The Impact of Co-Inoculation with Bradyrhizobium japonicum and Azospirillum brasilense on Cowpea Symbiosis and Growth

Biological nitrogen (N) fixation is a well-established practice in various legumes, such as soybeans. However, it has not been widely studied in cowpeas (Vigna unguiculata L. Walp). In this context, it is important to understand how the application of nitrogen-fixing bacteria, either alone or in association, can benefit the crop’s nitrogen demand. This study aimed to determine whether co-inoculation of Bradyrhizobium and Azospirillum favors nodulation and isoflavone production, and increases the nitrogen content, in cowpea crops. The experiment was set up using a randomized block design on two cowpea varieties, with seven treatments consisting of a control and the isolated application of Bradyrhizobium japonicum and Azospirillum brasilense, as well as different co-inoculation doses (75, 150, 225 and 300 mL per 50 kg of seed for each inoculant). There were four replications. Thirty days after emergence, the number of nodules and the dry masses of the nodules, roots and shoots of the plants were assessed. N content and isoflavone content in the fully developed third trifoliate leaf from the apex of the plants were also assessed. Statistical differences were observed between treatments for all analyzed variables, with higher values generally observed for co-inoculation treatments. Co-inoculation of B. japonicum and A. brasilense in cowpea seeds can be a viable and efficient practice. A dose of 75 mL of each inoculant favored nodule formation, root development and N content, as well as contributing to isoflavone production in the cowpea crop.