To study the effects of elevated ozone concentration on methane dynamics and a sedge species, Eriophorum vaginatum, we exposed peatland microcosms, isolated by coring from an oligotrophic pine fen, to double ambient ozone concentration in an open-air ozone exposure field for four growing seasons. The field consists of eight circular plots of which four were fumigated with elevated ozone concentration and four were ambient controls. At the latter part of the first growing season (week 33, 2003), the methane emission was 159 AE 14 mg CH 4 m À2 day À1 (mean AE SE) in the ozone treatment and 214 AE 8 mg CH 4 m À2 day À1 under the ambient control. However, towards the end of the experiment the ozone treatment slightly, but consistently, enhanced the methane emission. At the end of the third growing season (2005), microbial biomass (estimated by phospholipid fatty acid biomarkers) was higher in peat exposed to ozone (1975 AE 108 nmol g À1 dw) than in peat of the control microcosms (1589 AE 115 nmol g À1 dw). The concentrations of organic acids in peat pore water showed a similar trend. Elevated ozone did not affect the shoot length or the structure of the sedge E. vaginatum leaves but it slightly increased the total number of sedge leaves towards the end of the experiment. Our results indicate that elevated ozone concentration enhances the general growth conditions of microbes in peat by increasing their substrate availability. However, the methane production did not reflect the increase in the concentration of organic acids, probably because hydrogenotrophic methane production dominated in the peat studied. Although, we used isolated peatland microcosms with limited size as study material, we did not find experimental factors that could have hampered the basic conclusions on the effects of ozone.

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The effect of elevated UV-B radiation on CO 2 exchange of a natural flark fen was studied in open-field conditions during [2003][2004][2005]. The experimental site was located in Sodankylä in northern Finland (67122 0 N, 26138 0 E, 179 m a.s.l.). Altogether 30 study plots, each 120 cm  120 cm in size, were randomly distributed between three treatments (n 5 10): ambient control, UV-A control and UV-B treatment. The UV-B-treated plots were exposed to elevated UV-B radiation level for three growing seasons. The instantaneous net ecosystem CO 2 exchange (NEE) and dark respiration (R TOT ) were measured during the growing season using a closed chamber method. The wintertime CO 2 emissions were estimated using a gradient technique by analyzing the CO 2 concentration in the snow pack. In addition to the instantaneous CO 2 exchange, the seasonal CO 2 balances during the growing seasons were modeled using environmental data measured at the site. In general, the instantaneous NEE at light saturation was slightly higher in the UV-B treatment compared with the ambient control, but the gross photosynthesis was unaffected by the exposure. The R TOT was significantly lower under elevated UV-B in the third study year. The modeled seasonal (June-September) CO 2 balance varied between the years depending on the ground water level and temperature conditions. During the driest year, the seasonal CO 2 balance was negative (net release of CO 2 ) in the ambient control and the UV-B treatment was CO 2 neutral. During the third year, the seasonal CO 2 uptake was 43 AE 36 g CO 2 -C m À2 in the ambient control and 79 AE 45 g CO 2 -C m À2 in the UV-B treatment. The results suggest that the long-term exposure to high UV-B radiation levels may slightly increase the CO 2 accumulation to fens resulting from a decrease in microbial activity in peat. However, it is unlikely that the predicted development of the level of UV-B radiation would significantly affect the CO 2 balance of fen ecosystems in future.