Contrasting responses of surface and subsurface soil microbiome to ecological restoration in two types of steppe because of different changes in plant and soil properties

Grazing exclusion contributes to the restoration of degraded grassland, including its soil. Whether restoration merely affects the surface layer or also penetrates into the subsurface layer remains unclear. Therefore, taking the typical steppe and desert steppe in the Mongolian Plateau as cases, this study investigated the effects of grazing exclusion on soil microbial diversity, composition, and function in the soil surface (0–10 cm) and subsurface (10–30 cm). Bacterial diversity was higher in the surface layer than in the subsurface layer in the typical steppe, but the opposite was true in the desert steppe. Grazing exclusion significantly increased fungal diversity in the surface layer (typical steppe) or in both layers (desert steppe). Grazing exclusion significantly altered bacterial (Actinobacteria, Proteobacteria) and fungal (Basidiomycota, Ascomycota) community composition in both steppe types. In the typical steppe, both bacterial and fungal communities differed between soil layers, whereas only bacterial communities varied between soil layers in the desert steppe. Co-occurrence network stability correlated with bacterial (not fungal) community composition and was linked to soil nutrients (e.g., dissolved organic carbon, soil organic carbon, and total nitrogen) across steppe types and soil layers. Overall, our research showed that grazing exclusion showed higher proportion and stronger functioning of soil microbiome associated with plant growth promotion, nutrient acquisition, or pathogen suppression in the typical and desert steppes. The difference in microbiome between the surface layer and subsurface layer of the typical steppe depended on bacteria and fungi, while that difference only depended on bacteria for the desert steppe.