Intercropping of legumes to contribute nitrogen in low-input upland rice-based cropping systems

Upland rice is generally produced on acid soils of low inherent fertility, with no inorganic nutrients applied. The utility of incorporating grain or forage legumes into intercrops with upland rice was examined with the objective to increase the N availability to rice. Rice N yields, when intercropped with determinate early-maturing legumes, were about the same as with sole rice, while grain yields were between 70 and 100 percent of sole rice yields. The net nitrogen benefit and the rice yield compensation observed were due to the increased soil and aerial space available after the legume was removed. Direct N transfer did not appear to play a significant role in net N availability to rice. Indeterminant cultivars of cowpea and lablab were tested as intercrops with rice to supply green leaf manure to the rice crop. They supplied 21 and 93 kg N/ha to intercropped rice when clipped as green leaf manure. Rice yields in the intercrop were maintained compared to sole rice. N uptake increased 21 to 66 percent. The legumes yielded an additional 1.02 t/ha and 0.5 t/ha grain, respectively. When plowed down at the end of the subsequent dry season, prior to planting the following year's rice crop, the lablab supplied 19.0 t/ha fresh green manure biomass over 100 kg N/ha. The results suggest that a production system based on intercropping rice and indeterminate legumes may substantially improve the sustained yield of upland rice in low external input conditions.