Nodulation Is Maintained and Salinity Tolerance Enhanced in Two Soybean Cultivars Inoculated With Sinorhizobium fredii Under Brackish Water

Salinity is an increasing threat to agriculture, particularly in coastal regions affected by seawater intrusion and sea-level rise. This study evaluated the halotolerance and symbiotic potential of Sinorhizobium fredii USDA 208 in two soybean cultivars (includer and excluder) under three salinity levels—low (freshwater), moderate (brackish water), and high (seawater). The results demonstrated that S. fredii not only tolerates but also exhibits enhanced growth under moderate salinity. Nodulation was successfully established when salinity and inoculation occurred simultaneously. Nodulation was also maintained when salinity occurred after the inoculation, particularly in fresh and brackish water. Root development declined with increasing salinity, but the includer cultivar showed better root system architecture plasticity in brackish water, while the excluder cultivar exhibited higher shoot and root biomass across salinity levels. Bacterial inoculation improved shoot phosphorus uptake, the potassium: sodium ratio, and carotenoid retention, particularly in the includer cultivar, suggesting an enhanced physiological tolerance to moderate salinity. Inoculation also resulted in higher shoot nitrogen and maintained pigment content. Using a seawater recipe provides a better understanding of salinity than traditional NaCl-based studies and highlights the role of S. fredii USDA 208 in supporting soybean performance when salts accumulate in coastal agricultural soils.