Objectives The optimum annual nitrogen (N) application rate in the summer soybean system was studied in order to achieve high N use efficiency and yield of wheat-soybean rotation system in Ili valley.
Methods A field experiment was conducted in Yining County, Xinjiang Uygur Autonomous Region from 2016 to 2018. The N rates in previous wheat season were 0, 104, 173 and 242 kg/hm2, recorded as WN0, WN1, WN2 and WN3 in turn; On each wheat N rate treatment, three N rates for the following summer soybean were setup as 0, 69, 138 kg/hm2, and recorded as SN0, SN1 and SN2 in turn. Since the 20th day after germinating, soybean plant samples were collected every 10 days for the measurement of dry matter weight and N content. At harvest, the yield and yield components were investigated, and the nitrogen use efficiencies were calculated.
Results N application rate in the previous wheat season significantly affected the dry matter and N accumulation of summer soybean. When the N rate in wheat was in range of 0 to 173 kg/hm2, applying N in summer soybean could increase the dry matter accumulation and plant N uptake, and the lower N rate in wheat caused better effect of N application in soybean. When the N rate in wheat season was 173 kg/hm2, nitrogen application in soybean season increased yield, and the yield of SN1 treatment was the highest; when the N rates in wheat season were 104 and 242 kg/hm2, those between SN1 and SN2 were not significantly different. The nitrogen recovery rate, the agronomy utilization efficiency and partial productivity of nitrogen fertilizer in summer soybean decreased with the increase of nitrogen application rate.
Conclusions In the winter wheat-summer soybean rotation system in Ili Valley, when the N application rate in pre-wheat season is 173 kg/hm2, the suitable nitrogen rate in the following soybean season is 69 kg/hm2, which could meet the nitrogen requirement in all the growing stages and the high yield formation, and the nitrogen use efficiency of summer soybean.
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