Plant functional types, nutrients and hydrology drive carbon cycling along a transect in an anthropogenically altered Canadian peatland complex

Peatlands play an important role in global carbon cycling, however, the response of peatland carbon fluxes to anthropogenically changed hydrologic conditions and long-term infiltration of nutrients is still understudied. Along a transect of 4 study sites, spanning from largely pristine to strongly altered conditions within the Wylde Lake peatland complex in Ontario (Canada), we monitored carbon dioxide (CO 2 ) and methane (CH 4 ) fluxes at the soil/atmosphere interface and DIC and CH 4 concentrations in the peat profiles from April 2014 through September 2015. Moreover, we applied δ 13 C-CH 4 and δ 13 C-CO 2 stable isotope abundance analyses to examine CH 4 and CO 2 production and consumption as well as the dominant CH 4 emission pathways during the growing season of 2015. We found that a graminoid-moss dominated site, which was exposed to wet conditions and long-term infiltration of nutrients, was a great sink of CO 2 (2260 ± 480 g CO 2  m −2 ) but a great source of CH 4 (61.4 ± 32 g CH 4  m −2 ). Comparably low δ 13 C-CH 4 signatures (−62.30 ± 5.54 ‰) indicated only low mitigation of CH 4 emission by methanotrophic activity here. On the contrary, a shrub dominated site, which has been subjected to similarly high moisture conditions and loads of nutrients, was a much weaker sink of CO 2 (1093 ± 794 g CO 2  m −2 ) as compared with all other sites. The shrub dominated site featured notably low DIC concentrations in the peat as well as comparably 13 C enriched CH 4 (δ 13 C-CH 4 : −57.81 ± 7.03 ‰) and depleted CO 2 (δ 13 C-CO 2 : −15.85 ± 3.61 ‰) in a more decomposed and surficial aerated peat, suggesting a higher share of CH 4 oxidation. Plant mediated transport was the prevailing methane emission pathway throughout the summer of 2015 among all sites, even where graminoids covered only 10 % of the area. Our study provides insight into the accelerated carbon cycling of a strongly altered peatland and our results supported earlier findings, that strongly altered, shrub dominated peatlands may turn into weak carbon sinks or even sources, while a graminoid-moss dominance may maintain the peatland's carbon storage function.