Winter carbon dioxide effluxes from Arctic ecosystems: An overview and comparison of methodologies

The winter CO(2) efflux from subnivean environments is an important component of annual C budgets in Arctic ecosystems and consequently makes prediction and estimations of winter processes as well as incorporations of these processes into existing models important. Several methods have been used for estimating winter CO(2) effluxes involving different assumptions about the snowpack, all aiming to quantify CO(2) production. Here, four different methods are compared and discussed: (1) measurements with a chamber on the snow surface, F(snow), (2) chamber measurements directly on the soil, F(soil), after snow removal, (3) diffusion measurements, F(2-point), within the snowpack, and (4) a trace gas technique, F(SF6), with multiple gas sampling within the snowpack. According to measurements collected from shallow and deep snow cover in High Arctic Svalbard and subarctic Sweden during the winter of 2007-2008, the four methods differ by up to two orders of magnitude in their estimates of total winter emissions. The highest mean winter CO(2) effluxes, 7.7-216.8 mg CO(2) m(-2) h(-1), were observed using F(soil) and the lowest values, 0.8-12.6 mg CO(2) m(-2) h(-1), using F(SF6). The F(snow) and F(2-point) methods were both within the lower range, 2.1-15.1 and 6.8-11.2 mg CO(2) m(-2) h(-1), respectively. These differences result not only from using contrasting methods but also from the differences in the assumptions within the methods when quantifying CO(2) production and effluxes to the atmosphere. Because snow can act as a barrier to CO(2), F(soil) is assumed to measure soil production, whereas F(SF6), F(snow), and F(2-point) are considered better approaches for quantifying exchange processes between the soil, snow, and the atmosphere. This study indicates that estimates of winter CO(2) emissions may vary more as a result of the method used than as a result of the actual variation in soil CO(2) production or release. This is a major concern, especially when CO(2) efflux data are used in climate models or in carbon budget calculations, thus highlighting the need for further development and validation of accurate and appropriate techniques.