RESPONSE OF CARBON FLUXES TO SOIL MOISTURE VARIABILITY ACROSS AN ALASKAN TUNDRA LANDSCAPE

Soils in arctic and sub-arctic permafrost regions store large amounts of carbon (C), which is becoming more biologically available as soils warm and permafrost thaws. Microbial decay of organic forms of C can result in the production and emission of carbon dioxide (CO₂) and methane (CH₄), and the amount and form of C released into the atmosphere depend on organic matter composition and soil conditions. Soil moisture, which is a strong driver of microbial processes, varies spatially and temporally across tundra landscapes and may change dramatically as a result of permafrost thaw. The Yukon-Kuskokwim Delta (YKD) of Alaska is underlain by discontinuous permafrost and is particularly vulnerable to permafrost thaw and soil moisture changes associated with thaw. As permafrost thaws, some areas may dry as drainage increases with increasing thaw depth. Alternatively, permafrost thaw may lead to ground subsidence and saturation of previously dry soils. Our objective was to investigate patterns in C storage and processing across the landscape and in response to changes in soil moisture in the YKD. We analyzed soil C pools (0-30 cm) and CO₂ and CH₄ concentrations in soils from sites of different land cover and landscape position, including moist and dry peat plateaus, high and low intensity burned plateaus, fens, and drained lakes. We also conducted aerobic and anaerobic soil incubations to determine changes in CO₂ and CH₄ production under a range of soil moisture conditions.

Soils from burned plateaus, which were drier and had lower C content than unburned soils, had higher CO₂ production (per g soil) than unburned soils during aerobic incubations. Both increased and decreased moisture reduced CO₂ production from soils. Soil drying increased net CH₄ uptake in all aerobically-incubated burned soils, and wetting resulted in CH₄ emissions from low intensity burn soils. CO₂ and CH₄ production from fen soils were higher than from the other landscape positions analyzed here. Our results suggest that soil drying could lead to decreased microbial respiration, whereas subsidence may result in increased methanogenesis. Additionally, amplified CH₄ release from burned soils after rainfall events or subsidence may accompany the increased fire frequency projected in tundra regions.