Differences in the regulation of soil carbon quality and stability by topsoil and deep soil under grazing

The carbon pool management index (CPMI) is widely used to assess soil carbon stability and human activities impacts on soil carbon dynamics. Grazing is the dominant anthropogenic disturbance in grassland ecosystems, yet its effects on carbon stability across whole soil profiles and the underlying links among labile carbon fractions, microbial communities, and extracellular enzymes remain poorly understood. To address this knowledge gap, we investigated soil organic carbon (SOC), labile carbon fractions, enzyme activities, microbial diversity, and CPMI along a 0–100 cm soil profile in meadow grassland (MG) and typical grassland (TG) under fenced (G0) and grazing (G+) conditions in Inner Mongolia, China. The results revealed that grazing significantly altered the vertical distribution of SOC, labile carbon fractions, and CPMI, but the responses differed between grassland types. In MG, grazing reduced SOC, CPMI, and most labile carbon fractions in the 0–30 cm soil layer, but increased CPMI in deeper layers (50–100 cm). In contrast, grazing increased topsoil SOC in TG but generally reduced CPMI and labile carbon fractions in deeper soils. Grazing also modified the activity of carbon-acquiring enzymes and reshaped microbial community structure, with Actinobacteria and Ascomycota remaining the dominant bacterial and fungal taxa, respectively. The relationships among labile carbon fractions, microbial diversity, and enzyme activities varied markedly with soil depth and grazing treatment, indicating that the mechanisms regulating carbon stability differ between topsoil and deep soils. Network and regression analyses further showed that grazing strengthened the coupling among microbial communities, carbon fractions, and enzyme activities in MG, whereas these associations became weaker in TG. Network and regression analyses further showed that grazing strengthened the coupling among microbial communities, carbon fractions, and enzyme activities in MG, whereas these associations became weaker in TG. Our findings provide one of the depth-integrated assessments of CPMI responses to grazing across a 0–100 cm soil profile and demonstrate that grazing influences soil carbon stability through changes in microbial–enzyme–carbon interactions. These results advance understanding of carbon stabilization processes in grazed grasslands and highlight the importance of considering deep-soil responses when evaluating grassland carbon sequestration under grazing management.