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Ammonia and ammonium have received less attention than other forms of air pollution, with limited progress in controlling emissions at UK, European and global scales. By contrast, these compounds have been of significant past interest to science and society, the recollection of which can inform future strategies. Sal ammoniac ( nūshādir , nao sha ) is found to have been extremely valuable in long-distance trade ( ca AD 600-1150) from Egypt and China, where 6-8 kg N could purchase a human life, while air pollution associated with nūshādir collection was attributed to this nitrogen form. Ammonia was one of the keys to alchemy-seen as an early experimental mesocosm to understand the world-and later became of interest as 'alkaline air' within the eighteenth century development of pneumatic chemistry. The same economic, chemical and environmental properties are found to make ammonia and ammonium of huge relevance today. Successful control of acidifying SO 2 and NO x emissions leaves atmospheric NH 3 in excess in many areas, contributing to particulate matter (PM 2.5 ) formation, while leading to a new significance of alkaline air, with adverse impacts on natural ecosystems. Investigations of epiphytic lichens and bog ecosystems show how the alkalinity effect of NH 3 may explain its having three to five times the adverse effect of ammonium and nitrate, respectively. It is concluded that future air pollution policy should no longer neglect ammonia. Progress is likely to be mobilized by emphasizing the lost economic value of global N emissions ($200 billion yr -1 ), as part of developing the circular economy for sustainable nitrogen management. This article is part of a discussion meeting issue 'Air quality, past present and future'.

Although it is generally accepted that δ 15 N in lichen reflects predominating N isotope sources in the environment, confirmation of the direct correlation between lichen δ 15 N and atmospheric δ 15 N is still missing, especially under field conditions with most confounding factors controlled. To fill this gap and investigate the response of lichens with different tolerance to atmospheric N deposition, thalli of the sensitive Evernia prunastri and the tolerant Xanthoria parietina were exposed for ten weeks to different forms and doses of N in a field manipulation experiment where confounding factors were minimized. During this period, several parameters, namely total N, δ 15 N and chlorophyll a fluorescence, were measured. Under the experimental conditions, δ 15 N in lichens quantitatively responded to the δ 15 N of released gaseous ammonia (NH 3 ). Although a high correlation between the isotopic signatures in lichen tissue and supplied N was found both in tolerant and sensitive species, chlorophyll a fluorescence indicated that the sensitive species very soon lost its photosynthetic functionality with increasing N availability. The most damaging response to the different N chemical forms was observed with dry deposition of NH 3 , although wet deposition of ammonium ions had a significant observable physiological impact. Conversely, there was no significant effect of nitrate ions on chlorophyll a fluorescence, implying differential sensitivity to dry deposition versus wet deposition and to ammonium versus nitrate in wet deposition. Evernia prunastri was most sensitive to NH 3 , then NH 4 + , with lowest sensitivity to NO 3 - . Moreover, these results confirm that lichen δ 15 N can be used to indicate the δ 15 N of atmospheric ammonia, providing a suitable tool for the interpretation of the spatial distribution of NH 3 sources in relation to their δ 15 N signal., (Copyright © 2018. Published by Elsevier B.V.)

Little information exists concerning the long-term interactive effect of nitrogen (N) addition with phosphorus (P) and potassium (K) on Sphagnum N status. This study was conducted as part of a long-term N manipulation on Whim bog in south Scotland to evaluate the long-term alleviation effects of phosphorus (P) and potassium (K) on N saturation of Sphagnum (S. capillifolium). On this ombrotrophic peatland, where ambient deposition was 8 kg N ha -1 yr -1 , 56 kg N ha -1 yr -1 of either ammonium (NH 4 + , N red ) or nitrate (NO 3 - , N ox ) with and without P and K, were added over 11 years. Nutrient concentrations of Sphagnum stem and capitulum, and pore water quality of the Sphagnum layer were assessed. The N-saturated Sphagnum caused by long-term (11 years) and high doses (56 kg N ha -1 yr -1 ) of reduced N was not completely ameliorated by P and K addition; N concentrations in Sphagnum capitula for N red 56 PK were comparable with those for N red 56, although N concentrations in Sphagnum stems for N red 56 PK were lower than those for N red 56. While dissolved inorganic nitrogen (DIN) concentrations in pore water for N red 56 PK were not different from N red 56, they were lower for N ox 56 PK than for N ox 56 whose stage of N saturation had not advanced compared to N red 56. These results indicate that increasing P and K availability has only a limited amelioration effect on the N assimilation of Sphagnum at an advanced stage of N saturation. This study concluded that over the long-term P and K additions will not offset the N saturation of Sphagnum., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Main Conclusion: Different nitrogen forms affect different metabolic pathways in lichens. In particular, the most relevant changes in protein expression were observed in the fungal partner, with NO 3 - mostly affecting the energetic metabolism and NH 4 + affecting transport and regulation of proteins and the energetic metabolism much more than NO 3 - did. Excess deposition of reactive nitrogen is a well-known agent of stress for lichens, but which symbiont is most affected and how, remains a mystery. Using proteomics can expand our understanding of stress effects on lichens. We investigated the effects of different doses and forms of reactive nitrogen, with and without supplementary phosphorus and potassium, on the proteome of the lichen Cladonia portentosa growing in a 'real-world' simulation of nitrogen deposition. Protein expression changed with the nitrogen treatments but mostly in the fungal partner, with NO 3 - mainly affecting the energetic metabolism and NH 4 + also affecting the protein synthesis machinery. The photobiont mainly responded overexpressing proteins involved in energy production. This suggests that in response to nitrogen stress, the photobiont mainly supports the defensive mechanisms initiated by the mycobiont with an increased energy production. Such surplus energy is then used by the cell to maintain functionality in the presence of NO 3 - , while a futile cycle of protein production can be hypothesized to be induced by NH 4 + excess. External supply of potassium and phosphorus influenced differently the responses of particular enzymes, likely reflecting the many processes in which potassium exerts a regulatory function.

Long-term additions of nitrogen (N) to peatlands have altered bryophyte growth, species dominance, N content in peat and peat water, and often resulted in enhanced Sphagnum decomposition rate. However, these results have mainly been derived from experiments in which N was applied as ammonium nitrate (NH4NO3), neglecting the fact that in polluted areas, wet deposition may be dominated either by NO3(-) or NH4(+). We studied effects of elevated wet deposition of NO3(-) vs. NH4(+) alone (8 or 56kgNha(-1)yr(-1) over and above the background of 8kgNha(-1)yr(-1) for 5 to 11years) or combined with phosphorus (P) and potassium (K) on Sphagnum quality for decomposers, mass loss, and associated changes in hummock pore water in an ombrotrophic bog (Whim). Adding N, especially as NH4(+), increased N concentration in Sphagnum, but did not enhance mass loss from Sphagnum. Mass loss seemed to depend mainly on moss species and climatic factors. Only high applications of N affected hummock pore water chemistry, which varied considerably over time. Overall, C and N cycling in this N treated bog appeared to be decoupled. We conclude that moss species, seasonal and annual variation in climatic factors, direct negative effects of N (NH4(+) toxicity) on Sphagnum production, and indirect effects (increase in pH and changes in plant species dominance under elevated NO3(-) alone and with PK) drive Sphagnum decomposition and hummock C and N dynamics at Whim., (Copyright © 2016 Elsevier B.V. All rights reserved.)

The ability of Sphagnum moss to efficiently intercept atmospheric nitrogen (N) has been assumed to be vulnerable to increased N deposition. However, the proposed critical load (20kgNha(-1)yr(-1)) to exceed the capacity of the Sphagnum N filter has not been confirmed. A long-term (11years) and realistic N manipulation on Whim bog was used to study the N filter function of Sphagnum (Sphagnum capillifolium) in response to increased wet N deposition. On this ombrotrophic peatland where ambient deposition was 8kgNha(-1)yr(-1), an additional 8, 24, and 56kgNha(-1)yr(-1) of either ammonium (NH4(+)) or nitrate (NO3(-)) has been applied for 11years. Nutrient status of Sphagnum and pore water quality from the Sphagnum layer were assessed. The N filter function of Sphagnum was still active up to 32kgNha(-1)yr(-1) even after 11years. N saturation of Sphagnum and subsequent increases in dissolved inorganic N (DIN) concentration in pore water occurred only for 56kgNha(-1)yr(-1) of NH4(+) addition. These results indicate that the Sphagnum N filter is more resilient to wet N deposition than previously inferred. However, functionality will be more compromised when NH4(+) dominates wet deposition for high inputs (56kgNha(-1)yr(-1)). The N filter function in response to NO3(-) uptake increased the concentration of dissolved organic N (DON) and associated organic anions in pore water. NH4(+) uptake increased the concentration of base cations and hydrogen ions in pore water though ion exchange. The resilience of the Sphagnum N filter can explain the reported small magnitude of species change in the Whim bog ecosystem exposed to wet N deposition. However, changes in the leaching substances, arising from the assimilation of NO3(-) and NH4(+), may lead to species change., (Copyright © 2016 Elsevier B.V. All rights reserved.)

Gaseous elemental (GEM), particulate bound (PBM) and gaseous oxidised (GOM) mercury species were monitored between 2009 and 2011 at the rural monitoring site, Auchencorth Moss, Scotland using the Tekran speciation monitoring system. GEM average for the three year period was 1.40±0.19 ng m(-3) which is comparable with other northern hemisphere studies. PBM and GOM concentrations are very low in 2009 and 2010 with geometric mean (×/÷standard deviation) PBM values of 2.56 (×/÷3.44) and 0.03 (×/÷17.72) pg m(-3) and geometric mean (×/÷standard deviation) GOM values of 0.11 (×/÷4.94) and 0.09 (×/÷8.88) pg m(-3) respectively. Using wind sector analysis and air mass back trajectories, the importance of local and regional sources on speciated mercury are investigated and we show the long range contribution to GEM from continental Europe, and that the lowest levels are associated with polar and marine air masses from the north west sector.

Ammonia (NH3) empirical critical levels for Europe were re-evaluated in 2009, based mainly on the ecological responses of lichen communities without acknowledging the physiological differences between oligotrophic and nitrophytic species. Here, we compare a nitrogen sensitive lichen (Evernia prunastri) with a nitrogen tolerant one (Xanthoria parietina), focussing on their physiological response (Fv/Fm) to short-term NH3 exposure and their frequency of occurrence along an NH3 field gradient. Both frequency and Fv/Fm of E. prunastri decreased abruptly above 3 μg m(-3) NH3 suggesting direct adverse effects of NH3 on its photosynthetic performance. By contrast, X. parietina increased its frequency with NH3, despite showing decreased capacity of photosystem II above 50 μg m(-3) NH3, suggesting that the ecological success of X. parietina at ammonia-rich sites might be related to indirect effects of increased nitrogen (NH3) availability. These results highlight the need to establish NH3 critical levels based on oligotrophic lichen species., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Wet deposition of nitrogen (N) occurs in oxidized (nitrate) and reduced (ammonium) forms. Whether one form drives vegetation change more than the other is widely debated, as field evidence has been lacking. We are manipulating N form in wet deposition to an ombrotrophic bog, Whim (Scottish Borders), and here report nine years of results. Ammonium and nitrate were provided in rainwater spray as NH4 Cl or NaNO3 at 8, 24 or 56 kg N ha(-1)  yr(-1) , plus a rainwater only control, via an automated system coupled to site meteorology. Detrimental N effects were observed in sensitive nonvascular plant species, with higher cumulative N loads leading to more damage at lower annual doses. Cover responses to N addition, both in relation to form and dose, were species specific and mostly dependent on N dose. Some species were generally indifferent to N form and dose, while others were dose sensitive. Calluna vulgaris showed a preference for higher N doses as ammonium N and Hypnum jutlandicum for nitrate N. However, after 9 years, the magnitude of change from wet deposited N on overall species cover is small, indicating only a slow decline in key species. Nitrogen treatment effects on soil N availability were likewise small and rarely correlated with species cover. Ammonium caused most N accumulation and damage to sensitive species at lower N loads, but toxic effects also occurred with nitrate. However, because different species respond differently to N form, setting of ecosystem level critical loads by N form is challenging. We recommend implementing the lowest value of the critical load range where communities include sensitive nonvascular plants and where ammonium dominates wet deposition chemistry. In the context of parallel assessment at the same site, N treatments for wet deposition showed overall much smaller effects than corresponding inputs of dry deposition as ammonia., (© 2013 John Wiley & Sons Ltd.)

Here we investigate the response of soils and litter to 5 years of experimental additions of ammonium (NH4), nitrate (NO3), and ammonia (NH3) to an ombrotrophic peatland. We test the importance of direct (via soil) and indirect (via litter) effects on phosphatase activity and efflux of CO2. We also determined how species representing different functional types responded to the nitrogen treatments. Our results demonstrate that additions of NO3, NH4 and NH3 all stimulated phosphatase activity but the effects were dependent on species of litter and mechanism (direct or indirect). Deposition of NH3 had no effect on efflux of CO2 from Calluna vulgaris litter, despite it showing signs of stress in the field, whereas both NO3 and NH4 reduced CO2 fluxes. Our results show that the collective impacts on peatlands of the three principal forms of nitrogen in atmospheric deposition are a result of differential effects and mechanisms on individual components., (Copyright (c) 2010 Elsevier Ltd. All rights reserved.)

*Effects of nitrogen (N) enrichment on the heathland lichen Cladonia portentosa were quantified to test the hypothesis that modified N : phosphorus (P) relationships observed in this species in N-polluted natural environments are a direct effect of increased N deposition, and to evaluate potential confounding effects of N form and P availability. *Cladonia portentosa was harvested from experimental plots in lichen-rich peatland vegetation (background total N deposition of 8 kg N ha(-1) yr(-1)) treated for 4 yr with additional wet N deposition at 0, 8, 24 and 56 kg N ha(-1) yr(-1) as either NH(4)(+) or NO(3)(-), and with or without P added at either 0.6 or 4 kg P ha(-1) yr(-1). *Nitrogen enrichment increased thallus N concentration, N : P mass ratio and phosphomonoesterase (PME) activity by factors of up to 1.3, 1.4 and 1.7, respectively, effects being independent of N form. Phosphomonoesterase activity was tightly related to thallus N : P ratio with additions of P at 4 kg ha(-1) yr(-1) depressing PME activity by a factor of 0.4. *Nitrogen enrichment induces P-limitation in C. portentosa with attendant changes in chemical and physiological characteristics that could be used as sensitive biomarkers with which to detect low levels of N pollution.

The current critical level for ammonia (CLE(NH3)) in Europe is set at 8mug NH(3) m(-3) as an annual average concentration. Recent evidence has shown specific effects of ammonia (NH(3)) on plant community composition (a true ecological effect) at much smaller concentrations. The methods used in setting a CLE(NH3) are reviewed, and the available evidence collated, in proposing a new CLE(NH3) for different types of vegetation. For lichens and bryophytes, we propose a new CLE(NH3) of 1 microg NH(3) m(-3) as a long-term (several year) average concentration; for higher plants, there is less evidence, but we propose a CLE(NH3) of 3+/-1 microg NH(3) m(-3) for herbaceous species. There is insufficient evidence to provide a separate CLE(NH3) for forest trees, but the value of 3+/-1 microg NH(3) m(-3) is likely to exceed the empirical critical load for N deposition for most forest ecosystems.

The spatial relationship between the concentration and deposition of the major ions in precipitation and stemflow and their influence on the tissue nitrogen concentration of three epiphytic bryophytes on Quercus petraea (Matt) Liebl. and Q. robur L. was investigated at seven UK Atlantic oak woodland sites with a range of total N deposition of 55-250 mmol m(-2). The main driver of change in tissue N concentrations of three epiphytic bryophytes (Isothecium myosuroides Brid. (Eurhynchium myosuroides (Brid.) Schp.), Dicranum scoparium Hewd. and Thuidium tamariscinum (Hewd.) Schimp.) was total N deposition in stemflow, dominated by ammonium deposition. The three epiphytic species also showed strong relationships between tissue N concentration and total N deposition in rainfall but a poor correlation with total N ion concentration in rainfall. This study shows that epiphytic bryophytes utilise stemflow N and thus increase their risk from inputs of total N deposition compared to terricolous species at the same site.

Effects and implications of reduced and oxidised N, applied under 'real world' conditions, since May 2002, are reported for Calluna growing on an ombrotrophic bog. Ammonia has been released from a 10 m line source generating monthly concentrations of 180-6 microg m(-3), while ammonium chloride and sodium nitrate are applied in rainwater at nitrate and ammonium concentrations below 4mM and providing up to 56 kg N ha(-1) year(-1) above a background deposition of 10 kg N ha(-1) year(-1). Ammonia concentrations, >8 microg m(-3) have significantly enhanced foliar N concentrations, increased sensitivity to drought, frost and winter desiccation, spring frost damage and increased the incidence of pathogen outbreaks. The mature Calluna bushes nearest the NH3 source have turned bleached and moribund. By comparison the Calluna receiving reduced and oxidised N in rain has shown no significant visible or stress related effects with no significant increase in N status.

A selection of herbaceous plants representing the ground flora around a typical chemical installation in the UK was exposed continuously for 7 weeks to a mixture of six VOCs (acetone, acetonitrile, dichloromethane, ethanol, methyl t-butyl ether and toluene) in open-top chambers. Exposure concentrations were based on predictions of atmospheric dispersion from a single source, at a distance of approximately 2 km. The effects of continuous exposure, representing a worst-case, were measured in terms of uncontrolled water loss from leaves, leaf wettability, chlorophyll content and fluorescence, dry matter production and detailed observations of changes in plant growth and phenology. There were significant effects of VOC exposure on seed production, leaf water content and photosynthetic efficiency in some plant species. Such effects may be detectable in vegetation close to major industrial point sources of VOCs, or as a result of an accidental release of material during manufacture or transport. Some of the species tested e.g. birdsfoot trefoil (Lotus corniculatus L.) seem to be promising as potential bioindicators for VOCs, but there may be other even more sensitive species waiting to be discovered. However, the most obvious and conveniently measured response to VOCexposure in the birdsfoot trefoil (premature senescence i.e. advanced timing of seed pod production) could easily be confused in the field with climatic influences. It is also uncertain at this stage whether any of the effects observed would lead to longer term ecological changes in natural plant communities, through biased competition between sensitive and more tolerant species.

Increased deposition of atmospheric N largely from intensive agriculture is affecting biodiversity and the composition of natural and semi-natural vegetation in Europe. The value of species based bioindicators such as the Ellenberg N index and measurements of total tissue N and free amino acids in key plant species, is described with reference to a mixed woodland downwind of a livestock farm in the Scottish Borders, operated for over 20 years with a measured spatial gradient of ammonia concentration (29-1.5 microg m(-3)). All the indicators examined showed a relationship with N deposition and provided some indication of vegetation change. Total tissue N and arginine concentrations were most closely linked with ammonia concentrations and N deposition, with r(2) values of >0.97 and >0.78 respectively.

Volatile organic compounds (VOC) are of increasing environmental significance as a result of continually increasing volumes of traffic on European roads. An open-top chamber fumigation system has been devised to investigate how these contaminants transfer between the atmosphere and the ground, and how they partition between and within air-plant-soil systems. Variation in chamber temperature, solar radiation in the chamber and chamber flow rate were identified as factors that affected final air concentrations. These were assessed and quantified for all individual chambers used--effectively characterising each chamber. The real-life VOC concentrations generated were stable and readily reproducible. Grass exposed to benzene, toluene, 1,1,1-trichloroethane and tetrachloroethene, respectively, equilibrated in response to a change in air concentration within hours. The rate of equilibration in exposed grass in all cases was independent of air temperature. 1,1,1-Trichloroethane and tetrachloroethene appear to be biologically inert demonstrating a simple physico-chemical approach to equilibrium, however, benzene and toluene do not appear independent of plant metabolic activity. Aqueous solubility can account for all of the toluene and benzene in the fumigated plant material.

There has been increasing pressure on farmers in Europe to reduce the emissions of ammonia from their land. Due to the current financial climate in which farmers have to operate, it is important to identify ammonia control measures that can be adopted with minimum cost. The planting of trees around farmland and buildings has been identified as a potentially effective and low-cost measure to enhance ammonia recapture at a farm level and reduce long-range atmospheric transport. This work assesses experimentally what fraction of ammonia farm woodlands could potentially remove from the atmosphere. We constructed an experimental facility in southern Scotland to simulate a woodland shelterbelt planted in proximity to a small poultry unit. By measuring horizontal and vertical ammonia concentration profiles within the woodland, and comparing this to the concentration of an inert tracer (SF6) we estimate the depletion of ammonia due to dry deposition to the woodland canopy. Together with measurements of mean ammonia concentrations and throughfall fluxes of nitrogen, this information is used to provide a first estimate of the fraction of emitted ammonia that is recaptured by the woodland canopy. Analysis of these data give a lower limit of recapture of emitted ammonia, at the experimental facility, of 3%. By careful design of shelterbelt woodlands this figure could be significantly higher.

The deposition of atmospheric nitrogen can be enhanced at high altitude sites as a consequence of cloud droplet deposition and orographic enhancement of wet deposition on hills. The degree to which the increased deposition of nitrogen influences foliar nitrogen concentration in a range of upland plant species was studied in a series of field surveys in northern Britain. A range of upland plant species sampled along altitudinal transects at sites of known atmospheric nitrogen deposition showed marked increases in foliar nitrogen concentration with increasing nitrogen deposition and altitude (and hence with decreasing temperature). For Nardus stricta L., Deschampsia flexuosa (L.) Trin., Calluna vulgaris (L.) Hull, Erica cinerea L. and Hylocomium splendens (Hedw.) Br. Eur. on an unpolluted hill, foliar nitrogen increased by 0.07, 0.12, 0.15, 0.08 and 0.04% dry weight respectively for each 1 kg ha(-1) year(-1) increase in nitrogen deposition. Most species showed an approximately linear relationship between foliar nitrogen concentration and altitude but no trend with altitude for foliar phosphorus concentration. This provided evidence that the tissue nutrient status of upland plants reflects nutrient availability rather than the direct effects of climate on growth. However, differences in the relationship between foliar nitrogen concentration and atmospheric nitrogen deposition for N. stricta sampled on hills in contrasting pollution climates show that the possibility of temperature-mediated growth effects on foliar nitrogen concentration should not be ignored. Thus, there is potential to use upland plant species as biomonitors of nitrogen deposition, but the response of different species to nitrogen addition, in combination with climatic effects on growth, must be well characterised.

Native upland species, Nardus stricta, Eriophorum vaginatum, Erica cinerea and Vaccinium vitis-idaea were given 3 or 60 kg N ha -1 yr -1 , over 2 yr, applied as a mist (NH 4 NO 3 ). The high N treatment increased above-ground biomass in all four species, but only significantly in E. cinerea, E. vaginatum and N. stricta. Biomass increases in E. vaginatum and N. stricta resulted from enhanced tiller production rather than shoot elongation. Root growth increased in N. stricta, so that root∶shoot ratio in this species was unchanged by N. Root growth in E. vaginatum, E. cinerea and V. vitis-idaea did not respond to N and their root∶shoot ratios decreased. Tissue N concentrations increased in both shoots and roots of all species in response to N. The accumulated foliar N did not increase the proportion of N allocated to Rubisco and the photosynthetic capacities of N. stricta, E. vaginatum and V. vitis-idaea were unchanged. Thus growth responses to N were due to altered allocation rather than increased rate of photosynthesis per unit leaf area. The high N treatment increased flower production significantly in E. cinerea but not in the other species. Although in this experiment dwarf shrubs were more responsive than graminoids to N, in the field at current N inputs the enhanced tillering of the graminoids may be more competitively advantageous, especially where gaps develop in the canopy. Thus increasing N deposition may lead to increased grassiness of upland heath, and in particular, a spread of N. stricta.

Mature grafts of five Sitka spruce (Picea sitchensis (Bong.) Carr.) clones were exposed to simulated acid mist comprising an equimolar mixture of H(2)SO(4) and NH(4)NO(3) (1.6 and 0.01 mol m(-3)) at pH 2.5 and 5.0. Mist was applied to potted plants growing in open-top chambers on consecutive days, four times a week, at a precipitation equivalent of 1 mm per day. The total exposure to polluted mist was equivalent to three times that measured at an upland forest in SE Scotland. The aim of the experiment was to characterize the response of juvenile foliage produced by physiologically mature grafts (on seedling root stock) and compare it with the behaviour of juvenile foliage on seedlings. Development of visible foliar damage was followed through the growing season. Measurements of needle length, diameter, weight, surface area, surface was weight and wettability were made on current year needles to determine whether particular foliar characteristics increased susceptibility to injury. Significant amounts (> 10%) of visible needle damage was observed on only one of the five clones. Damage was most severe on the clone with the most horizontal branch and needle habit, but over the five clones there was no relationship between angle of branch display and damage. Likewise no combination of needle characteristics (length, width, area, amount of wax) was indicative of potential susceptibility. A comparison with previous acid misting experiments using seedlings suggests that juvenile foliage on physiologically mature trees is equally susceptible to visible injury as juvenile seedling foliage. Data of budburst differed among clones, and in this experiment exerted the over-riding influence on development of injury symptoms. Foliage exposed to a combination of strong acidity and high sulphate concentrations over the few weeks immediately following budburst suffered most visible injury. The absence of significant amounts of visible damage in UK forests probably reflects the general low susceptibility to visible injury of Sitka spruce exposed to acid mist.

Effects of elevated CO(2), clone and plant nutrition on bud dormancy of Sitka spruce (Picea sitchensis (Bong.) Carr.) were examined. Sitka spruce seedlings were fumigated with ambient or elevated (ambient + 350 micro mol mol(-1)) concentrations of CO(2) in open-top chambers for three growing seasons. In 1991 and 1992, elevated CO(2) delayed bud burst in the spring and advanced bud set in the autumn. The effect of the open-top chamber on the thermal requirement for bud burst was greater than the effect of elevated CO(2) (50 and 30 day degrees (D(d)), respectively). In a second study, four clones of Sitka spruce taken from two provenances, at 43 and 54 degrees N, were fumigated with ambient or elevated CO(2). There was a large natural variation in the timing of bud burst and bud set among the clones. Elevated CO(2) had no effect on bud dormancy of the Skidegate a clone, but it reduced the growing season of the North Bend b clone by 20 days. In a third study, Sitka spruce seedlings growing in ambient or elevated CO(2), were supplied with one of three nutrient regimes, low (0.1 x potential), medium (0.5 x potential) or high (2.0 x potential), using a method and solution based on the Ingestad technique. Elevated CO(2) did not affect bud dormancy in the high-nutrient treatment, but it reduced the growing season of plants in the low-nutrient treatment by 22 days. Increasing plant nutrient supply lengthened the growing season, plants flushed earlier in the spring and set bud later in the autumn. The effects of elevated CO(2) plus a 0, 2 or 4 degrees C climatic warming on the timing of bud burst and the subsequent risk of frost damage were assessed using a simulation model and meteorological data from three sites, Edinburgh, Braemar and Masset. The model predicted that (i) doubling the CO(2) concentration in the absence of climatic warming, will delay the onset of bud burst at all three sites, (ii) climatic warming in ambient CO(2) will hasten bud burst and (iii) climatic warming in elevated CO(2) will hasten bud burst at Edinburgh and Braemar but to a lesser extent than climatic warming alone. At Masset, a 4 degrees C warming was required to advance the date of bud burst of seedlings in the elevated CO(2) treatment. At all three sites, elevated CO(2) and climatic warming increased the mean daily temperature on the date of bud burst, thus reducing the risk of subsequent frost damage.

Current year needles from 5 yr-old Norway spruce trees, which had been exposed to either episodes of atmospheric O 3 , or periodic mistings with simulated acid rainwater throughout three summer periods, were-analyzed for changes in molar percentages and ratios of fatty acids isolated from different lipids at the time of maximum winter hardening. No significant changes due to acidic misting were detected but significant decreases in the degree of unsaturation off both C 16 and C 18 , fatty acids, the molar percentage of δ 5,9,12,15 , and the molar ratio δ 5,9 18: 2 to δ 9,12 18:2 in monogalactosyl diglyceride (MGDG) due to summer 0 3 exposures were found. Molar percentages and ratios of fatty acids did not change much in other lipids bur these changes in plastidie MGDG could be traced to a significant effect of summer O 3 on the δ 4 - and δ 12 -desaturases acting upon phosphatidyl choline (PC) in the endoplasmic reticulum. The replacement of the δ 6 -subset of C 18 fatty acids by an equivalent δ 5 -series throughout was confirmed by Gas chromatography and mass spectrometry Molecular modelling also showed that the δ 5 forms, which resembled the δ 5 -isomers, are very different in shape to the δ 5 -series and this may account, in part, for the extremely low winter temperatures from which Norway spruce needles may recover.

Mature grafts of five clones of Sitka spruce (Picea sitchensis Bong. Sarg.) were exposed to simulated acid mist composed of an equimolar mixture of sulphuric acid and ammonium nitrate at pH 2.5 and pH 5.0 in open-top chambers from May to November 1991. Treatments were applied on consecutive days, four times a week. The pH 2.5 treatment provided an overall dose three times higher than that received by forests in upland areas of Britain. Frost hardiness was assessed in November by freezing detached current year shoots at a range of temperatures and assessing the rate of electrolyte leakage Foliar nutrient concentrations were determined on the same shoots. Acid mist at pH 2.5 significantly reduced frost hardiness in four of the five clones; the temperature causing 50% shoot death (LT50) was increased by 0 to 7 degrees C. The clones varied in their level of hardiness, one clone being exceptionally frost sensitive. The frost hardiness of the frost sensitive clone was found to be less perturbed by acid mist than the hardiness of the more frost resistant clones. Mature grafts showed a smaller reduction in hardiness at an equivalent dose than that found previously with Sitka spruce seedlings. Compared with seedlings, grafts had lower absolute concentrations of foliar sulphur. Exposure to acid mist at pH 2.5 increased %S in current year foliage by <0.05% compared with absolute increases of more than 0.10% in current year foliage of seedlings. We conclude that the effect of acid mist on frost hardiness is likely to be less on mature trees than on seedlings and that the increased frost risk to mature trees of Sitka spruce from occult deposition alone is small.

Two-year-old red spruce (Picea rubens Sarg.) was grown in replicated open-top chambers supplied with charcoal-filtered air near Edinburgh, Scotland. Between May and November 1989, plants were exposed to four mist treatments, three containing sulphuric acid and ammonium nitrate in equimolar concentrations at 0.005 mol m -3 (pH 5) or 1.0 mol m -3 (pH 27), and a fourth treatment with sulphuric acid alone at 1.0 mol m 3 (equivalent to 2 mm precipitation). Two dose rates were used for the pH 2.7 treatment equivalent to 2 and 8 mm of rain per week. Three subtreatments (soil surface exposed to mist, addition of extra sulphuric acid to the soil surface, exclusion of mist from the soil) were included in each chamber. Frost hardiness was assessed by measuring rates of electrolyte leakage after controlled freezing of detached shoots. At the end of October, frost hardiness, expressed as the lethal temperature for 50% of shoots (LT 50 ), was decreased by 8 °C in the 8 mm wk -1 treatment at pH 27, compared to pH 5. The 2 mm wk -1 treatment at pH 2.7 had no effect on frost hardiness either when ammonium nitrate was present or absent (i.e. sulphuric acid only). Excluding mist from the soil, and adding extra sulphuric acid, both increased frost hardiness by about 3 °C when compared with uncovered soil. Excluding mist from the soil increased the amount of foliage initiated and produced inside the chambers but neither subtreatment, excluding the mist nor providing additional sulphuric acid to the soil affected foliar nutrient concentrations. Mist of pH 27 as sulphuric acid alone and in combination with ammonium nitrate both enhanced N uptake. Several observations concerning the effect of acidic mist on frost hardiness were confirmed by this study: (i) preventing mist from reaching the soil/roots, improving conditions for root growth can ameliorate the effects of acidic mist on shoot growth and frost hardiness; (ii) the effect was determined by the ion dose but not by the ion concentration in the mist; (iii) the effect was primarily mediated through foliar absorption; (iv) the presence of high foliar nitrogen concentrations did not increase frost hardiness when foliar sulphur concentrations were also high; (v) low N concentrations were more important for frost hardiness than high foliar N concentrations.

Two-year-old red spruce (Picea rubens Sarg.) of Pittston provenance and 3-yr-old plants of Chatham provenance were exposed to acid mist in replicated open-top chambers supplied with charcoal-filtered air near Edinburgh, Scotland. Plants of Chatham provenance had already been exposed to acid mist throughout the previous growing season. The plants were exposed to mist, equivalent to 4 mm rainfall per week, containing an equimolar mixture of sulphuric acid and ammonium nitrate at pH 2.5 or pH 5.0 (1.6 or 0.01 mol m 3 ) from May to November. This weekly dose was delivered at a low frequency (2 mm twice a week), or high frequency (1 mm on 4 consecutive days each week) to chambers fitted with ceilings to exclude rain. The low frequency dose was also applied to chambers without ceilings, to examine the effect of natural washing by rain. Frost hardiness, estimated by exposing detached shoots to controlled freezing and then measuring rates of electrolyte leakage, was determined during the misting period at the end of October and in December. Foliar nutrient concentrations were measured during the dormant period after treatment had ceased. At the end of October, plants which had received acid mist were less frost hardy than plants receiving mist at pH 5. The temperature causing 50% shoot death (LT 50 ) increased by 6 °C for low frequency application, and by 10 °C at high frequency, relative to the plants receiving mist at pH 5. Exclusion of ambient rainfall had no detectable effect on the frost hardiness response to acid mist. In December, 3 wk after the cessation of misting, all plants were more frost hardy than in October. Significant effects of the acid mist treatment could no longer be detected. Differences in nutrient concentrations were small among treatments, although K + concentrations in the low frequency treatment with acid mist with rain exclusion were 50 % below those in other treatments. Ca concentrations were 50% larger in the acid mist treatment with rain exclusion than without. The data suggested enhanced sulphate uptake resulting from increasing the frequency of exposure, but the increase was not significant. There was no clear relationship between the pattern of frost hardiness and nutrient concentrations except for S, which was 30% smaller in the control plants (pH 5) than in the high frequency pH 2.5 treatments. It is concluded that excluding rainfall, an experimental artifact introduced in evaluating effects of acid mist, does not influence the frost hardiness response of red spruce seedlings. The much greater effect of exposure to the same dose at double the frequency suggests that such experiments may underestimate effects in the field, if those trees are exposed to more frequent episodes of polluted cloud water than experimental plants.

Two-year-old spruce (Picea rubens Sarg.) seedlings were exposed to acid mists containing equimolar ammonium sulphate and nitric acid giving treatments of pH 2.5, 3.0 and 5.0. Acid mist treatments were applied twice weekly from late July until early October, 1987, when the plants were harvested. There were no significant differences in biomass accumulation or in height growth between treatments, but marked differences in root morphology were found. Significantly larger amounts of coarse roots were produced in the pH 5.0 acid mist treatment and plants in the pH 3.0 treatment produced significantly greater amounts of fine roots. Plants receiving acid mists of pH 3.0 or less had a greater frequency of fine root branches along their coarse roots. Production of mycorrhizal fruiting bodies of Thelephora terrestris Ehrenb.: Fr. was significantly greater (P < 0.001) at pH 5.0 than in the other treatments. Plants in the pH 2.5 and 3.0 treatments contained larger concentrations of N, 1.54 and 1.12% and S, 0.52 and 0.28% respectively, than those receiving acid mist at pH 5.0, i.e. 1.00 and 0.21% respectively. However, the between treatment differences of tissue nutrient concentrations were small relative to the differences in inputs between treatments. Foliar S exceeded concentrations recommended for nursery stock by 50 and 150% at pH 3.0 and 2.5 respectively.

Two cultivars of Igri and Gerbel winter barley Horteum vulgare L. were grown in open-top chambers in filtered and unfiltered air at a site with approximately 10 nl litre(-1) SO2 and 12 nl litre(-1) NO2 (seasonal mean). The experiment ran for three consecutive seasons 1982-1983, 1983-1984, 1984-1985, and significant effects of filtration were observed for each crop. In years 1982-1983 and 1984-1985, the crops in unfiltered air yielded larger grain dry matter, 9% in 1982-1983, and 8% in 1984-1985. For both crops, the differences were statistically significant at the 5% level. Differences were also observed for the remaining above-ground dry matter, and these were consistent in direction in each year but statistically significant only in 1984-1985. In both growing seasons (1982-1983 and 1984-1985), there were no major pest infestations and no long-term water stress or photochemical ozone episodes. In the remaining experiment (1983-1984) similar air concentrations of SO2 and NO2 produced effects of the opposite sign to those observed in 1982-1983 and 1984-1985. Significant reductions in grain yield (13%) were obtained in unfiltered air. The only major environmental difference for the 1983-1984 crop was a notable dry period in May and June 1984 with marked water stress in the crop, requiring irrigation. These results suggest that the relationship between yield and pollutant concentration may be confounded by additional stresses, many of which are a common component of the growing season for major crops.

This study examined ecological aspects of the distribution of tar spot disease caused by the fungus Rhytisma acerinum (Pers.) Fries on Acer pseudoplatanus L. (sycamore). The host is abundant in Edinburgh and surrounding areas yet no tar spots were found in the city, although they were abundant on sycamore in the surrounding countryside, especially at sheltered sites. The vertical distribution from a localized source of leaf litter from infected sycamore showed an approximately linear relationship between tar spot index of attached leaves (TSI = no. of tar spots per 100 cm 2 of leaf) and the log of height of such infected foliage above the ground. The distribution of tar spots was controlled primarily by the presence or absence of overwintered sycamore leaves infected with R. acerinum. In Edinburgh these leaves are actively removed by man and passively by wind, so reducing the potential source of inoculum by the following spring. The presence of SO 2 was unimportant in the distribution of R. acerinum. An experiment to study the relative importance of current air concentrations of SO 2 on the incidence of infection by R. acerinum showed that the average SO 2 concentration of < 50 μg m -3 hail no effect. The observed distribution of tar spot throughout the city and surrounding countryside could be explained entirely by distance from the nearest source of inoculum without involving a role for air pollutants.

Two-year-old red spruce seedlings [Picea rubens Sarg. syn. P. rubra (Du Roi) Link] were subjected to 6 simulated acid mist treatments (pH 2.5, 2.7, 3.0, 3.5, 4.0, 5.0) in a replicated experiment using open-top chambers. Acid mist solutions containing equimolar (NH 4 ) 2 SO 4 , and HNO 3 were applied twice weekly for 22 weeks, each application being equivalent to 2 mm of precipitation. Visible symptoms of foliar damage were observed on the 3 most acidic mist treatments (pH 2.5, 2.7, 3.0). The inputs of nitrogen, sulphur and acidity in the most acidic treatment were 55, 42, 1,3 kg ha -1 , respectively, over a 10 week period. The plants subjected to the pH 2.5 treatment were found to be most severely damaged with approximately 40% foliar necrosis after 10 weeks of treatment. On approximately 80% of seedlings, necrosis was confined to current year needles only. These damaged needles were initially light brown or light orange in colour turning a deeper red 3 to 5 weeks after initial necrosis. Percentage foliar damage was linearly related to concentration (of NH 4 + , NO 3 - , SO 4 2- and H + ) with 62% foliar damage in the pH 2.5 treatment after a 22-week period. Spray application stopped in December 1987. Observations during the following spring showed that the pH 2.5 and pH 3 treatments induced earlier Hushing, requiring 60 day °C less thermal time than the pH 5-0 treatment. In 1988, this decrease in thermal requirement was equivalent to flushing 11 days earlier. There was no evidence of acid mist treatments inducing bud mortality.

Seedlings of red spruce [Picea rubens Sarg. syn. P. rubra (Du Roi) Link] were exposed to mists containing equimolar (NH 4 , SO 4 and HNO 3 at pHs of 2.5, 2.7, 3.0, 35, 4.0 or 5.0. The mists were applied twice each week, amounting to 2 mm precipitation equivalent on each occasion, between July and December, to open-top chambers supplied with charcoal-filtered air. Frost hardiness of shoots excised from seedlings was determined on 6 occasions starting on 21 September, and was found to be strongly influenced by acid mist treatments, seedlings subject to the most acidic mists being the least frost hardy. On 21 September when the first sample was taken the lethal temperature for killing 50% of shoots (LT 50 ) was - 11 °C for the least acidic (pH 5.0) mist and - 7 °C for the most acidic (pH 2.5). By 19 October, the LT 50 s of pH 5.0 and pH 2.5 mists were -27 and -15 °C respectively. All intermediate treatments ranked according to treatment concentration, with the smaller concentrations causing lower LT. 50 values. The treatment at pH 30 provided S and N inputs to the seedlings similar to those experienced by red spruce at elevations of about 1000 m in the southern Appalachians. At pH 3.0, the frost hardiness LT 10 during October was typically 8 °C higher than the pH 5.0 treatment, leading to a significant increase in the probability of frost damage at the LT 10 level in an average October. The proximity of minimum night temperatures during September to December to the LT 10 temperatures of red spruce shoots receiving large inputs of SO 4 2- , NO 3 - , NH 4 + and H + suggests that decreases in frost hardiness caused by intercepted cloud water containing large concentrations of these ions may play a significant part in the observed decline at mountain-top locations.