Committee on air pollution effects research: 40 years of UK air pollution

The UK Committee on Air Pollution Effects Research (CAPER) was established 40 years ago. This special section was compiled to mark this anniversary. During this time there have been dramatic changes in the composition of the air over the UK. The four papers in this special section of Environmental Pollution represent the current air pollution effects research focus on ozone and nitrogen deposition, two related issues and are proving from a policy perspective to be quite intractable issues. The UK CAPER research community continues to advance the underpinning science and engages closely with the user community in government departments

Mapping portuguese Natura 2000 sites in risk of biodiversity change caused by atmospheric nitrogen pollution

In this paper, we assess and map the risk that atmospheric nitrogen (atN) pollution poses to biodiversity in Natura 2000 sites in mainland Portugal. We first review the ecological impacts of atN pollution on terrestrial ecosystems, focusing on the biodiversity of Natura 2000 sites. These nature protection sites, especially those located within the Mediterranean Basin, are under-characterized regarding the risk posed by atN pollution. We focus on ammonia (NH ) because this N form is mostly associated with agriculture, which co-occurs at or in the immediate vicinity of most areas of conservation interest in Portugal. We produce a risk map integrating NH emissions and the susceptibility of Natura 2000 sites to atN pollution, ranking habitat sensitivity to atN pollution using expert knowledge from a panel of Portuguese ecological and habitat experts. Peats, mires, bogs, and similar acidic and oligotrophic habitats within Natura 2000 sites (most located in the northern mountains) were assessed to have the highest relative risk of biodiversity change due to atN pollution, whereas Natura 2000 sites in the Atlantic and Mediterranean climate zone (coastal, tidal, and scrubland habitats) were deemed the least sensitive. Overall, results allowed us to rank all Natura 2000 sites in mainland Portugal in order of evaluated risk posed by atN pollution. The approach is of great relevance for stakeholders in different countries to help prioritize site protection and to define research priorities. This is especially relevant in countries with a lack of expertise to assess the impacts of nitrogen on biodiversity and can represent an important step up from current knowledge in such countriesinfo:eu-repo/semantics/publishedVersio

Woody stem methane emission in mature wetland alder trees

Methane (CH4) is an important greenhouse gas that is predominantly emitted to the atmosphere from anoxic wetland ecosystems. Understanding the sources and emissions of CH4 is crucially important for climate change predictions; however, there are significant discrepancies between CH4 source estimates derived via so-called bottom-up and top-down methods. Here we report CH4 emission from the stems of mature wetland alder (Alnus glutinosa) trees in the UK, a common tree of northern hemisphere floodplains and wetlands. The alder stems most likely behave as conduits for soil-produced CH4 either in the gaseous or aqueous phase, and may, therefore, help to reconcile methodological differences in the way the wetland CH4 source is estimated. Alder tree stems emitted average peak CH4 fluxes of 101 μg CH4 m−2 h−1 (on a stem area basis) in early October, a rate that is similar to that obtained from mature Japanese ash (Fraxinus mandshurica var. japonica) in Japan and amounting to approximately 20% of the measured CH4 flux from the soil surface. The finding suggests that trees, which occupy 60% of Earth's wetlands and are normally excluded from the measurement programmes that form the basis for bottom-up estimates of the global wetland source, could be important contributors to overall terrestrial ecosystem CH4 flux

Impact of long-term nitrogen deposition on the response of dune grassland ecosystems to elevated summer ozone

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

Evidence for differential effects of reduced and oxidised nitrogen deposition on vegetation independent of nitrogen load

Nitrogen (N) deposition impacts natural and semi-natural ecosystems globally. The responses of vegetation to N deposition may, however, differ strongly between habitats and may be mediated by the form of N. Although much attention has been focused on the impact of total N deposition, the effects of reduced and oxidised N, independent of the total N deposition, have received less attention. In this paper, we present new analyses of national monitoring data in the UK to provide an extensive evaluation of whether there are differences in the effects of reduced and oxidised N deposition across eight habitat types (acid, calcareous and mesotrophic grasslands, upland and lowland heaths, bogs and mires, base-rich mires, woodlands). We analysed data from 6860 plots in the British Countryside Survey 2007 for effects of total N deposition and N form on species richness, Ellenberg N values and grass:forb ratio. Our results provide clear evidence that that N deposition affects species richness in all habitats except base-rich mires, after factoring out correlated explanatory variables (climate and sulphur deposition). In addition, the form of N in deposition appears important for the biodiversity of grasslands and woodlands but not mires and heaths. Ellenberg N increased more in relation to NHx deposition than NOy deposition in all but one habitat type. Relationships between species richness and N form were habitat-specific: acid and mesotrophic grasslands appear more sensitive to NHx deposition while calcareous grasslands and woodlands appeared more responsive to NOy deposition. These relationships are likely driven by the preferences of the component plant species for oxidised or reduced forms of N, rather than by soil acidification

Fungal colonization patterns and enzymatic activities of peatland ericaceous plants following long-term nutrient addition

Northern peatlands are often dominated by ericaceous shrub species which rely on ericoid mycorrhizal fungi (ERM) for access to organic sources of nutrients, such as nitrogen (N) and phosphorus (P), and host abundant dark septate endophytes (DSE). Relationships between hosts and fungal symbionts may change during deposition of anthropogenic N and P. We studied the long-term effects of N and P addition on two ericaceous shrubs, Calluna vulgaris and Erica tetralix, at Whim Bog, Scotland by analyzing fungal colonization of roots, enzymatic activity, and fungal species composition. Unexpectedly, the frequency of typical ERM intracellular colonization did not change while the occurrence of ERM hyphae tended to increase and DSE hyphae to decrease. Our findings indicate that altered nutrient limitations shift root associated fungal colonization patterns as well as affecting ericaceous root enzyme activity and thereby decomposition potential. Reduction of recalcitrant fungal biomass in melanized DSE may have implications for peatland C sequestration under nutrient addition.Peer reviewe

Suppression of rice methane emission by sulfate deposition in simulated acid rain

Sulfate in acid rain is known to suppress methane (CH4) emissions from natural freshwater wetlands. Here we examine the possibility that CH4 emissions from rice agriculture may be similarly affected by acid rain, a major and increasing pollution problem in Asia. Our findings suggest that acid rain rates of SO2-4 deposition may help to reduce CH4 emissions from rice agriculture. Emissions from rice plants treated with simulated acid rain at levels of SO2-4 consistent with the range of deposition in Asia were reduced by 24% during the grain filling and ripening stage of the rice season which accounts for 50% of the overall CH4 that is normally emitted in a rice season. A single application of SO2-4 at a comparable level reduced CH4 emission by 43%. We hypothesize that the reduction in CH4 emission may be due to a combination of effects. The first mechanism is that the low rates of SO2-4 may be sufficient to boost yields of rice and, in so doing, may cause a reduction in root exudates to the rhizosphere, a key substrate source for methanogenesis. Decreasing a major substrate source for methanogens is also likely to intensify competition with sulfate-reducing microorganisms for whom prior SO2-4 limitation had been lifted by the simulated acid rain S deposition

Regional trends in soil acidification and exchangeable metal concentrations in relation to acid deposition rates

The deposition of high levels of reactive nitrogen (N) and sulphur (S), or the legacy of that deposition, remain among the world's most important environmental problems. Although regional impacts of acid deposition in aquatic ecosystems have been well documented, quantitative evidence of wide-scale impacts on terrestrial ecosystems is not common. In this study we analysed surface and subsoil chemistry of 68 acid grassland sites across the UK along a gradient of acid deposition, and statistically related the concentrations of exchangeable soil metals (1 M KCl extraction) to a range of potential drivers. The deposition of N, S or acid deposition was the primary correlate for 8 of 13 exchangeable metals measured in the topsoil and 5 of 14 exchangeable metals in the subsoil. In particular, exchangeable aluminium and lead both show increased levels above a soil pH threshold of about 4.5, strongly related to the deposition flux of acid compound

Response of a peat bog vegetation community to long-term experimental addition of nitrogen

1. We report results from a long‐term experiment in which additional nitrogen has been deposited on a peat bog in central Scotland for over 14 years, in three different forms: as ammonia (NH3) gas, as ammonium (NH4+) solution, or as nitrate (NO3-) solution. The automated experiment was designed to apply nitrogen in such a way that mimics real‐world nitrogen deposition. Background nitrogen deposition at the site was 0.8 g N m−2 year−1). 2. Observations of cover for 46 species were made. We analysed the change in six common species in relation to nitrogen dose and form. The responses differed among species and nitrogen forms, but five out of the six species declined, and NH3 produced the biggest change in cover per unit of nitrogen addition. The exception was the graminoid sedge Eriophorum vaginatum, which increased dramatically in the NH3 treatment. Multivariate analyses identified responses to nitrogen dose across treatments which were consistent with the univariate results. 3. We surmised that the larger experimental response to nitrogen observed in the NH3 treatment (cf. the NH4+ and NO3- treatments) was because of the higher nitrogen concentrations at the vegetation surface produced by dry deposition. NH4+ and NO3- were sprayed in solution, but much of this will enter the peat porewater, and be further diluted. Because NH3 deposits directly to the leaf, it stays contained within the small volume of water on and in the leaf, producing a high internal concentration of nitrogen ions. 4. Synthesis: Consistent trends with nitrogen were discernible across species. All species showed a decline with NH3 treatment, except for Eriophorum vaginatum which increased. In the absence of phosphorous and potassium (PK), all species declined with NH4+ and NO3-, except for Calluna vulgaris and Hypnum jutlandicum. The effect of PK was not consistent across species. Per unit of nitrogen deposited, NH3 generally had a larger impact on vegetation composition than NH4+ or NO3-. However, the actual deposition rate of NH3 on UK peat bogs is lower than the other forms. In the case of the most common species of the peat‐forming genus Sphagnum, we estimate that NH4+ deposition has the largest impact, followed by NO3- and NH3