AbstractsEarth & Environmental Science

Surface to atmosphere carbon exchange in peatlands with permanent open water pools

by Luc Pelletier

Institution: McGill University
Department: Department of Natural Resource Sciences
Degree: PhD
Year: 2015
Keywords: Earth Sciences - Biogeochemistry
Record ID: 2062683
Full text PDF: http://digitool.library.mcgill.ca/thesisfile130279.pdf


Peatland ecosystems are long-term net carbon (C) sinks but the open water pools that are characteristic of these temperate to subarctic wetlands have been found to be C sources to the atmosphere. Their contribution to the ecosystem level C budget has often been ignored even if they cover a significant area of the peatland surface. There is therefore a question as to whether peatlands containing pools are smaller net sinks of atmospheric C compared to peatlands without pools; the total C loss measured from pools is the same order of magnitude, but with the opposite sign, as the published average net ecosystem carbon balance for pool-free northern peatlands. In this study, I evaluated the annual C release from peatland open water pools, its impact on the net ecosystem carbon dioxide exchange (NEE-CO2) and assessed the NEE-CO2 variability and controls between peatlands with different pool cover.The annual C release from pools was estimated at 103 g C m-2 yr-1, of which 15% was released during the spring ice melt. Although this estimate is within the range of other published studies, my results suggest that previous assessments of C release underestimate the annual loss by not including the spring melt release and by using sporadic measurement techniques. The ecosystem scale measurements performed using the eddy covariance technique (EC) showed that the 30-min maximum CO2 uptake rates decreased by close to 25% for an increase in pool fraction within the EC tower source area from 0-10% to 20-30%. I found that pools lower the overall NEE-CO2 components (maximum photosynthetic uptake and ecosystem respiration rates) by reducing the ecosystem's aboveground vegetation biomass. The lower total vegetation biomass at the ecosystem scale reduced net CO2 uptake during the day but also reduced ecosystem respiration (ER) at night, as ER from the pools is mainly heterotrophic. The net effect was that the site remained a sink for CO2 during the measurement period despite the inclusion of pools. When comparing sites with different pool cover fraction and climate, I observed significant variability in NEE-CO2 and its components between sites, and found that biophysical controls on CO2 exchange previously identified for vegetated surface peatlands (leaf area index, plant functional type) apply to peatlands containing open-water pools. Overall, my results demonstrate that peatland open water pools can play a significant role in the ecosystem level surface to atmosphere C exchange. Measurements of interannual NEE-CO2 variability from different peatlands with pools will expand the validity of these findings. Future studies should also include an evaluation of the annual CH4 exchange, CO2 loss during the winter along with dissolved organic carbon export from peatland with pools in order to obtain a complete net ecosystem carbon balance. Les tourbières représentent des puits à long terme pour le carbone (C). Cependant, les mares observées à la surface de plusieurs de ces milieux humides des régions tempérées à subarctiques sont des sources…