Grazing-induced abiotic and resource changes drive distinct responses of soil bacterial and fungal community in temperate meadow steppe: Implications for carbon dynamics

• Our results demonstrate distinct patterns of changes in bacterial and fungal community. • Bacterial community showed a clear succession and were more sensitive to abiotic conditions than resource limitations. • Fungal community revealed a more discrete shift, limited by resources and structured by bottom-up forces. • Both bacterial and fungal composition shifted towards more oligotrophic and less copiotrophic with grazing increased. • Heavy grazing lead to a depleted substrate and inhibited ecosystem respiration, change soil carbon cycling and stocks. Soil microbial communities play a crucial role in maintaining grassland ecosystem functions and are strongly influenced by livestock grazing. However, the long-term responses and driving mechanisms of soil microbial communities to grazing intensity gradients, remain largely unexplored. In this study, we investigated the mechanism of different grazing intensities (i.e., ungrazed, light, moderate and heavy grazing) affect the diversity and composition of soil bacteria and fungi in the Hulunbuir Leymus chinensis meadow steppe. Using a Bipartite network to represent indicative species shifts, bacterial community presented a clear succession along the grazing intensity gradient, likely linked to soil abiotic conditions (e.g. soil temperature, silt). In contrast, fungal community exhibited a more discrete shift along the grazing intensity gradient, challenging the traditional view that fungal community is more stable under disturbance. The shifts in fungal community were closely related to the vegetation composition and aboveground biomass, reflecting a typical bottom-up resource-related regulation, which were more dynamic than changes caused by abiotic conditions along the grazing intensity gradient. Interestingly, indicator analysis showed that higher grazing intensity shifted bacterial and fungal composition towards more oligotrophic (e.g. Dothideomycetes, Sordariomycetes, Leotiomycetes, and Chloroflexi, Thermoleophilia) and less copiotrophic (e.g. Saprotrophs, Bacteroides and subgroup_6). This shift reflects the depleted substrate and is consistent with the observed inhibition of ecosystem respiration, implying lower organic matter decomposition. The distinct patterns of bacteria and fungi responses provides novel insights into the mechanisms, through which grazing alters soil bacterial and fungal communities with potential long-term consequences, including future growth-limiting resource and soil environment conditions to withstand future disturbances, which affect soil bacterial and fungal communities differently and consequently modulate soil organic carbon turnover. Moreover, the different substrate affinity of copiotrophic and oligotrophic groups altered available and recalcitrant C decomposition, which may change soil carbon cycling and stocks.