Thermokarsting Alters Belowground Biogeochemistry and Greenhouse Gas Fluxes

Climate change poses a significant risk to the function of Arctic ecosystems and the stability of frozen carbon (C) currently sequestered in Arctic permafrost. Substantial progress has been made in understanding and predicting how Arctic plant and soil C cycling will respond to warming. However, models remain uncertain, particularly with respect to the variable response of soil biogeochemistry and greenhouse gas (GHG) flux to warming within different plant functional types (PFTs) and landscape positions. The project’s overarching objective is to link soil GHG flux [carbon dioxide (CO₂), methylene, nitrous oxide (N₂O)] and soil biogeochemical variables to different landscape positions and PFTs. These findings will help resolve spatial variability in Arctic ecosystem response to warming. Here, the team presents data from field campaigns in Council, AK, in 2021 and 2023 and preliminary data from a laboratory mesocosm currently being conducted. Researchers found that a lowland thermokarst wetland containing aerenchymas graminoids acts as a net source of GHGs due to significant (p<0.05) emission of methane (CH₄), while the upland tundra containing shrub, lichen, and forb PFTs generally acts as a net sink of GHGs during the late thaw season. Upland PFTs correlate well with belowground chemistry (i.e., redox, pH, dissolved organic carbon) while belowground chemistry correlates well with soil GHG fluxes (CO₂, CH₄, N₂O). The project therefore postulates that PFT alters belowground biogeochemistry, which in turn affects how C is cycled and emitted as GHGs. Graminoids present at the lowland thermokarst wetland location likely contribute the most to rapid and sustained changes in belowground biogeochemistry, likely due to aerenchyma conducting GHG from the soil to the atmosphere as well as introducing oxygen to the otherwise anoxic rooting zone. The team additionally presents preliminary data on a rhizobox mesocosm experiment investigating interactions between this aerenchyma graminoid belowground biogeochemistry.