GRAIN LEGUMES AND GREEN MANURES AS PRE-RICE CROPS IN NORTHEAST THAILAND .1. LEGUME N-2-FIXATION, PRODUCTION AND RESIDUAL NITROGEN BENEFITS TO RICE

Two field experiments were conducted on farmers' fields in Northeast T hailand to examine the potential of several green manure and short-dur ation grain legume species as pre-rice green manures. The performance of species mixtures was also investigated. Experiments were conducted under rainfed conditions both with and without recommended application s of lime, P and K fertilizers. N-15-isotope methods were used to esti mate N-2-fixation in the legumes and N supplied by the legume residues to a following rice crop. When provided with lime, P and K fertilizer , all legumes grew well and fixed 65-85% of their N, amounting to 59-1 02 kg N ha(-1) wihin 90 days. The short-duration groundnut and a mixed Sesbania rostrata and multi-purpose cowpea treatment produced over 4 tonnes ha(-1) residue dry weight which contained 135-150 kg N ha(-1). In addition groundnut yielded almost 2 tonnes ha(-1) of grain, whilst the intercrop yielded 1 t ha(-1) of fresh pods of MCP, only 35% less t han the sole MCP which only produced 41 kg N ha(-1) in its stover. Ric e dry weight increases of over 50% were found after several of the leg ume treatments, and N-15-based estimates indicated up to 18 kg N ha(-1 ) was directly supplied by the legume stover to the rice. Without fert ilizer application residue yields of the S. rostrata + multi-purpose c owpea mixed crop were poor (580 kg ha(-1)) and N-2-fixation contribute d only 57% of the N accumulated (20 kg N ha(-1) fixed). The applicatio n of P and K fertilizers was found to approximately double residue yie lds with the combined addition of lime more than doubling these yields again, resulting in 88% of the legume N derived from N-2-fixation (84 kg N ha(-1)). The following rice recovered 9-17% of the added legume N and the yield response in rice reflected the differences in legume y ield. Application of P,K and lime resulted in a fivefold increase in t he amount of legume N recovered by rice, indicating the importance of alleviating nutrient deficiencies through fertilizer application if gr een manures are to be used successfully in Northeast Thailand.