RIZOBACTÉRIAS DE SOLOS AGRÍCOLAS COM POTENCIAL PARA A PROMOÇÃO DO CRESCIMENTO DE SOJA (Glycine max L.)

Abstract

The current production of major crops, such as Glycine max L., has become increasingly dependent of the use of bio-inputs, such as the application of plant growth-promoting inoculum. However, the evaluation of the potential of bacterial isolates from soils with different time histories of agricultural use are still scarce, and methods for isolation and testing of multifunctional microorganisms need to be continuously innovated and improved. This work aimed to select rhizobacteria from agricultural soils with potential to promote soybean growth. In Chapter I, G. max bait plants were used to attract rhizobacteria from soils obtained from three areas with different time histories for soybean cultivation and to evaluate the influence of management and soil characteristics on the phosphate-solubilizing bacterial population and the multifunctional potential of previously selected rhizobacteria for calcium phosphate (CaHPO4) and iron phosphate (FePO4) solubilization. In chapter II, an experiment in five areas of G. max cultivation were carried out to evaluate the hypothesis that PSRB, previously isolated from soybean seedlings, could be effective in promoting growth in crops of this oilseed (2019/20 and 2020/21), in soils with different histories of cultivation. This study compared results with those observed for BiomaPhos inoculant to test the effectiveness of PSRB (SAF9 - Brevibacillus sp., SAF11 - Brevibacillus sp., and SAC36 - Bacillus velezensis). Alternatively, it was also evaluated the hypothesis that the cultivation history of the planting areas may alter the effect of PSRB on growth promotion, and the hypothesis of superiority of the inoculation method by seeds compared to inoculation by soil. The present study also evaluated biometric growth, nutritional and yield parameters, and microbial biomass carbon (MBC). The use of bait plants enabled the isolation of 139 phosphate solubilizing rhizobacteria, including four strains with multifunctional potential. The largest number of solubilizing bacteria was obtained from the interaction of the bait plants with soil from an area with a history of soybean cultivation for thirty consecutive years. The high concentration of P, Fe, and K found in the soil were associated with this occurrence. In the in vitro tests, the pH values of the culture media had a moderate negative relationship with the amounts of P made available by the isolates, indicating that other processes besides the availability of organic acids, may underlie the solubilizing action of the isolates. The rhizobacteria SAF9 (Brevibacillus sp.), SAF11 (Brevibacillus sp.), BRC11 (Pseudomonas fluorescens), and SAC36 (Bacillus velezensis) stood out as multifunctional and are indicated within a perspective of obtaining bioinoculants to promote plant growth directly, indirectly, or synergistically, contributing to increasing the range of bio-inputs for soybean cultivation and more sustainable agricultural practices. PSRB were more effective or showed similar effectiveness to BiomaPhos for most of the biometric, nutritional and yield characteristics evaluated. In the areas tested in the summer 2019/20 crop, SAC36 and SAF9 strains stood out as growth promoters, while in the 2020/21 crop SAF11, SAC36, and BiomaPhos stood out. There does not seem to be a direct relationship between longer histories of soybean cultivation as a monoculture and lower yield in the area. Yield, however, seems to be associated with soil nutritional characters such as Ca, Mg, K, P, CEC, and OM contents. PSRB inoculation positively affected nodulation (NN) and nodule dry mass (MSN), in the evaluated areas in the 2019/20 crop, and aerial part dry mass (APDM), NN, NDM, yield and MBC of the evaluated areas in the 2020/21 crop. On the other hand, a strong effect of the inoculation method was observed on the APDM, RDM, NN, and MBC, since the plants inoculated by seed showed higher means compared to those in the plants inoculated by soil. This study attested to the growth-promoting potential of new phosphate solubilizing strains, which may be eventually incorporated by the bio-inputs market to compete freely with BiomaPhos.