Vertical Distribution and Stand-Type Differences in Soil Organic Carbon Stock in Plantations on the Bashang Plateau, China

Soil organic carbon (SOC) is an important component of forest ecosystem carbon storage, and its distribution may vary with plantation stand composition. However, information on the vertical distribution of SOC stocks among different plantation types on the Bashang Plateau remains limited. In this study, we investigated the vertical distribution of SOC concentration and SOC stock and their relationships with continuous vegetation and soil variables in three mid-aged plantation types: pure Larix principis-rupprechtii stands (LP), mixed Larix principis-rupprechtii–Betula platyphylla stands (LBM), and pure Betula platyphylla stands (BP). Three independent 25 × 25 m plots were established for each stand type, and soil samples were collected from five depth intervals (0–10, 10–20, 20–30, 30–50, and 50–70 cm). SOC concentration, bulk density, soil moisture, pH, sucrase activity, and catalase activity were determined, together with understory aboveground biomass, litter biomass, and fine-root biomass. SOC concentration (g/kg) and SOC stock (t C/ha) generally decreased with increasing soil depth, and more than 50% of the total 0–70 cm SOC stock occurred within the upper 0–30 cm. One-way ANOVA showed that total SOC stock differed significantly among stand types (p < 0.05), with 202.50 ± 4.82 t C ha−1 in BP, 190.25 ± 4.51 t C ha−1 in LBM, and 109.75 ± 4.64 t C ha−1 in LP. Pearson correlation analysis among continuous variables showed that SOC stock was positively associated with understory aboveground biomass, soil moisture, and sucrase activity, whereas bulk density was negatively associated with SOC stock. Neither litter biomass nor fine-root biomass showed significant associations with SOC stock. These results indicate that stand type significantly affected SOC stock on the Bashang Plateau, and broadleaf-containing stands had higher SOC stocks than the pure larch stand under the conditions examined. The findings provide field-based evidence for considering stand composition in forest soil carbon management and ecological restoration in semi-arid plateau ecosystems.