundance method. Our results reveal that hairy vetch and broad bean can supply 50–100% of N required for intensive wetland rice and can be a viable alternative N source to enhance soil fertility.