Biological N-2 fixation and residual N benefit of pre-rice leguminous crops and green manures

Rice yields in soils of low soil organic matter may benefit from preceding leguminous green manure crops. A pre-rice crop experiment, including ground nut (Arachis hypogaea), mungbean (Vigna radiata), Sesbania (Sesbania rostra ta), and a mixture of Sesbania and multipurpose cowpea ( Vigna unguiculata) was conducted on a characteristic sandy soil of North East Thailand. The S esbania-cowpea intercrop gave a similar total plant biomass as the Sesbania green manure alone (7 t ha (1)) but with the advantage to yield an edible product. The direct economic yield of cowpea was 1.3 t ha(-1) green beans a nd greater than that achieved with groundnut or mungbean. The Sesbania-cowp ea combination also proved to enhance rice yields by 0.8 t ha(-1). The bene fits in rice production were similar to the Sesbania green manure alone but surpassed the yields with the other grain crops or urea fertilizer of 30-6 0 kg N ha(-1). Sesbania dry matter production increased with increasing pla nting density. The resulting variation in plant quality, e.g. lignin, howev er, was low. Rice responses to treatments were more related to the total re sidue N yields than to changes in plant quality. Apart from mungbean (25%) the pre-rice leguminous crops were able to obtain a considerable (>39%) proportion of their N from N, fixation. The green ma nure Sesbania however fixed a larger proportion (79-89%) of its N than the grain crops (25-62%). This led not only to high amounts of N, fixed by Sesb ania but together with a N harvest index of zero yielded a large systems N benefit. With grain legumes this benefit was moderated by the N export in h arvestable products. In the case of mungbean this may even result in effect ive soil N mining. Residue N use efficiency varied between 19-29% and was s imilar to that obtained from a single application of chemical N fertilizer (17-28%). For the farmer the Sesbania-cowpea intercrop option seems thus the most pro mising one not only regarding rice yield benefits but also in terms of soil fertility enhancement and generation of edible products.