Effect of Sphagnum cover on GHG evolution and decomposition-chain exoenzyme activity of arctic thermokarst peat

As the global average temperature climbs, the poles are warming at a rate disproportionate to that of the equator, causing rapid permafrost melt. Arctic palsa mire ecosystems are an immensely valuable, and critically endangered, terrestrial carbon sink. The effects of climate warming on palsa mire ecosystems are capricious and complex, particularly in terms of decomposition dynamics in permafrost thaw features, like thermokarst pools that develop following palsa mound collapse. These dynamics can be explored by enzymatic chain components, and their subsequent effect on nutrient cycling and greenhouse gas emission. To study the biochemical mechanisms of permafrost melt in palsa mires, peat samples were taken from four palsa mires in the vicinity of Kilpisjärvi village, northernmost Finland. These samples were used to create thermokarst simulation mesocosms, from which experimental data was collected. Our experiment utilised fluorescent hydrolase substrate assays to determine the effects of 1) Sphagnum cover and 2) peat quality on enzyme activity within experimental chambers, and gas chromatography to assess the amount of methane and carbon dioxide dissolved in the water column. Quality of dissolved organic matter (absorbance spectra and phenolic content) as well as phenol substrate oxidation rate were measured. We considered these data against pH and redox measurements to understand the consequential changes in chemical pathways of arctic peatlands. It was found that recharacterization of palsa mire ecosystems by the formation of thermokarst pools can have an immediate effect on the emission of greenhouse gases (GHG) from the water column, with Sphagnum treatment significantly increasing dissolved GHG concentration. This effect is likely due to “priming” by fresh, labile organic compounds, as well as Sphagnum physically blocking gas exchange. Addition of Sphagnum stimulated phenol oxidase activity, contrary to our expectations. Hydrolytic enzyme activity increased with peat age, which is expected considering their ability to function in hypoxia. Further studies detailing greenhouse gas efflux in the field and specific microbial mechanisms should be carried out to more accurately project the full, long term effects of climate change on palsa mires and subsequent climate feedback.