Seasonal fluctuations in methane emissions from reservoirs: evidence from the methane metaboliccycle throughout the in situ sediment column profile.

Studying the issue of methane emissions from reservoirs, especially elucidating the mechanism of methane greenhouse gas production, revealing the seasonal flux changes of methane production, and providing scientific theories for China's low-carbon economic development strategy of optimizing energy structure and energy conservation and emission reduction. This paper combines geochemical and biological methods, with seasonal thermal stratification as the key entry point, to study the impact of methane production and oxidation processes in reservoir sediments under different thermal stratification conditions on the spatiotemporal dynamic changes of methane from the perspective of biological transformation. The ecological niche distribution of methane production/oxidation functional microorganisms in sedimentary column profiles is finely studied, revealing the ecological adaptation mechanism and production/oxidation efficiency of methane production/oxidation functional groups. Assuming that the methane dissolution in the sediment column profile of the reservoir has obvious seasonal characteristics, the seasonal thermal stratification of the Hongfeng Lake reservoir may be a key factor in the seasonal methane emission differences. The production and emission of methane from in-situ sedimentary columns may be influenced by other electron acceptors.
The distribution data of methane dissolution on sedimentary columns indicate that the methane dissolution in the in-situ sedimentary column profile of the reservoir has obvious seasonal characteristics, and the methane metabolism heat zone in summer sediments shifts upward. The results of indoor cultivation experiments further verified the distribution changes of methane heat zones in sediments, proving that seasonal thermal stratification in the Hongfeng Lake reservoir may be a key factor for the seasonal differences in methane emissions.
The enrichment of Methanosaeta, a specialized acetic acid trophic methanogenic archaea, was observed in the sedimentary columns during summer and winter. The fractionation coefficient results showed that the methane production pathway in the sedimentary columns of Hongfeng Lake Reservoir was mainly acetic acid trophic, and the methane production trend in this pathway was greater in summer than in winter. There is a very active methane anaerobic oxidation process in the sediment column of Hongfeng Lake reservoir in summer, and the methane production potential of Hongfeng Lake sediment in summer is lower than in winter, while the methane anaerobic oxidation potential in summer is slightly higher than in winter.
Functional microbial abundance data indicate that coupled sulfate reduction AOM occurs in in-situ sedimentary columns during summer, and coupled metal iron reduction AOM occurs in shallow and deep layers during summer. In winter sedimentary columns, the iron reducing bacteria Bacillus, a participant in M-AOM, was significantly enriched in the bottom layer.