To investigate the impact of periodic water-level fluctuations on soil microbial community structure in the riparian zone of the Three Gorges Reservoir Area, this study focused on the riparian zone at Zhangjiawan near the dam. Surface soil samples (0–20 cm) were collected in spring, summer, and autumn from three elevation gradients: below 160 m, 160–170 m, and above 170 m. Phospholipid fatty acid (PLFA) was employed to characterize the microbial community structure. The results indicated that microbial community distribution was significantly influenced by both elevation and season, with bacteria dominating community variations. In terms of elevation, the total microbial biomass and bacterial abundance were highest at the 160–170 m gradient, followed by the area above 170 m, and lowest below 160 m. Season and elevation exhibited a significant interactive effect on fungi. Seasonal patterns showed that total microbial biomass and bacterial abundance peaked in summer, whereas fungal abundance and the fungi-to-bacteria ratio were higher in autumn. In contrast, actinobacterial abundance remained stable across seasons. Soil organic carbon (SOC; 60.3%) was identified as the predominant environmental factor influencing microbial community structure, followed by total nitrogen (TN; 13.9%) and pH (2.4%). SOC was identified as the key driver of microbial community dynamics. These findings clarify the respective roles of water level fluctuation, seasonal variation, and environmental factors in shaping riparian microbial communities, providing critical baseline data and theoretical support for ecological protection and scientific management of the Three Gorges Reservoir Area.

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.