Accurate estimates of annual budgets of methane (CH₄) efflux in arctic regions are severely constrained by the paucity of non-summer measurements. Moreover, the incomplete understanding of the ecosystem-level sensitivity of CH₄ emissions to changes in tundra moisture makes prediction of future CH₄ release from the Arctic extremely difficult. This study addresses some of these research gaps by presenting an analysis of eddy covariance and chamber measurements of CH₄ efflux and supporting environmental variables during the autumn season and associated beginning of soil freeze-up at our large-scale water manipulation site near Barrow, Alaska (the Biocomplexity Experiment). We found that the autumn season CH₄ emission is significant (accounting for 21–25% of the average growing season emission), and that this emission is mostly controlled by the fraction of inundated landscape, atmospheric turbulence, and the decline in unfrozen water during the period of soil freezing. Drainage decreased autumn CH₄ emission by a factor of 2.4 compared to our flooded treatment. Flooding slowed the soil freezing process which has implications for extending elevated CH₄ emissions longer into the winter season

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Abstract. The tundra ecosystem is quite vulnerable to drastic climate change in the Arctic, and the quantification of carbon dynamics is of significant importance regarding thawing permafrost, changes to the snow-covered period and snow and shrub community extent, and the decline of sea ice in the Arctic. Here, CO2 efflux measurements using a manual chamber system within a 40 m × 40 m (5 m interval; 81 total points) plot were conducted within dominant tundra vegetation on the Seward Peninsula of Alaska, during the growing seasons of 2011 and 2012, for the assessment of driving parameters of CO2 efflux. We applied a hierarchical Bayesian (HB) model – a function of soil temperature, soil moisture, vegetation type, and thaw depth – to quantify the effects of environmental factors on CO2 efflux and to estimate growing season CO2 emissions. Our results showed that average CO2 efflux in 2011 was 1.4 times higher than in 2012, resulting from the distinct difference in soil moisture between the 2 years. Tussock-dominated CO2 efflux is 1.4 to 2.3 times higher than those measured in lichen and moss communities, revealing tussock as a significant CO2 source in the Arctic, with a wide area distribution on the circumpolar scale. CO2 efflux followed soil temperature nearly exponentially from both the observed data and the posterior medians of the HB model. This reveals that soil temperature regulates the seasonal variation of CO2 efflux and that soil moisture contributes to the interannual variation of CO2 efflux for the two growing seasons in question. Obvious changes in soil moisture during the growing seasons of 2011 and 2012 resulted in an explicit difference between CO2 effluxes – 742 and 539 g CO2 m−2 period−1 for 2011 and 2012, respectively, suggesting the 2012 CO2 emission rate was reduced to 27% (95% credible interval: 17–36%) of the 2011 emission, due to higher soil moisture from severe rain. The estimated growing season CO2 emission rate ranged from 0.86 Mg CO2 in 2012 to 1.20 Mg CO2 in 2011 within a 40 m × 40 m plot, corresponding to 86 and 80% of annual CO2 emission rates within the western Alaska tundra ecosystem, estimated from the temperature dependence of CO2 efflux. Therefore, this HB model can be readily applied to observed CO2 efflux, as it demands only four environmental factors and can also be effective for quantitatively assessing the driving parameters of CO2 efflux.

Abstract. Winter and spring soil CO 2 efflux measurements represent a significant component in the assessment of annual carbon budgets of tundra and boreal forest ecosystems, reflecting responses to climate change in the Arctic. This study was conducted in order to quantify CO 2 efflux, using a portable chamber system at representative sites along the Dalton Highway. Study sites included three tundra, two white spruce, and three black spruce forest locations during the winter and spring seasons of 2010-2012; the study of these sites promised better understanding of winter and spring carbon contributions to the annual carbon budget, as well as the respective ablation-ring effects during spring. Three-year spring CO 2 efflux depends on soil temperature at 5 cm depth on a regional scale. At their highest, Q 10 values were 4.2 × 10 6 , within the exposed tussock tundra of the upland tundra site, which tundra soils warmed from −0.9 to 0.5 • C, involving soil microbial activity. From the forest census (400 m 2 ) of the two white spruce forest sites, CO 2 emissions were estimated as 0.09-0.36 gC m −2 day −1 in winter and 0.14-4.95 gC m −2 day −1 in spring, corresponding to 1-3 % and 1-27 % of annual carbon, respectively. Contributions from spring CO 2 emissions are likely to increase as exposed soils widen in average length (major axis) from the east-, west-, south-, and north-side lengths (minor axis). Considering the periods of winter and spring seasons across tundra and boreal forests, average winter-and springseasonal CO 2 contributions to annual carbon budgets correspond roughly to 14-22 % for tundra and 9-24 % for boreal forest sites during 2011 and 2012. Spring carbon contributions, such as growing season CO 2 emissions, are sensitive to subtle changes at the onset of spring and during the snowcovered period in northern high latitudes, in response to recent Arctic climate change.

<p><strong>Abstract.</strong> Winter CO₂ flux is an important element to assess when estimating the annual carbon budget on regional and global scales. However, winter observation frequency is limited due to the extreme cold weather in sub-Arctic and Arctic ecosystems. In this study, the continuous monitoring of winter CO₂ flux in black spruce forest soil of interior Alaska was performed using NDIR CO₂ sensors at 10, 20, and 30 cm above the soil surface during the snow-covered period (DOY 357 to 466) of 2006/2007. The atmospheric pressure was divided into four phases: &amp;gt;1000 hPa (HP: high pressure); 985&amp;lt;<i>P</i>&amp;lt;1000 (IP: intermediate pressure); &amp;lt;986 hPa (LP: low pressure); and a snow-melting period (MP); for the quantification of the effect of the environmental factors determining winter CO₂ flux. The winter CO₂ fluxes were 0.22 ± 0.02, 0.23 ± 0.02, 0.25 ± 0.03, and 0.17 ± 0.02 gCO₂-C/m² d⁻¹ for the HP, IP, LP, and MP phases, respectively. Wintertime CO₂ emission represents 20 % of the annual CO₂ emissions in this boreal black spruce forest soil. Atmospheric temperature, pressure, and soil temperature correlate at levels of 56, 25, and 31 % to winter CO₂ flux, respectively, during the snow-covered period of 2006/2007, when snow depth experienced one of its lowest totals of the past 80 years. Atmospheric temperature and soil temperature at 5 cm depth, modulated by atmospheric pressure, were found to be significant factors in determining winter CO₂ emission and fluctuation in snowpack. Regional/global process-based carbon cycle models should be reassessed to account for the effect of winter CO₂ emissions, regulated by temperature and soil latent-heat flux, in the snow-covered soils of Arctic and sub-Arctic terrestrial ecosystems of the Northern Hemisphere.</p>