Groundwater as a limited carbon dioxide source in a large river (the Yangtze River).

Groundwater discharge to river networks makes up a major source of riverine CO emission, available evidence however comes mainly from headwater streams which are directly connected to terrestrial ecosystems and spatially limited in terms of system size. Here relying on coupled water and CO mass balances, we quantified the groundwater-mediated CO input to the Yangtze River mainstem on an annual basis, where the mass balance of water provided physical constraints on CO exchange between the river and groundwater. A landscape topographic control of the groundwater-river interaction was proposed where mountain reaches preferentially receive water and CO discharge from the groundwater while plain alluvial reaches predominantly lose water to the aquifers. Groundwater CO inputs were however small in magnitude on all reaches (0.3-14% of the total CO emission and transport by the river) and unable to account for the discrepancy between surface evasion and internal metabolism in the river. Minor direct groundwater discharge to the reaches in comparison to smaller streams (negative to < 3.5% of the surface water flows) was concluded to be the main reason for low groundwater-sourced CO in the large river reaches.