Isotopic Nitrogen and Carbon Allocation Among Soybean Plant Parts Under Impact of Bradyrhizobium japonicum Strains

Understanding how plants regulate nitrogen (N) and carbon (C) allocation among their organs under adverse environmental and climatic conditions remains a significant challenge, despite its direct impact on the value of plant residues and agricultural products. Therefore, this study aimed to examine the dynamics of N and C through their stable isotope ratios in two soybean varieties of differing maturity groups (Merlin and Laulema) inoculated with various nitrogen-fixing Bradyrhizobium japonicum bacterial strains. The contents of N and C as well as their isotopic ratios in soybean plant parts were analyzed at full-flowering (R2) and full-maturity (R8) stages. The results demonstrated overall compatibility between soybean varieties and selected B. japonicum strains, resulting in up to 32 nodules per plant; however, significant variation in root nodule numbers was observed. From a physiological perspective, both the soybean variety and the strain of nitrogen-fixing bacteria significantly influenced nitrogen stable isotope ratios across different plant organs, including roots, shoots, stems, pods, and seeds, with similar trends in δ(15N) variation among plant parts observed in both varieties. In contrast, the main differences in carbon stable isotope composition were observed among varieties less affected by the amendment strategy. N content was higher in roots and shoots during flowering and declined by twofold in roots and fivefold in aboveground biomass at maturity, reflecting extensive nitrogen remobilization to support seed formation. From an agronomic perspective, the highest yields were achieved by the inoculated soybean Merlin, with more than 3 t ha−1. However, the positive effects of symbiosis can improve yields in less productive varieties like Laulema, making them comparable to those of more productive varieties. Soybean inoculation not only influenced the isotopic redistribution within the plant but also proved to be an effective practice for increasing seed N content, with strain AGF78 producing the highest number of nodules and a significantly high amount of nitrogen in seeds, followed by SEMIA5079, the least effective being RF10.