Production of a liquid Bradyrhizobium japonicum inoculant with high impact on the mechanized sowing of soybean in Cuba

Symbiotic nitrogen fixation is critical for high soybean (Glycine max L. Merril) yields with protection to the environment. This paper describes a procedure for massive production of Bradyrhizobium japonicum in an orbital multi-platform shaker and the use of the bacterial culture for seed inoculation in soybean areas under mechanized sowing. The B. japonicum strain was selected based on its highly efficient symbiotic interaction with major soybean varieties cropped in Cuba. Optimization of the growth medium and the use of serial and scaled inoculations allowed maintaining the cultures in continuous logarithmic phase while increasing its volume 100-fold in 7 days. The final culture having average cell viability of 5 x 1010 c.f.u./mL and high exopolysaccharides content does not require additives and can be stably stored for at least 8 months at 4 oC. Soybean seeds were sprayed with the inoculant at equivalent dose of 150 mL/ha and sowed mechanically in production fields of Ciego de Avila province during the campaigns 2010-2013. The plants showed abundant nodulation and a proper nutritional state during the whole cycle. The bacterium strain was reisolated from root nodules of inoculated plants and its identity was confirmed by phenotype characterization. The high-cell density liquid inoculant rich in polysaccharides produced in this work is compatible with mechanized seed sowing, favors the bacterium survival in soil, and promotes the establishment of an efficient symbiotic interaction with the host plant.