An investigation into environmental modifiers of grassland response to ozone

Abstract

The thesis concerns the role of nitrogen and drought in modifying the responses of grassland plant species towards tropospheric ozone. The first part focuses on nitrogen with a pilot study conducted in green-house environment with closed chambers and four plant species from mesotrophic grasslands growing singly in pots. Nitrogen (up to 50 Kg/ha) and ozone (up to 90 ppb) showed that high nitrogen ameliorated detrimental ozone effects on visible injury in Cirsium arvense, Centaurea nigra and Trifolium pratense. Low nitrogen (0 Kg/ha) on the other hand, counterbalanced ozone effects by showing greater number of leaves as well as greater above ground, below ground and total biomass in Holcus lanatus. An expansion of the ozone x nitrogen interactions, exploring combined effects on photosynthesis and stomatal conductance, was carried out in open top chambers facility at Silwood Park (Ascot, UK), using four air treatments (Filtered Air, 40, 70 and 100 ppb ozone). Species representative of calcifugous grasslands, grown both as mixed and single species were used. The results indicted that ozone-induced decreases in stomatal conductance were greater in high-nitrogen-treated Trifolium repens grown as a single species. Similarly greater ozone-induced decreases were found for photosynthesis rates of high nitrogen treated Trifolium repens and Rumex acetosella grown as single and mixed species. The second part of the research evaluates the ecological impacts of rising tropospheric ozone levels and drought on grassland mesocosms characteristic of Silwood Park. The experiment was conducted in the open top chambers with rain exclusion roofs. Ozone concentrations used were 0 (Filtered Air), 30, 60 & 90 ppb. The results showed species level differences in response to both ozone and drought. Generally greater ozone effects were observed on well-watered plants and this could be explained through greater stomatal conductance when compared to droughted plants, leading to greater ozone flux. At community level, above ground biomass responses showed an increased dominance of grasses compared to forbs in high-ozone, well-watered treatments.