Optimal nitrogen application rate and nitrogen requirement characteristics of red kidney bean

Red kidney bean (Phaseolus vulgaris) is one of the most important miscellaneous grain crops in Shanxi Province. However, its nitrogen requirement pattern has not been investigated, which has impeded nitrogen management. This research evaluated the nitrogen requirement pattern of the red kidney bean cultivar 'British Red' in Shanxi Province, China. The field experiments were conducted at Dongyang Agricultural Experimental Station, Shanxi Academy of Agricultural Sciences (central area of Shanxi Province), and at Caojiagou Village in Kelan County (western part of Shanxi Province). Four nitrogen application rates: 0 kg·hm^-2 (N1), 60 kg·hm^-2 (N2), 120 kg·hm^-2 (N3), and 180 kg·hm^-2 (N4), were applied at Dongyang, and five application rates: 0 kg·hm^-2 (N1), 60 kg·hm^-2 (N2), 120 kg·hm^-2 (N3), 180 kg·hm^-2 (N4), and 240 kg·hm^-2 (N5), were applied at Kelan. The accumulation and distribution of biomass and nitrogen uptake in different parts of the plants were monitored during the growth period. The grain yields and nitrogen translocation efficiencies at different nitrogen application rates were also determined. The results showed that there were significant differences in grain yield, biomass, and nitrogen accumulation among different nitrogen application treatments. The grain yield against nitrogen application rates analysis produced an odd peak curve, and the highest grain yields were found in the N3 treatment, which were 2 359.89 kg·hm^-2 and 2 452.26 kg·hm^-2 at the two field experimental sites, respectively. The hundred-grain weight was the main contributory factor to the yield difference. The dry biomass accumulation showed a pattern of "slow-fast-slow" as red kidney bean growth progressed. The highest percentage of grain in the biomass was recorded in the N3 treatment, and reached 49.97% at Dongyang and 47.65% at Kelan, respectively. The nitrogen uptakes by grain in the N4 (Dongyang) and N3 (Kelan) treatments were 0.72 g·plant^-1 and 0.99 g·plant^-1, respectively, which were higher than in the other treatments. In conclusion, the application of reasonable rates of nitrogen improved the translocation of nitrogen from vegetative parts to grain and helped improve nitrogen uptake by grain and its translocation efficiency, which might be the mechanism underlying the grain yield increase. The appropriate nitrogen application rate is 110.36 kg·hm^-2 in the central area of Shanxi Province and 126.31 kg·hm^-2 in the western part of the province, which can be finely adjusted according to the actual soil fertility and variety.