Cascade dams amplify molecular transformation and compositional homogenization of dissolved organic matter on the...

River damming globally disrupts hydrological connectivity, transforming reservoirs into critical regulators of the global carbon cycling. However, the molecular characteristics and the specific transformation processes of dissolved organic matter (DOM) in cascade reservoirs remain unclear. To address this knowledge gap, we integrated water chemistry, stable carbon isotopes, and Fourier transform ion cyclotron resonance mass spectrometry to elucidate the molecular fate of DOM across cascade river-reservoir systems on the Eastern Tibetan Plateau. Results demonstrated that cascade damming enhanced autochthonous DOM production, as estimated by a simmr-based stable-isotope mixing model, increasing the phytoplankton-derived fraction of DOM from 27 ± 10% in rivers to 31 ± 12% in cascade reservoirs (posterior mean ± SD). Simultaneously, the favorable thermodynamic condition accelerated the turnover of allochthonous DOM by enhancing both photochemical and microbial decomposition processes within the warming lentic areas. These processes amplified DOM homogenization, evidenced by a decrease in the Jaccard dissimilarity coefficient along the flow path, resulting from consuming labile fractions. Furthermore, the reduced hydrological connectivity due to dam construction facilitated the downstream transport of autochthonous DOM. It is estimated that the global export flux of autochthonous dissolved organic carbon to the oceans amounts to 141 ± 2 Tg C yr^-1. Our results highlight that the global proliferation of reservoirs fundamentally alters the biogeochemical fate of DOM, with implications for regional and global carbon cycling.