Dynamics of riverine CO₂ in the Yangtze River fluvial network and their implications for carbon evasion

Abstract. Understanding riverine carbon dynamics is critical for not only better estimates of various carbon fluxes but also evaluating their significance in the global carbon budget. As an important pathway of global land–ocean carbon exchange, the Yangtze River has received less attention regarding its vertical carbon evasion compared with lateral transport. Using long-term water chemistry data, we calculated CO₂ partial pressure (pCO₂) from pH and alkalinity and examined its spatial and temporal dynamics and the impacts of environmental settings. With alkalinity ranging from 415 to > 3400 µeq L⁻¹, the river waters were supersaturated with dissolved CO₂, generally 2–20-fold the atmospheric equilibrium (i.e., 390 µatm). Changes in pCO₂ were collectively controlled by carbon inputs from terrestrial ecosystems, hydrological regime, and rock weathering. High pCO₂ values were observed spatially in catchments with abundant carbonate presence and seasonally in the wet season when recently fixed organic matter was exported into the river network. In-stream processing of organic matter facilitated CO₂ production and sustained the high pCO₂, although the alkalinity presented an apparent dilution effect with water discharge. The decreasing pCO₂ from the smallest headwater streams through tributaries to the mainstem channel illustrates the significance of direct terrestrial carbon inputs in controlling riverine CO₂. With a basin-wide mean pCO₂ of 2662 ± 1240 µatm, substantial CO₂ evasion from the Yangtze River fluvial network is expected. Future research efforts are needed to quantify the amount of CO₂ evasion and assess its biogeochemical implications for watershed-scale carbon cycle. In view of the Yangtze River's relative importance in global carbon export, its CO₂ evasion would be significant for global carbon budget.