The continuous cultivation of inoculated soybean in the same area can determine the soil colonization with a rhizobia population presenting low nitrogen fixation effectiveness. This fact can be a problem for the establishment of a more effective population. A greenhouse experiment was carried out to evaluate the symbiotic effectiveness and competitiveness of spontaneous mutants isolated from B. japonicum (SEMIA 5079 and SEMIA 5080) and B. elkanii (SEMIA 587 and SEMIA 5019) strains. The soybean biovar BR 16 was inoculated with the same proportion of inoculants (1:1; v/v) containing just one of the spontaneous mutants or in different combinations among them in comparison to a competitive strain (SEMIA 587 and 5019). Through the evaluation of nodulation, dry weight yields of plants, total N in plant and nodules occupancy, its was verified that the determinant factor for microorganisms symbiotic effectiveness in co-inoculated treatments was the synergism and the antagonistic interaction and not the nodule occupancy. It was also possible to select effective an competitive spontaneous mutants for soybean from parental strains recommended for commercial inoculants.
nitrogen fixation; nodule competition; B. japonicum; B. elkanii
