How do forest types regulate soil organic carbon quantity and quality in southeastern Xizang? Evidence from soil organic carbon and the carbon pool management index

The forest soil carbon pool is a core component of the terrestrial carbon cycle, and its quantity and quality are largely regulated by forest types, thereby influencing the carbon sequestration capacity of forest ecosystems. The Qinghai-Xizang Plateau is one of the regions most sensitive to global climate change, experiencing warming rates higher than the global average and pronounced ecological vulnerability. Among its subregions, southeastern Xizang is characterized by extensive forest cover and prominent ecological functions. Accordingly, elucidating the differentiation characteristics and driving mechanisms of soil carbon pool quantity and quality among different forest types in this region is of great significance for accurately evaluating regional carbon sink capacity. This study focused on multiple representative forest types in southeastern Xizang, including coniferous forests, broad-leaved forests, and conifer-broadleaf mixed forests, to systematically compare the distribution characteristics of soil organic carbon (SOC) and its fractions among different forest stands, and to explore the variation patterns and driving factors of the carbon pool management index (CPMI). The results showed significant differences in soil organic carbon and its fractions among forest types in southeastern Xizang (p < 0.05). The Cupressus gigantea forest exhibited consistently higher levels of soil organic carbon and its fractions, indicating a strong carbon accumulation capacity, whereas both Pinus densata forest and Pinus yunnanensis-Populus davidiana mixed forest showed relatively lower values overall. The carbon pool management index varied markedly among forest types. The Cupressus gigantea forest showed the highest carbon pool index (CPI) and carbon pool management index (CPMI) (p < 0.05), indicating the best soil carbon pool quality. Driving factor analysis revealed that soil organic carbon (SOC) and carbon pool index (CPI) were primarily regulated by available nitrogen (AN), available potassium (AK), electrical conductivity (EC), and soil water content (WC). The carbon pool management index (CPMI) was mainly driven by field capacity (FC) and total nitrogen (TN), whereas carbon pool activity (A) and the carbon pool activity index (AI) were more dependent on available phosphorus (AP) and total phosphorus (TP). Redundancy analysis (RDA) showed that the first two ordination axes together explained 92.8% of the total variation, with the first axis accounting for 88.1% and the second axis for 4.7%, indicating that environmental factors can effectively explain the variation in soil carbon pool quantity and quality. This study revealed the spatial differentiation patterns and distinct driving mechanisms of soil carbon pool quantity and quality in alpine forests, providing a scientific basis for evaluating forest carbon pool quality and guiding regional carbon sequestration enhancement and management.