Effects of Temperature, and Nitrogen and Sulfur Deposition, on Methane Emission From a Boreal Mire

To evaluate the effects of increased air temperature in combination with increased deposition of N and/or S on methane emission, we have raised in situ the temperature and rates of deposition of N and S in a poor fen lawn area of a boreal mixed mire. The experiment was laid out in a factorial design, where the mean daily air temperature 0.3 m above the vegetation surface was increased (by 3.6°C) using greenhouse enclosures. A significant increase in the cover of sedges was observed in the N-supplemented plots after the third year of treatment. All three experimental factors had significant effects on the methane emission. The effects of temperature and N deposition strongly interacted with the sedge cover, which was the single variable explaining most variation in methane emission. Raised temperature affected the emission positively when the sedge cover was high but showed no effect when the sedge cover was low. Nitrogen addition affected methane emission negatively when the sedge cover was high and had a zero or slightly positive effect at low sedge cover. These positive temperature and negative N interaction effects with sedge cover were likely due to changed biomass allocation patterns in the plants. The S additions had negative effects on methane emissions at ambient temperature but no effect at raised temperature. This interaction effect was possibly a result of different retention of S, since the total S concentration was higher in the S addition treatments at normal but not at raised temperature. The results stress the fact that a given variable may affect biogeochemical processes in different directions or to differing degrees depending on other variables, both experimental and natural. Most importantly, the effects of added nitrogen, but also of increased temperature, were critically dependent on the density of sedges.