This study investigated the effects of elevated background ozone on plant growth and carbon cycling in wetland ecosystems. Tropospheric ozone concentrations have been increasing since pre-industrial times and are currently approximately 30-40ppb in
Europe. Background concentrations of ozone are predicted to increase in Northern Europe as hemispheric transport of ozone and its precursor molecules occurs from Asia and America. At the same time, because of emissions legislation in Europe, peak concentrations of ozone are predicted to decrease.
Previous research has found that wetland vascular plants are relatively sensitive to ozone and that Sphagnum mosses may be less sensitive, despite their lack of a cuticle and their leaves that are only one cell thick. Short-term exposure to high peak ozone
concentrations has previously shown that ozone may cause an increase in methane emissions from wetland mesocosms, but longer-term, chronic ozone exposure has been shown to cause a decrease in methane emissions.
The effects of elevated ozone on the growth and physiology of wetland species was assessed on individual species and on naturally germinating communities within mesocosms collected from North Welsh fen and bog habitats. Gas exchange of methane and carbon dioxide from the wetland mesocosms was measured using headspace sampling and pore water carbon content was also measured. During the first year of study the effects of high peaks of ozone on wetland mesocosms and plants was assessed whilst during the second and third years of study the impacts of
increasing background ozone was assessed over the growing season.
Vascular plant senescence was found to increase following both short and long-term exposure to ozone in individual species and in the communities. However, these increases in senescence were not always accompanied by significant decreases in biomass. Short-term ozone exposure consisting of five days of 150ppb peaks followed by two days background of 20ppb for four weeks did not significantly change methane emissions from wetland mesocosms. However, there was a trend towards an increase in methane emissions under elevated ozone during this experiment which, when measured again in bog mesocosms over a longer-term ozone exposure simulating predicted increases in background ozone (eight treatments ranging from a mean of 16ppb to 102ppb) was significant. When vascular plants were removed from bog mesocosms as they germinated by excision below the water-table methane emissions no longer increased as background ozone concentrations increased, suggesting that vascular plants control a large part of the increase in methane emissions that had been measured. Methane emissions also dropped if the water-table was lowered, irrespective of ozone exposure or wetland type. Carbon dioxide exchange however, was not consistently affected by elevated ozone and in the majority of results, carbon dioxide exchange was unchanged as background ozone concentrations were increased.
Methane production and consumption from fen and bog mesocosms, both in-situ and when measured using laboratory assays, was not significantly affected by elevated ozone suggesting that the increase in methane emissions from bog mesocosms is linked to the increases in plant senescence induced by ozone. The majority of methane fluxes through wetland plants occur as a result of pressurised ventilation where gas flows from high-pressure regions such as photosynthesising green leaves,
down into the roots to supply them with oxygen and then flows out through low-pressure, senesced, "leaky" leaves. As plant senescence is increased by elevated ozone it is supposed that this could increase the gas flow through the plants resulting in higher methane emissions.