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Nitrogen deposition and tropospheric ozone are important drivers of vegetation damage, but their interactive effects are poorly understood. This study assessed whether long-term nitrogen deposition altered sensitivity to ozone in a semi-natural vegetation community. Mesocosms were collected from sand dune grassland in the UK along a nitrogen gradient (5–25 kg N/ha/y, including two plots from a long-term experiment), and fumigated for 2.5 months to simulate medium and high ozone exposure. Ozone damage to leaves was quantified for 20 ozone-sensitive species. Soil solution dissolved organic carbon (DOC) and soil extracellular enzymes were measured to investigate secondary effects on soil processes. Mesocosms from sites receiving the highest N deposition showed the least ozone-related leaf damage, while those from the least N-polluted sites were the most damaged by ozone. This was due to differences in community-level sensitivity, rather than species-level impacts. The N-polluted sites contained fewer ozone-sensitive forbs and sedges, and a higher proportion of comparatively ozone-resistant grasses. This difference in the vegetation composition of mesocosms in relation to N deposition conveyed differential resilience to ozone. Mesocosms in the highest ozone treatment showed elevated soil solution DOC with increasing site N deposition. This suggests that, despite showing relatively little leaf damage, the 'ozone resilient' vegetation community may still sustain physiological damage through reduced capacity to assimilate photosynthate, with its subsequent loss as DOC through the roots into the soil. We conclude that for dune grassland habitats, the regions of highest risk to ozone exposure are those that have received the lowest level of long-term nitrogen deposition. This highlights the importance of considering community- and ecosystem-scale impacts of pollutants in addition to impacts on individual species. It also underscores the need for protection of 'clean' habitats from air pollution and other environmental stressors. Image 1 • The N-polluted sites contained fewer ozone-sensitive forbs and sedges. • Vegetation composition in relation to N deposition conveyed differential ozone-resilience. • Mesocosms in the highest ozone treatment showed elevated soil solution DOC. For dune grassland habitats, the regions of highest risk to ozone exposure are those that have received the lowest level of long-term nitrogen deposition. [ABSTRACT FROM AUTHOR]

Growing evidence suggests that tropospheric ozone has widespread effects on vegetation, which can contribute to alter ecosystem carbon (C) dynamics and belowground processes. In this study, we used intact soil mesocosms from a semi-improved grassland and investigated the effects of elevated ozone, alone and in combination with nitrogen (N) fertilization on soil-borne greenhouse gas emissions and ecosystem C fluxes. Ozone exposure under fully open-air field conditions was occurred during the growing season. Across a one-year period, soil methane (CH 4) and nitrous oxide (N 2 O) emissions did not differ between treatments, but elevated ozone significantly depressed soil CH 4 uptake by 14% during the growing season irrespective of N fertilization. Elevated ozone resulted in a 15% reduction of net ecosystem exchange of carbon dioxide, while N fertilization significantly increased ecosystem respiration during the growing season. Aboveground biomass was unaffected by elevated ozone during the growing season but significantly decreased by 17% during the non-growing season. At the end of the experiment, soil mineral N content, net N mineralization and extracellular enzyme activities (i.e., cellobiohydrolase and leucine aminopeptidase) were higher under elevated ozone than ambient ozone. The short-term effect of single application of N fertilizer was primarily responsible for the lack of the interaction between elevated ozone and N fertilization. Therefore, results of our short-term study suggest that ozone exposure may have negative impacts on soil CH 4 uptake and C sequestration and contribute to accelerated rates of soil N-cycling. • Temperate grassland mesocosms were exposed to ozone with or without N fertilizer. • Elevated ozone had negative effects on soil CH 4 uptake and net ecosystem CO 2 exchange. • Aboveground biomass was reduced during the non-growing but not the growing season. • A positive feedback of soil N-cycling to ozone exposure was detected. [ABSTRACT FROM AUTHOR]

Abstract To investigate whether nitrogen (N) load affects the ozone (O 3) stomatal flux-effect relationship for birch biomass, three-year old birch saplings were exposed to seven different O 3 profiles (24 h mean of 35–66 ppb) and four different N loads (10, 30, 50 and 70 kg ha−1 yr−1) in precision-controlled hemispherical glasshouses (solardomes) in 2012 and 2013. Stomatal conductance (g s) under optimal growth conditions was stimulated by enhanced N supply but was not significantly affected by enhanced O 3 exposure. Birch root, woody (stem + branches) and total biomass (root + woody) were not affected by the Phytotoxic Ozone Dose (POD 1 SPEC) after two seasons of O 3 exposure, and enhanced N supply stimulated biomass production independent of POD 1 SPEC (i.e. there were no POD 1 SPEC × N interactions). There was a strong linear relationship between the stem cross-sectional area and tree biomass at the end of the experiment, which was not affected by O 3 exposure or N load. Enhanced N supply stimulated the stem cross-sectional area at the end of season 2, but not at the end of season 1, which suggests a time lag before tree biomass responded to enhanced N supply. There was no significant effect of POD 1 SPEC on stem cross-sectional area after either the first or second growing season of the experiment. Contrasting results reported in the literature on the interactive impacts of O 3 and N load on tree physiology and growth are likely due to species-specific responses, different duration of the experiments and/or a limitation of the number of O 3 and N levels tested. Graphical abstract Unlabelled Image Highlights • O 3 flux-effect relationships investigated for birch at 7 O 3 profiles and 4 N loads • g s under optimal conditions was stimulated by N but not affected by O 3. • O 3 flux-effect relationships for biomass were not affected by N supply. • O 3 reduced stem cross-sectional area after the first season only. • N stimulation of stem cross-sectional area increased in the second season. [ABSTRACT FROM AUTHOR]

Abstract Reduced belowground carbon (C) allocation by plants exposed to ozone may change properties and activities of the microbial community in soils. To investigate how soil microbial biomass and extracellular enzyme activities respond to elevated ozone, we collected soils from a temperate grassland after four years of ozone exposure under fully open-air field conditions. We measured soil microbial biomass, the metabolism of low molecular weight C substrates and hydrolytic extracellular enzyme activities in both bulk soil and isolated aggregates to assess changes in microbial activity and community function. After four years of elevated ozone treatment, soil total organic C was reduced by an average of 20% compared with ambient condition. Elevated ozone resulted in a small but insignificant reduction (4–10%) in microbial biomass in both bulk soil and isolated aggregates. Activities of extracellular enzymes were generally not affected by elevated ozone, except β -glucosidase, whose activity in bulk soil was significantly lower under elevated ozone than ambient condition. Activities of β -glucosidase, leucine aminopeptidase and acid phosphatase were higher in microaggregates (<0.25 mm) as compared to macroaggregates (>0.25 mm). Elevated ozone had no effects on mineralization rates of low molecular weight C substrates in both bulk soil and isolated aggregates. We therefore conclude that the size and activity rather than function of the soil microbial community in this semi-natural grassland are altered by elevated ozone. Graphical abstract Unlabelled Image Highlights • A temperate grassland was exposed to ozone for 4 years under field conditions. • Soil total carbon and β-glucosidase activity were decreased under elevated ozone. • Neither ozone nor its interaction with aggregate size affected microbial activities. • Elevated ozone could alter the size and activity of soil microbial community. [ABSTRACT FROM AUTHOR]

Recent decades have seen a changing temporal profile of ground-level ozone (O 3 ) in Europe. While peaks in O 3 concentrations during summer months have been declining in amplitude, the background concentration has gradually increased as a result of the hemispheric transport of O 3 precursors from other world regions. Ground-level O 3 is known to adversely affect O 3 -sensitive vegetation, including reducing the yield of O 3 -sensitive crops such as common wheat ( Triticum aestivum L.). The reduction in wheat yield has been shown to be linearly related to the phytotoxic O 3 dose above a flux threshold of Y (POD Y ) accumulated over a specific period. In the current study, we tested whether the flux-effect relationships for wheat yield and 1,000-grain weight were affected by the temporal profile of O 3 exposure. A modern wheat cultivar (Skyfall) was exposed to eight different realistic O 3 profiles repeated weekly: four profiles with increasing background O 3 concentrations (ca. 30–60 ppb) including small peaks and four profiles with increasing O 3 peak concentrations (ca. 35–110 ppb). Both wheat yield and 1,000-grain weight declined linearly with increasingPOD Y . The slope of the flux-effect relationships was not affected significantly by the profile of O 3 exposure. Hence, flux-effect relationships developed for wheat based on exposure to enhanced peak O 3 concentrations are also valid for the changing European O 3 profile with higher background and lower peak concentrations. The current study also shows that the modern wheat cultivar Skyfall is more sensitive to O 3 than European wheat varieties tested for O 3 sensitivity in the 1980s and 1990s. [ABSTRACT FROM AUTHOR]

Ground-level ozone (O 3 ) concentrations and atmospheric nitrogen (N) deposition rates have increased strongly since the 1950s. Rising ground-level O 3 concentrations and atmospheric N deposition both affect plant physiology and growth, however, impacts have often been studied in isolation rather than in combination. In addition, studies are often limited to a control treatment and one or two elevated levels of ozone and/or nitrogen supply. In the current study, three-year old Betula pendula saplings were exposed to seven different O 3 profiles (24 h mean O 3 concentration of 36–68 ppb in 2013, with peaks up to an average of 105 ppb) in precision-controlled hemispherical glasshouses (solardomes) and four different N loads (10, 30, 50 or 70 kg N ha −1 y −1 ) in 2012 and 2013. Here we report on the effects of enhanced O 3 concentrations and N load on leaf traits and gas exchange in leaves of varying age and developmental stage in 2013. The response of leaf traits to O 3 (but not N) vary with leaf developmental stage. For example, elevated O 3 did not affect the chlorophyll content of the youngest fully expanded leaf, but it reduced the chlorophyll content and photosynthetic parameters in aging leaves, relatively more so later than earlier in the growing season. Elevated O 3 enhanced the N content of senesced leaves prior to leaf fall, potentially affecting subsequent N cycling in the soil. Enhanced N generally stimulated the chlorophyll content and photosynthetic capacity. Whilst elevated O 3 reduced the light-saturated rate of photosynthesis (A sat ) in aging leaves, it did not affect stomatal conductance (g s ). This suggests that photosynthesis and g s are not closely coupled at elevated O 3 under-light saturating conditions. We did not observe any interactions between O 3 and N regarding photosynthetic parameters (V c,max , J max , A sat ), chlorophyll content, g s , N content in senesced leaves and leaf number. Hence, the sensitivity of these leaf traits to O 3 in young silver birch trees is neither reduced nor enhanced by N load. [ABSTRACT FROM AUTHOR]

In this study we have demonstrated that rising background ozone has the potential to reduce grassland forage quality and explored the implications for livestock production. We analysed pasture samples from seven ozone exposure experiments comprising mesotrophic, calcareous, haymeadow and sanddune unimproved grasslands conducted in open-top chambers, solardomes and a field release system. Across all grassland types, there were significant increases in acid detergent fibre, crude fibre and lignin content with increasing ozone concentration, resulting in decreased pasture quality in terms of the metabolisable energy content of the vegetation. We derived a dose-response function for metabolisable energy of the grassland with ozone concentration, applicable to a range of grassland types, and used this to predict effects on pasture quality of UK vegetation at 1 km resolution using modelled ozone data for 2007 and for predicted higher average ozone concentrations in 2020. This showed a potential total reduction in lamb production in the UK of approximately 4% in 2020 compared to 2007. The largest impacts were in geographical areas of modest ozone increases between the two years, but where large numbers of lambs were present. For an individual farmer working to a very small cost margin this could represent a large reduction in profit, both in regions where the impacts per lamb and those where the impacts per km 2 of grazing land are largest. In the short term farmers could adapt their lamb management in response to changed forage quality by additional supplementary feed of high metabolisable energy content. Nationally this increase in annual additional feed in 2020 compared to 2007 would be 2,166 tonnes (an increase of 0.7%). Of added concern are the longer-term consequences of continual deterioration of pasture quality and the implications for changes in farming practices to compensate for potential reductions in livestock production capacity. [ABSTRACT FROM AUTHOR]

This paper provides a process-oriented perspective on the combined effects of ozone (O 3 ), climate change and/or nitrogen (N) on vegetation. Whereas increasing CO 2 in controlled environments or open-top chambers often ameliorates effects of O 3 on leaf physiology, growth and C allocation, this is less likely in the field. Combined responses to elevated temperature and O 3 have rarely been studied even though some critical growth stages such as seed initiation are sensitive to both. Under O 3 exposure, many species have smaller roots, thereby enhancing drought sensitivity. Of the 68 species assessed for stomatal responses to ozone, 22.5% were unaffected, 33.5% had sluggish or increased opening and 44% stomatal closure. The beneficial effect of N on root development was lost at higher O 3 treatments whilst the effects of increasing O 3 on root biomass became more pronounced as N increased. Both responses to gradual changes in pollutants and climate and those under extreme weather events require further study. [ABSTRACT FROM AUTHOR]

Four genotypes of snap bean ( Phaseolus vulgaris L.) were selected to study the effects of ambient ozone concentration at a cropland area around Beijing by using 450 ppm of ethylenediurea (EDU) as a chemical protectant. During the growing season, the 8 h (9:00–17:00) average ozone concentration was very high, approximately 71.3 ppb, and AOT40 was 29.0 ppm.h. All genotypes showed foliar injury, but ozone-sensitive genotypes exhibited much more injury than ozone-tolerant ones. Compared with control, EDU significantly alleviated foliar injury, increased photosynthesis rate and chlorophyll a fluorescence, Vc max and J max , and seed and pod weights in ozone-sensitive genotypes but not in ozone-tolerant genotypes. EDU did not significantly affect antioxidant contents in any of the genotypes. Therefore, EDU effectively protected sensitive genotypes from ambient ozone damage, while protection on ozone-tolerant genotypes was limited. EDU can be regarded as a useful tool in risk assessment of ambient ozone on food security. [ABSTRACT FROM AUTHOR]

Impacts of tropospheric ozone on sweet potato (Ipomoea batatas) are poorly understood despite being a staple food grown in locations deemed at risk from ozone pollution. Three varieties of sweet potato were exposed to ozone treatments (peaks of: 30 (Low), 80 (Medium), and 110 (High) ppb) using heated solardomes. Weekly measurements of stomatal conductance (gs) and chlorophyll content (CI) were used to determine physiological responses, along with final yield. gs and CI were reduced with increasing ozone exposure, but effects were partially masked due to elevated leaf senescence and turnover. Yield for the Erato orange and Murasaki varieties was reduced by ∼40% and ∼50% (Medium and High ozone treatments, respectively, vs Low) whereas Beauregard yield was reduced by 58% in both. The DO 3 SE (Deposition of Ozone for Stomatal Exchange) model was parameterized for gs in response to light, temperature, vapour pressure deficit and soil water potential. Clear responses of gs to the environmental parameters were found. Yield reductions were correlated with both concentration based AOT40 (accumulated ozone above a threshold of 40 ppb) and flux based POD 6 (accumulated stomatal flux of ozone above a threshold of 6 nmol m− 2 s− 1) metrics (R2 0.66 p = 0.01; and R2 0.44 p = 0.05, respectively). A critical level estimate of a POD 6 of 3 (mmol m−2 Projected Leaf Area−1) was obtained using the relationship. This study showed that sweet potato yield was reduced by ozone pollution, and that stomatal conductance and chlorophyll content were also affected. Results from this study can improve model predictions of ozone impacts on sweet potato together with associated ozone risk assessments for tropical countries. [Display omitted] • Leaf stomatal conductance and chlorophyll reduced with higher ozone (O 3) exposures. • Elevated O 3 up to 110 ppb reduced yields by ∼50% in three varieties. • O 3 flux model (DO 3 SE) parametrised for sweet potato. • Critical level (at 5% yield loss) estimated using the O 3 dose/response relationship. • Results can improve risk assessments of O 3 yield impacts for this staple food crop. [ABSTRACT FROM AUTHOR]

Tropospheric (ground-level) ozone is a harmful phytotoxic pollutant, and can have a negative impact on crop yield and quality in sensitive species. Ozone can also induce visible symptoms on leaves, appearing as tiny spots (stipples) between the veins on the upper leaf surface. There is little measured data on ozone concentrations in Africa and it can be labour-intensive and expensive to determine the direct impact of ozone on crop yield in the field. The identification of visible ozone symptoms is an easier, low cost method of determining if a crop species is being negatively affected by ozone pollution, potentially resulting in yield loss. In this study, thirteen staple African food crops (including wheat (Triticum aestivum), common bean (Phaseolus vulgaris), sorghum (Sorghum bicolor), pearl millet (Pennisetum glaucum) and finger millet (Eleusine coracana)) were exposed to an episodic ozone regime in a solardome system to monitor visible ozone symptoms. A more detailed examination of the progression of ozone symptoms with time was carried out for cultivars of P. vulgaris and T. aestivum , which showed early leaf loss (P. vulgaris) and an increased rate of senescence (T. aestivum) in response to ozone exposure. All of the crops tested showed visible ozone symptoms on their leaves in at least one cultivar, and ozone sensitivity varied between cultivars of the same crop. A guide to assist with identification of visible ozone symptoms (including photographs and a description of symptoms for each species) is presented. Image 1 • Thirteen African crop species were exposed to an episodic ozone regime in solardomes. • All crops tested showed visible ozone symptoms in at least one cultivar. • Ozone induced early leaf loss (P. vulgaris) and accelerated senescence (T. aestivum). • Visible symptoms can be used to indicate potential ozone impacts on crop yield. Thirteen African crop species exposed to an episodic ozone regime showed ozone-induced effects on leaves in at least one cultivar, including visible symptoms, early leaf loss and accelerated senescence. [ABSTRACT FROM AUTHOR]

In this study we have demonstrated that plants originating from upland peat bogs are sensitive to increasing background concentrations of ozone. Peatland mesocosms from an upland peat bog in North Wales, UK were exposed to eight levels of elevated background ozone in solardomes for 4 months from May to August, with 24 h mean ozone concentrations ranging from 16 to 94 ppb and cumulative AOT0 24hr ranging from 45.98 ppm h to 259.63 ppm h. Our results show that plant senescence increased with increasing exposure to ozone, although there was no significant effect of increasing ozone on plant biomass. Assessments of carbon dioxide and methane fluxes from the mesocosms suggests that there was no change in carbon dioxide fluxes over the 4 month exposure period but that methane fluxes increased as cumulative ozone exposure increased to a maximum AOT 0 24hr of approximately 120 ppm h and then decreased as cumulative ozone exposure increased further. [ABSTRACT FROM AUTHOR]