Elucidating the Role of Geochemical and Microbial Processes in Arctic System Carbon Dynamics.

Soils from high-latitude ecosystems have an estimated storage of nearly 2000 Pg of carbon. Climate change scenarios predict the highest magnitude of global warming in these latitudes, causing accelerated permafrost thawing that could lead to a catastrophic release of carbon from these soils. The source-sink feedback effects in the Arctic are poorly understood in terms of the rates of organic matter degradation and transformation of carbon pools in a warming scenario. The rates of organic matter degradation and the proportion of CH₄ and CO₂ greenhouse gasses released are governed by the microbial response to warming, organic matter senitivity and availability, pH and redox conditions. To enhance the process based understanding and improve representation of Arctic subsur­face responses to climate change in land models, we intensively examined soil organic matter transformations from middle, ridge and trough areas of a low-centered polygon on the Barrow Environmental Observ­atory (Barrow, Alaska). Significant amounts of iron(II) in organic and mineral soils of the active layer and groundwater indicate anoxic conditions. Unamended incubations of soils using relevant anoxic conditions at -2, +4 or +8 °C produced both CH₄ and CO₂, with different response curves. CO₂ formation followed Monod kinetics with a short lag phase. CH₄ production followed exponential kinetics, with a longer lag characteristic of microbial growth and adaptation. Rates of formation for both CH₄ and CO₂ were substantially higher in microcosms containing organic horizon samples (38-43% total carbon), compared to B horizon samples (17-18% carbon) or permafrost (16-18% carbon) that produced CO₂ but not CH₄ during incubations.