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

Rivers are considered as an important source of methane (CH) to the atmosphere, but our understanding for the methanogenic pathway in rivers and its linkage with CH emission is very limited. Here, we investigated the diffusive flux of CH and its stable carbon isotope signature (δC-CH) along the river-estuary continuum of the Yangtze River. The diffusive CH flux was estimated to 27.9 ± 11.4 μmol/m/d and 36.5 ± 24.4 μmol/m/d in wet season and dry season, respectively. The δC-CH values were generally lower than -60‰, with the fractionation factor (α) higher than 1.055 and the isotope separation factor (ε) 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 CH concentration by adding CO/H 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 CH 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 CH flux was calculated and results showed that CH 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 CH emission.