Biological Nitrogen Fixation in Soybean: Mechanisms, Benefits, Sustainability, and Future Prospects

Soybean is a globally important legume crop which fulfills most of its nitrogen (N) requirement through Biological Nitrogen Fixation (BNF) in symbiosis with Bradyrhizobium species, thereby reducing dependence on synthetic fertilizers and supporting more sustainable production systems. This review synthesizes current knowledge on the mechanism, capacity, and regulation of BNF in soybean, including nodule formation, nitrogenase activity and response to soil and environmental conditions. The evidence shows that BNF can provide a substantial share of the crop’s N uptake, although high-yielding systems frequently experience the “N gap”, which is a difference between a higher crop demand and a lower N supplied from BNF and existing soil reserves. This can be partially managed with strategies like inoculation, co-inoculation, re-inoculation or judicial application of N. This review further highlights the advances in microbial inoculant technologies, plant growth-promoting rhizobacteria (PGPR), soybean breeding and genetic engineering aimed at improving BNF stability, efficiency and capacity across different soil environments. Overall, the maximization of soybean BNF has strong potential to reduce synthetic fertilizer use, improve yield and seed quality, and enhance the economic and environmental sustainability of soybean-based systems.