Ecological role of bacterial inoculants and their potential impact on soil microbial diversity

Microbial inoculants are an alternative method of increasing crop productivity that can reduce the use of chemical fertilizers, which is one of the more controversial agricultural practices that affect the environment. Beneficial bacteria, collectively known as plant growth-promoting bacteria (PGPB), enhance plant growth and protect plants from disease and abiotic stresses through a wide variety of mechanisms. Bacterial inoculation efficiency is associated with the beneficial features of the inoculated bacterium, as well as with the complex network of interactions occurring in the soil. Beneficial bacteria have previously been examined for interactions with different plant hosts, soil types, and agricultural practices, but there is limited information concerning the potential effects of the release of microorganisms on soil functionality. Despite the plant growth promotion characteristics, the survival, abundance, and persistence of inoculant in soil or plant roots are characteristics that could potentially lead to its invasiveness. Inoculants can also interfere with soil health and microbial and faunal community composition. This review presents an overview of plant-PGPB interactions and their impacts on microbial communities, hypothesizing about the potential of these interactions to promote positive disturbances in soil, mainly in poor environments. The inoculation of free-living bacteria seems to cause a short-term impact to agricultural soils, while rhizobia-based inoculants or bacterial inoculations performed under stress conditions are long-term processes. However, there is great variability amongst results concerning the effects of bacterial inoculation into different plant and soil conditions.