Long-Term Release of Carbon Dioxide from Arctic Tundra Ecosystems in Alaska
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Releases of the greenhouse gases carbon dioxide (CO2) and methane (CH4) from thawing permafrost are expected to be among the largest feedbacks to climate from arctic ecosystems. However, the current net carbon (C) balance of terrestrial arctic ecosystems is unknown. Recent studies suggest that these ecosystems are sources, sinks, or approximately in balance at present. This uncertainty arises because there are few long-term continuous measurements of arctic tundra CO2 fluxes over the full annual cycle. Here, we describe a pattern of CO2 loss based on the longest continuous record of direct measurements of CO2 fluxes in the Alaskan Arctic, from two representative tundra ecosystems, wet sedge and heath tundra. We also report on a shorter time series of continuous measurements from a third ecosystem, tussock tundra. The amount of CO2 loss from both heath and wet sedge ecosystems was related to the timing of freeze-up of the soil active layer in the fall. Wet sedge tundra lost the most CO2 during the anomalously warm autumn periods of September–December 2013–2015, with CH4 emissions contributing little to the overall C budget. Losses of C translated to approximately 4.1 and 1.4% of the total soil C stocks in active layer of the wet sedge and heath tundra, respectively, from 2008 to 2015. Increases in air temperature and soil temperatures at all depths may trigger a new trajectory of CO2 release, which will be a significant feedback to further warming if it is representative of larger areas of the Arctic.
Keywordsarctic tundra net ecosystem exchange permafrost soil temperature carbon dioxide methane
This work was funded by the National Science Foundation Division of Polar Programs Arctic Observatory Network Grant Numbers 856864, 1304271, 0632264, and 1107892. This study was also partially funded by the NSF Alaska Experimental Program to Stimulate Competitive Research award number OIA-1208927. Logistical support was provided by CH2M Hill Polar Services. We thank Diane Huebner, Jackson Drew, and Lola Oliver for assistance with soil C samples. F.S. Chapin III, S.C. Wofsy, and two anonymous reviewers provided valuable comments on an earlier draft of this manuscript.
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