Isotopic insights into methane production, oxidation, and emissions in Arctic polygon tundra.

Abstract

Arctic wetlands are currently net sources of atmospheric CH4 . Due to their complex biogeochemical controls and high spatial and temporal variability, current net CH4 emissions and gross CH4 processes have been difficult to quantify, and their predicted responses to climate change remain uncertain. We investigated CH4 production, oxidation, and surface emissions in Arctic polygon tundra, across a wet-to-dry permafrost degradation gradient from low-centered (intact) to flat- and high-centered (degraded) polygons. From 3 microtopographic positions (polygon centers, rims, and troughs) along the permafrost degradation gradient, we measured surface CH4 and CO2 fluxes, concentrations and stable isotope compositions of CH4 and DIC at three depths in the soil, and soil moisture and temperature. More degraded sites had lower CH4 emissions, a different primary methanogenic pathway, and greater CH4 oxidation than did intact permafrost sites, to a greater degree than soil moisture or temperature could explain. Surface CH4 flux decreased from 64nmolm(-2) s(-1) in intact polygons to 7nmolm(-2) s(-1) in degraded polygons, and stable isotope signatures of CH4 and DIC showed that acetate cleavage dominated CH4 production in low-centered polygons, while CO2 reduction was the primary pathway in degraded polygons. We see evidence that differences in water flow and vegetation between intact and degraded polygons contributed to these observations. In contrast to many previous studies, these findings document a mechanism whereby permafrost degradation can lead to local decreases in tundra CH4 emissions.