Diffusional flux of CO₂ through snow: Spatial and temporal variability among alpine‐subalpine sites

Abstract

Three alpine and three subalpine sites were monitored for up to 4 years to acquire data on the temporal and spatial variability of CO2 flux through snowpacks. We conclude that the snow formed a passive cap which controlled the concentration of CO2 at the snow‐soil interface, while the flux of CO2 into the atmosphere was controlled by CO2 production in the soil. Seasonal variability in the flux at all sites was characterized by early winter minima followed by a rise in flux that averaged 70% above the minima over about a 1‐month period. The seasonal variability was not related to soil temperatures which remained relatively constant. Interannual variability was small, and spatial variability was smaller than previously reported. Spatial variability on a scale of 1 to 10 m was less than 30% of the average fluxes and not significantly greater than estimated error in most cases. Spatial variability on a scale of 10‐ to 100‐m was about a factor of 2 and on a scale of 100 to 1000 m was about a factor of 4. The 100‐ to 1000‐m variability was complicated by the fact that the sites were in different ecosystems, alpine and subalpine, and at different elevations. We attribute the small variability at the 1‐ to 10‐m scale to the deep snow cover, from 1.4 to 5 m. We hypothesize that horizontal diffusion under the snow cover reduced small‐scale horizontal gradients, while the insulating effect of the deep snow cover kept the soil temperature and moisture relatively constant. Equivalent annual wintertime flux averaged about 95 g C m−2 yr−1 in the alpine and about 232 g C m−2 yr−1 in the subalpine sites. Measurements of CO2 concentrations at 0.2 and 0.5 m in the soil of one of the subalpine sites indicated that production early in the snow season occurred at or below 0.5 m while production between 0.5 m, and the surface became important after the start of the melt season.

Surface gas-exchange processes of snow algae

“…Presumably, curves such as these occur because the algae's photosynthetic activity acts against a background of constant CO 2 efflux from the snow surface. Such efflux certainly does occur: in winter and spring, CO 2 effluxes from the snow ranging from 0.2 to 0.5 mol CO 2 ⅐m Ϫ2 ⅐s Ϫ1 have been calculated from a location quite close to our field site (21,22). As a rule, our more negative gas-exchange rates came from algal patches growing in relatively thin snow, which is consistent with these studies.…”

Winter fluxes of CO2 and CH4 from subalpine soils in Rocky Mountain National Park, Colorado

“…An early winter minimum in CO2 fluxes was also observed by Sommerfeld et al [1996], who suggested that temporal trends in winter gas fluxes might be driven by changes in soil moisture. Data from the soil moisture probe at the edge of the wetland indicate that soil moisture during the winter was at a minimum at the time a permanent snow cover was established in early November, then increased gradually through the midwinter until the onset of snowmelt in early May (Figure 1).…”

“…Because uncertainties in 0, 'r, and gas concentrations generally do not introduce more than a 15% error in gradient flux estimates [Sommerfeld et al, 1996], the discrepancy between these two methods indicates that chamber measurements at the snow surface probably underestimate gas flux through snow. One possible explanation for lower chamber fluxes is that an adequate seal was not achieved between the chamber and the snow surface [Livingston and Hutchinson, 1994].…”

Winter fluxes of greenhouse gases from snow‐covered agricultural soil:intra‐annual and interannual variations

“…Although overall winter emissions of N20 from agricultural soils may be important, large interannual variations in winter gas fluxes (Tables 4 and 5) [Nakane, 1978;Mariko et al, 1994;Sommerfeld et al, 1993Sommerfeld et al, , 1996. However, these winter CO2 fluxes from barley were higher than those (2.8 to 7.5 [tg CO2 m -2 s 'l) estimated under snow cover from barley plots in Finland [Koizumi et al, 1996].…”

“…Some studies have reported N loss by nitrification and denitrification processes in soils during the summer and spring thaw [Williams et al, 1992], but few refer to agricultural soils covered by snow. This has been due, in part, to difficulties in the methodology of flux measurement during winter [Sommerfeld et al, 1996] and to the Copyright 2000 by the American Geophysical Union.…”