| In order to explore the influence rules of the postponement of nitrogen topdressing on the amounts of fertilizer nitrogen, symbiotic nitrogen fixation, total nitrogen accumulation, and yield of soybeans. In 2022 and 2023, a split-plot experiment design was adopted, with three nitrogen topdressing treatments, namely 0 kg.hm-2 (N0), nitrogen topdressing of 120 kg.hm-2 at the initial flowering stage (R1N120), and nitrogen topdressing of 120 kg.hm-2 at the initial podding stage (R3N120), set as the main plots, and two soybean varieties, Xindadou 27 and Xinnongdou 2, as the sub-plots. The influence rules of the postponement of nitrogen topdressing on the number of root nodules, dry weight of root nodules, nitrogenase activity, leghemoglobin content, nitrogen accumulation amount, and yield were studied. The results showed that compared with R1N120, R3N120 significantly increased the number of root nodules and the dry weight of root nodules (P＜0.05); it significantly increased the nitrogenase activity of root nodules and the leghemoglobin content, and the performance of each treatment was N0＞R3N120＞R1N120. Compared with R1N120, R3N120 significantly increased the total nitrogen accumulation amount, symbiotic nitrogen fixation amount, fertilizer nitrogen uptake amount, and agronomic nitrogen use efficiency (NUE); the total nitrogen accumulation amounts of Xindadou 2 and Xinnongdou 2 increased by 8.98% and 8.96% respectively (P＜0.05); the symbiotic nitrogen fixation amounts increased by 7.65% and 12.87% respectively (P＜0.05); the fertilizer nitrogen uptake amounts increased by 42.39% and 26.23% respectively (P＜0.05). The average yields of Xindadou 27 and Xinnongdou 2 under the R3N120 treatment reached 3 921.60 kg.hm-2 and 4 161.92 kg.hm-2 respectively, which were 7.42% and 9.34% higher than those of R1N120 respectively (P＜0.05). In conclusion, the postponement of the nitrogen topdressing period from the R1 stage to the R3 stage significantly increases the yield of soybeans and the nitrogen use efficiency. This is the result of the postponement of nitrogen topdressing, which increases the number of root nodules and the dry weight of root nodules, and increases the symbiotic nitrogen fixation amount of root nodules and the total nitrogen accumulation amount in the plants. |