Enhanced soybean biomass by co-inoculation of Bradyrhizobium japonicum and plant growth promoting rhizobacteria and its effects on microbial community structures.

Co-inoculation of rhizobia with plant growth promoting rhizobacteria (PGPR) plays an important role in both promotion of nodulation and plant growth of leguminous plants. In this study, rhizobacteria were screened for co-inoculation with Bradyrhizobium japonicumon soybean for their capacity to promote the nodulation under aseptic condition. The obtained rhizobacteria were further screened in soybean-nodulating bradyrhizobia-free soils to evaluate their co-inoculation effects on soybean nodulation, plant growth and rhizosphere soil microbial community structures. Under pot conditions, co-inoculation of either B. japonicum CB 1809 or USDA 110 with Azospirillum sp. gave more benefits in nodulation and plant growth than Bacillus solisalsi did. Under field conditions, Azospirillumsp. co-inoculation with either B. japonicum CB 1809 or USDA 110 gave 32.23 and 16.85% nodulation, and 26.51 and 18.83% nodule dry weight increased over single inoculation of CB 1809 and USDA 110, respectively. In addition, each co-inoculation significantly increased 23.65 and 34.92% seed yield over single inoculation of CB 1809 and USDA 110, respectively, and three to six times seed yield over non-inoculated control.  Denaturing gradient gel electrophoresis (DGGE) and principle component analysis (PCA) results revealed that soybean rhizosphere eubacterial community structures in both pot and field experiments were shifted by plant growth stages but not by bacterial inoculation. In contrast, neither inoculation of tested bacteria nor plant growth stages shifted the rhizosphere soil fungal community structures.   Key words: Soybean, Bradyrhizobium japonicum, plant growth promoting rhizobacteria (PGPR), co-inoculation, nodulation, rhizosphere soil microbial community structures.