Downstream carbon transport and surface CO₂ evasion in the Hanjiang River Network and their implications for regional...

Fluvial carbon fluxes have been increasingly recognized as important components of the global carbon budget. However, it is challenging to accurately quantify carbon fluxes in river networks; therefore, the role of carbon fluxes in the regional carbon budget remains poorly understood. The Hanjiang River Network (HRN) is located in a subtropical monsoon climate zone, and its material transport has a notable impact on the Changjiang River. In this study, it was hypothesized that the total fluvial carbon fluxes from the river network in the subtropical monsoon climate zone are dominated by vertical CO₂ evasion and account for a large fraction of terrestrial net primary productivity (NPP) (e.g., 10 %) and fossil CO₂ emissions (e.g., 30 %), which is roughly equivalent to the global average. Therefore, the downstream export of three carbon fractions and CO₂ evasion were estimated in the HRN over the last two decades and the findings were compared with NPP and fossil CO₂ emissions in the basin. The results suggest that approximately 2.14-6.02 Tg C year^-1 (1 Tg = 10¹² g) of carbon is exported in the HRN. Vertical CO₂ evasion represents the largest destination at 1.22-5.34 Tg C year^-1 or 68 % of the total fluvial carbon flux component, corresponding to 1.5 %-11 % of the fossil CO₂ emissions. Downstream export of dissolved inorganic carbon is the second largest destination with a magnitude of 0.56-1.92 Tg C year^-1. Downstream organic carbon export plays a relatively small role with a magnitude of 0.04-0.28 Tg C year^-1. The findings also indicate that the offset of total fluvial carbon fluxes from terrestrial NPP is unexpectedly small (2.0 %-5.4 %). Data availability and the simplification of carbon processes introduced uncertainty; therefore, future research should incorporate a fuller representation of fluvial carbon processes and fractions to improve regional-scale carbon accounting.