Sources and composition of riverine dissolved organic matter to marginal seas from mainland China

Rivers are an important connection between terrestrial ecosystems and the ocean, and are hotspots of organic carbon transformation. Riverine dissolved organic matter (DOM) concentration and composition are sensitive to both regional and global change processes. The comparative studies on riverine DOM among different rivers, especially when it comes to seasonal changes, were critical to better constrain the river carbon cycle. In this study, we analyzed organic carbon concentration, DOM sources and composition from the estuary locations of 48 Chinese rivers using the stable isotopic analysis and Fourier transform ion cyclotron resonance mass spectrometry. Both dissolved organic carbon (DOC) and particulate organic carbon (POC) concentrations, displayed significantly higher mean values in autumn than in summer (p < 0.05). Most of the DOM in these rivers entering marginal seas in four seasons came from the terrigenous sources (60.6%-68.4%), followed by the autochthonous matters (18.6%-25.6%), and the contribution of the anthropogenic sewage ranged from 7.6% to 13.7%. The proportion contributions of terrigenous and anthropogenic sources to DOM were similar in all seasons, but the contribution of autochthonous inputs showed the highest value in autumn than other seasons (p > 0.05). Although the highly unsaturated compounds are the most abundant molecular formulas detected in all riverine DOM, the molecular composition of riverine DOM shows a large difference among seven river basins. The human activity disturbance may be one of the major reasons for DOM composition differences. The contribution of sewage discharge on Chinese riverine DOM flux could be roughly estimated as 16.11–17.74 Gg C /yr. This work seeks to provide a foundation for ongoing research of regional and global riverine carbon cycling, and put forward the method for evaluating the potential impact of human disturbances on riverine organic carbon components in Chinese rivers.