Variations in rainfall DOM as a driver of surface water concentration in the Chinese Loess Plateau-Qinling Mountains transition zone

Wet deposition of dissolved organic matter (DOM) is important yet understudied for fluvial carbon cycling. To investigate rainwater DOM sources and their contributions to riverine systems in the Chinese Loess Plateau (CLP)-Qinling Mountains (QM) transition zone, this study captured 57 rainwater samples with varying rainfall intensities over one year, accompanying monthly surface water sampling from an adjacent river. Three major fluorescent DOM (FDOM) components were observed in rainwater, including a polycyclic aromatic hydrocarbon-like component (C1), a low-aromaticity humic-like component (C2), and a tryptophan-like proteinaceous component (C3). C1 dominated FDOM and exhibited seasonal variations, highlighting the importance of anthropogenic inputs, especially in summer. Higher aromaticity and humification in autumn and winter reflected the combined effects of anthropogenic inputs and photochemical processes. The results confirmed the influence of CLP dust on rainwater DOM, particularly during spring dust episodes. Backward trajectories revealed the contributions of air masses originating from dust source regions to increased particulate matter and associated organic compound inputs. The high-altitude QM acts as a geographical-climatic barrier regulating DOM transport. Rainwater-derived DOM contributed <1% to surface water directly, highlighting rainstorm-induced terrestrial runoff and in-stream biogeochemical processes for riverine DOM, consistent with the higher humification and aromaticity of riverine FDOM and their smaller seasonal fluctuation. The discrepancies in DOM between rainwater and surface river indicate different dominant source controls and processing pathways, while likely remaining linked through rainfall-runoff and post-precipitation transport. This study highlights the significance of underlying surface characteristics across CLP-QM regions in affecting rainwater DOM and its contribution to terrestrial riverine systems. Rainstorm-induced soil washing and runoff processes deserve attention for quantifying riverine DOM sources in the future.