Unravelling riverine dissolved organic matter sources using molecular fingerprints and FEAST model in a multi-tributary mountain river basin

Revealing the sources, composition and fate of riverine dissolved organic matter (DOM) is fundamental to understanding the biogeochemical cycles of aquatic ecosystems. This study aimed to reveal the impact of land uses and wastewater treatment plants (WWTPs) on riverine DOM. Spatiotemporal variations in molecular characteristics of riverine DOM in the river network containing 15 tributaries in the mainstream of upper Hanjiang River were studied. Differences in molecular characteristics of DOM in soil leachates of various land uses and the effluent of WWTPs were analyzed and their contributions to riverine DOM in both dry and wet seasons were calculated using FEAST model. DOM in soil leachates was primarily composed of lignin, protein and lipid-like compounds but was dominated by lignin and tannin-like compounds in the effluent of WWTPs. Contribution rates of the soil leachate of each land use calculated by FEAST model showed a significant positive linear correlation with the area-based proportion of each land use in the basins of tributaries. Contributions of area-based proportion of each land use to riverine DOM followed the order of grassland > forest > cropland for both seasons. DOM in the upstream of tributaries contributed more than 50 % to the molecular composition of DOM in the downstream of tributaries but the contribution of the effluent of WWTPs to riverine DOM did not exceed 3 %. These results demonstrated that FEAST model could be used for source identification of riverine DOM based on molecular fingerprint data. Accordingly, this study provides new insights into the carbon cycling and ecological health within the watershed.