Bridging the Gap From Peat to Ponds: Terrestrial and Aquatic Greenhouse Gas Emissions in an Evolving Permafrost‐Associated Subarctic Peatland

Abstract Rapid permafrost thaw in peatlands transforms the subarctic landscape into a mosaic of ponds and peat soil, varying in moisture, carbon and nutrients, and time since thaw. As subarctic permafrost degrades, ponds emerge and mature and can gradually infill with Sphagnum peat. These ponds are considered hotspots of methane (CH 4 ) and carbon dioxide (CO 2 ) emissions, but their potential for nitrous oxide (N 2 O) emissions is understudied. Moreover, how greenhouse gas (GHG) fluxes vary between these aquatic and terrestrial environments is unclear, largely because these ecosystems are typically studied separately. We quantified N 2 O, CO 2 , and CH 4 emissions along four transects from a degrading terrestrial palsa to ponds varying in age within a sporadic‐permafrost subarctic peatland near Whapmagoostui‐Kuujjuarapik, QC. We also characterized depth‐dependent GHG concentrations in snow and peat. Waterlogged Sphagnum peat emitted, on average, 361% more C than drier bare peat above permafrost. GHG emissions from Sphagnum peat were comparable to pond emissions, with a greater proportion of CH 4 , suggesting waterlogged systems represent key emission hotspots. Terrestrial N 2 O fluxes were generally low, but the eroding palsa emitted more N 2 O than saturated Sphagnum peat. In ponds, N 2 O was consumed, indicating complete denitrification. Depth profiles of peat and snowpack GHG concentrations indicate likely emissions during winter. Terrestrial GHG fluxes, but not pond fluxes, were strongly associated with nitrogen availability. Our results suggest that future permafrost thaw, leading to bare peat degradation into ponds and the subsequent development of Sphagnum peat with presumably more N 2 fixation potential, will lead to sustained elevated C emissions from peatlands.