S-containing molecular markers of dissolved organic carbon attributing to riverine dissolved methane production across different land uses.

The emission of methane (CH) from streams and rivers contributes significantly to its global inventory. The production of CH is traditionally considered as a strictly anaerobic process. Recent investigations observed a "CH paradox" in oxic waters, suggesting the occurrence of oxic methane production (OMP). Human activities promoted dissolved organic carbon (DOC) in streams and rivers, providing significant substrates for CH production. However, the underlying DOC molecular markers of CH production in river systems are not well known. The identification of these markers will help to reveal the mechanism of methanogenesis. Here, Fourier transform ion cyclotron mass spectrometry and other high-quality DOC characterization, ecosystem metabolism, and in-situ net CH production rate were employed to investigate molecular markers attributing to riverine dissolved CH production across different land uses. We show that endogenous CH production supports CH oversaturation and positively correlates with DOC concentrations and gross primary production. Furthermore, sulfur (S)-containing molecules, particularly S-aliphatics and S-peptides, and fatty acid-like compounds (e.g., acetate homologs) are characterized as markers of water-column aerobic and anaerobic CH production. Watershed characterization, including riverine discharge, allochthonous DOC input, turnover, as well as autochthonous DOC, affects the CH production. Our study helps to understand riverine aerobic or anaerobic CH production relating to DOC molecular characteristics across different land uses.