Title: Temporal, Spatial, and Temperature Controls on Organic Carbon Mineralization and Methanogenesis in Arctic High-Centered Polygon Soils

Warming temperatures in continuous permafrost zones of the Arctic will alter both hydrological and geochemical soil conditions, which are strongly linked with heterotrophic microbial carbon (C) cycling. Heterogeneous permafrost landscapes are often dominated by polygonal features formed by expanding ice wedges: water accumulates in low centered polygons (LCPs), and water drains outward to surrounding troughs in high centered polygons (HCPs). These geospatial differences in hydrology cause gradients in biogeochemistry, soil C storage potential, and thermal properties. Presently, data quantifying carbon dioxide (CO ₂ ) and methane (CH ₄ ) release from HCP soils are needed to support modeling and evaluation of warming-induced CO ₂ and CH ₄ fluxes from tundra soils. This study quantifies the distribution of microbial CO ₂ and CH ₄ release in HCPs over a range of temperatures and draws comparisons to previous LCP studies. Arctic tundra soils were initially characterized for geochemical and hydraulic properties. Laboratory incubations at −2, +4, and +8°C were used to quantify temporal trends in CO ₂ and CH ₄ production from homogenized active layer organic and mineral soils in HCP centers and troughs, and methanogen abundance was estimated from mcrA gene measurements. Results showed that soil water availability, organic C, and redox conditions influence temporal dynamics and magnitude of gas production from HCP active layer soils during warming. At early incubation times (2–9 days), higher CO ₂ emissions were observed from HCP trough soils than from HCP center soils, but increased CO ₂ production occurred in center soils at later times (>20 days). HCP center soils did not support methanogenesis, but CH ₄ -producing trough soils did indicate methanogen presence. Consistent with previous LCP studies, HCP organic soils showed increased CO ₂ and CH ₄ production with elevated water content, but HCP trough mineral soils produced more CH ₄ than LCP mineral soils. HCP mineral soils also released substantial CO ₂ but did not show a strong trend in CO ₂ and CH ₄ release with water content. Knowledge of temporal and spatial variability in microbial C mineralization rates of Arctic soils in response to warming are key to constraining uncertainties in predictive climate models.