Integrated human and natural regulation of carbon concentrations and fluxes dynamics in the Yangtze River mainstream over the past two decades.

Rivers play a key role in the global carbon cycle, yet the long-term spatiotemporal dynamics of dissolved organic carbon (DOC), particulate organic carbon (POC), and dissolved inorganic carbon (DIC) in large rivers remain uncertain, particularly for large Asian rivers where simultaneous assessments of all three forms are scarce. We compiled a monthly database (2000-2022) of DOC, POC, and DIC concentrations at 10 sites along the Yangtze River mainstream by integrating 2022 field campaigns with a systematic literature synthesis, and applied three machine learning algorithms to reconstruct continuous monthly concentrations. Fluxes were derived by combining the best-performing concentration estimates with discharge. From headwaters to estuaries, all three fluxes increased, driven by precipitation-controlled runoff growth, whereas concentrations diverged: DOC increased downstream through elevated CODMn (permanganate index) from runoff flushing; POC declined via vegetation interception and reservoir retention; and DIC decreased through runoff dilution. Over the past two decades, DOC and DIC fluxes were primarily regulated by precipitation-driven discharge, but DOC flux also rose with increasing DOC concentration, while POC flux declined synchronously with POC concentration, and DIC flux showed no significant trend. The corresponding concentration changes-a rise in DOC and declines in POC and DIC-were primarily driven by cascade reservoir construction and climate warming. Consequently, the Yangtze River mainstream is transitioning from a system historically dominated by POC and DIC export toward one increasingly characterized by DOC export. This transition has important implications for carbon burial efficiency and greenhouse gas emissions.