The Unimodal Distribution Pattern of Soil Organic Carbon Across Elevation Gradients in the Three Gorges Reservoir

Soil organic carbon (SOC) and its active fractions—labile organic carbon (Lab-C), dissolved organic carbon (DOC), and microbial biomass carbon (MBC)—govern soil carbon stability and climate feedback mechanisms. To investigate the distribution patterns and regulatory mechanisms of SOC and those active fractions along elevational gradients in the riparian zone of the Three Gorges Reservoir Area (subjected to intense waterlogging stress), soil sampling and analysis were conducted across four zones of the Longtanping: below 160 m, 160–170 m, 170–180 m, and above 180 m in early September 2021. Results indicated that as elevation increases, the content of SOC and active components exhibited a unimodal distribution pattern showing initial increases followed by decreases; moreover, this pattern can be attributed to the pH-riven changes in bacterial abundance under varying inundation stress conditions. The peak values occurred at elevations of 160–170 m, with the overall distribution pattern being as follows: 160–170 m > 170–180 m > above 180 m > below 160 m. Correlation analysis revealed significant positive correlations among SOC, DOC, MBC, Lab-C, pH, TN, and bacterial abundance (p < 0.05). Lab-C demonstrated the strongest explanatory power for SOC variations, serving as a sensitive indicator of SOC turnover and persistence dynamics. This study provides critical insights into the carbon cycling mechanism and regional carbon sink assessment in reservoir riparian ecosystems.