15N balance studies in a rice - green gram - rice cropping system

Field exptriments were conducted to study the effect of extraneous application of green manure (Sc bania aculeata) and urea on 15N balance in a rice-green gram-rice cropping sequence. Respor,se to green manuring and also to urea-N up to 80 kg ha−1 was observed in the first crop of the sequence. No carry-over benefit of green manuring could be observed in the succeeding crops. Urea-N was better utilized than the green manure-N whether applied alone or in combination up to a total of 80 kg N ha−1. Higher nitrogen use efficiency was associated with lower rate of N application. Combined use of green manure and urea resulted in higher nitrogen use efficiency up to 80 kg N ha−1. N loss through leaching and ammonia volatilization were small. Succeeding rice utilized more of residual fertilizer N/GM-N as compared to green gram. Crop recovery by the sequence went up to 31–34% while about 26–28% remained in the soil after the sequence. A total of 37–43% of the applied N could not be accounted for in the study. Green manuring encouraged fertilizer N recovery by crops and higher soil N retentivity.