Climatic predictors hierarchically regulate soil carbon components and microbial communities along an elevational gradient

Mountain soils play a critical role in global carbon cycling, yet how elevational gradients regulate soil organic carbon (SOC) component formation is unclear. We selected three elevation zones along the Qilian Mountains: the forest-grassland zone (2500–3250 m), the subalpine shrub-meadow zone (3250–3600 m), and the alpine meadow zone (3600–3800 m). Surface soil samples (0–10 cm) were collected from multiple plots within each zone. Soil properties, microbial community structure (via phospholipid fatty acid (PLFA) analysis), and SOC components (including microbial necromass carbon (MNC) via amino sugar analysis and plant residue carbon (PRC) via lignin phenol analysis) were measured. Results showed soil organic carbon, total nitrogen, and total microbial biomass increased with elevation. Conversely, the ratio of Gram-positive to Gram-negative bacteria (G + /G − ), microbial biomass per unit SOC, and the contributions of microbial necromass carbon (MNC) and plant residue carbon (PRC) to SOC declined. Structural equation modeling revealed that mean annual temperature and precipitation are primary drivers. They directly and indirectly regulate MNC and PRC formation by altering soil properties and microbial communities (e.g., fungal/bacterial ratio (F/B)). The findings demonstrate a clear hierarchical regulatory pathway: mean annual temperature (MAT) and mean annual precipitation (MAP) → soil properties (e.g., alkali-hydrolyzable nitrogen (AN), available phosphorus (AP), bulk density (BD)) → microbial community structure (PLFA/SOC, F/B) → SOC components (MNC/SOC, PRC/SOC). This indirect pathway had a much greater explanatory power than the direct influence of microbial processes. These findings highlight the overarching control of macro-climate in mountain soil carbon dynamics, providing key insights for predicting soil carbon stability under climate change.