Long‐term ozone effects on vegetation, microbial community and methane dynamics of boreal peatland microcosms in open‐field conditions

Abstract

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.

How does elevated ozone reduce methane emissions from peatlands?

“…Ozone exposure has also been observed to make stomata sluggish, increasing nocturnal transpiration and O 3 uptake (Davison & Barnes, 2002;Hoshika et al, 2013). Although the air temperature was on average 3.8˚C higher in the OTCs than at the Scottish field site, the mean summer However, the results from both this and our previous study in open-top chambers (OTCs) (Toet et al 2011) were not consistent with the findings of the four-year mire open-air fumigation study of Mörsky et al (2008), who reported no overall long-term responses of CH 4 emissions to elevated O 3 , which were comparable to our NFA+35/10 treatment. The use of OTCs, rather than a field release system, may modify the size of the effect of a given ozone concentration, but these different findings may also be due to a range of other factors, including local climate, soil microbiota, and peat chemistry.…”

“…Ozone is also unlikely to penetrate through the aerenchyma, as it is expected to react rapidly on contact with moist plant surfaces. We cannot exclude the possibility of reduced transport of CH 4 through the aerenchyma of sedge plants due to elevated O 3 , although Mörsky et al (2008) reported no significant effect of O 3 on the proportion of aerenchymatous tissue in Eriophorum vaginatum leaves, and there was no significant effect of O 3 on sedge green leaf density of the sedges in our study; furthermore, sedge leaf density did not correlate positively with CH 4 emissions.…”

“…25%. In contrast, Mörsky et al (2008) reported that open-field exposure of boreal peatland microcosms to a similar increase in O 3 concentration in Central Finland only caused a decrease in CH 4 emission at the end of the first growing season, which was lost in the three subsequent growing seasons. Recently, Williamson et al…”

“…These values were, however, probably lowered, indicating more prevalence towards acetoclastic methanogenesis, due to C isotopic fractionation of CH 4 during aerobic, anaerobic or facultative CH 4 oxidation when the CH 4 passed the peat profile to the moss surface (Semrau et al, 2011;Smemo & Yavitt, 2011;Whiticar, 1999). Dominance of the hydrogenotrophic pathway has been observed in several sedge-dominated peatlands (Whiticar et al, 1986;Mörsky et al, 2008;Holmes et al, 2015) and may also explain the lack of increase in CH 4 emission despite the increase in organic acid concentrations (including acetate) in the peat pore water at elevated O 3 in the peatland microcosm study of Mörsky et al (2008). Transient shifts in pathway dominance are less likely as similar contributions of recent photosynthates to CH 4 emission and also no effect of elevated O 3 were observed in the previous summer (data not shown), but seasonal shifts cannot be excluded.…”

“…The potential for such effects was shown by Jones et al (2009), who found a rapid decrease in dissolved organic carbon (DOC) concentrations in fen mesocosms after O 3 exposure, with a change in molecular composition of DOC indicating a switch in the substrate for microorganisms from root-derived carbon (C) to soil C; similar effects were not found in mesocosms dominated by Sphagnum moss. Such indirect effects of elevated O 3 in peatlands might be expected to affect CH 4 production, although both Rinnan et al (2003) and Mörsky et al (2008) reported that elevated O 3 had no significant effect on CH 4 production potential.…”