Effects of rhizobial inoculation on soybean nodulation and rhizocompartment bacterial community under different soil conditions

Rhizobial inoculation in soybean is an effective strategy in sustainable agriculture to reduce chemical fertilizer application and to increase crop production. It not only provides nitrogen sources for host plants but also improves the rhizosphere soil environment. However, the inoculation efficiency of rhizobia remains to be improved. In this study, we investigated the nodulation efficiency of Bradyrhizobium and Sinorhizobium strains under different soil conditions and evaluated their impacts on the rhizocompartment bacterial community. We found that inoculation with Bradyrhizobium diazoefficiens UASD 110 increased the number of soybean nodules in acidic soil, while Sinorhizobium fredii CCBAU 45436 was more effective in alkaline soil. However, inoculation with neither strain significantly affected nodulation in neutral soil. Then, we demonstrated that UASD 110 was more competitive in nodulation than CCBAU 45436, which was related to its higher abundance in the rhizosphere. Furthermore, we showed that single inoculation with UASD 110 nor CCBAU 45436 failed to alter the bacterial diversity, these two strains differentially influenced the rhizosphere microbial composition. Finally, we identified the main rhizosphere microorganisms that were affected by these two strains. Our findings revealed that the nodulation capacity of rhizobia and their colonization of rhizosphere and nodules are soil-type dependent, yet its impact on the rhizobacterial community exhibited consistent patterns. These findings provide valuable insights in optimizing rhizobial inoculation strategies to enhance nitrogen fixation efficiency.