Spatial and temporal variability in soil CO2 fluxes during the snow covered season on Niwot Ridge

Until recently, microbial activity in seasonally snow covered ecosystems was thought to be limited during the snow-covered season.  However, in the last decade many studies have shown that the gas fluxes through the snowpack in winter can be a significant component of the annual flux.  Because of the logistical difficulties of winter measurements, these cumulative fluxes have often been based on weekly or even more infrequent measurements.  Further, measurements are often made in only one location. Here we present data from a subalpine meadow on Niwot Ridge, Colorado.  Air samples were collected automatically from the snowpack and analyzed for carbon dioxide (CO₂) in a nearby underground laboratory. Samples were collected hourly from either eight inlets along a vertical profile on one tower or eight inlets at a height of 30 cm along a horizontal profile spanning the variability in snow depth across the meadow. The diffusive CO₂ flux was then calculated based on the concentration gradient using Fick’s law from the sampling height of 30 cm to the snowpack surface. The average CO₂ flux from January to June from the 30 cm inlet on the vertical tower was 0.78 umol CO₂/m2/s with maximum fluxes near the time of maximum snow depth. This is similar to measurements made in previous years at this site which ranged from 0.71 to 0.86 umol CO₂/m2/s.  Fluxes generally increased as snow depth increased with maximum fluxes of 1.6 umol CO₂/m2/s near the time of maximum snow depth. Hourly measurements across the horizontal profile were made during four 3-7 day periods between January and April.  The mean flux at the eight sampling locations ranged from 0.05 to 0.65 umol CO₂/m2/s.  The mean flux at vertical tower was only 0.53 umol CO₂/m2/s at these times.  The mean flux across the horizontal profile was highly correlated with snow depth (R2=0.75).  This relationship may be related to differences in advective flux (i.e. wind pumping) across the snow depth gradient.  However, as the relationship between wind speed and diffusive CO₂ flux was not related to snow depth, wind pumping is not likely to be the main explanation for the relationship of CO₂ flux and snow depth.  This suggests that other factors that vary with snow depth and affect biological processes, such as soil temperature and soil carbon, may explain more of the variation.  Further research will help elucidate which of these factors can better explain CO₂ emissions from snow covered soils.