In tropical rice lowlands, vegetation during the dry-to-wet season transition (DTW) facilitates in situ recycling of N from soil or legume biological nitrogen fixation (BNF) to wet-season flooded rice. As little is known, we examined the fates of soil N and BNF N in DTW vegetation in a 2-yr study on a Philippine Tropudalf using 15N-labeled residues produced and soil-incorporated in situ. During DTW, Sesbania rostrata (Bremek. & Oberm.), mungbean [Vigna radiata (L.) R. Wilczek var. radiata], weeds, and a weed-free fallow were subplots in dry-season main plots of weedy, weed-free, and frequently tilled fallows. Rice yield and N uptake were not influenced by dry-season fallows, which did influence soil N and BNF N in DTW vegetation and, therefore, the amount of soil N lost, removed in a product or recycled. Build-up and decline of soil NH4−N occurred within 5 wk of residue incorporation, before significant N uptake by rice. Rice yield and N uptake responded to greater recycled DTW N; N uptake averaged 103 kg ha−1 with 208 kg S. rostrata N ha−1, 79 kg ha−1 with 62 kg mungbean N ha−1, 61 kg ha−1 with 41 kg weed N ha−1, and 44 kg ha−1 with no residue N. Nitrogen-15 estimates of N recovery by rice (20% of S. rostrata N, 27% of mungbean N, 16% of weed N) were lower than the actual increase in rice N uptake due to residues. High proportions of residue N remained in soil, but N loss of 32% was estimated for S. rostrata N. As green manure (GM) N is ineffective beyond the first few weeks of incorporation, incorporating much legume N to flooded rice wastes valuable BNF N. Unmet rice N demand beyond early crop stage is better supplied with fertilizer N synchronized with rice N demand. A mixture of native weeds and GM legume is likely to prevent build-up of soil NO3−N and allow BNF while limiting total N accumulation in the DTW vegetation for use as GM.
Joint contribution from the International Rice Research Institute (IRRI), the Nitrogen Fixation and Tropical Agricultural Legumes Center (NifTAL Center, Univ. HI, 1000 Holomua Ave., Paia, HI 96779), and the International Fertilizer Development Center (IFDC, P.O. Box 2040, Muscle Shoals, AL 35662).