Legume decomposition and nitrogen release when applied as green manures totropical vegetable production systems

For legume green manures (GM) to be effective, environmentally sound N sour ces for horticultural crops in the tropics, their N release must be in sync hrony with crop N demand. Decomposition and N release of surface applied (m ulch) or incorporated soybean [Glycine max (L.) Merr.] and indigofera (Indi gofera tinctoria L.) GM were studied in six field studies conducted at thre e locations in Taiwan and the Philippines between 1993 and 1995. Litter bag s and inorganic N soil samplings were used in order to understand tomato (L ycopersicon esculentum Mill.) crop responses to GM N. Resulting soil N cont ents were compared with a control (no GM, no fertilizer). The N content of 60 to 74 d soybean GM varied between 110 and 140 kg N ha-L and that of indi gofera between 5 and 40 kg N ha(-1). Nitrogen-15-labeled soybean GM was tra ced in the soil and in organic matter fractions (humic acids, calcium humat es, humins) in one of the field studies. Soybean and indigofera decomposed rapidly, losing 30 to 70% of their biomass within 5 wk after application, d epending on GM placement, season (wet vs. dry), and location. Soil nitrate contents increased corresponding to GM N release at all locations and seaso ns, with a maximum increase of 80 to 100 kg NO3-N ha(-1) with incorporated soybean. The peak N release occurred 2 to 6 wk after GM application in two of the three locations, and 5 to 8 wk in the third location. The apparent d ecline of GM N release at all locations and seasons 8 wk after application was only partly caused by tomato N uptake. At tomato harvest, 30 to 60% of the GM N-15 was found in the soil, and was found mostly in humins. Comparab le N release dynamics across seasons and locations suggest a possible N fer tilizer substitution by incorporated soybean GM for basal N application and first side dressing to tomato. With respect to season and location, GM N s hould be supplemented with N fertilizer starting after 8 wk to ensure optim al tomato yields.