Emissions of CH4, N2O and CO2 from a virgin fen and a fen drained for grassland in Finland

Greenhouse gas fluxes (CH4, N2O and CO2) at a managed Finnish peat soil site were compared with those in a virgin fen in 1991 and 1992. The field site was drained about 60 years ago, and as it was used for pasturing and fodder production, the CH4 released by the cattle was estimated on the basis of data in the literature. The annual budgets of the gases were estimated and their GWP values calculated to assess the climatic impact of the current agricultural activity. Methane emission from the field site, 1 kg CH4-C ha(-1) yr(-1) was low, compared with that from the natural fen site, 260 kg CH4-C ha(-1) yr(-1). Annual N2O emission at the field site was 8 and 9 kg N2O-N ha(-1) yr(-1) in 1991 and 1992, respectively. Summer N2O emission in 1992 from vegetationless soil was five times higher than that from actual grassland. The N2O emissions from the natural fen site were low, close to the detection limit. The GWP for the field CH4 release (including cattle) corresponded to 0.3 X 10(4) kg CO2 ha(-1). The GWP for the annual N2O production was 0.2-0.4 x 10(4) kg CO2 ha(-1), while the grassland soil released 2.2 x 10(4) kg CO2 ha(-1) yr(-1). In its virgin state the field site may have accumulated 600 kg CO2 ha(-1) yr(-1). Although there must be a compensating carbon input into the soil in the form of root litter and exudates, a considerable carbon loss occurred (1.5 x 10(4) kg ha(-1) yr(-1)). The total GWP associated with cultivation was estimated to be 2.6 x 10(4) kg CO2 ha(-1) yr(-1).