NITROGEN UPTAKE AND RECOVERY FROM UREA AND GREEN MANURE IN LOWLAND RICE MEASURED BY 15N AND NONISOTOPE TECHNIQUES (VOL 148, PG 91, 1993)

In the recent past considerable attention is paid to minimize dependen ce on purchased inputs such as inorganic nitrogen fertilizer. Green ma nure in the form of flood-tolerant, stem-nodulating Sesbania rostrata and Aeschynomene afraspera is an alternative N source for rice, which may also increase N use efficiency. Therefore research was conducted t o determine the fate of N applied to lowland rice (Oryza sativa L.) in the form of Sesbania rostrata and Aeschynomene afraspera green manure and urea in two field experiments using N-15 labeled materials. N-15 in the soil and rice plant was determined, and N-15 balances establish ed. Apparent N recoveries were determined by non-tracer method. N-15 r ecoveries averaged 90 and 65% of N applied for green manure and urea t reatments, respectively. High partial pressures of NH3 in the floodwat er, and high pH probably resulted from urea application and favoured l osses of N from the urea treatment. Results show that green manure N c an supply a substantial proportion of the N requirements of lowland ri ce. Nitrogen released from Sesbania rostrata and Aeschynomene afrasper a green manure was in synchrony with the demand of the rice plant. The effect of combined application of green manure and urea on N losses f rom urea fertilizer were also investigated. Green manure reduced the N losses from N-15 labeled urea possibly due to a reduction in pH of th e floodwater. Positive added N interactions (ANIs) were observed. At h arvest, an average of 45 and 25% of N applied remained in the soil for green manure and urea, respectively.