Spatial heterogeneity of riverine dissolved organic matter and its molecular relationship with dissolved organic carbon dynamics.

Dissolved organic matter (DOM) in rivers is a chemically complex continuum and plays a key role in riverine biogeochemical processes. However, the molecular-scale heterogeneity of DOM and its relationship with dissolved organic carbon (DOC) concentration remain poorly understood. Here, we combine ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry with machine-learning and molecular network analyses to characterize the spatial distribution of DOM molecular composition in the Hetao Basin of the Yellow River and to explore how DOC dynamics are associated with DOM. Riverine DOM was dominated by lignin-like and saturated reduced compounds, whereas trace halogen-containing molecules provided clear evidence of anthropogenic inputs. Spatially distinct molecular assemblages, particularly among heteroatom-rich compounds, revealed pronounced heterogeneity in DOM sources and transformation pathways. Elevated DOC concentrations were consistently associated with reduced average molecular mass and aromaticity, increased saturation, and enhanced intermolecular connectivity, suggesting an increased contribution of low-molecular-weight and saturated organic carbon under high DOC conditions. Among molecular descriptors, the N/C ratio and double bond equivalent emerged as important regulators of DOM-DOC coupling. These findings provide a molecular-level framework for understanding the association between DOC variability and DOM composition in a major river system, offering new insight into the spatial heterogeneity of riverine DOM.