Isotopic insights into methane production, oxidation, and emissions in Arctic polygon tundra

Abstract

Arctic wetlands are currently net sources of atmospheric CH4 . Due to their complex biogeochemical controls and high spatial and temporal variability, current net CH4 emissions and gross CH4 processes have been difficult to quantify, and their predicted responses to climate change remain uncertain. We investigated CH4 production, oxidation, and surface emissions in Arctic polygon tundra, across a wet-to-dry permafrost degradation gradient from low-centered (intact) to flat- and high-centered (degraded) polygons. From 3 microtopographic positions (polygon centers, rims, and troughs) along the permafrost degradation gradient, we measured surface CH4 and CO2 fluxes, concentrations and stable isotope compositions of CH4 and DIC at three depths in the soil, and soil moisture and temperature. More degraded sites had lower CH4 emissions, a different primary methanogenic pathway, and greater CH4 oxidation than did intact permafrost sites, to a greater degree than soil moisture or temperature could explain. Surface CH4 flux decreased from 64 nmol m(-2) s(-1) in intact polygons to 7 nmol m(-2) s(-1) in degraded polygons, and stable isotope signatures of CH4 and DIC showed that acetate cleavage dominated CH4 production in low-centered polygons, while CO2 reduction was the primary pathway in degraded polygons. We see evidence that differences in water flow and vegetation between intact and degraded polygons contributed to these observations. In contrast to many previous studies, these findings document a mechanism whereby permafrost degradation can lead to local decreases in tundra CH4 emissions.

Ice wedge degradation and CO2 and CH4 emissions in the Tuktoyaktuk Coastlands, NT

“…It is clear from this study and others (Sachs et al 2010, Vaughn et al 2016, 400 Wainwright et al 2015, Lara et al 2015 Map of the study area showing study sites, nearby communities, and ecoregion boundaries. The red line shows the limit the 1:1 tree: upland tundra cover ratio as mapped by (Timoney et al 1992).…”

“…Melt ponds in high-centred polygonal terrain in the Tuktoyaktuk Coastlands were 365 larger sources of CO 2 and CH 4 relative to other forms of thermokarst and other northern 366 water bodies (Hamilton et al 1994;Kling et al 1991;Laurion et al 2010;Negandhi et al 367 2013 and from non-thermokarst water bodies in Alaska (Kling et al 1991, -0.23 Vaughn et al (2016) and from polygon rims (3.6 mmol CO 2 m -2 h -1 ) and low center 384 polygons (2.16 mmol CO 2 m -2 h -1 ) observed by Zona et al (2011). They were lower than 385 the emissions observed by Lara et al (2015) Vaughn et al 2016).…”

Impacts of temperature and soil characteristics on methane production and oxidation in Arctic polygonal tundra

“…Acetate was most abundant and exhibited the most dynamic concentration changes among individual organic 10 acids measured from the anaerobic microcosms, which is consistent with previous studies on LCP and HCP carbon decomposition (Yang et al, 2016). The consumption of acetate correlated with increases of CH 4 and CO 2 concentrations in previous incubation experiments suggested either acetoclastic methanogenesis or syntrophic acetate oxidation coupled to hydrogenotrophic methanogenesis, which are both consistent with isotopic analyses of CH 4 from the site (Throckmorton et al, 2015;Vaughn et al, 2016). Using reaction stoichiometry for acetoclastic methanogenesis and anaerobic respiration 15 through iron reduction (Istok et al, 2010), we estimated the amount of acetate being consumed by these parallel processes in the transition zone and permafrost (Fig.…”

“…Although a number of factors, including vegetation height and plant composition 25 (von Fischer et al, 2010), soil inundation (Sturtevant et al, 2012), thaw depth (Sturtevant and Oechel, 2013;Grant et al, 2017), and season (Chang et al, 2014) were suggested as explanatory factors for CH 4 flux variations, the huge differences in CH 4 flux between polygon types could not be fully explained by variations in moisture or temperature (Sachs et al, 2010;Vaughn et al, 2016). High concentrations of dissolved CH 4 in the deeper active layer, contrasted with low surface emissions from FCPs and HCPs, suggest the importance of methane oxidation in these landscape features might be underestimated 30 (Vaughn et al, 2016).…”

Thaw Transitions and Redox Conditions Drive Methane Oxidation in a Permafrost Peatland

“…Thus, as permafrost is thawing due to atmospheric warming at a global scale (Olefeldt et al, 2016;Turetsky et al, 2019), it is vital to understand the potential of MOB to attenuate CH 4 emissions from thawing permafrost. Previous work comparing the abundance/activity of MOB using intact and thawed sites (Liebner & Wagner, 2007;Vaughn et al, 2016) or through in vitro experiments thawing permafrost cores (Mackelprang et al, 2011;Voigt et al, 2019) suggests that permafrost thaw can increase CH 4 oxidation. However, these studies focused on comparing end-member thaw states, which omit CH 4 oxidation in transitional thaw stages that can cover a considerable area of thawing permafrost peatlands (Malmer et al, 2005;Palace et al, 2018).…”