Increased wintertime CO₂ loss as a result of sustained tundra warming

Permafrost soils currently store approximately 1672 Pg of carbon (C), but as high latitudes warm, this temperature‐protected C reservoir will become vulnerable to higher rates of decomposition. In recent decades, air temperatures in the high latitudes have warmed more than any other region globally, particularly during the winter. Over the coming century, the arctic winter is also expected to experience the most warming of any region or season, yet it is notably understudied. Here we present nonsummer season (NSS) CO 2 flux data from the Carbon in Permafrost Experimental Heating Research project, an ecosystem warming experiment of moist acidic tussock tundra in interior Alaska. Our goals were to quantify the relationship between environmental variables and winter CO 2 production, account for subnivean photosynthesis and late fall plant C uptake in our estimate of NSS CO 2 exchange, constrain NSS CO 2 loss estimates using multiple methods of measuring winter CO 2 flux, and quantify the effect of winter soil warming on total NSS CO 2 balance. We measured CO 2 flux using four methods: two chamber techniques (the snow pit method and one where a chamber is left under the snow for the entire season), eddy covariance, and soda lime adsorption, and found that NSS CO 2 loss varied up to fourfold, depending on the method used. CO 2 production was dependent on soil temperature and day of season but atmospheric pressure and air temperature were also important in explaining CO 2 diffusion out of the soil. Warming stimulated both ecosystem respiration and productivity during the NSS and increased overall CO 2 loss during this period by 14% (this effect varied by year, ranging from 7 to 24%). When combined with the summertime CO 2 fluxes from the same site, our results suggest that this subarctic tundra ecosystem is shifting away from its historical function as a C sink to a C source.