Uptake of N-15 by wetland rice in response to application of N-15-labelledSesbania rostrata and urea

Pot experiments were carried out to evaluate the response of rice to Sesban ia rostrata green manure N as compared to urea fertilizer N under flooded c onditions. After growing S. rostrata for 21 days with a N-15-labelled N sou rce, the labelled Sesbania was applied to wetland rice as a green manure an d the uptake of N-15 from this substrate was compared to that from labelled urea. Rice was cultivated twice in the same pots. The rice was grown for a period of 49 days in each case, separated by a period of 21 days when the soil was allowed to dry. The N-15 content of the soil and shoots and roots of rice was determined and N-15 balances established. The total N content o f the shoots and roots of rice was determined by a non-tracer method. The p ercentage recovery of N-15 from shoot material which was derived from urea N was more than twice that from S. rostrata. The recovery of N-15 from the pots receiving both green manure and urea was low, and not significantly di fferent from that recovered from the green manure treatment. As much as 64. 5-73.5% and 40.1-41% of the N-15 remained in the soil which had received gr een manure or urea, respectively. The overall recoveries of N-15 varied bet ween 86.5% and 94.4%. At the second harvest, the oven-dry weight of shoots was significantly (P<0.05) higher in green-manure treated pots, but the tot al N content did not differ significantly. Labelled N remaining in the soil after amendment with the green manure was much more available to the rice crop than that remaining after the addition of urea-N. The total recovery o f labelled N (shoots plus roots) amounted to 65.5% and 74%, respectively of the residual labelled N in the two S. rostrata treatments (i.e. 19.55 mg N -15 pot(-1) and 39.10 mg N-15 pot(-1)) and 23.2% and 23.2% of the residual labelled N in the two urea treatments (i.e. 19.55 mg N-15 pot and 39.10 mg N-15 pot(-1)), respectively.