Carbon dioxide and methane fluxes in boreal peatland microcosms with different vegetation cover--effects of ozone or ultraviolet-B exposure.

O(3) concentrations in the troposphere are rising and those in the stratosphere decreasing, the latter resulting in higher fluxes of solar ultraviolet-B (UV-B) radiation to the earth's surface. We assessed whether the fluxes of CO(2) and CH(4) are altered by enhanced UV-B radiation or elevated tropospheric O(3) concentrations in boreal peatland microcosms (core depth 40 cm, diameter 10.5 cm) with different vegetation cover. At the end of the UV-B experiment which lasted for a growing season, net CO(2) exchange (NEE) and dark ecosystem respiration ( R(TOT)) were sevenfold higher, and CH(4) efflux 12-fold higher, in microcosms with intact vegetation dominated by Eriophorum vaginatum L. and Sphagnum spp., compared to microcosms from which we removed E. vaginatum. Vegetation treatment had minor effects on CH(4) production and consumption potentials in the peat, suggesting that the large difference in CH(4) efflux is mainly due to efficient CH(4) transport via the aerenchyma of E. vaginatum. Ambient UV-B supplemented with 30% and elevated O(3) concentrations (100 and 200 ppb, for 7 weeks) significantly increased R(TOT) in both vegetation treatments. Elevated O(3) concentrations reduced NEE over time, while UV-B had no clear effects on the fluxes of CO(2) or CH(4) in the cloudy summer of the study. Field experiments are needed to assess the significance of increasing UV-B radiation and elevated tropospheric O(3) concentration on peatland gas exchange in the long-term.