NITROGEN LOSSES AND LOWLAND RICE YIELD AS AFFECTED BY RESIDUE NITROGEN RELEASE

Synchronizing N supply from incorporated plant residues with N demand of rice (Oryza sativa L.) may increase the plant's N use efficiency and reduce soil N losses. This hypothesis was tested under flooded lowland conditions in a three-season held experiment. Leguminous green manures and rice straw with various lignin to N ratios (L/N) and urea were compared using N release, rice N uptake, N use efficiency, grain yield, and total N-15 balance as criteria. Basally applied urea resulted in high initial soil NH4+. Where residues were applied, exchangeable NH4+-N varied as a function of L/N. Daily N uptake by rice peaked at approximate to 4 wk. Matching of soil NH4+-N with daily rice N uptake was less apparent in urea and Sesbania rostrata Brem. S. Oberm. treatments than in the S. rostrata-rice straw mixture treatment. Nitrogen-15 balances indicated that the mismatch between supply and demand may have caused the measured N losses of 35% from urea and 6 to 10% from S. rostrata applied at 60 kg N ha(-1). Synchronized N supply and rice N uptake resulted in negligible N Loss and increased the portion of applied N remaining in the son, but it did not cause a yield increase. Sesbania rostrata-rice shaw mixture depressed yield and N use efficiency in the dry season when yield potential was high. A residual effect was observed in the S. rostrata-rice straw treatment in the third unfertilized crop, resulting in a 10% increase in grain yield. Apparently, synchronizing soil N supply with N demand by incorporating residues with suitable chemical composition may not immediately increase rice grain yields, but it improves long-term soil fertility.