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 an d reduce soil N losses. This hypothesis was tested under flooded lowla nd conditions in a three-season held experiment. Leguminous green manu res 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 yiel d, and total N-15 balance as criteria. Basally applied urea resulted i n high initial soil NH4+. Where residues were applied, exchangeable NH 4+-N varied as a function of L/N. Daily N uptake by rice peaked at app roximate to 4 wk. Matching of soil NH4+-N with daily rice N uptake was less apparent in urea and Sesbania rostrata Brem. S. Oberm. treatment s than in the S. rostrata-rice straw mixture treatment. Nitrogen-15 ba lances 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. ros trata applied at 60 kg N ha(-1). Synchronized N supply and rice N upta ke resulted in negligible N Loss and increased the portion of applied N remaining in the son, but it did not cause a yield increase. Sesbani a rostrata-rice shaw mixture depressed yield and N use efficiency in t he dry season when yield potential was high. A residual effect was obs erved in the S. rostrata-rice straw treatment in the third unfertilize d crop, resulting in a 10% increase in grain yield. Apparently, synchr onizing soil N supply with N demand by incorporating residues with sui table chemical composition may not immediately increase rice grain yie lds, but it improves long-term soil fertility.