Co-inoculation of Azospirillum brasilense strains and Bradyrhizobium sp. in soybean crops and development of a new technology of application

Abstract

The success of soybeans, in large part, is justified by the benefits arising from the inoculation of seeds with Bradyrhizobium japonicum, an association capable of supplying the demand for nitrogen by the crop, since these bacteria supply nitrogen to the plants through biological nitrogen fixation (BNF). A new technology has been increasingly adopted by soybean producers, called co-inoculation, which aims to promote plant growth by combining different mechanisms of different microorganisms. In the case of soybeans, co-inoculation occurs with the mixed inoculation of B. japonicum and/or B. elkanii and A. brasilense AbV5/AbV6. Seeking to enhance even more the positive responses with the co-inoculation in soybean, this work aims to test new strains of A. brasilense in the co-inoculation of the soybean crop, under field conditions. Three new strains, A. brasilense HM053, HM210 and IH1 were evaluated. A. brasilense HM053 and HM210 which excrete ammonium constitutively, and A. brasilense IH1 which is more resistant to oxidative stress. The experiments were carried out in the cities of Dois Vizinhos, Mandaguaçu, Rio Verde, Indiara and Pindorama, seeking different soil and climate conditions in Brazil to evaluate soybean growth and productivity improvement under different conditions. Co-inoculation with the new strains showed promising results for the characteristics evaluated, sometimes being statistically equal to the use of the AbV5/6 strain, sometimes being superior. At Mandaguaçu and Dois Vizinhos, in the flowering period, the HM053 strain stood out, presenting values higher than those found with the AbV5/6 strain, for the parameters of nodulation, dry weight and nitrogen content of the aerial part. However, it is notable that there is a lack of co-inoculation response pattern in the different sites evaluated. Therefore, a study in order to improve the efficiency of the inoculation was developed, with the objective of encapsulating the bacteria, protecting them from biotic and abiotic stresses, thus increasing the viability and stability of the bacterial cells. The A. brasilense AbV5/6 strain was encapsulated in a double cross-linked hydrogel granule based on cationic starch, using the swelling diffusion method followed by desiccation. The survival evaluation of A. brasilense AbV5/6 was carried out up to sixty days after loading and storage, and the average of viable bacteria remained in the order of magnitude of 107 CFU g, indicating that the hydrogel spheres are adequate to protect the A. brasilense for a period of at least 60 days. In addition, an experiment was carried out in a plant growth chamber and greenhouse to observe the effects of inoculation of encapsulated bacteria in corn seeds. In a growth chamber, the association of the inoculation with the hydrogel showed increments of 17% in the fresh weight of the aerial part and of 19% in the length of the roots. In the greenhouse, inoculation with A. brasilense AbV5/6 increased the levels of chlorophyll a and b, however, only in the presence of the hydrogel. The presence of the hydrogel improved the fresh weight of shoots by 6% and the height of corn plants by 7%. The results showed that this technology can enhance the effects of A. brasilense AbV5/6 in promoting maize growth under controlled conditions. More studies must be carried out to validate this technology in other crops and under field conditions.