Biogeochemical Controls on Microbial CO2 and CH4 Production in Polygonal Soils From the Barrow Environmental Observatory

Abstract:

Organic matter buried in Arctic soils and permafrost will become accessible to increased microbial degradation as the ground warms due to climate change. The rates of organic matter degradation and the proportion of CH₄ and CO₂ greenhouse gasses released in a potential warming feedback cycle depend on the microbial response to warming, organic carbon structure and availability, the pore-water quantity and geochemistry, and available electron acceptors. Significant amounts of iron(II) ions in organic and mineral soils of the active layer in low-centered ice wedge polygons indicate anoxic conditions in most soil horizons. To adapt and improve the representation of these Arctic subsurface processes in terrestrial ecosystem models for the NGEE Arctic project, we examined soil organic matter transformations from elevated and subsided areas of low- and high-centered polygons from interstitial tundra on the Barrow Environmental Observatory (Barrow, AK). Using microcosm incubations at fixed temperatures and controlled thawing systems for frozen soil cores, we investigated the microbiological processes and rates of soil organic matter degradation and greenhouse gas production under anoxic conditions, at ecologically relevant temperatures of -2, +4 or +8 °C. In contrast to the low-centered polygon incubations representing in situ water-saturated conditions, microcosms with unsaturated high-centered polygon samples displayed lower carbon mineralization as either CH₄ or CO₂. Substantial differences in CH₄ and CO₂ response curves from different microtopographic samples separate the thermodynamic controls on biological activity from the kinetic controls of microbial growth and migration that together determine the temperature response for greenhouse gas emissions in a warming Arctic.