Effect in soil and rhizosphere microbiota of Brachiaria inoculated with Azospirillum brasilense: a pilot trial in two Oxisol types

Context The Brachiaria genus includes several species of pastures distributed in tropical and subtropical regions. Plant growth-promoting bacteria (PGPB), such as Azospirillum brasilense, have been used as inoculants to increase crop production. Aims This study explored the effect of A. brasilense on Brachiaria seedlings, rhizosphere, and soil. Methods We inoculated A. brasilense on Brachiaria seeds sown in two types of soil mainly varying in texture (medium texture-Mt and clayey-C soils). We then collected the rhizosphere to evaluate the microbiota adhered to the plants by high-throughput 16S sequencing using bioinformatic tools. Shoot and root biomass were also evaluated. Key results Inoculation increased the aerial biomass of Brachiaria plants. However, it did not increase root biomass. Soil texture is a critical element in shaping rhizosphere communities. A. brasilense decreased the abundance of Firmicutes, mainly in C Oxisols. Network analysis showed that Proteobacteria, Acidobacteria, Actinobacteria, Firmicutes, and Bacteroidetes were the main phyla in the vicinity of Brachiaria roots. Furthermore, the abundance of specific phyla, such as Armatimonadetes, Tenericutes, and Fusobacteria (Mt) and Latescibacteria, Rokubacteria, and WS2 (C) increased in the bulk fraction. In the rhizosphere, Chlamydiae was exclusively related to Mt Oxisols. By contrast, Verrumicrobia and Fusobacteria were only found in the C soils. Conclusions Relative abundance of Acidobacteria and Actinobacteria increased after inoculation in the rhizosphere of both types of Oxisols. Implications These results indicate that inoculation can affect Brachiaria plants and their rhizospheric bacterial communities. The effect of taxonomic groups altered through inoculation and the relationship between the functional capacities of each group within the microbiota are yet to be elucidated.