Soil CO₂ Controls Short-Term Variation but Climate Regulates Long-Term Mean of Riverine Inorganic Carbon

The evasion of CO₂ from inland waters, a major carbon source to the atmosphere, depends on dissolved inorganic carbon (DIC) concentrations. Our understanding of DIC dynamics across gradients of climate, geology, and vegetation conditions however have remained elusive. To understand its large-scale patterns and drivers, we collated instantaneous and mean (multiyear average) DIC concentrations from about 100 rivers draining minimally-impacted watersheds in the contiguous United States. Within individual sites, instantaneous concentrations (C) measured at daily to seasonal scales exhibit a near-universal response to changes in river discharge (Q) in a negative power law form. High concentrations occur at low discharge when DIC-enriched groundwater dominates river discharge; low concentrations occur under high flow when relatively DIC-poor shallow soil water predominates river discharge. Such patterns echo the widely observed increase of soil CO₂ and DIC with depth and the shallow-and-deep hypothesis that emphasizes the importance of flow paths and source water chemistry. Across sites, mean concentrations (C_m) decrease with increasing mean discharge (Q_m), a long-term climate measure, and reachs maxima at around 200 mm/yr. A parsimonious model reveals that high mean DIC arises from soil CO₂ accumulation when rates of DIC-generating reactions are relatively high compared to its export fluxes in arid climates. Although instantaneous and mean DIC concentrations similarly decrease with increasing discharge, results here highlight their distinct drivers: daily to seasonal-scale instantaneous concentration variations (C) are controlled by subsurface CO₂ distribution over depth (from below), whereas long-term mean concentrations (C_m) are regulated by climate (from above). The results emphasize the significance of land-river connectivity via subsurface flow paths. They also underscore the importance of characterizing subsurface CO₂ distribution to illuminate belowground processes in order to project the future of water and carbon cycles in a warming climate.