Responses of Soil Microorganisms to Vegetation and Soil under Different Grazing Practices

This study set four grazing practices (no grazing (NG), summer grazing (SG), winter grazing (WG), and all year-round grazing (AG)) in an alpine meadow pasture in the northeastern Tibetan Plateau, China. The effects of different grazing practices on soil microbial biomass and community composition were comprehensively evaluated using the phospholipid fatty acid method (PLFA), Mantel test analysis, and structural equation modeling. The results showed that total PLFA content under four grazing modes significantly correlates with plant cover, aboveground biomass, and soil moisture content (P < 0.01). Bacterial content initially increases and decreases, and fungal content continuously increases, while actinomycetes, Gram-positive, and Gram-negative bacteria decrease. The bacterial-to-fungal ratio is lowest during SG and highest during AG, with significant differences between all types of grazed and NG grasslands (P < 0.05). AG reduced microbial biomass overall, with bacterial, fungal, actinomycete, Gram-positive, and Gram-negative biomass reduced by 18, 14, 5, 16, and 4%, respectively. The effects of soil factors were characterized by an increasing trend in soil pH and opposite trends in soil water content and soil bulk weight, with the former decreasing and the latter increasing. Concerning soil nutrients, grazing activities led to a decrease in soil organic carbon and total nitrogen content while total phosphorus content increased. This study showed that grazing practices indirectly affected soil microbial community structure by directly influencing aboveground biomass, soil organic carbon content, and soil total nitrogen. Conversely, aboveground biomass significantly enhanced the structure of soil microbial communities, demonstrating a clear positive correlation (P < 0.001). While SG promotes plant community diversity and soil microbial community diversity to a certain extent, AG tends to be significantly lower. It will likely lead to grassland degradation and nutrient loss, thus negatively affecting the ecosystem. This study is the first to evaluate the impact of grazing practices on soil microorganisms in alpine meadows on the Qinghai-Tibet Plateau, providing new insights into understanding the relationship between the ecosystem level or aboveground and underground parts, and the management of alpine grasslands.