Critical role of mid-elevation in microbial regulation of soil carbon dynamics on the southern foothills of the Qinling Mountains

Soil microbial communities play a crucial role in regulating the spatial distribution of soil carbon along elevational gradients in mountain ecosystems. However, consensus remains limited on how habitats at different elevations influence microbial regulation of soil organic carbon (SOC) and its constituent fractions. In this study, we investigated microbial and environmental controls on SOC dynamics along an elevational gradient (1100–2200 m a.s.l.) on the southern foothills of the Qinling Mountains, China. We assessed the composition and abundance of soil microbial communities, key carbon fractions (dissolved organic carbon [DOC], microbial biomass carbon [MBC], easily oxidizable carbon [EOC], and recalcitrant organic carbon [ROC]), and associated environmental parameters across three soil depths (0–20, 20–40, and 40–60 cm). Regression analyses revealed that SOC, MBC, EOC, and ROC exhibited quadratic (concave-down) relationships with elevation across all soil layers. In contrast, DOC concentrations remained relatively stable in surface soils (0–20 cm) but increased with elevation in deeper layers (40–60 cm). Soil microbial communities displayed distinct spatial patterns along the elevational gradient. Redundancy analysis and stepwise regression models indicated that DOC variation was more strongly regulated by soil physicochemical properties than by biological factors. Soil total nitrogen, soil moisture, bacteria, and fungi were identified as important regulators of other SOC fractions. Notably, fungi demonstrated greater environmental tolerance than bacteria, and stable mid-elevation habitats promoted soil C accumulation by fungi. However, surface runoff loss and microbial consumption of available substrates were greater at both high and low elevations, leading to decreased soil C contents at these elevations. Overall, this study highlights the importance of comprehending the role of elevational gradients in regulating microbe-mediated soil C sequestration in mountain ecosystems in response to climate change.