Winter production of CO₂ from soils in winter-dominated ecosystems represents a large source of CO₂ to the atmosphere. Most studies on soil respiration have focused on growing season processes, based on the assumption that biological activity virtually ceases in cold or frozen soils, but recent research indicates that soil organisms may be very active in snow-covered soils. We explored the wintertime soil respiration in a high-elevation (3300 m) system characterized by a deep, continuous snowpack. We compared CO₂ flux through the snowpack and rates of soil respiration in a subalpine forest and a nearby meadow in the Snowy Mountains, Wyoming.  We measured the CO₂ concentration, carbon isotope ratio, snow density and temperature at regular depths within the snowpack.  Soil samples were collected and analyzed for microbial biomass carbon (MBC), and subsamples were incubated in the lab in a 2x2 factorial design with temperature (0.5°C and 22.5°C) and with or without dextrose. Sampling was done mid-winter and spring 2008, 2009 and 2010.  The field and lab data were analyzed within a Bayesian framework incorporating a gas diffusion model to estimate wintertime soil respiration.

Results/Conclusions

Snowpack density increased throughout the winter from 279 to 408.6 kg m^-3, and the temperature difference between the top and bottom of the snowpack decreased from 9.5 to 0.5°C.  The snow was on average 51.7 cm deeper and soils were 1°C warmer in the meadow compared to the forest.  MBC was higher in meadow soils (10-30 μg C g^-1 soil) than forest soils (4-11 μg C g^-1 soil).  Correspondingly, laboratory incubations showed that meadow soil respiration (0.001-0.0015 μmol g^-1 s^-1) was higher than forest (5e^-4-.001 μmol g^-1 s^-1).  Incubations resulted in an interesting interaction between temperature and dextrose; at 22.5°C, dextrose resulted in increased respiration, but did not increase MBC, while at 0.5°C, dextrose resulted in a small increase in respiration, but a large increase in MBC.  Soil respiration calculated using the CO₂ and ¹³C gradient was similar in the meadow and forest in mid-winter (0.4 μmol m² s^-1) but only the forest soils respired more (0.6 μmol m² s^-1) in spring.  This difference may be explained by increased fungal biomass in the forest litter, or by root respiration associated with rising air temperatures.   By including the physical characteristics of the snowpack in our estimate of CO₂ production and comparing that to biological mechanisms driving soil respiration, we are developing a better understanding of wintertime CO2 fluxes.