Autochthonous organic matter input in reservoirs: limited methane oxidation in sediments fails to suppress methane emission

The interception of rivers leads to the accumulation of substantial organic matter in reservoirs, exerting a significant influence on greenhouse gas emissions. The diverse imported organic matter, coupled with sedimentary heterogeneity and intricate microbial processes, gives rise to seasonal variations in methane emissions from reservoirs. In this study, sediment cores were supplemented with terrestrial and autochthonous carbon to emulate reservoir carbon input across different seasons, thereby investigating methane emission potential and associated microbial mechanisms within the sediment cores. Results demonstrate that autochthonous organic matter enhances sediment organic content, thereby providing more substrates for the methanogenic process and fostering the proliferation of methanogens (with a relative abundance of 47.17% to 60.66%). Notably, the dominant genera of Methanosaeta, Methanosarcina, and Candidatus Methanomethylicus were boost on the surface layer of sediment. Concurrently, the introduction of autochthonous organic carbon spurs an increase in methane-oxidizing microbe, reaching up to 5.59%, with Methylobacter and Candidatus Methanoperedens as the predominant species, which has led to a downward migration of the functional groups in the sediment. Under the priming impact of autochthonous carbon, however, the methane oxidation probably doesn’t consume the substantial methane produced in sediment. Consequently, the sediment functions as a hotspot for methane release into the overlying water, highlighting the necessity to include summer as critical periods for integrated assessments, particularly during algae bloom.

河流截留作用会导致水库中大量有机质积累，进而对温室气体排放产生显著影响。多样的输入型有机质，加之沉积异质性与复杂的微生物过程，共同导致水库甲烷排放呈现季节波动。本研究通过向沉积物岩芯中添加陆源碳（terrestrial carbon）与内源碳（autochthonous carbon），模拟不同季节下水库的碳输入模式，以此探究沉积物岩芯内的甲烷排放潜力及其相关微生物机制。研究结果显示，内源有机质可提升沉积物有机质含量，为产甲烷过程提供更多底物，并促进产甲烷古菌（Methanogens）的增殖，其相对丰度可达47.17%至60.66%。值得注意的是，甲烷鬃毛菌属（Methanosaeta）、甲烷八叠球菌属（Methanosarcina）以及候选甲烷甲基菌属（Candidatus Methanomethylicus）等优势菌属在沉积物表层得到富集。与此同时，内源有机碳的输入会刺激甲烷氧化微生物的丰度提升，最高可达5.59%，其中甲基杆菌属（Methylobacter）与候选甲烷还原菌属（Candidatus Methanoperedens）为优势类群，这使得沉积物中的功能菌群出现向下迁移现象。但在内源碳的激发效应（priming effect）作用下，甲烷氧化作用大概率无法消耗沉积物中产生的大量甲烷。因此，沉积物成为甲烷向上覆水体释放的热点区域，这凸显了将夏季（尤其在藻类水华暴发期）纳入综合评估关键时段的必要性。