Co-inoculation effects of 'Bradyrhizobium japonicum' and 'Azospirillum' sp. on competitive nodulation and rhizosphere eubacterial community structures of soybean under rhizobia-established soil conditions

Bradyrhizobial inoculants used for soybean seed inoculation to maximize the benefit of N₂-fixation should include bradyrhizobial strain with high N₂-fixation rates and ability to compete with the indigenous rhizobial populations. In this study, co-inoculation of plant growth promoting rhizobacteria (PGPR) 'Azospirillum' sp. with either of 'Bradyrhizobium japonicum' CB 1809 or USDA 110 increased shoot and root dry weight of soybean over non-inoculated control under pot condition with no indigenous soybean nodulating bradyrhizobia. Moreover, competition for nodulation and the effects on rhizosphere soil eubacterial community structures by using single or co-inoculation of 'B. japonicum' and 'Aspirillum' sp. under rhizobia-established Myanmar and Thailand soils were investigated. By inoculation of gus-marked USDA 110 singly or its co-inoculation gave 93.21 to 94.75% and 74.21 to 100% in nodule occupancy, and 23.50 to 41.95% and 50.37 to 73.24% promotion in biomass dry weight over non-inoculated control in Myanmar and Thailand soil samples, respectively. Each of all the tested inoculum levels, that is 106, 107 and 108 cfu/ml of 'Azospirillum' sp. enhanced nodulation in combination with USDA 110 with a corresponding increase in 73.8, 62.25 and 95.34%; and 51.52, 62.38 and 79.46% over non-inoculated control, respectively in Myanmar and Thailand soil, respectively. In addition, soybean rhizosphere soil eubacterial community structures were not shifted by bacterial inoculation. Therefore, 'Azospirillum' sp. could be used in co-inoculant production with 'B. japonicum' for soybean.