NITROGEN UPTAKE AND RECOVERY FROM UREA AND GREEN MANURE IN LOWLAND RICE MEASURED BY N-15 AND NONISOTOPE TECHNIQUES

In the recent past considerable attention is paid to minimize dependence on purchased inputs such as inorganic nitrogen fertilizer. Green manure 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 to 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 established. Apparent N recoveries were determined by non-tracer method. N-15 recoveries averaged 90 and 65% of N applied for green manure and urea treatments, respectively. High partial pressures of NH, in the floodwater, and high pH probably resulted from urea application and favoured losses of N from the urea treatment. Results show that green manure N can supply a substantial proportion of the N requirements of lowland rice. Nitrogen released from Sesbania rostrata and Aeschynomene afraspera 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 from urea fertilizer were also investigated. Green manure reduced the N losses from N-15 labeled urea possibly due to a reduction in pH of the floodwater. Positive added N interactions (ANIs) were observed. At harvest, an average of 45 and 25% of N applied remained in the soil for green manure and urea, respectively.