Rhizobacteria and arbuscular mycorrhizal fungus presented distinct and specific effects on soybean growth when inoculated with organic compost

How organic compost interacts with plant growth-promoting microorganisms and stimulates the growth of crops are not well understood. Thus, organic compost containing the plant growth-promoting rhizobacteria Bacillus subtilis and the arbuscular mycorrhizal fungus Rhizophagus irregularis could be a potential strategy to promote the growth of soybean. We hypothesized that using both B. subtilis and R. irregularis in combination would improve our compost amendment with synergistic or emerging properties that would provide additional growth enhancements to the crop, compared to using each inoculum separately. In general, plants grown in soil with organic compost presented highest growth than plants grown in unamended soil. Comparing to unamended soil, plants grown in soil with organic compost containing B. subtilis increased the root dry weight (∼50%), while plants grown in soil with R. irregularis showed higher nodule biomass (∼30%), symbiotic efficiency (∼50%), chlorophyll content (∼20%), and nitrogen accumulation (∼45%). Our results show that although compost amendment containing both inoculant microorganisms provided complementary crop growth promotion properties, we did not observe additional synergistic or emergent benefits.