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

Abstract

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 (Cm) decrease with increasing mean discharge (Qm), 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 (Cm) 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.

High Dissolved Carbon Concentration in Arid Rocky Mountain Streams

“…This is consistent with recent work that shows long-term mean DIC concentrations are driven by climate but short-term, instantaneous concentrations are regulated by the distribution of soil CO 2 depth distribution. 39 The data indicate one to two orders of magnitude higher DIC concentrations and fluxes than DOC, meaning most watersheds and streams generate and carry much more DIC than DOC. DOC can be transformed into DIC along river corridors, which can further become CO 2 gas and evade to the atmosphere.…”

“…Concentrations of DOC and DIC (or alkalinity) have been observed to rise across the globe, although warming effects often intertwine with the impacts of human activities. , Remote mountain areas often have minimal human impacts and experience faster warming, making them particularly suited for studying warming effects. , While the impacts of warming on water availability have been examined extensively, warming effects on water quality have remained poorly understood. Recent work has shown generally higher mean solute concentrations in arid climates in the United States and across the globe. ,− Here we ask the question: What are the patterns and drivers of DOC and DIC concentrations and fluxes in Rocky Mountain streams across gradients of climate, vegetation, and terrains? Answers to this question may offer an early glimpse in low lying areas when they inevitably experience a similar magnitude of warming in the future. , We compile data from across the Rockies to identify patterns and drivers and use a watershed reactor model to understand the mechanisms.…”

“…We chose to use arithmetic mean over the commonly used flow- (also discharge- or flux-) weighted average concentrations, similar to our previous work. , Concentrations of DOC and DIC typically have strong dependence on discharge − ,− so flow-weighted average depends on flow conditions (hydrological regimes) at the time of sampling. Discharge varies by orders of magnitude such that concentrations measured at high discharge are weighted more than concentrations at low discharge in flow-weighted calculations.…”

“…Concentrations of DOC commonly increase with increasing discharge (flushing pattern), often attributed to the escalating dominance of shallow, DOC-enriched soil water as water table rises at high discharge. − For the same reason, DIC or alkalinity often decreases with increasing discharge (dilution pattern) due to the rising dominance of shallow DIC-poor soil water (compared to deeper groundwater) at high discharge . Concerted measurements of DIC, 13 C isotopes, and instantaneous river discharge have revealed the dominant delivery of DIC-rich deep groundwater at low discharge and DIC-poor shallow soil water to streams at high discharges, respectively. − These instantaneous concentration–discharge relationships reflect the response of stream chemistry to day-to-day weather variations and are often regulated by their subsurface distribution over depth. ,− …”

Long‐term changes in dissolved inorganic carbon across boreal streams caused by altered hydrology

“…Specifically, while DOC typically decreases with soil depth in forested ecosystems (Lyon et al 2011), DIC shows the opposite pattern (Fig. 8) (Tang et al 2003; Öquist et al 2009; Stewart et al 2022). Increases in DIC concentration with depth reflect, in part, the flux of CO 2 to atmosphere, which creates a vertical CO 2 concentration gradient with values close to atmospheric levels in near‐surface strata.…”