Background
The authors write that "only three previous published studies have assessed the impacts of O3 on CH4 and CO2 fluxes in peatlands." Niemi et al. (2002), as they describe it, "reported that CH4 emissions more than doubled when peatland microcosms were exposed to 100 ppb O3 over 4-7 weeks during summer in controlled-environment chambers." In contrast, they state that "Rinnan et al. (2003) reported no significant effect on CH4 emissions of a 7-week exposure of peat microcosms to 100 or 200 ppb O3." Last of all, they indicate that "Morsky et al. (2008) reported that open-field exposure of boreal peatland microcosms in central Finland to a doubling of ambient O3 concentrations caused a decrease in CH4 emission at the end of the first growing season," but they note that the decrease "was lost in the three subsequent growing seasons." Thus, it is clear that prior work on the subject has not provided a definitive answer to the core question of whether rising O3 concentrations have a significant impact, one way or the other, on methane emissions from peatlands.

What was done
In a study that was more reflective of reality in terms of scale, Toet et al. moved up in size from microcosms to mesocosms, which they collected from a lowland raised bog on the northern shore of Morecambe Bay, Cumbria, UK (54°13'N, 3°1'W), which they placed into open-top chambers situated on a level gravel base at Newcastle University's field station (54°59'N, 1°48'W). And there, for the next two years, they observed what happened in four ambient and four O3-enriched chambers, the latter of which had their atmospheric O3 concentrations raised by 50 ppb for eight hours of each day during the summer period (April-early October) and by 10 ppb for eight hours of each day throughout the winter.

What was learned
The four UK researchers found that "methane emissions were significantly reduced, by about 25%, by elevated ozone during midsummer periods of both years," but that "no significant effect of ozone was found during the winter periods."

What it means
After lengthy discussion of their findings, as well as those of other researchers they cite, Toet et al. conclude, in the final sentence of their paper, that "increased O3 could be a significant brake on the increased flux of CH4 that is expected as these northern peatlands warm."

References
Morsky, S.K., Haapala, J.K., Rinnan, R., Tiiva, P., Saarnio, S., Silvola, J., Holopainen, T. and Martikainen, P.J. 2008. Long-term ozone effects on vegetation, microbial community and methane dynamics of boreal peatland microcosms in open-field studies. Global Change Biology 14: 1891-1903.

Niemi, R., Martikainen, P.J., Silvola, J. and Holopainen, T. 2002. Ozone effects on Sphagnum mosses, carbon dioxide exchange and methane emission in boreal peatland microcosms. Science of the Total Environment 289: 1-12.

Rinnan, R., Impio, M., Silvola, J., Holopainen, T. and Martikainen, P.J. 2003. Carbon dioxide and methane fluxes in boreal peatlands with different vegetation cover -- effects of ozone or ultraviolet-B exposure. Oecologia 137: 475-483.