Greenhouse gas balance of thermokarst fens and bogs in the discontinuous permafrost zone during extreme drought years

The greenhouse gas (GHG) balance of northern peatlands is being influenced by the combined effects of rapid permafrost thaw and increasing frequency and severity of droughts. Our soil chamber flux measurements revealed that thermokarst fens in the discontinuous permafrost zone of boreal western Canada (Lutose, Alberta) acted as stronger net sources of carbon dioxide (CO2) and consistently emitted more methane (CH4) than thermokarst bogs during two extreme-drought years. Peatlands in the discontinuous permafrost zone store large amounts of soil carbon, but thawing permafrost results in changes to hydrology and vegetation, which have the potential to substantially impact their greenhouse gas balance. As permafrost thaws, thermokarst bogs and fens develop and expand, further modifying GHG dynamics. Although thermokarst fens account for approximately 30% of peatlands in the discontinuous zone, they have received little attention, and there is limited understanding of how the GHG balance varies along the trophic gradient from poor fens to extreme-rich fens.

This study explored the spatial and temporal variability of greenhouse gas fluxes across four sites, including a thermokarst bog, poor fen, rich fen, and an extreme-rich fen. Trophic status of each site was classified based on pH, electrical conductivity (EC), vegetation, and concentrations of magnesium (Mg) and calcium (Ca). Fluxes were measured over two growing seasons and one winter (June 2023 to October 2024). Controls on greenhouse gas fluxes were explored using data on vegetation composition, water chemistry, hydrology, and climatic conditions. Our analysis showed that CH4 emissions generally increased along the trophic gradient, with the exception of the extreme-rich fen, where high sulfate concentrations suppressed emissions. Non-growing season CH4 emissions were also a significant contributor to annual emissions across all sites.

Overall, our findings indicate that trophic status plays an important role in determining the greenhouse gas balance of thermokarst bogs and fens following permafrost thaw. Understanding the drivers of the carbon dioxide balance and methane emissions in these ecosystems during extreme drought years is essential for refining models of peatland carbon dynamics and predicting their future role in the global carbon cycle as climate change continues.