Effect of co-inoculation with Bradyrhizobium japonicum and Azospirillum brasilense on soybean plants

In order to assess the effects of co-inoculation with both Bradyrhizobium japonicum and Azospirillum brasilense on soybean plants, a pot experiment under growth controlled conditions was carried out. Four-week-old plants were harvested and analyzed for shoot, root and nodule dry weight, nitrogen content, nodule number and distribution, nodule protein and leghaemoglobin content, and nitrogenase and glutamine synthetase activities. Nodule number and nodule dry weight were not affected by dual inoculation; however, co-inoculated plants showed a significantly higher proportion of nodules attached to the main root and located in the upper 3 cm of the root system. Although no significant difference in total dry matter production could be detected, nitrogen content of dual inoculated plants was significantly increased (23 % over B. japonicum-single inoculated plants). Accordingly, a strong stimulation of acetylene reduction activity was observed for this treatment. Likewise, leghaemoglobin content significantly increased (39 %). These results suggest that co-inoculation with B. japonicum and A. brasilense leads to an increased number of the most active nodules, therefore, to a greater nitrogen fixation and assimilation. In addition, an enhancement of nodule effectiveness due to A. brasilense presence in the root system is suggested. (C) Elsevier, Paris.