Carbon dioxide (CO2) emissions were measured over a period of 3 years at the sub-alpine Swiss CARBOMONT site Rigi Seebodenalp. Here we show, that winter respiration contributes larger than expected to the annual CO2 budget at this high altitude, rich in belowground organic carbon grassland (7-15% C by mass). Furthermore the contribution of winter emissions to the annual CO2 budget is highly dependent on the definition of "winter” itself. Cumulative winter respiration determined over a 6 month period from 15th of October until 15th of April contributed 23.3±2.4 and 6.0±0.3% to the annual respiration during the years under observation, respectively. The insulation effect of snow and a lowering of the freezing point caused by high concentrations of soil organic solutes prevented the soil from freezing. These conditions favored higher soil temperatures resulting in relatively high respiratory losses. The duration of snow cover and micrometeorological conditions determining the photosynthetic activity of the vegetation during snow-free periods influenced the size and the variability of the winter CO2 fluxes. Seasonal values are strongly influenced by the days at the end and the beginning of the defined winter period, caused by large variations in length of periods with air temperatures below freezing. Losses of CO2 from the snow-covered soil were highest in winter 2003/2004. These high losses were partially explained by higher temperatures in the topsoil, caused by higher air temperatures just before snowfall. Thus, losses are not a consequence of higher soil temperatures registered during the summer heat wave 2003. However, water stress in summer 2003 might have caused an increment in dead organic matter in the soil providing additional substrate for microbial respiration in the following winter. Although considerable day-to-day fluctuations in snow effluxes were recorded, no conclusive and generally valid relationship could be found between CO2 losses from the snow pack and snow depth, rate of snow melt, wind speed or air pressure. This suggests that time lags and hysteresis effects may be more important for understanding winter respiration than concurrent environmental conditions in most ecosystems of comparable typ
