Spatiotemporal dynamics of dissolved organic matter in Asia's longest river: Linking isotopes, land use, and anthropogenic impacts.

Large river basins integrate the three major ecosystems of atmosphere, land, and oceans, serving as crucial mediators in the global carbon cycle. Nevertheless, pronounced variations occur in geographic locations, climatic conditions, land-use patterns, and anthropogenic activities across sub-catchments of these large basins. As a result, systematic investigations into the spatial heterogeneity of dissolved organic matter (DOM) composition, abundance, and their driving factors are still lacking. In this study, we combined Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS), Ultraviolet-Visible Spectroscopy (UV-vis), excitation-emission matrix spectroscopy (EEM), and water isotopes (δH-HO and δO-HO) to investigate DOM characteristics across sub-catchments of Asia's longest river (Yangtze River). The results demonstrated that dissolved organic carbon (DOC) concentration in the Yangtze River is relatively low (mean: 3.17 ± 0.95 mg L) compared with global major rivers. The predominance of humic-like and lignin-like components in DOM across all sub-catchments highlights the dominance of allochthonous DOM sources. Notably, chromophoric dissolved organic matter (CDOM) concentration peaked in the Poyang Lake (PYL) catchment. Meanwhile, the Yangtze River mainstem (YRM) catchment, featuring extensive spatial coverage and complex environmental dynamics, showed substantial variability in DOM optical and molecular characteristics. Among sub-catchments, the Jialing River (JLR) exhibited the highest proportion of humic-like compositions (C1+C3), suggesting DOM originating primarily from forest-derived macromolecules and soil organic matter. Conversely, the Taihu Lake Catchment (THL) catchment displayed elevated abundance of protein-like components (C2) and lipid/aminoglycan-related compounds, indicative of significant contributions from autochthonous organic matter associated with eutrophication driven by intensive anthropogenic activities. These findings underscore that terrestrial carbon inputs constitute a pivotal component of riverine carbon cycling.