| To clarify the relationship between symbiotic nitrogen fixation in soybeans and their yield and quality formation, as well as the regulatory effect of nitrogen fertilizer, a split-plot pot experiment was conducted from 2023 to 2024 using Dongnongdou252 as the material. The main plots were soil sterilization (NOR) and control (R) treatments, while the subplots were nitrogen application rates of 0 g/pot (N0), 1.04 g/pot (N1), 2.07 g/pot (N2), and 3.15 g/pot (N3), totaling 8 treatments. The experiment systematically investigated the effects of the combined treatments of soil sterilization and nitrogen application rates on the dry matter accumulation of soybean roots, the length and surface area of lateral roots, the number of nodules, the nitrogen accumulation of plants, grain yield, and protein content. The results showed that in the NOR treatment at the R6 stage, the total dry matter of roots, the length and surface area of lateral roots in N0, N1, N2, and N3 were reduced by 8.93% to 17.82%, 10.02% to 20.90%, and 8.55% to 25.58%, respectively, compared with the R treatment. No nodules were formed in the NOR treatment, and the nitrogen accumulation in the NOR treatment at N2 was reduced by 11.32% to 29.67% compared with the R treatment. The yields of the NOR treatment at N0, N1, N2, and N3 were reduced by 18.83% to 24.00% compared with the R treatment, and the protein content of the grains was significantly reduced. Increasing nitrogen fertilizer application could not compensate for the loss of nitrogen accumulation, yield, and protein content in grains caused by the loss of symbiotic nitrogen fixation capacity due to soil sterilization. With the increase in nitrogen application rates in both NOR and R treatments, the dry weight of roots, the length and surface area of lateral roots, nitrogen accumulation, yield, and nitrogen fertilizer agronomic efficiency all showed a trend of increasing first and then decreasing, with the highest values at N2. In the R treatment at N2, soil nitrogen, fertilizer nitrogen, and symbiotic nitrogen fixation accounted for 1.58% to 21.44%, 30.44% to 33.82%, and 48.10% to 54.60% of the nitrogen accumulation, respectively. It can be seen that symbiotic nitrogen fixation promotes the growth of soybean roots and plays an important role in yield and quality formation that cannot be replaced by nitrogen fertilizer. To achieve high yield, quality, and efficiency in soybeans, it is necessary to fully utilize the potential of symbiotic nitrogen fixation. |