Persistent net release of carbon dioxide and methane from an Alaskan lowland boreal peatland complex

Abstract

Permafrost degradation in peatlands is altering vegetation and soil properties and impacting net carbon storage. We studied four adjacent sites in Alaska with varied permafrost regimes, including a black spruce forest on a peat plateau with permafrost, two collapse scar bogs of different ages formed following thermokarst, and a rich fen without permafrost. Measurements included year-round eddy covariance estimates of net carbon dioxide (CO₂ ), mid-April to October methane (CH₄ ) emissions, and environmental variables. From 2011 to 2022, annual rainfall was above the historical average, snow water equivalent increased, and snow-season duration shortened due to later snow return. Seasonally thawed active layer depths also increased. During this period, all ecosystems acted as slight annual sources of CO₂ (13-59 g C m^-2 year^-1 ) and stronger sources of CH₄ (11-14 g CH₄ m^-2 from ~April to October). The interannual variability of net ecosystem exchange was high, approximately ±100 g C m^-2 year^-1 , or twice what has been previously reported across other boreal sites. Net CO₂ release was positively related to increased summer rainfall and winter snow water equivalent and later snow return. Controls over CH₄ emissions were related to increased soil moisture and inundation status. The dominant emitter of carbon was the rich fen, which, in addition to being a source of CO₂ , was also the largest CH₄ emitter. These results suggest that the future carbon-source strength of boreal lowlands in Interior Alaska may be determined by the area occupied by minerotrophic fens, which are expected to become more abundant as permafrost thaw increases hydrologic connectivity. Since our measurements occur within close proximity of each other (≤1 km² ), this study also has implications for the spatial scale and data used in benchmarking carbon cycle models and emphasizes the necessity of long-term measurements to identify carbon cycle process changes in a warming climate.

Keywords: Interior Alaska; boreal lowland ecosystems; carbon cycling; methane emissions; net ecosystem exchange; precipitation; thawing permafrost.