Snow melt stimulates ecosystem respiration in Arctic ecosystems

Abstract

Cold seasons in Arctic ecosystems are increasingly important to the annual carbon balance of these vulnerable ecosystems. Arctic winters are largely harsh and inaccessible leading historic data gaps during that time. Until recently, cold seasons have been assumed to have negligible impacts on the annual carbon balance but as data coverage increases and the Arctic warms, the cold season has been shown to account for over half of annual methane (CH 4) emissions and can offset summer photosynthetic carbon dioxide (CO 2) uptake. Freeze-thaw cycle dynamics play a critical role in controlling cold season CO 2 and CH 4 loss, but the relationship has not been extensively studied. Here, we analyze freeze-thaw processes through in situ CO 2 and CH 4 fluxes in conjunction with soil cores for physical structure and porewater samples for redox biogeochemistry. We find a movement of water toward freezing fronts in soil cores, leaving air spaces in soils, which allows for rapid infiltration of oxygen-rich snow melt in spring as shown by oxidized iron in porewater. The snow melt period coincides with rising ecosystem respiration and can offset up to 41% of the summer CO 2 uptake. Our study highlights this important seasonal process and shows spring greenhouse gas emissions are largely due to production from respiration instead of only bursts of stored gases. Further warming is projected to result in increases of snowpack and deeper thaws, which could increase this ecosystem respiration dominate snow melt period causing larger greenhouse gas losses during spring.

Much stronger tundra methane emissions during autumn freeze than spring thaw

“…The difference in CH 4 emissions during spring thaw and autumn freeze can be explained by the different freeze-thaw and thaw-freeze processes ( Figure 6). During autumn freeze, soil freezes downward from the top soil layers and upward from the permafrost surfaces (Arndt et al, 2020;Bing et al, 2015).…”

“…Soil water migrates upward and downward to freezing fronts leaving "air pockets" in between, even in the inundated active layer (Arndt et al, 2020;Bing et al, 2015;Shoop & Bigl, 1997).…”

“…Therefore, large-scale CH 4 oxidation is less likely to happen during autumn freeze with the barrier effect of surface ice. By contrast, spring soil thaw can directly expose CH 4 to more oxygen, and melting of the oxygen-rich ice also provides an indirect route for oxygen delivery (Arndt et al, 2020). (Dai et al, 2019).…”

“…Furthermore, Arctic warming may be more apparent in the non‐growing season than in the growing season (Bekryaev et al., 2010; IPCC, 2014). Due to the relatively short active growing season and low biomass of tundra vegetation (Chapin, 1987), CH 4 emissions in the non‐growing season, including shoulder and cold seasons, will account for a substantial proportion of the annual budget (Arndt et al., 2020; Pirk et al., 2016; Schädel et al., 2016; Schuur et al., 2015; Treat et al., 2018; Zona et al., 2016).…”

Simulating shrubs and their energy and carbon dioxide fluxes in Canada's Low Arctic with the Canadian Land Surface Scheme Including biogeochemical Cycles (CLASSIC)

“…There are large uncertainties as to whether the Arctic tundra is currently an annual source or sink of CO 2 (McGuire et al, 2012). Recent studies have highlighted the importance of CO 2 emissions during the long winter and shoulder season periods, which may offset CO 2 gains by Arctic ecosystems during the short growing season Oechel et al, 2014;Euskirchen et al, 2017;Arndt et al, 2020). Long-term trends in winter CO 2 fluxes are generally difficult to ascertain due to a scarcity of year-round measurements, but several studies have suggested winter CO 2 emissions are changing.…”