Efflux of carbon dioxide from snow-covered forest floors

The release of CO₂ from the snow surface in winter and the soil surface in summer was directly or indirectly measured in four cool-temperate deciduous broadleaved and evergreen needle forests. The closed chamber method (CC-method) and Fick's diffusion model (DM-method) were used for the direct and indirect measurements, respectively. The winter soil temperatures from the soil surface to 10 cm depth were between 0 and 2°C. The concentration of CO₂ within snowpack increased linearly with increasing snow depth. The average effluxes of CO₂ calculated from the gradients of CO₂ concentration in the snow using the DM-method ranged from 20 to 75 mg CO₂ m^-2 h^-1, while the CC-method showed the average effluxes of 20 to 50 mg CO₂m^-2h^-1. These results reveal that the snow thermally insulates the soil, allowing CO₂ production to continue at soil temperatures a little above freezing throughout the winter. Carbon dioxide formed in the soil can move across snowpack up to the atmosphere. The winter/summer ratio of CO₂ emission was estimated to be higher than 7%. Therefore, the snow-covered soil served as a source of CO₂ in the winter and the effluxes represent an important part of the annual CO₂ budget in snowy regions.