Hydrogenotrophic pathway dominates methanogenesis along the river-estuary continuum of the Yangtze River

Rivers are considered as an important source of methane (CH₄) to the atmosphere, but our understanding for the methanogenic pathway in rivers and its linkage with CH₄ emission is very limited. Here, we investigated the diffusive flux of CH₄ and its stable carbon isotope signature (δ¹³C-CH₄) along the river-estuary continuum of the Yangtze River. The diffusive CH₄ flux was estimated to 27.9 ± 11.4 μmol/m²/d and 36.5 ± 24.4 μmol/m²/d in wet season and dry season, respectively. The δ¹³C-CH₄ values were generally lower than -60‰, with the fractionation factor (α_c) higher than 1.055 and the isotope separation factor (ε_c) ranged from 55 to 100. In situ microbial composition showed that hydrogenotrophic methanogens accounts for over 70% of the total reads. Moreover, the incubation test showed that the headspace CH₄ concentration by adding CO₂/H₂ to the sediment was orders of magnitude higher than that by adding trimethylamine and sodium acetate. These results jointly verified the river-estuary continuum is a minor CH₄ source and dominated by hydrogenotrophic pathway. Based on the methanogenic pathway here and previous reported data in the same region, the historical variation of diffusive CH₄ flux was calculated and results showed that CH₄ emission has reduced 82.5% since the construction of Three Gorges Dam (TGD). Our study verified the dominant methanogenic pathway in river ecosystems and clarified the effect and mechanism of dam construction on riverine CH₄ emission.