Managing Nitrogen Sources in Soybean–Rhizobium Symbiosis During Reproductive Phenological Stage: Partitioning Symbiotic and Supplemental N with 15N

Understanding how supplemental nitrogen (N) interacts with biological N2 fixation (BNF) in modern soybean cultivars is essential for designing fertilization strategies that avoid unnecessary N inputs. We investigated N partitioning among soil, fertilizer and symbiotic sources in soybean grown in a greenhouse pot experiment on a tropical Oxisol. Plants were inoculated with Bradyrhizobium and subjected to four N managements: no external N, soil-applied 15N-urea (20 kg N ha−1), foliar 15N-urea (2 kg N ha−1, 0.7% w/v), and the combination of soil + foliar N. Using 15N isotope dilution, we quantified N derived from the atmosphere (NDFA), fertilizer (NDFF) and soil (NDFS) at organ and whole-plant scales, and related these fractions to nodulation, nitrogenase activity and yield. In the absence of external N, NDFA exceeded 97% in all organs, indicating a strong reliance on BNF and efficient internal N remobilization during grain filling, accompanied by higher leaf nitrate reductase activity. Soil and soil + foliar N markedly increased NDFF and NDFS while suppressing nodulation (particularly at V4) and reducing nitrogenase activity, yet they did not improve grain yield or vegetative biomass. Foliar N alone had only modest effects on N partitioning and did not enhance yield. Under these tropical soil conditions, symbiotic fixation and internal N remobilization were sufficient to meet grain N demand, highlighting the limited agronomic benefit and potential ecological cost of supplemental N during reproductive growth.