Carbon redistribution driven by autochthonous processes in the world's largest water transfer project: Disrupting...

Natural rivers play a crucial role in regulating the global carbon cycle. However, as critical engineered waterways, inter-basin water transfer projects (IBWTs) substantially alter regional hydrological connectivity and biogeochemical processes, yet their contribution to the carbon cycle remains poorly understood. Here, we address this knowledge gap with a comprehensive field investigation of the South-to-North Water Diversion Middle Route Project (SNWD-MRP), the world's largest IBWT. The results show that carbon dynamics within the canal are dominated by autochthonous processes, primarily algal photosynthesis and microbial metabolism, which establish a seasonal pattern of "summer consumption and winter storage" for dissolved carbon species and are synergistically modulated by anthropogenic hydrological management. From summer to winter, the concentrations of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) increased by 20.26% and 30.95%, respectively, whereas the partial pressure of carbon dioxide (pCO₂) and the dissolved methane concentration (dCH₄) decreased by 49.19% and 42.06%, respectively. Using 2024 as an example, we found that the canal exported 236.94 Gg C yr^-1 laterally and emitted 33.77 Gg C yr^-1 vertically as carbon dioxide (CO₂) and methane (CH₄), representing only about 2.57% of the total flux from large natural rivers. Notably, the decadal cumulative lateral DOC export (146.78 Gg C) rivals the annual DOC export of large river systems such as the Yellow River. Algal-fixed CO₂ was microbially converted into potent greenhouse gases like CH₄, further intensifying the greenhouse effect caused by emissions from the entire canal. Overall, our findings suggest that the large-scale IBWT can create a novel aquatic corridor that disrupt natural carbon boundaries and drive regional-scale carbon redistribution, thereby providing critical insights for coordinating water resource management strategies with carbon neutrality objectives.