Stable Carbon Isotopes Reveal Soil‐Stream DIC Linkages in Contrasting Headwater Catchments

Abstract

Large CO2 evasion to the atmosphere occurs as dissolved inorganic carbon (DIC) is transported from soils to streams. While this physical process has been the focus of multiple studies, less is known about the underlying biogeochemical transformations that accompany this transfer of C from soils to streams. Here we used patterns in stream water and groundwater δ13C‐DIC values within three headwater catchments with contrasting land cover to identify the sources and processes regulating DIC during its transport. We found that although considerable CO2 evasion occurs as DIC is transported from soils to streams, there were also other processes affecting the DIC pool. Methane production and mixing of C sources, associated with different types and spatial distribution of peat‐rich areas within each catchment, had a significant influence on the δ13C‐DIC values in both soils and streams. These processes represent an additional control on δ13C‐DIC values and the catchment‐scale cycling of DIC across different northern landscape types. The results from this study demonstrate that the transport of DIC from soils to streams results in more than just rapid CO2 evasion to the atmosphere but also represents a channel of C transformation, which questions some of our current conceptualizations of C cycling at the landscape scale.

Thermokarst Effects on Carbon Dioxide and Methane Fluxes in Streams on the Peel Plateau (NWT, Canada)

“…Given that the importance of nutrients and water temperature for regulating CH 4 varies among study locations (Stanley et al, 2016), these results indicate that CH 4 production in streams was more strongly coupled with organic carbon availability across the Peel Plateau landscape than with the exposure of permafrost material by RTSs. This aligns with the general understanding that CH 4 in streams is often associated with conditions that promote the accumulation of organic matter and enhance hydrologic connectivity between streams and C gas-rich soil porewaters in the surrounding landscape (Campeau et al, 2018;Crawford et al, 2013;Crawford, Stanley, et al, 2017;Jones & Mulholland, 1998;Wallin et al, 2014).…”

“…Efforts to refine these watershed‐scale flux estimates should include larger stream orders and new RTSs, characterize the effects of impounded streams (St. Pierre et al, ) and sedimentation (Kokelj, Tunnicliffe, et al, ) on C gas, explore p CO 2 ‐ p CH 4 dynamics across the soil‐stream interface (Campeau et al, ), and also work to understand the effects of thermokarst on fluvial C gas efflux in regions where permafrost contains a greater proportion of organic carbon (e.g., Hugelius et al, ; Olefeldt et al, ). Integrating terrestrial processes that can affect C gas in soils (e.g., vegetation‐nutrient dynamics, active layer deepening, and mineral weathering; Berner, ; Schuur & Mack, ; Striegl et al, ) into studies of fluvial C gas dynamics will also help to advance understanding of the northern C cycle.…”

Large‐Scale Landscape Drivers of CO2, CH4, DOC, and DIC in Boreal River Networks

“…Here we have reported a relationship between riverine p CH 4 and large‐scale landscape drivers that could help fill the gap in boreal riverine measurements by using easily obtained remotely sensed properties to extrapolate CH 4 concentrations at the whole network scale. There is evidence that groundwaters are a major source of CH 4 to streams (Campeau, Bishop, et al, ; Jones & Mulholland, ; Rasilo et al, ), in addition to that produced internally (Schindler & Krabbenhoft, ), and this would explain in part the covariation between CH 4 and CO 2 , since both may share a predominantly soil origin. In contrast to the pattern observed for CO 2 , however, p CH 4 did not consistently decline with downstream distance, despite the fact that both gases are subject to the physical forcing throughout the network, suggesting that the replenishment of CH 4 from local stream sources, likely the stream bed, roughly offsets degassing throughout the fluvial continuum.…”

High‐Resolution Spatial Sampling Identifies Groundwater as Driver of CO2 Dynamics in an Alpine Stream Network

“…This is, among other reasons, because of the typically high hydrological connectivity between groundwater with their adjacent soils in headwater streams (Butman & Raymond, 2011;Crawford et al, 2013;Peter et al, 2014). Shallow groundwater transports CO 2 derived from soil respiration (e.g., Campeau et al, 2018), while deeper groundwater can entrain dissolved inorganic carbon (DIC) from carbonaceous rock weathering (Amiotte Suchet et al, 2003) into the streams. The upwelling of groundwater may therefore locally enrich streamwater in CO 2 (and DIC) and subsequently increase CO 2 evasion fluxes along downstream reaches (Crawford et al, 2014;Duvert et al, 2018).…”

“…The sources of CO 2 in streamwater can be identified using stable carbon isotope analyses (Campeau et al, 2018;Kendall & Caldwell, 1998;Venkiteswaran et al, 2014). δ 13 C values of streamwater CO 2 depend on the relative contribution of carbon from respiration, weathering, and atmospheric exchange (e.g., Finlay, 2001).…”