Seasonal N uptake and N2fixation by common and adzuki bean at various spacings

The adzuki bean (Vigna angularis (Wild.) Ohwi and Ohashi) and common bean (Phaseolus vulgaris L.) have a high physiological demand for N. A 2-year field study was conducted to investigate the seasonal change of available soil N and symbiotic N-2 fixation usage. The beans were seeded at two densities, 22.2 plants m(-2) with a row spacing of 0.3 m and 11.1 plants m(-2) with a row spacing of 0.6 m. The amount of fixed N-2 in the shoot was calculated using the N-15 natural abundance method. The common bean demonstrated low N-2 fixation and the ability to accumulate high levels of soil N. Soil nitrate under the common bean was continually absorbed. The adzuki bean, on the other hand, had a remarkable peak of N accumulation in the early reproductive stage. This was mainly due to N-2 fixation, though the soil nitrate level was high. Narrowing the plant row spacing increased the dry matter yield of both species, but the origin of the increased N differed between the species. For the first 77 DAP in 1999 ( 73 DAP in 2000) the N increase for both beans was due to both soil and atmospheric N-2. At harvest, though, the increase of N in common bean was mainly due to soil N, while that in adzuki bean was mainly due to atmospheric N-2. It can be concluded that the low symbiotic N-2 fixation ability of common bean was due to its high soil N uptake ability and constant N accumulation, which enabled an efficient soil N absorption. Adzuki bean absorbed N mainly for a short period and depended more on symbiotically fixed N-2 and, in contrast to common bean, left a high level of NO3-N remaining in the soil after cropping.