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 in creased deposition of N and/or S on methane emission. we have raised in sit u the temperature and rates of deposition of N and S in a poor fen lawn are a of a boreal mixed mire. The experiment was laid out in a factorial design , where the mean daily air temperature 0.3 In above the vegetation surface was increased (by 3.6 degreesC) using greenhouse enclosures. A significant increase in the cover of sedges was observed in the N-supplemented plots af ter the third year of treatment. All three experimental factors had signifi cant effects on the methane emission. The effects of temperature and N depo sition strongly interacted with the sedge cover, which was the single varia ble explaining most variation in methane emission. Raised temperature affec ted the emission positively when the sedge cover was high but showed no eff ect when the sedge cover was low. Nitrogen addition affected methane emissi on negatively when the sedge cover was high and had a zero or slightly posi tive effect at low sedge cover. These positive temperature and negative N i nteraction effects with sedge cover were likely due to changed biomass allo cation patterns in the plants. The S additions had negative effects on meth ane emissions at ambient temperature but no effect at raised temperature. T his interaction effect was possibly a result of different retention of S, s ince the total S concentration was higher in the S addition treatments at n ormal but not at raised temperature. The results stress the fact that a giv en variable may affect biogeochemical processes in different directions or to differing degrees depending on other variables. both experimental and na tural. Most importantly, the effects of added nitrogen, but also of increas ed temperature. were critically dependent on the density of sedges.