Increased wintertime CO₂ loss as a result of sustained tundra warming

Abstract

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) CO2 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 CO2 production, account for subnivean photosynthesis and late fall plant C uptake in our estimate of NSS CO2 exchange, constrain NSS CO2 loss estimates using multiple methods of measuring winter CO2 flux, and quantify the effect of winter soil warming on total NSS CO2 balance. We measured CO2 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 CO2 loss varied up to fourfold, depending on the method used. CO2 production was dependent on soil temperature and day of season but atmospheric pressure and air temperature were also important in explaining CO2 diffusion out of the soil. Warming stimulated both ecosystem respiration and productivity during the NSS and increased overall CO2 loss during this period by 14% (this effect varied by year, ranging from 7 to 24%). When combined with the summertime CO2 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.

Long-Term Release of Carbon Dioxide from Arctic Tundra Ecosystems in Alaska

“…We are aware of only a few published studies of year-round arctic tundra eddy covariance measurements of NEE (Oechel et al, 2014;Lüers et al, 2016), while other NEE eddy coviarance measurements take into account a subset of the winter months in alpine, subarctic tundra (e.g., Webb et al, 2016). Over one year of full annual measurements of Alaskan tussock tundra Oechel et al (2014) find a source of 13.6 g C m -2 yr -1 , which agrees well with the two full annual measurements of tussock tundra NEE we present here, estimated as a source of 15 g C m -2 yr -1 in 2013 and 7 g C m -2 yr -1 (Fig.…”

“…However, the net C balance of these ecosystems will be controlled by the balance between potential gains in plant productivity, which may also increase under warming (Shaver et al, 1991;Natali et al, 2012) and losses through SOM decomposition and lateral export of carbon. budget in arctic ecosystems because microbes can continue to respire under the snowpack in temperatures close to and even below freezing (Coyne & Kelley, 1971;Panikov et al, 2006;Drotz et al, 2010), but the magnitude and interannual variability of this winter flux remains uncertain (Björkman et al, 2010;Webb et al, 2016). Due to a harsh, remote environment and the difficulties of collecting measurements in areas without line power, continuous measurements of arctic tundra CO2 fluxes over the full annual cycle across numerous years have not existed until recently (Euskirchen et al, 2012;Lüers et al, 2014;Oechel et al, 2014).…”

Detecting regional patterns of changing CO 2 flux in Alaska

“…Climate warming threatens to increase permafrost thaw and release soil carbon back to the atmosphere as a positive feedback promoting additional warming (2). It is unclear whether the observed intensification of the northern high-latitude carbon cycle is dominated by plant productivity or microbial decomposition, both of which seem to be increasing (3)(4)(5)(6). Although warming temperatures and C/N fertilization promote greening and higher summer productivity during the short, intense growing season, these same factors also drive increased emissions during the long cold season (3)(4)(5).…”

“…In the last decade, researchers have recognized the importance of year-round landatmosphere CO 2 flux observations (3)(4)(5). Synthesis studies of these data show that increasing growing season uptake has been offset by stronger winter respiration.…”

“…Spatial sampling with significantly greater density than available from current ground-based and aircraft observing systems is needed. (v) Limited seasonal coverage: climate warming has intensified seasonal carbon exchange in the ABZ, with increasing summer uptake offset by increasing winter emissions (3)(4)(5)(6). Observations restricted to the growing season fail to capture this differential temporal response, such that key drivers of present and future carbon balance may go undetected.…”