October 1994
Prepared at the request of the Department of the Environment

Index of reports

Executive Summary

and their transformation products can cause a wide range of environmental effects on a local, national and global level including:

• acidification
  
• eutrophication
  
• the formation of tropospheric ozone
  
• contributions to greenhouse gases and climate change through radiative forcing.
  

Consequently, both national and international discussions are moving towards the control of gaseous N compounds leading to the implementation of controls in the areas of industry, transport and agriculture.

In view of the importance and complexity of the problem, the Department of Environment (DOE) has commissioned a number of independent scientists to produce a report of current understanding of N deposition and its impact on the environment and to give an indication of major gaps in knowledge. The report is structured to outline the current scientific consensus beginning with the emissions, atmospheric transport and transformations, followed by deposition to terrestrial surfaces and impacts on vegetation, soils and freshwaters. It draws on a wide range of science from the DOE funded research programme and other United Kingdom, European and North American sources. It also reviews the evidence for direct effects of gaseous N compounds on crops, natural vegetation and trees and summarises the current situation regarding the mapping and modelling of N critical loads and levels. The report shows that while significant gaps in knowledge exist in some of these areas, sufficient is already known to allow the first attempts to be made to map N critical loads and levels.

The definition of critical loads adopted by the United Nations Economic Commission for Europe (UNECE), is 'a quantitative estimate of exposure to one or more pollutant below which significant harmful effects on sensitive elements of the environment do not occur according to present knowledge'. The term critical load refers only to the deposition of pollutants. Threshold gaseous concentration exposure are termed critical levels and are defined as 'the concentration in the atmosphere above which direct adverse effects on receptors such as plants, ecosystems or materials, may occur according to present knowledge'.

Emissions, Transformation and Deposition

• Emissions of oxidised N (NO_x) which are dominated by vehicle and power station sources currently amount to 846 kt yr^-1 and have increased by about 20% over the period 1970-1990 due mainly to vehicle emissions.
  
• Ammonia emissions are largely from UK agriculture and have been estimated at 180-440 kt yr^-1 of N by recent authors. it is likely that emissions lie towards the upper end of this range at 310-400 kt yr^-1.
  
• 75% of the nitrous oxide (N₂O) emission in the UK arises from fertilised land and adipic acid production for nylon manufacture.
  
• Amounts of wet deposition inputs of nitrate (NO₃^-) and ammonium (NH₄⁺) in the UK are similar being 108 kt N yr^-1 and 131 kt N yr^-1, respectively. The largest inputs are found in the uplands of Wales, northern England and western Scotland at about 30 kg ha^-1 of N annually (NO₃^- + NH₄⁺).
  
• Dry deposition of NO₂ in the UK has been estimated to contribute 100 kt of N annually, with the largest inputs in the Midlands and south east England where 10-15 kg ha^-1 of N are deposited annually.
  
• Dry deposition of NH₃ in the UK amounts to 100 kt of N annually but this estimate is very uncertain (possibly as large as ±50%). Woodland in the Border counties of Wales and in East Anglia receive the largest inputs, up to 70 kg ha^-1 of nitrogen annually as NH₃.
  
• The total deposition of reduced and oxidised N in the UK are similar. Reduced N deposition (NH₃ + NH₄⁺) provides 231 kt annually while oxidised N provides 223 kt. However, more of UK oxidised N emissions are exported from the country (74%) than reduced N emissions (34%) showing that reduced N has a much shorter residence time in the atmosphere than oxidised N.
  
• Agricultural sources currently contribute more to deposited nitrogen in the UK than either vehicles or power station sources.
  

Soils

• In natural and semi-natural (very low-management) ecosystems, plant N requirements are met from soil reserves or from atmospheric N inputs. In contrast, for agricultural systems and some managed forests, fertiliser inputs provide a major proportion of the N requirements of the crop.
  
• Rates of transformation of N in soils are dependent on microbial populations and a range of soil conditions including pH, temperature, aeration, water content and carbon to nitrogen ratios of soil organic material.
  
• Nitrogen transformation and plant uptake of mineral N involve the production and consumption of protons and can, therefore, contribute to soil acidification.
  
• Nitrate is mobile in soil water and if present in excess of the demands of plant uptake and the microbial population, will be leached to surface waters. it will be accompanied by calcium and other base cations in near-neutral soils, but predominantly by aluminium in highly acid mineral soils.
  
• Ammonium present in soils in excess of the demands of plant uptake and the microbial population can be retained on exchange surfaces or in mineral lattices over the short term.
  
• Land use and management can greatly affect N transformations and mobility in soils. For example, mineralisation rates are increased by cultivation, drainage, and burning. Grazing animals also increase available NO₃^- concentrations in the soil.
  
• The impacts of enhanced inputs of N on N availability and decomposition processes will result in short-term beneficial effects on plant growth, but long-term reduction in nutrient cycling rates and possible adverse effects on forest health through acidification and nutrient imbalances.
  
• Dutch research provides clear evidence of an effect of enhanced N deposition on soil Severe acidification of forest soils downwind of intensive farming areas (receiving >100 kg N ha^-1 yr^-1 chiefly as NH₃) has occurred. Large amounts of NO₃^- and in extreme cases NH₄⁺, are being leached to ground waters, and high soil NH₄⁺ levels are influencing available base cations for plant uptake.
  
• Increased NO₃^- leaching from forest systems throughout Europe has been reported. In mainland Europe signs of forest damage are linked to large atmospheric N inputs and the consequent acidification and nutrient imbalances.
  
• In the UK, enhanced NO₃^- leaching has occurred at Welsh sites but appear to be related to forest age. In plantation forests in Scotland, unexpectedly large accumulation rates of N and organic matter may represent a destabilisation caused by acidic deposition and enhanced N inputs.
  

Vegetation

• The varied impact of atmospheric N compounds on plant growth reflects great differences in the N requirements of plants.
  
• The plant species most vulnerable to N deposition appear to be mosses and lichens and the poor recovery of sensitive species of Sphagnum moss in the southern Pennines is largely due to toxicity of NH₄⁺ and NO₃^- deposition. Loss of moss and lichen species in Cumbria over the past 30-40 years may also be due to increased N deposition over that period.
  
• High concentrations of NH₄⁺ and NO₃^- in fog and cloudwater may directly affect the growth of trees. In Germany an adverse effect of NH₄⁺ on conifer foliage vitality has been suggested through direct foliar uptake which may cause increased leaching of base cations, particularly magnesium.
  
• In Dutch forests, large N inputs to the soil of 80 kg N ha^-1 yr^-1 are having a major impact on soil solution chemistry. The large contribution of NH₄⁺ to N inputs is causing acidification and adversely affecting uptake of base cations.
  
• A key effect of N deposition on vegetation is the potential to alter tolerance towards several natural stress conditions such as frost, drought and herbivory. Indeed, changes in patterns of insect feeding on plant may be one of the most sensitive indicators of increased N deposition.
  
• Floristic changes recorded in a number of European countries coincide with long term increases in N deposition. In some cases, other environmental factors have changed over the same period, particularly those involving management by grazing, cutting or burning.
  
• In the Netherlands, experiments have provided strong evidence of a link between atmospheric N deposition and vegetative changes in heathlands and calcareous grasslands.
  
• In the UK, there is evidence that strongly implicates atmospheric N deposition in changes in species composition and biodiversity:
  
• The foliar N content of heather and mosses have been shown to increase with increases in annual N deposition. Largest concentrations were found in East Anglia and Cumbria and lowest concentrations in north west Scotland.
  
• In permanent plots in Cumbria, numbers of moss and lichen species have declines and in the Breckland heaths of East Anglia, heather is declining. Both areas received large inputs of atmospheric N.
  
• The expansion of the grass Brachypodium pinnatum in calcareous grasslands of southern England at the expense of species richness is partly due to reduced grazing pressure. However, increased N input may be contributing to recent invasions and increases.
  
• At a community level, N fertiliser experiments have shown that increased N inputs can cause changes in species composition. Changes in management regimes can result in changes in species composition emphasising the importance of including a management variable in critical loads of N for semi-natural plant communities.
  

Freshwaters

• Nitrate levels in UK upland waters are generally correlated with atmospheric inputs of N. Limited data show no trends in concentrations with time in these waters.
  
• Nitrate, derived predominantly from agricultural fertiliser runoff, if the major form of inorganic N in lowlands freshwaters in the UK, with concentrations increasing in a north-south and west-east direction.
  
• Upland, oligotrophic waters are likely to be most susceptible to acidification and enrichment due to increased atmospheric inputs of N.
  
• Major land use change, eg afforestation, causes both short and long-term changes in surface water NO₃^- levels. The few data that are available suggest that organic N in modifying chemical and biological responses may be important.
  
• Phosphorus (P) is generally more limiting to productivity in upland waters than N. Monitoring of surface waters where N/P ratios are already high and increasing is advisable, especially in relation to eutrophication.
  
• Surface water acidification could intensify if NO₃^- levels in upland waters were to increase. Nitrate-driven acidification may be important during short term episodes generated by rainstorms and snowmelt, but the real biological impact of episodes remains to be assessed.
  

Mapping of Critical Loads and Levels

• The critical loads and levels approach has become an accepted method for considering the control of pollutant emissions both within the UK and in Europe.
  
• Whereas critical loads for sulphur have been considered simply in terms of acidification effects, critical loads for N are less easily calculated since N may acidify and act as a nutrient to terrestrial and freshwater ecosystems. Critical loads must be calculated for both acidity and nutrient effects for the sensitive receptors in an area. In order to protect the area, the lowest value of the two should be used.
  
• In determining critical loads for vegetation, an empirical approach is being used. Existing databases such as national biological recording schemes enable sensitive plant communities to be identified.
  
• For N eutrophication in soils, a more theoretical mass balance approach is being used to calculate critical loads, and these will be compared with critical load values for acidification.
  
• Acidification effects of N are also expected to be important for freshwaters. Two methods have been proposed for estimating the N critical loads for acidity, one using the steady state water chemistry approach and the other a first order mass balance model.
  
• Critical loads based on steady state conditions take no account of timescales of changes. Dynamic models can be used in estimating rates of recovery in response to different deposition scenarios which are used in designing abatement strategies.
  
• To protect vegetation from direct adverse effects of nitrogen oxides, a mean annual concentration of 30 m g m^-3, has been set as the critical level for nitrogen oxides for all vegetation types.
  

• Significant changes in UK emissions of NO_x will occur over the next few years as a result of the introduction of vehicle exhaust catalysts.
  
• In 1996, a protocol for the control of NO_x emissions will be proposed to control regional eutrophication, acidic deposition and production of photochemical oxidants.
  
• UK agricultural emissions of NH₃ and NH₄⁺ contribute as much to total N deposition throughout the country as NO_x emissions from industry and vehicles. Thus emission controls to reduce problems of total N deposition must therefore address emissions from industry, vehicles and agriculture.
  
• The critical loads concept provides an integrated approach for the formulation of control strategies. However, while impacts of enhanced N deposition on vegetation, soils and surface waters have been assessed qualitatively, quantifying the impacts presents more problems.
  
• In the wider context of environmental protection, the application of the critical loads approach to N deposition requires the links between the problems of regional acidification, eutrophication and global change to be quantified.
  

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