How does elevated ozone reduce methane emissions from peatlands?

The effects of increased tropospheric ozone (O₃) pollution levels on methane (CH₄) emissions from peatlands, and their underlying mechanisms, remain unclear. In this study, we exposed peatland mesocosms from a temperate wet heath dominated by the sedge Schoenus nigricans and Sphagnum papillosum to four O₃ treatments in open-top chambers for 2.5years, to investigate the O₃ impacts on CH₄ emissions and the processes that underpin these responses. Summer CH₄ emissions, were significantly reduced, by 27% over the experiment, due to summer daytime (8hday^-1) O₃ exposure to non-filtered air (NFA) plus 35ppb O₃, but were not significantly affected by year-round, 24hday^-1, exposure to NFA plus 10ppb or NFA plus 25ppb O₃. There was no evidence that the reduced CH₄ emissions in response to elevated summer O₃ exposure were caused by reduced plant-derived carbon availability below-ground, because we found no significant effect of high summer O₃ exposure on root biomass, pore water dissolved organic carbon concentrations or the contribution of recent photosynthate to CH₄ emissions. Our CH₄ production potential and CH₄ oxidation potential measurements in the different O₃ treatments could also not explain the observed CH₄ emission responses to O₃. However, pore water ammonium concentrations at 20cm depth were consistently reduced during the experiment by elevated summer O₃ exposure, and strong positive correlations were observed between CH₄ emission and pore water ammonium concentration at three peat depths over the 2.5-year study. Our results therefore imply that elevated regional O₃ exposures in summer, but not the small increases in northern hemisphere annual mean background O₃ concentrations predicted over this century, may lead to reduced CH₄ emissions from temperate peatlands as a consequence of reductions in soil inorganic nitrogen affecting methanogenic and/or methanotrophic activity.