Effects of Contact and Non-Contact Application of Exogenous Nitrogen on Nodulation and Nitrogen Fixation of Soybean

Nitrogen (N) fertilizers can promote soybean growth, nodulation, and nitrogen fixation to a certain extent. However, excessive nitrogen application inhibits the nitrogen fixation capacity of soybean nodules. In this study, three experimental materials were used to investigate the direct and indirect effects of localized exogenous nitrogen (Ammonium Nitrate, NH4NO3) on nodule nitrogen fixation in soybean. Three nitrogen supply methods were applied: bilateral nodulation dual-root soybeans, unilateral nodulation dual-root soybeans, and upper- and lower-layered soybeans. The root nitrogen accumulation of direct contact with exogenous nitrogen reached 72.61 mg/plant, 30.59 mg/plant, and 88.48 mg/plant, respectively, and its nitrogen accumulation ability was higher. Exogenous nitrogen inhibited nodule growth and nitrogen accumulation. Nodule development and nitrogenase activity were regulated both directly and indirectly by exogenous nitrogen, with a more pronounced inhibitory effect observed in the roots directly exposed to nitrogen. Experiment I demonstrated that the number and dry weight of nodules on the nitrogen supply side decreased by 35.04% and 40.00%, respectively, while the difference was not significant on the non-nitrogen supply side. Furthermore, the nodule system exhibited a substantial buffering effect on exogenous nitrogen. In Experiment I, no significant differences were observed in the number, dry weight, or nitrogenase activity of nodules on the non-nitrogen-supplying side. The number and dry weight of nodules in Experiment II decreased by 61.55% and 35.91%, respectively. The specific nitrogenase activity (SNA) and acetylene reduction assay (ARA) also decreased by 32.28% and 67.20%, respectively, showing significant differences. In Experiment III, the number and dry weight of nodules in the upper layers decreased by 23.70% and 15.12%, respectively. Furthermore, significant differences in nitrogenase activity were detected, indicating that the nodules exposed to exogenous nitrogen spontaneously initiated the nitrogen regulation mechanism. This partially offsets the inhibitory effect on the nitrogen fixation function of nodules on the indirectly exposed side. This study revealed that exogenous nitrogen supply significantly affected the growth efficiency and nodule nitrogen fixation function of soybean plants by regulating nitrogen absorption and resource allocation. The use of deep unilateral fertilization can ensure the nitrogen fixation capacity of nodules and nitrogen accumulation in soybean plants and provide theoretical support for improving nitrogen use efficiency and realizing scientific fertilization.