Altitudinal Variation in Soil Organic Carbon Turnover: Decoupling Climate and Edaphic Drivers in Changbai Mountain Forest Wetlands

Understanding how soil and climatic factors jointly regulate soil organic carbon (SOC) turnover in forested wetlands is crucial for assessing their role as carbon sources or sinks. This study investigated δ¹³C-based carbon dynamics along an altitudinal gradient (700–1,818 m) in the Changbai Mountain to identify key environmental drivers and inform regional carbon management strategies. We analyzed δ¹³C and β values in soil and litter samples across six elevations, alongside SOC and nutrient concentrations (TN, TP, TK), using principal component analysis to determine the dominant controls on SOC turnover. Results showed that SOC concentrations declined with depth, while δ¹³C values increased. Notably, the 700 m site exhibited unusually high SOC and δ¹³C enrichment, likely due to persistent waterlogging. β values peaked at mid-elevations (700–1,300 m) and were consistently higher in litter than in soil, indicating more active turnover. Edaphic factors accounted for 86% of the variation in β values, far exceeding the influence of climate variables (26.8%). Overall, mid-elevation wetlands exhibited the most favorable conditions for SOC turnover due to optimal moisture and nutrient availability, while high-elevation zones (> 1,500 m) functioned as cold-driven carbon sinks. δ¹³C is demonstrated to be an effective tracer of SOC turnover, underscoring the need for hydrological restoration and nutrient management in mid-altitude wetland ecosystems.