Symbiotic interaction between a mixture of Bradyrhizobium japonicum strains and different soybean cultivars

Aim. To study the mutual relations between different soybean cultivars and two Bradyrhizobium japonicum strains after mixed inoculation, to evaluate the impact of the introduced microorganisms on the local rhizobial communities in soil, the formation and functioning symbiotic systems, and productivity of soybean. Methods. Microbiological and serological methods; field experiment, gas chromatography and mathematical-statistical methods. Results. A small- plot field experiment demonstrated that the use of a mixture of B. japonicum 46 and B. japonicum КВ11 strains pro- moted a more even distribution of the local and introduced rhizobia in the nodule populations of 12 soybean cultivars of different geographic origin. There was no domination of particular strains in the nodules, and the symbiotic systems formed were more balanced than the control without inoculation. After inoculation of the above-mentioned mixture, in most cultivars there was a considerable increase in the number of nodules (10–45 % more) and their mass (11–86 % increase). There was also an increase in the level of symbiotic nitrogen fixation with 1.2–4.2 times and an increase in the weight of grain per plant by 6–29 % (depending on the cultivar). The efficiency of a mixture of B. japonicum 46 and КВ11 strains in a preparation Rizogumin was confirmed in extensive field trials with soybean on an area of about 60 thousand ha in different regions of Ukraine. In the background of the local populations of the soybean nodulating bacteria in the areas, the application of the formulated mixture of the two rhizobia strains ensured a stable increase by 15–33 % in the soybean yield as compared with the control without the inoculation. Conclusions. A novel approach was investigated, which lies in the application of a mixture of B. japonicum 46 (genetic group USDA 6) and B. japoni- cum КВ11 (genetic group USDA 123) strains for different soybean cultivars inoculation. Combining these two strains and their introduction into agrocenoses ensured the formation of a balanced symbiotic systems (without clear domina- tion of some strains in the nodules). This was expressed in an intensified nodulation, symbiotic nitrogen fixation, and increased yield of different cultivars of soybeans by 15–33 % as compared to the control (without inoculation).