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

Abstract

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 N2 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 N2 in the shoot was calculated using the 15N natural abundance method. The common bean demonstrated low N2 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 N2 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 N2. 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 N2. It can be concluded that the low symbiotic N2 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 N2 and, in contrast to common bean, left a high level of NO3-N remaining in the soil after cropping.