A reduction in the length of the snow-covered season in response to a warming of high-latitude and high-elevation ecosystems may increase soil carbon availability both through increased litter fall following longer growing seasons and by allowing early winter soil frosts that lyse plant and microbial cells. To evaluate how an increase in labile carbon during winter may affect ecosystem carbon balance we investigated the relationship between carbon availability and winter CO2 fluxes at several locations in the Colorado Rockies. Landscape-scale surveys of winter CO2 fluxes from sites with different soil carbon content indicated that winter CO2 fluxes were positively related to carbon availability and experimental additions of glucose to soil confirmed that CO2 fluxes from snow-covered soil at temperatures between 0 and −3°C were carbon limited. Glucose added to snow-covered soil increased CO2 fluxes by 52–160% relative to control sites within 24 h and remained 62–70% higher after 30 days. Concurrently a shift in the δ13C values of emitted CO2 toward the glucose value indicated preferential utilization of the added carbon confirming the presence of active heterotrophic respiration in soils at temperatures below 0°C. The sensitivity of these winter fluxes to substrate availability, coupled with predicted changes in winter snow cover, suggests that feedbacks between growing season carbon uptake and winter heterotrophic activity may have unforeseen consequences for carbon and nutrient cycling in northern forests. For example, published winter CO2 fluxes indicate that on average 50% of growing season carbon uptake currently is respired during the winter; changes in winter CO2 flux in response to climate change have the potential to reduce substantially the net carbon sink in these ecosystems.