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

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

Evidence of widespread effects of ozone on crops and (semi-)natural vegetation in Europe (1990 – 2006) in relation to AOT40– and flux-based risk maps

Records of effects of ambient ozone pollution on vegetation have been compiled for Europe for the years 1990–2006. Sources include scientific papers, conference proceedings, reports to research funders, records of confirmed ozone injury symptoms and an international biomonitoring experiment coordinated by the ICP Vegetation. The latter involved ozone-sensitive (NC-S) and ozone-resistant (NC-R) biotypes of white clover (Trifolium repens L.) grown according to a common protocol and monitored for ozone injury and biomass differences in 17 European countries, from 1996 to 2006. Effects were separated into visible injury or growth/yield reduction. Of the 644 records of visible injury, 39% were for crops (27 species), 38.1% were for (semi-) natural vegetation (95 species) and 22.9% were for shrubs (49 species). Owing to inconsistencies in reporting effort from year to year it was not possible to determine geographical or temporal trends in the data. Nevertheless, this study has shown effects in ambient air in 18 European countries from Sweden in the north to Greece in the south. These effects data were superimposed on AOT40 (accumulated ozone concentrations over 40 ppb) and POD3gen (modelled accumulated stomatal flux over a threshold of 3 nmolm2 s1) maps generated by the EMEP Eulerian model (50km 50km grid) that were parameterized for a generic crop based on wheat and NC-S/NC-R white clover. Many effects were found in areas where the AOT40 (crops) was below the critical level of 3ppmh. In contrast, the majority of effects were detected in grid squares where POD3gen (crops) were in the mid-high range (412 mmolm2). Overall, maps based on POD3gen provided better fit to the effects data than those based on AOT40, with the POD3gen model for clover fitting the clover effects data better than that for a generic crop

Dominance of biologically produced nitrate in upland waters of Great Britain indicated by stable isotopes

Atmospheric deposition of nitrogen (N) compounds is the major source of anthropogenic N to most upland ecosystems, where leaching of nitrate (NO3−) into surface waters contributes to eutrophication and acidification as well as indicating an excess of N in the terrestrial catchment ecosystems. Natural abundance stable isotopes ratios, 15N/14N and 18O/16O (the “dual isotope” technique) have previously been used in biogeochemical studies of alpine and forested ecosystems to demonstrate that most of the NO3− in upland surface waters has been microbially produced. Here we present an application of the technique to four moorland catchments in the British uplands including a comparison of lakes and their stream inflows at two sites. The NO3− concentrations of bulk deposition and surface waters at three sites are very similar. While noting the constraints imposed by uncertainty in the precise δ18O value for microbial NO3−, however, we estimate that 79–98% of the annual mean NO3− has been microbially produced. Direct leaching of atmospheric NO3− is a minor component of catchment NO3− export, although greater than in many similar studies in forested watersheds. A greater proportion of atmospheric NO3− is seen in the two lake sites relative to their inflow streams, demonstrating the importance of direct NO3− deposition to lake surfaces in catchments where terrestrial ecosystems intercept a large proportion of deposited N. The dominance of microbial sources of NO3− in upland waters suggests that reduced and oxidised N deposition may have similar implications in terms of contributing to NO3− leaching

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

Sampling systems for isotope ratio mass spectrometry of atmospheric ammonia

Passive and active ammonia (NH3) sampling devices have been tested for their nitrogen (N) capture potential and 15N fractionation effects. Several sampling techniques produced significantly different 15NH3 signals when sampling the same NH3 source released from field site fumigation campaigns. Conventional passive NH3-monitoring systems have shown to provide insufficient N for isotope-ratio mass spectrometry and various modified devices have been developed, based on existing diffusion tube designs, to overcome this problem. The final sampler design was then tested in a wind tunnel to verify that sampling NH3 in different environmental conditions did not significantly fractionate the 15N signal

Chronic exposure to increasing background ozone impairs stomatal functioning in grassland species.

Two species found in temperate calcareous and mesotrophic grasslands (Dactylis glomerata and Leontodon hispidus) were exposed to eight ozone treatments spanning preindustrial to post-2100 regimes, and late-season effects on stomatal functioning were investigated. The plants were grown as a mixed community in 14 L containers and were exposed to ozone in ventilated solardomes (dome-shaped greenhouses) for 20 weeks from early May to late September 2007. Ozone exposures were based on O3 concentrations from a nearby upland area, and provided the following seasonal 24 h means: 21.4, 39.9 (simulated ambient), 50.2, 59.4, 74.9, 83.3, 101.3 and 102.5 ppb. In both species, stomatal conductance of undamaged inner canopy leaves developing since a midseason cutback increased linearly with increasing background ozone concentration. Imposition of severe water stress by leaf excision indicated that increasing background ozone concentration decreased the ability of leaves to limit water loss, implying impaired stomatal control. The threshold ozone concentrations for these effects were 15–40 ppb above current ambient in upland UK, and were within the range of ozone concentrations anticipated for much of Europe by the latter part of this century. The potential mechanism behind the impaired stomatal functioning was investigated using a transpiration assay. Unlike for lower ozone treatments, apparently healthy green leaves of L. hispidus that had developed in the 101.3 ppb treatment did not close their stomata in response to 1.5 μm abscisic acid (ABA); indeed stomatal opening initially occurred in this treatment. Thus, ozone appears to be disrupting the ABA-induced signal transduction pathway for stomatal control thereby reducing the ability of plants to respond to drought. These results have potentially wide-reaching implications for the functioning of communities under global warming where periods of soil drying and episodes of high vapour pressure deficit are likely to be more severe