Fluvial CO₂ and CH₄ patterns across wildfire‐disturbed ecozones of subarctic Canada: Current status and implications for future change

Abstract

Despite occupying a small fraction of the landscape, fluvial networks are disproportionately large emitters of CO2 and CH4, with the potential to offset terrestrial carbon sinks. Yet the extent of this offset remains uncertain, because current estimates of fluvial emissions often do not integrate beyond individual river reaches and over the entire fluvial network in complex landscapes. Here we studied broad patterns of concentrations and isotopic signatures of CO2 and CH4 in 50 streams in the western boreal biome of Canada, across an area of 250,000 km2. Our study watersheds differ starkly in their geology (sedimentary and shield), permafrost extent (sporadic to extensive discontinuous) and land cover (large variability in lake and wetland extents). We also investigated the effect of wildfire, as half of our study streams drained watersheds affected by megafires that occurred 3 years prior. Similar to other boreal regions, we found that stream CO2 concentrations were primarily associated with greater terrestrial productivity and warmer climates, and decreased downstream within the fluvial network. No effects of recent wildfire, bedrock geology or land cover composition were found. The isotopic signatures suggested dominance of biogenic CO2 sources, despite dominant carbonate bedrock in parts of the study region. Fluvial CH4 concentrations had a high variability which could not be explained by any landscape factors. Estimated fluvial CO2 emissions were 0.63 (0.09–6.06, 95% CI) and 0.29 (0.17–0.44, 95% CI) g C m−2 year−1 at the landscape scale using a stream network modelling and a mass balance approach, respectively, a small but potentially important component of the landscape C balance. These fluvial CO2 emissions are lower than in other northern regions, likely due to a drier climate. Overall, our study suggests that fluvial CO2 emissions are unlikely to be sensitive to altered fire regimes, but that warming and permafrost thaw will increase emissions significantly.

Thermokarst amplifies fluvial inorganic carbon cycling and export across watershed scales on the Peel Plateau, Canada

“…7av). Further, immediately downstream of the RTS FM2 inflow to Dempster Creek, the decrease in CO 2 and shift in δ 13 C-CO 2 away from a biotic source suggest that CO 2 degassing to the atmosphere was more prominent than respiration of permafrost DOC (Doctor et al, 2008;Drake et al, 2018b;Kendall et al, 2014). Thus, immediately downstream of RTSs, microbial respiration of permafrost DOC does not appear to generate substantial CO 2 .…”

“…While respiration likely produced some CO 2 in RTS FM2 runoff (Littlefair et al, 2017), observed δ 13 C-CO 2 (−11 ‰) more strongly reflected H 2 SO 4 weathering of regional carbonate bedrock (−0.7 ‰ to −5.6 ‰; Hitchon and Krouse, 1972) when accounting for isotopic fractionation of ∼ 8 ‰ between carbonate and CO 2 at the temperature of FM2 runoff (18 • C; Clark and Fritz, 1997). Along the lower reach of the FM2 runoff transect, the increase in δ 13 C-CO 2 aligned with the preferential loss of 12 C in the CO 2 phase via DIC fractionation and degassing (Doctor et al, 2008;Drake et al, 2018b;Kendall et al, 2014). 13 C enrichment of the CO 2 pool by methanogenesis (Campeau et al, 2018), photosynthesis (Descolas-Gros and Fontungne, 1990), and/or calcite precipitation (Turner, 1982) was unlikely as CH 4 in FM2 runoff was relatively low (pCH 4 = 3.6 ± 1.9 µatm, mean ± SD, n = 6), the high turbidity of FM2 runoff likely inhibited photosynthesis (Levenstein et al, 2018), and calcite was below saturation (SI = −0.79).…”

“…Finally, ε was subtracted from δ 13 C-HCO − 3 to obtain theoretical δ 13 C-CO 2 . Similarity between observed and theoretical δ 13 C-CO 2 values was interpreted as δ 13 C-CO 2 variability driven by carbonate equilibrium reactions, whereas dissimilarity was taken to reflect effects from CO 2 degassing (Zhang et al, 1995) and/or biotic CO 2 production (Kendall et al, 2014). Stream networks and watershed areas were delineated using the ArcHydro tools in ArcGIS 10.5 from the gridded (30 m) Canadian Digital Elevation Model (CDEM).…”

Net landscape carbon balance of a tropical savanna: Relative importance of fire and aquatic export in offsetting terrestrial production

“…NEP savanna and NEP wetland are the annual net terrestrial C productivity for the savanna and wetland components, respectively, and NEP is the area-weighted average for the whole catchment. Percentages in parentheses for each landscape component at the top of the diagram correspond to area weightings [Colour figure can be viewed at wileyonlinelibrary.com] et al, Horgby et al, 2019;Hutchins, Tank, et al, 2020). (Iversen et al, 2019).…”

Isotope applications to soil science at the University of Alberta — an historical perspective

“…By combining measurements of 14 C of dissolved organic carbon and CO 2 released from northern permafrost peatlands to 13 C-CO 2 analyses, D. Olefeldt and his group were able to co-currently determine the age and sources of CO 2 to streams and to differentiate between CO 2 released from weathering vs. soil organic matter degradation (Burd et al 2018;Estop-Aragonés et al 2018). In addition, 13 C-CH 4 chamber measurements were used to assess whether methane was produced via acetoclastic or hydrogenotrophic pathways in wetland soils of different permafrost conditions and from thermokarst ponds in peatlands (Hutchins et al 2020). Lastly, 13 Clabelled glucose was added to peat during anaerobic incubations to test hypotheses of primingagain to study whether peat following permafrost thaw is vulnerable to rapid decomposition (unpublished results).…”