Nitrogen deposition

Nitrogen deposition is the term used to describe the input of reactive nitrogen species from the atmosphere to the biosphere. Most concern has addressed the impacts of nitrogen deposition to terrestrial ecosystems, but impacts may also occur in the marine environment. The pollutants that contribute to nitrogen deposition derive mainly from nitrogen oxides (NOX) and ammonia (NH3) emissions. In the atmosphere NOX is transformed to a range of secondary pollutants, including nitric acid (HNO3), nitrates (NO3- ) and organic compounds, such as peroxyacetyle nitrate (PAN), while NH3 is transformed to ammonium (NH4+). Both the primary and secondary pollutants may be removed by wet deposition (scavenging of gases and aerosols by precipitation) and by dry deposition (direct turbulent deposition of gases and aerosols) (Fowler et al. 1989, Hornung et al. 1995).

While these pollutants may have lead to acidification (see also acid deposition), nitrogen deposition refers to the pollutant dose that may lead to nitrogen eutrophication. It should be noted that other nitrogen compounds occur in the atmosphere (N2, N2O), but these are not readily available for use by plants, and are therefore not included in the assessment of nitrogen deposition.

For assessment of effects it has been assumed that nitrogen originating from NH3 or NOX has the same ecological effect (Sutton and Fowler 1993, Hornung et al. 1995). This assumption is now being challenged, as both UK wide survey work (Stevens et al, and manipulation studies (Sheppard et al 2008) have found stronger correlations between detrimental effects on semi-natural plant species, particularly among lower plants, and the concentration or dose of reduced N.(NHy). However, it is clear that NOX emissions are much more widely dispersed than NH3, with the latter often deposited in high quantities to semi-natural vegetation in intensive agricultural areas. Reduced N (NHx) is primarily emitted from intensive animal units and more recently vehicles with the introduction of catalytic converters. Thus effects of NH3 are most common close to urban highway and roadside verges, and within 1-500m of the point source depending on the size of the source of the source. Aerosols of ammonia, by comparison, are carried much further and contribute to wet deposition. The loading of N in wet deposition will depend on the amount of precipitation and the amount of N. In the east, N concentrations can be quite high due to the low rainfall, whereas in the west the rainfall is much higher but the concentrations tend to be lower.

Effects via acidification usually result from the mobile anion effect, leaching base cations from exchange sites of the soil. This effect is associated with NO3-, an anion, that is not readily fixed in the soil due too its negative charge, and as it leaches through the soil profile it attracts, and takes with it, positively charged cations. In soils above pH4 NH4+ ions can be be nitrified to NO3-, so both forms can give rise to base cation leaching. Excess NO3- ie. when deposition exceeds plant demand due to too much, or insufficiency of P and K, or other factors that restrict plant growth can lead to NO3- leaching and enrichment of water sources. In more acid soils and carbon enriched soils a large proportion of the NH4+ will be fixed, adsorb to the negatively charged sites, immobilised and remain in the soil. N eutrophication can also increase the mineralization of organic N by microbes through a reduction in the C:N ratio, so that the eutrophication effect is exacerbated. This effect may be eclipsed by the immobilisation, uptake of the mineral N by an expanding microbial community.

Communities most at risk from N eutrophication are those rich in bryophytes and where species richness is comprised of slow growing species. Competition from invasive species - often grasses poses a threat for many communities but the type of species invading will depend on the proximity of a seed source (arable, farmed land). N deposition can also increase the risk of damage from abiotic factors, e.g. drought (summer and winter) and frost. Where N deposition leads to enhanced foliar N concentrations there is increased risk of damage from pests and pathogens both above and below ground. Detrimental impacts of N below-ground include loss of species diversity with respect to ectomycorrhiza and reductions in decomposer populations e.g.enchytraeid worms. Nitrogen can also increase litter fall, reducing the amount of light passing through to ground dwelling species.

While direct toxicity from wet N deposition is rare, dry deposition of NH3 can cause toxicity (Marschner 1995). Plants can only control uptake through the stomata. However, mechanisms can be damaged by NH3 which facilitate greater uptake. Light and assimilated carbon products are required to detoxify NH3 via assimilation and the production of N compounds such as amino acids. Understorey plants may be particularly at risk from NH3 toxicity as growth of upperstorey plants is stimulated with consequent reductions in light transmission. Ammonia is an alkaline gas with a high water solubility and is thus attracted to acidic wet surfaces. Some plant species particularly among the lower plants Sphagnum moss and some lichens are characterised by large surface area to mass, wet acidic surfaces which may make them especially susceptible to ammonia deposition.

Because the availability of nitrogen is often the main growth limitation in many semi-natural ecosystems the response of the majority of plants is positive initially, ie they grow better. However such communities exist in balance because their growth rates are contained by the level of available N. When the availability of N increases this balance is upset and some, especially the lower plants, lose out from too little light or other resources. Management intervention can help maintain the balance by removing, cutting down etc the faster growing species. Grazing is often used to maintain the balance between slow and fast growing species in some systems. However, getting the balance right between management intervention and N deposition is a complex issue of optimising positive and negative outcomes.


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