Divergent Association Pathways of Soil Organic Carbon Variation Under Grazing Exclusion Across Three Grassland Sites: Relationships with Microbial Network Structure and Community Assembly

Grazing exclusion is a widely implemented restoration strategy for degraded grasslands, but its effects on soil organic carbon (SOC) remain highly variable, and the ecological pathways associated with these changes—particularly the roles of soil microorganisms—are still insufficiently understood. To investigate context-dependent SOC responses, we conducted a 12-year grazing exclusion study across three contrasting grassland sites representing an environmental gradient (temperate desert, temperate steppe, and mountain meadow) on the northern slope of the Tianshan Mountains. By integrating multidimensional vegetation–soil–microbe observations with microbial network analysis and community assembly approaches, we examined how plant, soil, and microbial characteristics were associated with SOC variation under grazing exclusion. Our results showed that grazing exclusion was associated with significant increases in surface SOC content across all three investigated sites (16–81%), but these increases corresponded to distinct site-specific ecological association patterns. At the temperate desert site, SOC variation was mainly associated with soil conditions and microbial characteristics, where soil moisture and nutrient availability, together with fungal network complexity, showed significant associations with SOC content. At the temperate steppe site, vegetation recovery represented an important ecological component associated with SOC variation, while bacterial community assembly patterns were also related to SOC differences. At the mountain meadow site, plant, soil, and microbial characteristics showed multiple associations with SOC variation. Across all sites, grazing exclusion was consistently associated with changes in microbial network structure and community assembly processes, and these microbial ecological characteristics showed significant statistical relationships with SOC content. Our findings indicate that SOC responses to grazing exclusion are context-dependent and involve different ecological association pathways across contrasting grassland ecosystems, highlighting the importance of considering local environmental conditions when evaluating grassland restoration outcomes and carbon management strategies.