Nutrient Controls on Methane Emissions in a Permafrost Thaw Subarctic Peatland

Abstract:

Permafrost peatlands in northern latitudes are large reservoirs of sequestered carbon that are vulnerable to climate change. While peatlands account for a small fraction of total global land surfaces, their potential to release sequestered carbon in response to higher temperatures is of concern. Of particular relevance is the conversion of these carbon stores into methane (CH₄), a strong greenhouse gas with a global warming potential 20 times greater than that of CO2 over a 100-year time frame. Here, we explore how key nutrients impact the consumption of CH₄ at the Stordalen Mire in Abisko, Sweden, a discontinuous permafrost peatland with expanding thaw over the last century. Peatland CH4 emissions are highly spatially variable due to multiple emission pathways and strong dependence on several environmental factors. Among controls on CH₄ emissions, such as temperature and water table depth, primary production of wetland vegetation is also a strong factor in the variability of CH₄ emissions. Plant community shifts among permafrost thaw stages subsequently change nutrient cycling and availability, which in turn impacts primary production. Early stages of permafrost thaw are mosaicked with a variety of vascular plants and mosses. We analyzed potential enzymatic activities of chitinase, glucosidase, and phosphatase as proxies for organic nitrogen, carbon, and phosphorus cycling, respectively, in tandem with potential CH₄ oxidation rates. In addition, stoichiometric ratios of carbon, nitrogen, and phosphorus concentrations are used to illustrate nutrient limitation controls on CH₄ oxidation rates. While CH₄ emissions are low throughout initial thaw stages, < 7 CH₄ mg m^-2 day^-1, we found they had the highest rates of potential CH₄ oxidation. These permafrost thaw-induced CH₄ oxidation rates are 5 and 11 times higher, in the surface and depth of the peat profile respectively, than subsequent aerobic permafrost thaw stages. As CH₄ emissions are low in intact permafrost peatlands, these high rates of potential CH₄ oxidation indicate the importance of plant communities and the methanotrophic microbes they harbor.