Ian D. Leith

Centre for Ecology & Hydrology, Wallingford, England, United Kingdom

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Publications (95)233.44 Total impact

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    ABSTRACT: 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.
    Environmental science. Processes & impacts. 04/2014;
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    ABSTRACT: 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.
    Environmental Pollution 01/2014; 187:206–209. · 3.73 Impact Factor
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    ABSTRACT: Wet N deposition occurs in oxidised (nitrate) and reduced (ammonium) forms, in proportions that vary spatially with source and topography. Whether one form drives vegetation change more than the other is widely debated, as we lack corroboratory field evidence. We have manipulated N form in wet deposition to an ombrotrophic bog, Whim, for nine years. Ammonium and nitrate were provided in rainwater spray as NH4 Cl or NaNO3 at 8, 24 or 56 kg N ha(-1) y(-1) , plus a rainwater only control, via an automated system coupled to site meteorology. Cover of key species fluctuated considerably, displaying temporal increases, declines or both, independent of N. Detrimental N effects were observed in sensitive non-vascular plant species, with higher cumulative N loads leading to more damage at lower annual doses, but overall the effect on moss cover was small. Cover responses to N, both form and dose were species specific, and mostly dependent on N dose. Some species were generally indifferent to N form and dose, Eriophorum vaginatum, Erica tetralix, while others: Pleurozium schreberi > Cladonia portentosa > Sphagnum capillifolium were dose sensitive. Calluna vulgaris showed a preference for higher N as reduced N and Hypnum jutlandicum for oxidised N. However, after nine years, the magnitude of change from wet deposited N on overall species cover (HOF model) is small, indicating only a slow decline in key species. Differences in soil N availability were similarly muted and rarely, directly related to species cover. Ammonium caused most N accumulation and damage to sensitive species at lower N loads, but toxic effects also occurred with nitrate. Generic N form effects were absent, making ecosystem specific critical load separation by form problematic. However, we recommend implementing the lowest value of the critical load range where communities include sensitive non-vascular plants and ammonium dominates wet deposition chemistry. This article is protected by copyright. All rights reserved.
    Global Change Biology 08/2013; · 8.22 Impact Factor
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    ABSTRACT: a b s t r a c t Nitrogen pollution affects many peatlands with consequences for their biodiversity and ecosystem function. Microorganisms control nutrient cycling and constitute most of the biodiversity of peatlands but their response to nitrogen is poorly characterised and likely to depend on the form of deposition. Using a unique field experiment we show that ammonia exposure at realistic point source levels is associated with a general shift from heterotrophic (bacteria and fungi) to autotrophic (algal) dominance and an increase in total biomass. The biomass of larger testate amoebae increased, suggesting increased food supply for microbial predators. Results show the widespread impacts of N pollution and suggest the potential for microbial community-based bioindicators in these ecosystems. Ombrotrophic peatlands, which receive their nutrients from the atmosphere, are naturally oligotrophic and highly sensitive to atmospheric deposition of acids, nutrients, and other pollutants. Nitrogen deposition affects many peatlands in industrialised regions and may shift peatlands from carbon sinks to sources with important consequences for the global carbon cycle (Aerts et al., 1992; Bragazza et al., 2006). Such processes are microbially-mediated but the overall microbial community response to N is poorly characterised. Microbes constitute most of the biodiversity of peatlands (Gilbert and Mitchell, 2006) but studies of the biodi-versity impacts of N pollution in terrestrial ecosystems are almost entirely restricted to plants. The form of N deposition is an important determinant of ecosystem impacts (Stevens et al., 2011; Liu and Greaver, 2009), with evidence that the impacts of gaseous ammonia can be particularly acute (Sheppard et al., 2011). Ammonia is a significant threat to peatlands in areas of intensive agriculture (Sutton et al., 2011) but existing research on the microbial response to N has been limited to wet NH 4 NO 3 applica-tion (Gilbert et al., 1998). In this study, we use a morphological, functional group approach to investigate the response of micro-organisms to ammonia dry deposition in a field experiment. The Whim Moss experiment is a globally-unique pollution experiment on a Calluna vulgariseEriophorum vaginatum blanket bog in southern Scotland (3 16 0 W, 55 46 0 N: Leith et al., 2004). NH 3 is released from a pipe 1 m above the ground when air temperature exceeds 0 C, wind speed exceeds 2.5 m s À1 and wind direction is within the range 180e215 . Monthly average NH 3 concentrations are determined using passive ALPHA samplers (Sutton et al., 2001) positioned at 0.1 m above the vegetation along a downwind tran-sect (Sheppard et al., 2011). After nine years of treatment (September 2011), the upper 50 mm of ten stems of Sphagnum capillifolium (Ehrh.) Hedw. were extracted at seven locations along this transect and placed in 10% formaldehyde. Additional samples were taken off the main transect and from an area receiving only ambient deposition. Samples were prepared by multiple cycles of agitation and washing following Jasey et al., 2011 and examined under inverted microscopy. All larger micro-organisms were
    Soil Biology and Biochemistry 03/2013; 57:936. · 4.41 Impact Factor
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    ABSTRACT: Peatlands' vast carbon reserves accumulated under low nitrogen availability. Carbon and nitrogen cycling are inextricably linked, so what are the consequences of increased reactive nitrogen deposition for the sustainability and functioning of peatlands, and does the form of the nitrogen deposition make a difference? We have addressed these questions for an ombrotrophic peatland, Whim bog in SE Scotland, using a globally unique field simulation of reactive N deposition as dry deposited ammonia and wet deposited reduced N, ammonium and oxidised N, nitrate, added as ammonium chloride or sodium nitrate. The effects of 10 yr of reactive N additions, 56 kg N ha-1 yr-1, depended on the N form. Ammonia-N deposition caused the keystone Sphagnum species, together with the main shrub Calluna and the pleurocarpous mosses to disappear, exposing up to 30% of the peat surface. This led to a significant increase in soil water nitrate and nitrous oxide emissions. By contrast wet deposited N, despite significantly reducing the cover of Sphagnum and Pleurozium moss, did not have a detrimental effect on Calluna cover nor did it significantly change soil water N concentrations or nitrous oxide emissions. Importantly 10 yr of wet deposited N did not bare the peat surface nor significantly disrupt the vegetation, enabling the N to be retained within the carbon rich peatland ecosystems. However, given the significant role of Sphagnum in maintaining conditions that retard decomposition this study suggests that all nitrogen forms will eventually compromise carbon sequestration by peatlands through loss of some keystone Sphagnum species.
    Biogeosciences Discussions 07/2012; 9(7):8141-8171.
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    ABSTRACT: • Peat bogs have accumulated more atmospheric carbon (C) than any other terrestrial ecosystem today. Most of this C is associated with peat moss (Sphagnum) litter. Atmospheric nitrogen (N) deposition can decrease Sphagnum production, compromising the C sequestration capacity of peat bogs. The mechanisms underlying the reduced production are uncertain, necessitating multifactorial experiments. • We investigated whether glasshouse experiments are reliable proxies for field experiments for assessing interactions between N deposition and environment as controls on Sphagnum N concentration and production. We performed a meta-analysis over 115 glasshouse experiments and 107 field experiments. • We found that glasshouse and field experiments gave similar qualitative and quantitative estimates of changes in Sphagnum N concentration in response to N application. However, glasshouse-based estimates of changes in production--even qualitative assessments-- diverged from field experiments owing to a stronger N effect on production response in absence of vascular plants in the glasshouse, and a weaker N effect on production response in presence of vascular plants compared to field experiments. • Thus, although we need glasshouse experiments to study how interacting environmental factors affect the response of Sphagnum to increased N deposition, we need field experiments to properly quantify these effects.
    New Phytologist 04/2012; 195(2):408-18. · 6.74 Impact Factor
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    ABSTRACT: In this study, we compare annual fluxes of methane (CH4), nitrous oxide (N2O) and soil respiratory carbon dioxide (CO2) measured at nine European peatlands (n = 4) and shrublands (n = 5). The sites range from northern Sweden to Spain, covering a span in mean annual air temperature from 0 to 16 °C, and in annual precipitation from 300 to 1300 mm yr-1. The effects of climate change, including temperature increase and prolonged drought, were tested at five shrubland sites. At one peatland site, the long-term (>30 yr) effect of drainage was assessed, while increased nitrogen deposition was investigated at three peatland sites. The shrublands were generally sinks for atmospheric CH4 whereas the peatlands were CH4 sources, with fluxes ranging from -519 to +6890 mg CH4-C m-2 yr-1 across the studied ecosystems. At the peatland sites, annual CH4 emission increased with mean annual air temperature, while a negative relationship was found between net CH4 uptake and the soil carbon stock at the shrubland sites. Annual N2O fluxes were generally small ranging from -14 to 42 mg N2O-N m-2 yr-1. Highest N2O emission occurred at the sites that had highest concentration of nitrate (NO3-) in soil water. Furthermore, experimentally increased NO3- deposition led to increased N2O efflux, whereas prolonged drought and long-term drainage reduced the N2O efflux. Soil CO2 emissions in control plots ranged from 310 to 732 g CO2-C m-2 yr-1. Drought and long-term drainage generally reduced the soil CO2 efflux, except at a~hydric shrubland where drought tended to increase soil respiration. When comparing the fractional importance of each greenhouse gas to the total numerical global warming response, the change in CO2 efflux dominated the response in all treatments (ranging 71-96%), except for NO3- addition where 89% was due to change in CH4 emissions. Thus, in European peatlands and shrublands the feedback to global warming induced by the investigated anthropogenic disturbances will be dominated by variations in soil CO2 fluxes.
    Biogeosciences 01/2012; 9:3739-3755. · 3.75 Impact Factor
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    ABSTRACT: Atmospheric nitrogen (N) deposition is a global and increasing threat to biodiversity and ecosystem function. Much of our current understanding of N deposition impacts comes from field manipulation studies, although interpretation may need caution where simulations of N deposition (in terms of dose, application rate and N form) have limited realism. Here, we review responses to simulated N deposition from the UKREATE network, a group of nine experimental sites across the UK in a diversity of heathland, grassland, bog and dune ecosystems which include studies with a high level of realism and where many are also the longest running globally on their ecosystem type. Clear responses were seen across the sites with the greatest sensitivity shown in cover and species richness of bryophytes and lichens. Productivity was also increased at sites where N was the limiting nutrient, while flowering also showed high sensitivity, with increases and declines seen in dominant shrub and forb species, respectively. Critically, these parameters were responsive to some of the lowest additional loadings of N (7.710 kg ha-1 yr-1) showing potential for impacts by deposition rates seen in even remote and unpolluted regions of Europe. Other parameters were less sensitive, but nevertheless showed response to higher doses. These included increases in soil %N and plant available KCl extractable N, N cycling rates and acidbase status. Furthermore, an analysis of accumulated dose that quantified response against the total N input over time suggested that N impacts can build up within an ecosystem such that even relatively low N deposition rates can result in ecological responses if continued for long enough. Given the responses have important implications for ecosystem structure, function, and recovery from N loading, the clear evidence for impacts at relatively low N deposition rates across a wide range of habitats is of considerable concern.
    Global Change Biology 01/2012; 18(4):1197-1215. · 8.22 Impact Factor
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    ABSTRACT: We transplanted Sphagnum ‘turfs’ containing abundant Drosera rotundifolia into an existing nitrogen deposition experiment at Whim Moss near Edinburgh. These mesocosms received simulated N deposition as either NH 4+ or NO 3-, to give total N deposition rates of approximately 8, 16 or 32, or 64 kg N ha-1 year-1. Simulated N deposition was added in a realistic way (i.e., with rainfall throughout the year). The δ15N of this added N was elevated relative to background N. We measured the tissue chemistry and δ15N of Sphagnum papillosum and D. rotundifolia over two years after transplant. Our aim was to determine uptake of the deposited N and the impact on S. papillosum tissue chemistry and D. rotundifolia tissue chemistry and ecology. We found clear, significant impacts of N deposition on S. papillosum, with increased capitula N content and reduced C:N ratio. Increased δ15N indicated uptake of deposited N. The response of D. rotundifolia was less clear with impacts only at the highest rate of N deposition. There was no evidence of differential uptake of reduced or oxidized wet N deposition by either S. papillosum or D. rotundifolia. Using the natural abundance stable isotope method we estimated the minimum contribution of prey N to the total N in D. rotundifolia to be 35%. The results suggest that differences in the uptake of reduced or oxidized wet N deposition might not be ecologically significant when wet N deposition is added realistically. They also support the suggestion that a model of N dynamics in Sphagnum-dominated ecosystems that includes the role of Sphagnum as a small-scale ecosystem engineer, is required to predict vascular plant responses to N deposition accurately.
    Folia Geobotanica 01/2012; 47(2). · 1.57 Impact Factor
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    ABSTRACT: Although the effects of atmospheric nitrogen deposition on species composition are relatively well known, the roles of the different forms of nitrogen, in particular gaseous ammonia (NH3), have not been tested in the field. Since 2002, we have manipulated the form of N deposition to an ombrotrophic bog, Whim, on deep peat in southern Scotland, with low ambient N (wet + dry = 8 kg N ha−1 yr−1) and S (4 kg S ha−1 yr−1) deposition. A gradient of ammonia (NH3, dry N), from 70 kg N ha−1 yr−1 down to background, 3–4 kg N ha−1 yr−1 was generated by free air release. Wet ammonium (NH4+, wet N) was provided to replicate plots in a fine rainwater spray (NH4Cl at +8, +24, +56 kg N ha−1 yr−1). Automated treatments are coupled to meteorological conditions, in a globally unique, field experiment. Ammonia concentrations were converted to NH3-N deposition (kg N ha−1) using a site/vegetation specific parameterization. Within 3 years, exposure to relatively modest deposition of NH3, 20–56 kg NH3-N ha−1 yr−1 led to dramatic reductions in species cover, with almost total loss of Calluna vulgaris, Sphagnum capillifolium and Cladonia portentosa. These effects appear to result from direct foliar uptake and interaction with abiotic and biotic stresses, rather than via effects on the soil. Additional wet N by contrast, significantly increased Calluna cover after 5 years at the 56 kg N dose, but reduced cover of Sphagnum and Cladonia. Cover reductions caused by wet N were significantly different from and much smaller than those caused by equivalent dry N doses. The effects of gaseous NH3 described here, highlight the potential for ammonia to destroy acid heathland and peat bog ecosystems. Separating the effects of gaseous ammonia and wet ammonium deposition, for a peat bog, has significant implications for regulatory bodies and conservation agencies.
    Global Change Biology 07/2011; 17(12):3589 - 3607. · 8.22 Impact Factor
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    ABSTRACT: Peatlands in the northern hemisphere have accumulated more atmospheric carbon (C) during the Holocene than any other terrestrial ecosystem, making peatlands long-term C sinks of global importance. Projected increases in nitrogen (N) deposition and temperature make future accumulation rates uncertain. Here, we assessed the impact of N deposition on peatland C sequestration potential by investigating the effects of experimental N addition on Sphagnum moss. We employed meta-regressions to the results of 107 field experiments, accounting for sampling dependence in the data. We found that high N loading (comprising N application rate, experiment duration, background N deposition) depressed Sphagnum production relative to untreated controls. The interactive effects of presence of competitive vascular plants and high tissue N concentrations indicated intensified biotic interactions and altered nutrient stochiometry as mechanisms underlying the detrimental N effects. Importantly, a higher summer temperature (mean for July) and increased annual precipitation intensified the negative effects of N. The temperature effect was comparable to an experimental application of almost 4 g N m(-2)  yr(-1) for each 1°C increase. Our results indicate that current rates of N deposition in a warmer environment will strongly inhibit C sequestration by Sphagnum-dominated vegetation.
    New Phytologist 03/2011; 191(2):496-507. · 6.74 Impact Factor
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    ABSTRACT: To understand the implications of atmospheric nitrogen deposition on carbon turnover in peatlands, we conducted a 13C pulse labeling experiment on Calluna vulgaris and Eriophorum vaginatum already receiving long-term (5 years) amendments of 56 kg N ha−1 y−1 as ammonium or nitrate. We examined shoot tissue retention, net ecosystem respiration returns of the 13C pulse, and soil porewater DOC content under the two species. 13C fixation in Eriophorum leaves was enhanced with nitrogen addition and doubled with nitrate supply. This newly fixed C appeared to be relocated below-ground faster with nitrogen fertilization as respiration returns were unaffected by N inputs. By contrast, increases in 13C fixation were not observed in Calluna. Instead, net ecosystem respiration rates over Calluna increased with N fertilization. There was no significant label incorporation into DOC, suggesting a conservative strategy of peatland vegetation regarding allocation of C through root exudation. Greater concentrations of total DOC were identified with nitrate addition in Calluna. Given the long-term nature of the experiment and the high N inputs, the overall impacts of nitrogen amendments on the fate of recently synthesized C in Eriophorum and Calluna in this ombrotrophic peatland were surprisingly more moderate than originally hypothesized. This may be due to N being effectively retained within the bryophyte layer, thus limiting, and delaying the onset of, below-ground effects.
    Soil Biology and Biochemistry 01/2011; 43(3):495-502. · 4.41 Impact Factor
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    ABSTRACT: Sphagna are vulnerable to enhanced nitrogen (N) deposition. This article reports how the green (shade, under Calluna) and red (open grown) Sphagnum capillifolium respond to ammonium and nitrate additions of 56 kg N ha−1 y−1 over the background of 8–10 kg N ha−1 y−1 on an ombrotrophic bog in the Scottish Borders after seven years. Samples and measurements were made during a range of hydrated and desiccated conditions in the summer of 2009. Both ammonium and nitrate increased moss N concentration, but while ammonium decreased cross-sectional area of leaf hyaline cells and the leaf hyaline/chlorophyllose cell area ratio, nitrate increased both of them and capitulum pH. The changes in leaf morphology have not previously been reported to our knowledge. Especially the red S. capillifolium was affected by ammonium with significant changes in shoot N concentration (+71%) and the cross-sectional area of leaf chlorophyllose cells (+67%), and reductions in shoot dry weight (−30%) and fresh weight (−42%), the cross-sectional area of leaf hyaline cells (−24%), the leaf hyaline/chlorophyllose cell area ratio (−54%), as well as in chlorophyll fluorescence (measured as Fv/Fm) of desiccated capitulum (−65%) (all p < 0.05). These observations show that N deposition may affect moss physiology also through changes in leaf anatomy and morphology. The results also highlight potential sampling issues and causes of variability in N responses when collecting variably pigmented Sphagna.Highlights► N enhancement affects leaf morphology, especially size of hyaline cells. ► The effects on leaf morphology differ between wet-deposited ammonium and nitrate. ► Shade (green) and open (red) grown Sphagnum capillifolium differ in the rate of N responses.
    Environmental and Experimental Botany 01/2011; 72(2):140-148. · 2.58 Impact Factor
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    ABSTRACT: 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.
    Environmental Pollution 10/2010; 158(10):3157-63. · 3.73 Impact Factor
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    ABSTRACT: *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.
    New Phytologist 03/2010; 186(4):926-33. · 6.74 Impact Factor
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    ABSTRACT: The effects of 4 years of simulated nitrogen deposition, as nitrate (NO3−) and ammonium (NH4+), on microbial carbon turnover were studied in an ombrotrophic peatland. We investigated the mineralization of simple forms of carbon using MicroResp™ measurements (a multiple substrate induced respiration technique) and the activities of four soil enzymes involved in the decomposition of more complex forms of carbon or in nutrient acquisition: N-acetyl-glucosaminidase (NAG), cellobiohydrolase (CBH), acid phosphatase (AP), and phenol oxidase (PO). The potential mineralization of labile forms of carbon was significantly enhanced at the higher N additions, especially with NH4+ amendments, while potential enzyme activities involved in breakdown of more complex forms of carbon or nutrient acquisition decreased slightly (NAG and CBH) or remained unchanged (AP and PO) with N amendments. This study also showed the importance of distinguishing between NO3− and NH4+ amendments, as their impact often differed. It is possible that the limited response on potential extracellular enzyme activity is due to other factors, such as limited exposure to the added N in the deeper soil or continued suboptimal functioning of the enzymes due to the low pH, possibly via the inhibitory effect of low phenol oxidase activity.
    Global Change Biology 09/2009; 16(8):2307 - 2321. · 8.22 Impact Factor
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    ABSTRACT: Increased emissions of ammonia from intensive farming have been identified as the major cause of many of the changes in species composition in plant communities in Europe (Bobbink et al. 1993; Sutton et al. 1993). Transect studies at intensive livestock farms in the UK, have provided important information on the impact of NH3 and deposited N on woodland groundflora (Pitcairn et al. 1998, 2002, 2003, 2005). Several parameters have been measured to understand the basis of species composition change in woodland vegetation and to identify possible biomonitors of vegetation change (Pitcairn et al. 2003; Sutton et al. 2004; Leith et al. 2005). The value of 3 biomonitors of N impacts, are described in this paper, for 2 different woodland sites impacted by poultry farm ammonia emissions, Earlston, a Scots pine open plantation in southern Scotland, and Piddles Wood, a mixed deciduous woodland in south west England.
    12/2008: pages 59-69;
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    ABSTRACT: • The existing Critical Levels (CLEs) for NH3 require revision in the light of new experimental evidence from field-based experiments and surveys. • The existing annual CLE (8 mg NH3 m−3), when expressed as an equivalent deposition of N to an ecosystem, is less protective than the current Critical Load for most, if not all, European ecosystems and habitats. • Field-based evidence relating effects on vegetation to NH3 concentrations measured over one year or longer show that the current annual CLE is set too high. • A new long-term CLE for the most sensitive vegetation type (lichens and bryo-phytes) is proposed, based on observed changes to species composition in the field. • Most of the evidence comes from studies in the UK, but there is corroborative evidence from Switzerland, Italy and Portugal. • The proposed long-term CLE for NH3 for ecosystems in which lichens and bryo-phytes are important is 1 mg NH3 m−3. • There is less evidence for long-term effects of NH3 on species changes in communities of higher plants; on the basis of expert judgement we propose a long-term CLE for higher plants of 3 mg NH3 m−3. • No assumptions have been made on the mechanism by which NH3 exposure leads to changes in species composition. • Several recommendations are made, to address uncertainties relating to the lack of observational data and long-term NH3 concentration measurement, particularly in southern and eastern Europe. • There is also need for better understanding of the mechanisms whereby NH3 affects plants, so that predictive models can be constructed for extrapolation to other types of vegetation and land use in different climatic zones.
    12/2008: pages 15-40;
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    ABSTRACT: A line source of ammonia, simulating NH3 emissions from an intensive livestock unit, was established in 2002 on an ombrotrophic bog, Whim bog in the Scottish Borders. The site is at 55° 46'N, 3° 16'W and (based on 4.5 years) has a mean monthly temperature of 8.8°C and mean annual rainfall 971 mm, supporting a Calluna Eriophorum vegetation, NVC M19. Release of NH3 is controlled by meteorological conditions i.e. wind direction and speed. NH3 concentrations were measured at >10 distances along the transect of release using passive ALPHA samplers, fixed at two or more heights above the vegetation. Effects were recorded for the ericoids, mosses, Sphagnum and Cladonia lichens with respect to cover changes, visible damage, nutrients and interactions with abiotic stress. These data, collected over >4 years, have been used to evaluate the Critical Level (CLE) for NH3 and compared with values derived from short—term (weeks) exposures at high concentrations in controlled conditions. Results suggest the annual Critical Level of 8 μg m−3 is too high to protect sensitive species experiencing long-term exposures to NH3. In particular, they show exposure periods longer than 1 year lead to detrimental effects at lower mean NH3 concentrations through relationships between CLE and exposure period that are still linear after 4.5 years exposure. These results highlight the need to set a CLE representing the mean concentration of NH3 in air for long term protection of sensitive habitats (e.g. over 20–30 years), that will be substantially smaller than the current annual CLE.
    12/2008: pages 49-58;
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    ABSTRACT: Substrate nitrogen and soluble ammonium represent newly recognized parameters in the context of the bioindication of atmospheric nitrogen concentrations and deposition. The interest in these parameters originates from the analysis of plant carbon and nitrogen dynamics in ecosystem models in relation to the potential for plants to absorb or emit ammonia from the atmosphere, as regulated by the ammonia ‘compensation point’ (Sutton et al. 2004). The ammonia compensation point is a function of both agricultural and atmospheric N inputs. Enhanced nitrogen deposition raises the ammonia ‘compensation point’ and provides a limitation to further nitrogen deposition. In this context, the rates of atmospheric ammonia deposition, and hence total N deposition, depend partly on the extent to which the ecosystem has already responded to N deposition. As a consequence of these interactions, the level of substrate N or soluble ammonium (NH4-N) may be a convenient indicator of accumulated nitrogen deposition and ecosystem response (Sutton et al. 2004). Previous studies (Sutton et al. 2004; Leith et al. 2005) suggest that the relationship to atmospheric N is more precise for soluble ammonium (NH4-N) than for total tissue N in plants. The steeper slope of the line in Fig. 17.1 for soluble NH4-N (= free ammonium) indicates that NH4-N is more sensitive to changes in ammonia air concentration than total tissue N (%N) and total soluble nitrogen (substrate nitrogen).
    12/2008: pages 281-289;

Publication Stats

1k Citations
233.44 Total Impact Points

Institutions

  • 2001–2014
    • Centre for Ecology & Hydrology
      • Centre for Ecology and Hydrology Bush Estate (Edinburgh)
      Wallingford, England, United Kingdom
  • 2011
    • The University of Edinburgh
      • School of GeoSciences
      Edinburgh, SCT, United Kingdom
  • 2006
    • The University of York
      York, England, United Kingdom
  • 2005
    • Manchester Metropolitan University
      Manchester, England, United Kingdom
  • 2000
    • University of Aberdeen
      Aberdeen, Scotland, United Kingdom
  • 1993
    • Building Ecology Research Group
      Santa Cruz, California, United States