Annual CO₂ balance of a subarctic fen in northern Europe: Importance of the wintertime efflux

Abstract

The annual balance of CO 2 was measured on a subarctic flark fen located in northern Finland (69°08′N, 27°17′E). Measurements were conducted using the micrometeorological eddy covariance method during the period from April 1998 to April 1999. Special emphasis was put on the importance of the wintertime CO 2 balance as a component of the annual budget. The CO 2 fluxes observed during the winter were relatively small (0.0055 mg CO 2 m −2 s −1 ), but due to the long duration of the snow cover period, they play an important role in the annual balance of CO 2 . The efflux during the coldest periods, when soil surface temperatures fell below −10 °C, was enabled by markedly higher peat temperatures (close to 0 °C) in deeper layers. The typical nighttime respiration observed in summer was 0.10 mg CO 2 m −2 s −1 . The net photosynthesis had a typical daytime peak value of about −0.20 mg CO 2 m −2 s −1 in July. Corresponding with the instantaneous CO 2 fluxes, the highest daily sink balances of about −9 g CO 2 m −2 d −1 were observed in July, while the highest daily respiration balances of about 2.5 g CO 2 m −2 d −1 were observed before and after the sink period. The CO 2 balance for the winter period (105 g CO 2 m −2 ) was greater than the absolute value of the total annual balance (−68 g CO 2 m −2 ), which shows the importance of the wintertime efflux. After taking into account estimates for CH 4 efflux and carbon leaching, an annual carbon balance of −7 ± 5 g C m −2 was obtained for the fen.