Response of Soil Organic Carbon Components in Pinus yunnanensis Stand to Altitude Variation

Under global climate change, the response of mountain forest soil carbon pools to elevation is central to carbon cycle research, and Pinus yunnanensis stands, which span a wide elevation range, serve as a typical subject for studying how soil properties in mountain ecosystems respond to elevation gradients. To reveal the variation patterns and underlying regulatory mechanisms of soil nutrients and organic carbon components in Pinus yunnanensis stands across different altitudinal gradients, this study took Pinus yunnanensis stands at three altitude gradients (1604 m, 2377 m, 3206 m) within Yunnan Province as research objects, collected stratified soil samples, and determined soil chemical properties, organic carbon components, enzyme activity, and microbial biomass. The results showed that changes in elevation significantly influence soil nutrient content: soil pH gradually decreases with increasing elevation; soil organic carbon, total nitrogen, alkali-hydrolyzable nitrogen, available phosphorus, and readily available potassium first increase then decrease with elevation, reaching their highest levels at Jin’an Town (JA); total phosphorus and total potassium gradually increase with elevation, peaking at Xiaozhongdian Town (XZD); particulate organic carbon, mineral-bound organic carbon, and microbial biomass carbon follow similar patterns to organic carbon, all showing enrichment in the surface layer; JA exhibits the highest carbon cycle enzyme activity and bacterial biomass, while XZD shows dominant fungal biomass. Partial Least Squares Path Modeling (PLS-PM) analysis indicates that elevation strongly positively drives microbial biomass, indirectly regulating enzyme activity and chemical properties, ultimately jointly influencing organic carbon components. In conclusion, soil properties varied markedly, and under stable precipitation, the thermal gradient emerged as the primary driver; the mid-elevation site (2377 m) showed optimal soil functioning, with peak nutrient and carbon stocks linked to heightened microbial and enzymatic activity, and path modeling confirmed that temperature, via microbial mediation, is the key regulator of soil organic carbon dynamics in these pine forests.