Effects of Permafrost Thaw on Net Ecosystem Carbon Balance in a Subarctic Peatland

Abstract:

This research is to assess changes in net ecosystem carbon balance (NECB) with permafrost thaw in northern peatland: in particular how changes in C biogeochemistry influence NECB. Thawed transects associated with varying stages of permafrost thaw: from palsas with intact permafrost (P), through edge of palsa (EP), dry lawn (DL), wet lawn (WL), edge of thawed pond (ET), pond sedges (PS), to several thawed ponds (TP) in a subarctic peatland in northern Quebec were sampled in the snow free seasons of 2013 and 2014. The exchange of CO₂ and CH₄, vegetation, dissolved organic C (DOC) concentration and biodegradability, active layer depth, air and peat temperatures, water table depth (WT), pH, and conductivity were measured. Peat temperatures were quite similar among different locations, but the WT decreased significantly along the transect creating varied environmental conditions that supporting different plant communities. From dry to wet area, vegetation abundance and biomass showed reductions of shrubs and lichens, and increases of Sphagnum, grasses and sedges. Pore water pH increased from dry to wet area, and conductivity slightly decreased. Wet thaw area WL, ET and PS had relatively higher season gross ecosystem production (GEP) and higher season ecosystem respiration (ER), but relative similar net ecosystem CO₂ exchange (NEE). Only TP had a significant higher positive season NEE. Palsa was the only CH₄ sink, and quite high CH₄ emissions were found after it thawed. CH₄-C release significantly increased from dry to wet in thawed area, which even several times bigger than total C exchange in ET and PS. Generally, wet area had higher DOC concentration and higher DOC biodegradability indicated by lower SUVA₂₅₄ (except PS which received great influence from pond). All components in the NECB (GEP, ER, CH₄, DOC) increased significantly in magnitude from palsa to wet thawed area, and ecosystem C sink turned into source as palsa thawed into PS and TP. These results demonstrated WT and vegetation change lead the changes in peatland NECB with permafrost thaw. Large amount and more labile DOC produced in thawed area may indicate a ‘fast’ C uptake–decomposition loop was adapted in thawed peatland, and associated with the high CO₂ and CH₄ efflux.