Abstract

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 ofBradyrhizobium 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 Azospirillum 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. 

Key words: Bradyrhizobium, plant growth promoting rhizobacteria (PGPR), soybean, co-inoculation, competition, rhizosphere eubacterial community structure.