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

Changes in vegetation and soil characteristics in coastal sand dunes along a gradient of atmospheric nitrogen deposition

A field survey was conducted to detect signals of atmospheric nitrogen (N) in 11 dune systems along a nitrogen deposition gradient in the United Kingdom. In the mobile and semi-fixed dunes, above-ground biomass was positively related to N inputs. This increase was largely due to increased height and cover of Ammophila arenaria. In the long term, this increased biomass may lead to increased organic matter accumulation and consequently accelerated soil development. In the fixed dunes, above ground biomass also showed a positive relationship with N inputs as did soil C : N ratio while soil available N was negatively related to N inputs. Plant species richness was negatively related to N inputs. In the dune slacks, while soil and bulk vegetation parameters showed no relationship with N inputs, cover of Carex arenaria and Hypochaeris radicata increased. Site mean Ellenberg N numbers showed no relationship with N deposition either within habitats or across the whole dataset. Neither abundance-weighting nor inclusion of the Siebel numbers for bryophytes improved the relationship. The survey reveals that the relationships of soil and vegetation with atmospheric N deposition vary between sand dune habitats but, despite this variability, clear correlations with N inputs exist. While this survey cannot establish causality, on the basis of the relationships observed we suggest a critical load range of 10 - 20 kg N ha(-1) yr(-1) for coastal sand dunes in the UK

Does a simulated upland grassland community respond to increasing background, peak or accumulated exposure of ozone?

Tropospheric ozone concentrations are increasing, which may result in elevated background concentrations at rural high-altitude sites. In this study simulated upland grassland communities containing seven species were exposed to ozone treatments in solardomes for 12 weeks in each of two consecutive summers. Ozone profiles, based on future ozone predictions, were of elevated background concentrations, episodic peaks of ozone and a combination of the two. During the winter between the two exposures the communities were kept outdoors in ambient air. Whereas previous studies have demonstrated that peaks of ozone cause detrimental effects to vegetation, this study shows that for simulated grassland communities an increase in background ozone concentration in the absence of peaks of ozone also corresponded with increased senescence. In many cases senescence was further increased when peaks of ozone were also present. The species used showed no acclimation to ozone and the same relationship between senescence and ozone dose occurred in both years of the study. A decrease in cumulative biomass was demonstrated for Anthoxanthum odoratum, which contributed to a decrease in total community biomass and grass:forb ratio. These results indicate that current and future ozone concentrations could cause detrimental effects on growth and vitality of natural grassland communities and that for some species the consequences of increased background ozone concentration are as severe as that of increased peaks

Species-specific effects of elevated ozone on wetland plants and decomposition processes

Seven species from two contrasting wetlands, an upland bog and a lowland rich fen in North Wales, UK, were exposed to elevated ozone (150 ppb for 5 days and 20 ppb for 2 days per week) or low ozone (20 ppb) for four weeks in solardomes. The rich fen species were: Molinia caerulea, Juncus subnodulosus, Potentilla erecta and Hydrocotyle vulgaris and the bog species were: Carex echinata, Potentilla erecta and Festuca rubra. Senescence significantly increased under elevated ozone in all seven species but only Molinia caerulea showed a reduction in biomass under elevated ozone. Decomposition rates of plants exposed to elevated ozone, as measured by carbon dioxide efflux from dried plant material inoculated with peat slurry, increased for Potentilla erecta with higher hydrolytic enzyme activities. In contrast, a decrease in enzyme activities and a non-significant decrease in carbon dioxide efflux occurred in the grasses, sedge and rush species. Short-term, episodic ozone exposure increased senescence and changed decomposition processes in wetland plant species

Surface ozone concentrations and ecosystem health: past trends and a guide to future projections

This paper reviews current understanding of the sources and sinks of ozone in the troposphere, recent studies of long-term trends, and the factors which have to be taken into consideration when constructing and interpreting future models of ozone concentration. The factors controlling surface O3 concentrations are discussed initially to provide a basis for the ensuing discussion, followed by a summary of the evidence for recent trends in ground-level ozone concentrations, i.e. over the past 3 decades, which have shown a significant increase in the annual average in ‘background’ air typical of the unpolluted northern hemisphere. Closer to precursor sources, although urban winter concentrations have increased, rural peak spring and summer concentrations during ozone ‘episodes’ have decreased markedly in response to emissions reductions. In order to determine whether such trends are meaningful, the statistical techniques for determining temporal trends are reviewed. The possible causes of long-term trends in ozone are then discussed, with particular reference to the use of chemistry-transport models to interpret past trends. Such models are also used to make predictions of future trends in surface ozone concentrations, but few are comprehensive in integrating future climate changes with changes in land use and in emissions of ozone precursors. Guidance is given on the likely effects of climate/precursor/chemistry interactions so that model predictions can be judged

Nitrogen deposition effects on plant species diversity; threshold loads from field data

National-scale plant species richness data for Great Britain in 1998 were related to modelled contemporary N deposition (Ndep) using a broken stick median regression, to estimate thresholds above which Ndep definitely has had an effect. The thresholds (kgN ha-1 a-1) are 7.9 for acid grassland 14.9 for bogs, 23.6 for calcareous grassland, 7.8 for deciduous woodland and 8.8 for heath. The woodland and heath thresholds are not significantly greater than the lowest Ndep, which implies that species loss may occur over the whole range of contemporary Ndep. This also applies to acid grassland if it is assumed that Ndep has substituted for previous N fixation. The thresholds for bog and calcareous grassland are both significantly above the lowest Ndep. The thresholds are lower than the mid-range empirical Critical Loads for acid grassland, deciduous woodland and heath, higher for bogs, and approximately equal for calcareous grassland

Terricolous lichens as indicators of nitrogen deposition: evidence from national records

Large areas of Great Britain currently receive nitrogen (N) deposition at rates which exceed the thresholds above which there is risk of damage to sensitive components of the ecosystem (critical loads for nutrient nitrogen and critical levels for ammonia), and are predicted to continue to do so. Excess N can damage semi-natural ecosystems. Lichens are potentially sensitive to air quality because they directly utilise nutrients deposited from the atmosphere thus may be good indicators of air quality. We used data from the British Lichen Society (BLS) database, which records the presence of all lichen taxa growing in Britain at 10 km resolution. The probability of presence of a taxa at a given level of N deposition was analysed together with driver data for climate, change in sulphur deposition, land-use and N deposition using generalised additive models (GAMs). Many taxa showed negative responses to N deposition with reductions in the probability of presence as N deposition increased. In all of the habitats, there were a mix of terricolous taxa which showed negative or no significant relationship with N deposition. Most of the taxa with negative relationships with N deposition started to decline in prevalence at the lowest levels of deposition found in this study. Levels of deposition over which a negative response apparently occurs are lower than those at which critical loads have been set for some habitats. These findings suggest that some terricolous lichen taxa are sensitive to atmospheric N deposition and even low levels of nitrogen deposition could be damaging terricolous lichen communities making then potentially good indicators of N deposition

Modelling the impacts of atmospheric nitrogen deposition on Calluna-dominated ecosystems in the UK

1. The increased deposition of nitrogen (N) from the atmosphere over the last century has been associated in Europe with changes in species composition, including replacement of characteristic ericaceous shrubs such as Calluna vulgaris by grasses in heathlands and moorlands. However, these changes may also be associated with changes in management practices and environmental stresses, which may interact with changes in N deposition. Policies have now been implemented to reduce N deposition, but whether, and over what time scale, changes in vegetation composition will be reversed is uncertain. 2. A model was developed to simulate competitive growth between Calluna vulgaris and the grass species Deschampsia flexuosa and Molinia caerulea, driven by light and N availability. The model was parameterized for application to UK heath and moorland systems, and tested using a synthesis of data from 10 years of three field manipulation experiments in the UK. New routines to simulate management (burning, mowing, sheep grazing) were incorporated, and the model included a stochastic treatment of heather beetle Lochmaea suturalis responses. The effects of increases and decreases in N deposition over a period of 250 years were simulated under different management regimes. 3. Model runs demonstrated that changes in species composition in response to step changes in N deposition may occur over several decades and management cycles. The simulations showed a strong effect of management intensity, and in particular litter removal, in modifying the long-term impact of N deposition: recovery of Calluna dominance in lowland heaths was predicted within two to three decades of a decrease in N deposition under high-intensity management, but over five decades under low-intensity management. The timing of outbreaks of heather beetles, which were modelled stochastically, also had a strong effect on the balance between Calluna and grass species. Sensitivity analysis demonstrated the importance of mineralization rates, and Calluna growth rates and mortality rates, in influencing model outcomes, and also demonstrated significant interactions between these three factors and the probability of heather beetle outbreaks. 4. Synthesis and applications. Using a simulation model of competition between heather and grasses for light and N, scenarios were run for 250 years. Nitrogen deposition above 30 kg ha year−1 initially increased Calluna biomass, consistent with field experiments, but after several decades led to grass dominance rather than heather dominance. The effect of N deposition depended on grazing pressure and the degree of litter removal through mowing, burning or sod-cutting. The benefits of policies to reduce N deposition, in terms of restoring heather dominance, may only be realized after several decades, and active site management may be needed to capture the full benefits of such policies