Non-Negligible Allochthonous Contributions to Dissolved Organic Matter Biodegradability in the Yangtze River

Biodegradable dissolved organic carbon (BDOC) in rivers is crucial for regulating organic carbon degradation and greenhouse gas emissions during carbon transport from land to ocean. BDOC is closely linked to the biolability of riverine dissolved organic matter (DOM). However, the bioavailability of allochthonous DOM, the dominant source of DOM in large rivers, remains unclear. In this study, stable carbon isotope, excitation–emission matrix fluorescence, and Fourier transform ion cyclotron resonance mass spectrometry analyses were performed to investigate the effects of DOM sources on DOM bioavailability in the Yangtze River. The results indicate that BDOC is an important fraction of dissolved organic carbon (DOC) in the Yangtze River (29.82 ± 15.30%). Autochthonous source contributed 38.21 ± 25.42% to total biolabile DOM, exhibited saturated characteristics, and primarily comprising CHOP and CHOS compounds. Surprisingly, allochthonous biolabile DOM accounted for 34.41 ± 27.53%, emphasizing relatively high oxidation state and aromaticity with enriched heteroatomic contributions. Prolonged water retention in the Three Gorges Dam region promotes allochthonous biolabile DOM degradation, whereas increased human activity downstream leads to autochthonous biolabile DOM accumulation. This study highlights allochthonous contributions to DOM biolability and indicates that continued increases in terrigenous inputs can enhance riverine BDOC, thereby influencing CO₂ release from rivers.