Inoculant Formulation for Bradyrhizobium spp.: Optimizing CMC/Starch Blends for Improved Performance

Inoculating legumes with nitrogen-fixing bacteria, such as Bradyrhizobium, can significantly reduce reliance on synthetic nitrogen fertilizers. To optimize this process, a suitable rhizobial strain must be carefully selected and formulated. This study aimed to develop a biopolymer blend formulation for Bradyrhizobium pachyrhizi strain BR 3262. From four commercial starches and two carboxymethylcelluloses (CMC), we developed CMC/starch blends compatibilized or not with MgO at concentrations from 0.1% to 1.0% and subjected them to autoclaving for either 30 or 60 min. The resulting inoculants were stored for 168 days. Generally, blends compatibilized with 1.0% MgO exhibited a significant decrease in cell numbers, likely due to the observed pH values of approximately 10. The best performance was observed for CMC-I/starch B blends autoclaved for 60 min, and CMC-II/starch C blends autoclaved for 30 min, both compatibilized with 0.3% MgO. These blends maintained a cell viability of 108 CFU mL−1 for approximately 130 days at room temperature. Blend optimization depends on the selection of specific interactions and quantities of each component in order to achieve a given functionality; in the conditions of this study, the capacity to maintain Bradyrhizobium cell viability for at least four months.