Bradyrhizobium japonicum S36 and Sinorhizobium fredii S65 Improve Soil Nutrient Status and Promote Soybean Yield Formation in Cereal–Soybean Intercropping Environments

Rhizobial inoculation has the potential to improve soybean N nutrition and productivity in cereal–soybean intercropping systems, yet the performance of different strains across contrasting field environments remains insufficiently understood. In this study, six soybean rhizobial isolates were initially screened under greenhouse sand-culture conditions, and two promising strains, Bradyrhizobium japonicum S36 and Sinorhizobium fredii S65, were subsequently evaluated in maize–soybean and sorghum-soybean field environments. Relative to the uninoculated control, both strains were associated with higher plant N status, greater chemical soil fertility, increased dry-matter accumulation, and higher grain yield. B. japonicum S36 and S. fredii S65 increased grain yield by 23.8% and 18.6%, respectively, in the maize–soybean system and by 49.2% and 40.4%, respectively, in the sorghum-soybean system. S. fredii S65 was more strongly associated with increases in two-seeded pod number and total seed number, whereas B. japonicum S36 showed broader advantages in chemical soil fertility, biomass accumulation, 100-seed weight, grain yield, and grain Ca, Fe, and Mo concentrations. Multivariate analyses further revealed coordinated associations among rhizobial inoculation, chemical soil fertility, plant N status, biomass accumulation, pod formation, and grain yield. Overall, both strains showed promising agronomic potential, with B. japonicum S36 exhibiting a more comprehensive response profile. These findings provide field-based evidence for the development of resource-efficient cereal–soybean intercropping systems and have potential relevance to sustainable food production, improved agricultural resource-use efficiency, and climate-resilient agriculture, corresponding to the United Nations Sustainable Development Goals of Zero Hunger, Responsible Consumption and Production, Climate Action, and Life on Land.