Characteristics of CO2 Emissions in the Near-dam Hydro-fluctuation Zone of Three Gorges Reservoir under Water Level Fluctuation

[Objective] Water-level fluctuation zone (WLFZ) represents a key challenge for managing carbon emissions from global reservoirs. The Three Gorges Reservoir (TGR) has become a key focus for investigating the emission of greenhouse gases, such as carbon dioxide (CO2). Previous studies have not yielded consistent findings regarding the correlation between different elevations and soil carbon release — a gap that limits an accurate understanding of carbon cycling mechanisms in the reservoir’s WLFZ and hinders effective carbon emission management. [Methods] This study explored soil carbon emission characteristics in the near-dam WLFZ of TGR under fluctuating water levels. Soil respiration rates were measured using the Li-8100 Automated Soil CO2 Flux System. Two representative WLFZs—Longtanping and Lanlingxi—were selected, and within each area, three elevation intervals were established: below 160 m, 160-170 m, and above 170 m. This design ensured that the data would reflect the impact of water level fluctuations on soil carbon emissions across different WLFZ segments. One-way analysis of variance (ANOVA) in SPSS 25.0 was applied to examine differences in soil respiration across elevations and seasons. [Results] No positive correlation was found between elevation and soil respiration. Instead, as elevation increased, soil respiration across the entire study area exhibited a trend of first rising and then falling, with the maximum rate observed under moderate flooding stress. Specifically, the peak soil respiration rate reached 3.91 μmol/m2/s in Longtanping and 2.69 μmol/m2/s in Lanlingxi, with an average of 3.30 μmol/m2/s. This suggested that moderate flooding created optimal conditions for soil microbial activity and organic matter decomposition—two processes that drove carbon emission—whereas excessive or insufficient flooding inhibited these biological activities, reducing respiration rates. When the two WLFZs were analyzed comprehensively and Lanlingxi individually, no significant difference in soil respiration was found between the below-160 m and above-170 m intervals. However, in Longtanping, soil respiration above 170 m was slightly higher than that below 160 m. This regional discrepancy might be attributed to differences in local environmental factors, such as soil texture, organic matter content, vegetation coverage, or microbial community composition. Soil respiration exhibited significant temporal variability. Overall, the seasonal trend showed rates in July and August being highest, followed by September, June, and May. Minor differences existed between the two WLFZs: Longtanping showed a pattern where rates in July and August were highest, followed by September and June, and then May, while Lanlingxi displayed a pattern where rates in July, August, and September were equal and higher than June and May. Nevertheless, both areas recorded their peak soil respiration in August, with the highest rates occurring in the 160-170 m interval: 6.97 μmol/m2/s in Longtanping and 4.58 μmol/m2/s in Lanlingxi. The elevated summer respiration rates (especially in July and August) were primarily linked to vigorous vegetation growth and metabolic activity during this period. Vegetation contributed to carbon emission by releasing organic matter through root exudation and litterfall (providing substrates for microbes) and enhancing soil aeration via root respiration (facilitating microbial decomposition). [Conclusion] Moderate dry-wet alternation (i.e., moderate flooding stress) maximizes soil carbon emissions in the study area, while extreme flooding (either too high or too low) suppresses emission intensity. Summer, characterized by robust vegetation growth and metabolism, shows significantly higher soil respiration than other seasons—with July and August showing particularly high rates, and the moderately flooded zones in August recording the peak. The findings of this study have both theoretical and practical value. Theoretically, they enhance the understanding of carbon cycling in large reservoir WLFZ and contribute to global carbon cycle research. Practically, they provide a scientific basis for the quantitative analysis of carbon emissions in the Three Gorges Reservoir’s WLFZs and support future studies on carbon cycling following WLFZ ecological restoration. This information can further guide water level management strategies to regulate soil carbon emissions, aiding global carbon neutrality efforts and the sustainable development of the reservoir ecosystem.