Net ecosystem CO 2 exchange measured by autochambers during the snow‐covered season at a temperate peatland

Net ecosystem exchange of CO 2 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 CO 2 from November 2000 through March 2001. The fen was covered in snow for most of this period and CO 2 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. CO 2 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 CO 2 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 CO 2 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 CO 2 emissions upon thaw. Although most of the observed CO 2 flux was efflux from the ecosystem, occasional CO 2 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 CO 2 emissions. However, if less snow results in a higher frequency of freeze–thaw events, then winter CO 2 emissions could be larger with a warmer climate. Copyright © 2002 John Wiley & Sons, Ltd.