Process-level controls on CO2 fluxes from a seasonally snow-covered subalpine meadow soil, Niwot Ridge, Colorado

Fluxes of CO 2 during the snow-covered season contribute to annual carbon budgets, but our understanding of the mechanisms controlling the seasonal pattern and magnitude of carbon emissions in seasonally snow-covered areas is still developing. In a subalpine meadow on Niwot Ridge, Colorado, soil CO 2 fluxes were quantified with the gradient method through the snowpack in winter 2006 and 2007 and with chamber measurements during summer 2007. The CO 2 fluxes of 0.71 μmol m −2 s −1 in 2006 and 0.86 μmol m −2 s −1 in 2007 are among the highest reported for snow-covered ecosystems in the literature. These fluxes resulted in 156 and 189 g C m −2 emitted over the winter, ~30% of the annual soil CO 2 efflux at this site. In general, the CO 2 flux increased during the winter as soil moisture increased. A conceptual model was developed with distinct snow cover zones to describe this as well as the three other reported temporal patterns in CO 2 flux from seasonally snow-covered soils. As snow depth and duration increase, the factor controlling the CO 2 flux shifts from freeze–thaw cycles (zone I) to soil temperature (zone II) to soil moisture (zone III) to carbon availability (zone IV). The temporal pattern in CO 2 flux in each zone changes from periodic pulses of CO 2 during thaw events (zone I), to CO 2 fluxes reaching a minimum when soil temperatures are lowest in mid-winter (zone II), to CO 2 fluxes increasing gradually as soil moisture increases (zone III), to CO 2 fluxes decreasing as available carbon is consumed. This model predicts that interannual variability in snow cover or directional shifts in climate may result in dramatically different seasonal patterns of CO 2 flux from seasonally snow-covered soils.