Greenhouse gas fluxes in a drained peatland forest during spring frost-thaw event

Abstract. Fluxes of greenhouse gases (GHG) carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) were measured during a two month campaign at a drained peatland forest in Finland by the eddy covariance (EC) technique (CO₂ and N₂O), and automatic and manual chambers (CO₂, CH₄ and N₂O). In addition, GHG concentrations and soil parameters (mineral nitrogen, temperature, moisture content) in the peat profile were measured. The aim of the measurement campaign was to quantify the GHG fluxes during freezing and thawing of the top-soil, a time period with potentially high GHG fluxes, and to compare different flux measurement methods. The forest was a net CO₂ sink during the two months and the fluxes of CO₂ dominated the GHG exchange. The peat soil was a small sink of atmospheric CH₄ and a small source of N₂O. Both CH₄ oxidation and N₂O production took place in the top-soil whereas CH₄ was produced in the deeper layers of the peat, which were unfrozen throughout the measurement period. During the frost-thaw events of the litter layer distinct peaks in CO₂ and N₂O emissions were observed. The CO₂ peak followed tightly the increase in soil temperature, whereas the N₂O peak occurred with a delay after the thawing of the litter layer. CH₄ fluxes did not respond to the thawing of the peat soil. The CO₂ and N₂O emission peaks were not captured by the manual chambers and hence we conclude that high time-resolution measurements with automatic chambers or EC are necessary to quantify fluxes during peak emission periods. Sub-canopy EC measurements and chamber-based fluxes of CO₂ and N₂O were comparable, although the fluxes of N₂O measured by EC were close to the detection limit of the system. We conclude that if fluxes are high enough, i.e. greater than 5–10 μg N m⁻² h⁻¹, the EC method is a good alternative to measure N₂O and CO₂ fluxes at ecosystem scale, thereby minimizing problems with chamber enclosures and spatial representativeness of the measurements.