Depressed or Elevated: Microtopography Controls Carbon Loss Processes in Arctic Tundra.

Large reserves of organic carbon (C) in the active layer and permafrost affected soils of the Arctic tundra ecosystems are vulnerable to accelerated microbial degradation and release as greenhouse gases like methane (CH₄) and carbon dioxide (CO₂). In the present microcosm study, we determine dynamics and fluxes of CH₄ and CO₂ from depressed, elevated and subsided areas of low- and high-centered polygons from interstitial tundra on the Barrow Environmental Observatory (Barrow, AK). Temperature sensitivities of anaerobic respiration and methanogenesis were determined for organic, mineral and permafrost horizons incubated at -2, +4, or +8 °C up to 60 days. Production rates for both CO₂ and CH₄ were substantially higher for organic horizons (20 to 40 % wt. C) than the mineral horizons (< 18 % wt. C). Permafrost soils (~12 % wt. C) produced CO₂ but negligible CH₄. A characteristic lag phase, temperature threshold for methanogenesis, and temporal dynamics indicated that a constant Q₁₀ relationship is inadequate to explain temperature responses from a range of -2 to +8 °C. Temperature response of methanogen mcrA genes (encoding the alpha subunit of methyl coenzyme M reductase) were correlated to the relative abundance of active methanogens in the active layer vs. permafrost. qPCR analysis revealed negligible abundance of mcrA gene copies in the permafrost. Time course measurements in this study were used to estimate continuous, differentiable functions for CH₄ and CO₂ production at relevant temperatures. These functions enable rate calculations that will help parameterize Arctic terrestrial ecosystem models for anaerobic biogeochemical processes.