Co-inoculation of PGPB and AMF: Impacts on common bean yield, nutrition, and selective root taxon co-occurrence

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Although inoculation with plant growth-promoting bacteria (PGPB) has shown promise for cleaner production with reduced reliance on synthetic fertilizers in common bean, field evidence regarding its combined use with arbuscular mycorrhizal fungi (AMF) remains quite limited. This study investigated the effects of PGPB and their co-inoculation with the AMF Rhizophagus clarus on grain yield, plant nutrition, and microorganisms associated with common bean roots. The experiment was conducted in a randomized block design with an 8 x 2 factorial scheme and four replicates, and was repeated in the same plots over two winter cropping seasons. The treatments consisted of inoculation with the PGPBs Azospirillum brasilense, Azomonas macrocytogenes, Azotobacter beijerinckii, Bacillus amyloliquefaciens, Bacillus aryabhattai, Burkholderia vietnamiensis, Nitrospirillum amazonense , and an uninoculated control, combined or not with R. clarus . The results demonstrated that co-inoculation efficacy is highly context- and strain-dependent. significant increases in grain yield were achieved when R. clarus was applied alone. Furthermore, true synergistic increments were observed under co-inoculation with Bur. vietnamiensis , Az. brasilense , or Azot. beijerinckii . Significant grain biofortification was observed, with specific PGPBs enhancing nitrogen and magnesium export, while the AMF increased iron export. Conversely, the incompatibility of certain co-inoculations was evidenced by antagonistic effects, such as the reduction in yield following the co-inoculation of R. clarus + A. macrocytogenes . Finally, the analysis of the root-associated microbiome - restricted to a subset of three selected PGPB strains and their co-inoculations with R. clarus ( Az. brasilense, Bur. vietnamiensis, and N. amazonense ) - revealed that the inoculations did not alter the overall diversity but were observed in association with exclusive genera, driving specific qualitative changes in the microbial community without altering its global structure. In conclusion, under the conditions of this single cultivation site, this study demonstrates that R. clarus can benefit winter common bean by increasing yield and inducing qualitative changes in low-abundance root-associated taxa. Similarly, PGPB and their co-inoculations with AMF can benefit crop yield and nutrition, depending on the specific microbial combination.