A Carbon Source in a Carbon Sink: Carbon Dioxide and Methane Dynamics in Open-Water Peatland Pools

Peatlands store organic carbon available for decomposition and transfer to neighboring water bodies, which can ultimately generate carbon dioxide (CO2) and methane (CH4) emissions. The objective of this study was to clarify the biogeochemical functioning of open-water peatland pools and their influence on carbon budgets at the ecosystem and global scale. Continuously operated automated equipment and monthly manual measurements were used to describe the CO2 and CH4 dynamics in boreal ombrotrophic peatland pools and porewater (Quebec, Canada) over the growing seasons 2019 and 2020. The peat porewater stable carbon isotope ratios (delta 13C) for both CO2 (median delta 13C-CO2: -3.8 parts per thousand) and CH4 (median delta 13C-CH4: -64.30 parts per thousand) suggested that hydrogenotrophic methanogenesis was the predominant degradation pathway in peat. Open-water pools were supersaturated in CO2 and CH4 and received most of these dissolved carbon greenhouse gases (C-GHG) from peat porewater input. Throughout the growing season, higher CO2 concentrations and fluxes in pools were measured when the water table was low-suggesting a steady release of CO2 from deep peat porewater. Higher CH4 ebullition and diffusion occurred in August when bottom water and peat temperatures were the highest. While this study demonstrates that peatland pools are chimneys of CO2 and CH4 stored in peat, it also shows that the C-GHG concentrations and flux rates in peat pools are comparable to other aquatic systems of the same size. Although peatlands are often considered uniform entities, our study highlights their biogeochemical heterogeneity, which, if considered, substantially influences their net carbon balance with the atmosphere. Peatland pools are supersaturated in CO2 and CH4 and represent net carbon greenhouse gas sources to the atmosphere CO2 and CH4 concentrations and fluxes fluctuated over the seasons and were explained by water table level fluctuations and temperature changes Net peatland carbon budgets must include open water surface emissions to avoid overestimating carbon removal from peatlands