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Nitrogen fixation associated with 16 rice (Oryza saliva) varieties or lines of short and long durations, grown in a flooded field, during the 1984 wet season and 1985 dry season was measured by a simple and rapid acetylene reduction assay. Assaying the plants at heading stage for 3 consecutive days was found suitable for determining varietal differences in associative Ns fixation. Significant differences in acetylene-reducing activity (ARA) per plant and unit plant dry weight among short-and long-duration varieties were found. Plant ARA significantly correlated with root dry weight, and total plant drY weight at heading, and total plant dry weight at harvest during both dry and wet seasons. Plant ARA also showed significant partial correlation coefficients with root dry weight, keeping the shoot dry weight constant which indicates that the root biomass (dry weight) is an important plant character affecting ARA. Generally, ARA per plant was proportional to plant biomass. Although IR50, a shortduration variety had a lower biomass, ARA was similar to that of some long-duration varieties with a high biomass. On the other hand, Peta and Rodjolele, which are tall varieties produced a high biomass but low ARA. IR42, a long-duration variety. had the highest ARA per plant and per unit weight of plant. Harvest index (H!), nitrogen harvest index (NHI), and nitrogen remobiIization efficiency (NRE) of the varieties were also determined in the wet season. NHI was significantly different among long-and short-duration varieties and NRE differed among the short duration varieties. No correlations between ARA per plant and HI, NHI, and NRE were found. The possibility of using rice plant characters as criteria for screening varieties for their ability to stimulate biological NI fixation is discussed.

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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 NO∐3−N and allow BNF while limiting total N accumulation in the DTW vegetation for use as GM.