Net ecosystem CO2 exchange measured by autochambers during the snow-covered season at a temperate peatland

Net ecosystem exchange of CO2 was measured at a temperate peatland in southeastern New Hampshire. Classified as a mineral-poor fen owing to deep, water-logged peats that are influenced to a limited extent by groundwater, the ecosystem is dominated by plants such as sedges (Carex spp.) and evergreen shrubs. Ten automatic chambers measured fluxes every 3 h by sampling changes in headspace concentration of CO2 from November 2000 through March 2001. The fen was covered in snow for most of this period and CO2 was emitted from the snow pack throughout the winter. The largest fluxes were associated with ground temperatures of 0 °C and with declining atmospheric pressure. CO2 effluxes up to 3 μmol m 2 s 1 were recorded when the ground temperature reached the thaw point. Fluxes were lower when the ground temperature rose above 0 °C, however, suggesting that the large fluxes were associated with a build up and release of stored CO2 degassing as soon as the ground thawed, or by enhanced microbial activity associated with freeze–thaw dynamics. The large number of thaw events coupled with frequent short-term releases of CO2 suggest that degassing occurred on a regular basis with changes in atmospheric pressure and/or microbial decomposition occurred beneath the snowpack. The extent of soil freezing prior to thaw was also an important factor, with colder soils yielding smaller CO2 emissions upon thaw. Although most of the observed CO2 flux was efflux from the ecosystem, occasional CO2 uptake by the ecosystem of up to 1 μmol m 2 s 1 was also observed, indicating small rates of photosynthesis even during winter. Photosynthesis occurred only when the ground temperature was >0 °C. The implications for a warmer climate are unclear. If warmer winter temperatures yield less snow in the temperate region, then soils could freeze more deeply and result in lower CO2 emissions. However, if less snow results in a higher frequency of freeze–thaw events, then winter CO2 emissions could be larger with a warmer climate. Copyright  2002 John Wiley & Sons, Ltd.