Enhancing Salinity Tolerance in Black Soybean Genotypes Through Synergistic Rhizobacterial Inoculation

Soil salinity is a major environmental constraint that limits soybean productivity by disrupting plant growth, nutrient uptake, and biological nitrogen fixation. Beneficial microorganisms have emerged as a sustainable approach to improve crop tolerance to saline conditions. This study aimed to identify the most salt-tolerant black soybean phenotype and evaluate the individual and combined effects of symbiotic nitrogen-fixing bacteria (SNFB) and plant growth-promoting rhizobacteria (PGPR) under salinity stress. This study consisted of two stages, the first identified the most salt-tolerant black soybean phenotype (BS 87, BS 99, and BS 114), and the second evaluated the effects of symbiotic nitrogen-fixing bacteria, plant growth-promoting rhizobacteria (PGPR) co-inoculation, and their consortium under four saline conditions (0, 2, 4, and 6 dS m⁻¹). Microbial inoculation significantly interacted with salinity levels. Under low salinity (0–2 dS m⁻¹), SNFB enhanced chlorophyll index, leaf area, and effective nodulation, whereas PGPR improved plant height, chlorophyll index, leaf area (38.66%), and plant biomass by 52.46%, respectively. Under high salinity (4–6 dS m⁻¹), SNFB–PGPR co-inoculation produced the greatest improvements in plant height, root length, chlorophyll index, leaf area, and plant biomass (37.20%), outperforming individual inoculation treatments. Black soybean phenotypes BS 87 demonstrated superior salinity tolerance, while combined SNFB and PGPR inoculation showed promise for mitigating salinity-induced growth limitations, particularly under severe stress. These findings highlight the potential of rhizobacterial inoculation as a sustainable strategy for improving black soybean performance in saline environments.