Effects of permafrost thaw and changes in soil moisture on CH4 and CO2 fluxes in Alaskan upland tundra

The magnitude and rate of greenhouse gas emissions (methane (CH₄) or carbon dioxide (CO₂)) are highly uncertain in Arctic ecosystems. The fraction of C released as CH₄ may increase with ground subsidence from thawing permafrost, where newly waterlogged areas may shift even relatively well-drained upland tundra areas to stronger CH₄ sources.

To examine how total C emissions change with variable thaw depth and soil moisture conditions, measurements were made from the Carbon in Permafrost Experimental Heating Research (CiPEHR) project located in Interior Alaska. This site has undergone experimental soil warming from 2009 - 2018 and water table manipulation since 2011. Methane and CO₂ fluxes were measured in 2017 across a gradient of active layer thickness (ALT) and water table depths (WTD) that fell into 3 categories: control (WTD -21 cm; ALT -70 cm), wetter + deeper thaw (WTD -11 cm; ALT -75 cm) to reflect sites where increased drainage would result in drier, warmer soil conditions, and inundated + deeply thawed (WTD +1 cm; ALT -108 cm) as a result of ground subsidence and resulting in warmer, wetter soils.

The wetter + deeper thaw treatment experienced highest ecosystem respiration rates (532.0 ± 79.6 g C m^-2) relative to inundated + deeply thawed (327.5 ± 38.2 g C m^-2) and control (399.5 ± 33.9 g C m^-2). Methane emissions were highest in the inundated + deeply thawed treatment (1050.1 ± 433.5 mg C m^-2) relative to control (82.6 ± 15.4 mg C m^-2), or wetter + deeper thaw conditions (164.8 ± 57.5 mg C m^-2). Methane emissions response to soil warming was variable, with fluxes ranging from net uptake of CH₄ to relatively strong emissions rates. Plots with strongest CH₄ emissions were inundated and had high graminoid biomass (Eriophorum vaginatum).

Total C respiration rates (CO₂ + CO₂ equivalent of CH₄) were calculated by accounting for the mass difference between the gases and multiplying CH₄ by its 100 year sustained emissions global warming potential. Highest total C respiration rates occur with wetter + deeper thaw (539.4 g C m^-2), relative to respiration rates in inundated + deeply thawed (374.7 g C m^-2) or control (403.2 g C m^-2). These results highlight the effects of warming on C cycling in permafrost regions, where increased thaw and thaw-driven hydrologic changes play a substantial role in determining the form and magnitude of C release.