Unraveling the carbon fraction heterogeneity in China's Rivers: Hydrology, nutrients, and dam regulation.

Representatively, the river carbon (C) cycle involves complex processes and interactions, leading to pronounced spatiotemporal C concentration heterogeneity that effectively complicates its accurate assessment. It is critical to resolve this issue to reconcile discrepancies in terrestrial C sink evaluations. Accordingly, we used a comprehensive dataset encompassing 1670 water quality stations across 944 rivers in China as well as multiple models based on water quality parameters to resolve the spatiotemporal heterogeneity of different C species at a national scale. The C fraction in these representative China's rivers exhibited significant heterogeneity across basins, stream orders, headwaters-to-estuaries, and seasonal patterns, wherein dissolved inorganic C (DIC), dissolved organic C (DOC), and particulate organic C (POC) concentrations were measured at 25.68 ± 12.96, 3.59 ± 1.29, and 1.07 ± 0.57 mg/L, respectively. Natural processes dominated, with precipitation and available nutrients jointly explaining 39.56-52.60 % of the variability. Specifically, because of C source differences, 61 % of rainstorms reduced DIC concentrations via dilution while 68 % and 77 % enhanced DOC and POC concentrations through flushing, respectively. Total phosphorus (TP) was the dominant organic C (OC) predictor. Additionally, correlations between C and nutrients varies from sub-catchment to systems. Notably, widespread damming has enhanced river retention and regulated hydrological regimes, removing 24.46 ± 24.79 % POC within regulated segments. This removal is unrelated to reservoir characteristics, instead being primarily determined by upstream POC concentrations. Given the ongoing reservoir infrastructure expansion taking place in China, greater attention should be directed toward the implications of dam construction on riverine C cycling and its alignment with national dual C goals.