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

Pot experiments were carried out to evaluate the response of rice to Sesbania rostrata green manure N as compared to urea fertilizer N under flooded conditions. After growing S. rostrata for 21 days with a (15)N-labelled N source, the labelled Sesbania was applied to wetland rice as a green manure and the uptake of (15)N 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 (15)N content of the soil and shoots and roots of rice was determined and (15)N balances established. The total N content of the shoots and roots of rice was determined by a non-tracer method. The percentage recovery of (15)N from shoot material which was derived from urea N was more than twice that from S. rostrata. The recovery of (15)N from the pots receiving both green manure and urea was low, and not significantly different from that recovered from the green manure treatment. As much as 64.5-73.5% and 40.1-41% of the (15)N remained in the soil which had received green manure or urea, respectively. The overall recoveries of (15)N varied between 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 total 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 of 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 (15)N pot(-1) and 39.10 mg (15)N pot(-1)) and 23.2% and 23.2% of the residual labelled N in the two urea treatment (i.e. 19.55 mg (15)N pot(-1)), respectively.