Effects of microbial inoculants on the biomass and diversity of soil microbial communities: a meta-analysis

Microbial inoculants, transplanting microorganisms from their natural habitat to improve plant performance, hold promise for sustainable agriculture and ecological restoration but also raise a potential concern as a purposeful invasion to alter soil resident communities. Current studies have mainly focused on the impact of microbial inoculants on altering soil microbial communities in various single soil conditions. However, the comprehensive impact of microbial inoculants on soil microbial community (biomass, diversity, structure, and network) under a large scale of soil resource conditions remains unknown. Through a meta-analysis of 335 studies, we found significant and positive effects of microbial inoculants on microbial biomass. More importantly, we discovered that environmental stress weakened their positive effects, while fertilizer application and the use of native microbial inoculants enhanced them. Moreover, increased initial soil nutrients amplified the positive impact of microbial inoculants on fungal biomass, actinomycete biomass, microbial biomass carbon, and microbial biomass nitrogen. Although microbial inoculants did not significantly alter microbial diversity, they induced changes in microbial community structure and bacterial composition significantly. Lastly, we showed a reduction in the complexity of bacterial networks induced by microbial inoculants, along with increased stability. Our study highlights the overall positive impact of microbial inoculants on soil microbial biomass, emphasizing the benefits of native inoculants and the importance of considering soil nutrient levels and environmental stress.