Abstract
Studies on biomass production, N accumulation, symbiotic effectiveness and mineralization of cowpea, sunhemp, sesbania and clusterbean in relation to yield of wetland rice were conducted for two years under field conditions. The mean green and dry matter production of shoots of 60-day-old cowpea, sunhemp, sesbania and clusterbean plants were 24 and 6.9, 21 and 5.4, 20 and 5.0 and 17 and 3.8 t ha-1, respectively. The corresponding N additions through these green manures were 113, 110, 108 and 87 kg ha-1, respectively. Sesbania had the most nodules and their highest fresh weight plane -1, followed by cowpea, sunhemp and clusterbean. The symbiotic effectiveness of nodules at 15, 25, 35 and 55 days were 55-100, 60-95, 25-95 and 16-50% in cowpea, sesbania, sunhemp and clusterbean, respectively. All green manures decomposed rapidly and about 40% of the added carbon was lost as CO2 in 7-15 days. The mineralization rate constant of green manures was 0.022-0.013 day-1. A peak in the formation of KCl -extractable NH4+ -N from the soils amended with different green manures was observed between 7-15 days period. There was not much difference in the mineralization of N among different green manures. Green manures were found equally effective and resulted in significantly higher rice yield compared with fallow treatments. Rice yield with green manuring alone was comparable to 120 kg N ha-1; green manuring with 60 kg N ha-1 resulted in yields equivalent to 180 kg N ha-1. These results indicate a supplementation of 120 kg ha-1 of fertilizer N with green manuring in rice.
