Seasonal dynamics and environmental drivers of dissolved organic matter distribution in heterogeneous river terrains

Dissolved organic matter (DOM) in rivers plays a key role in the global carbon cycle and aquatic ecosystems, yet its spatiotemporal dynamics across complex terrains remains inadequately characterized. To address this gap, we conducted seasonal sampling along the Luan River and examined how DOM composition varies over space and time in response to environmental drivers. Using parallel factor analysis (PARAFAC), we identified two humic‐like components (C1 + C2, Em > 380 nm) and one protein‐like component (C3, Em < 380 nm). DOM fluorescence intensity was lowest in winter, while C1 and C3 levels peaked in autumn and C2 in summer ( p < 0.05). Spatially, C1 and C3 levels were highest downstream regions, whereas C2 peaked in the midstream section ( p < 0.05). Microbial sources dominated DOM in spring, winter, and in both headwater and downstream areas (FI > 1.9). Principal coordinates analysis (PCoA) combined with multi‐response permutation process (MRPP) confirmed significant spatiotemporal differences in DOM composition ( p < 0.05). Random forest modeling showed humic‐like components were more sensitive to environmental changes. Redundancy analysis (RDA) and Mantel tests identified temperature as the dominant seasonal driver of DOM variation ( p < 0.05), while dissolved total phosphorus (DTP) and nitrate nitrogen (NO₃⁻‐N) were the key spatial determinants. Significant correlations were also observed between NO₃⁻‐N and ammonium nitrogen (NH₄⁺‐N), as well as between DTP and DOM, suggesting shared origins and compositional linkages ( p < 0.05). Our findings highlight the pronounced seasonal and spatial heterogeneity of DOM in multi‐terrain river systems, offering valuable insights into carbon dynamics and ecosystem functioning in river systems.