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Chemical N fertilizers are usually applied to modern rice ( Oryz.a sativa L.) cultivars, but yield responses are obtained from green manuring as well. To be considered as a viable alternative to chemical N fertilizer in tropical Asia, however, green manures must tolerate the ephemeral waterlogging that precedes rice cultivation. Experiments on an Aerie Tropaqualf soil during three early wet seasons determined green manure N accumulation in 60 d or less, and rice grain yield response to it. In the first experiment (1983), cowpea (Vigna unguiculata (L.) Walp.) and Sesbania cannabina ((Retz.) Poir.) were grown 24, 36, and 48 d before incorporation. Nitrogen accumulation was greatest (79 kg N ha-1 ) in 48-d S. cannabina. Accumulation in cowpea was apparently depressed by waterlogging. Rice grain yield response to green manure N was unaffected by location of green manure cultivation (in situ vs. transported from a nearby field). In the second (1984) and third (1985) experiments, Sesbania sp. were flooded during the last 25 d of growth to assure early wet-season waterlogging. Mean N accumulation in flooded S. rostrata (Brem. and Obrem.), which nodulates on its stem, was 90 kg ha-1 in 48 d and 164 kg ha-1 in 60 d. Nitrogen accumulations in nonflooded S. rostrata were 128 kg ha-1 in 48 d and 198 kg ha-1 in 60 d. Accumulation in a Nepalese S. cannabina cultivar averaged 84% of that in S. rostrata. Rice grain yield response was a function of green manure N regardless of species or age. Both S. rostrata and S. cannabina tolerate waterlogging and accumulate sufficient N in less than 60 d to increase rice yields more than 2 Mg ha-1 • R.R. Furoc and M.A. Dizon, Multiple Cropping Dep., and E.P. Marqueses, Agronomy Dep., Int. Rice Res. Inst. (IRRI),

For many Asian farmers, inorganic N fertilizer for rice (Oryza sativaL.) is costly. Biologically fixed sources of N are being reexamined as alternatives to inorganic N fertilizer. Land that is fallow during the first 5 to 7 weeks of the wet season, before monsoon rains intensify, can be used to accumulate N in a green manure. This experiment determined quantities of N that well‐adapted tropical legumes accumulated in 20 to 45 days and rice yield responses to green manuring with legumes grown for 45 or fewer days. The experiment, repeated in 1981, 1982, 1983, and 1984, was conducted on a fine, mixed, isohyperthermic Aeric Tropaqualf at IRRI, Los Baños, Philippines. Rice was transplanted in factorial arrangements of inorganic N rates (0 and 80 kg N ha 1) and green manure growth durations. In 1981, 1982, and 1983, mungbean [Vigna rudiata(L.) Wilczek] was the green manure, and growth durations were 0 (clean fallow), 20, 30, and 40 days. In 1984, cowpea [Vigna Unguiculata(L.) Walp.] was the green manure, and durations were 0,25,35, and 45 days. Average N accumulation in the oldest green manure was 83 kg N ha 1. Mean rice yield response to 80 kg inorganic N ha 1was 1.0 Mg ha1. Mean response to the oldest green manure was 2.0 Mg ha−1. The study demonstrated that a fast‐growing tropical legume can accumulate more than 80 kg N ha−1in 45 days and that rice yield responses exceeding 2 Mg ha−1are possible from green manure incorporation.

Intercropping of upland rice (Oryza sativa L.) with short‐duration grain legumes has shown promising productivity and resource use efficiency. To better understand intercrop relationships, we used aboveand underground partitions, residue removal, and plant removal to investigate the interactions between upland rice (120‐d crop duration) and mungbean [Vigna radiata (L) Wilczek, 65‐d crop duration]. Treatments were evaluated during two rainy seasons on an unfertilized Typic Tropudalf at Los Baños, Philippines. Nitrogen uptake by intercropped rice (33.4 and 41.1 kg N ha−1) approximated that of sole rice (35.4 and 38.1 kg N ha−1). Intercropped rice yielded 73 to 87% of sole rice and intercropped mungbeans yielded 59 to 99% of sole mungbean. Root barriers did not affect rice N uptake or dry matter accumulation prior to the maturity of the mungbean, but reduced N uptake, dry matter, and grain yields substantially by the time of rice harvest. Sole rice with every third row removed at mungbean harvest had N, grain, and dry matter yields similar to the intercropped rice with every third row occupied by the legume. Sole rice with every third row vacant during the entire growing season yielded similarly (2.6 Mg h−1) to sole rice (2.3 Mg h−1) and intercropped rice (2.0 Mg h−1). There was no evidence that N transfer from the legume to the rice increased N availability to rice above that expected with a sole rice crop with the same planting scheme. Rice yield compensation in the intercrop was apparently due to the increased soil volume for N extraction and increased aerial space available after mungbean harvest.