Effects of heavy grazing on soil microbial diversity and its drivers in different types of grasslands

Microorganisms are the most active components of soil and are very sensitive to external disturbances, so they are often used as an important indicator for evaluating the function and health of soil ecosystems. Many studies have investigated the effects of grazing on soil microorganisms, however, how the grazing effects on soil microbes vary among different grassland types and the underlying mechanism are still largely unexplored. In this study, we selected four different types of grasslands in northern China, i.e., meadow steppe, typical steppe, desert steppe and alpine meadow and compared the variation of heavy-grazing effects on soil microbial community composition and diversity and explored the potential drivers of soil microbial community composition and diversity in each grassland type. The results showed that within the same grassland type, the effects of heavy grazing on the α diversity of soil bacterial and fungal communities were not significant compared with those of the control. Similarly, we found that the β diversity of soil bacterial and fungal communities from the heavy-grazing plots was not significantly different from those from the control plots. However, we found that the α diversity and β diversity of soil fungal and bacterial communities varied significantly among the four grassland types. The ASV richness and Shannon diversity of soil bacterial community in meadow steppe was consistently higher than those of the other three grassland types. In contrast, alpine meadow generally had the highest fungal α-diversity. The fungal ASV richness in alpine meadow was significantly higher than that of meadow steppe and desert steppe and the fungal Shannon diversity in alpine meadow was significantly higher than that of typical steppe. The bacterial and fungal community structures of typical steppe and meadow steppe were similar, and differed significantly from those of desert steppe and alpine meadow. The random forest results showed that the key drivers of soil bacterial and fungal α-diversity and β-diversity differed among the different grassland types. Soil organic carbon, C:N ratio, available phosphorus, nitrate nitrogen and plant belowground biomass were the most important predictors driving the α diversity of soil bacterial communities. Soil available nitrogen, total nitrogen, available phosphorus, pH and plant aboveground biomass were the most important predictors driving the α diversity of soil fungal communities. Soil organic carbon, plant belowground biomass and total nitrogen were the most important predictors driving the β diversity of soil bacterial communities. Soil organic carbon and available nitrogen were the most important predictors driving the β diversity of soil fungal communities. This study compared the difference of heavy grazing on soil microbial community diversity in different grassland types, revealed the key drivers of soil microbial diversity in different types of grasslands, and provided a theoretical basis for the management of natural grasslands as well as the restoration of degraded grasslands.