Nitrogen Deposition

This article is about the unseen ‘elephant in the room’ – nitrogen deposition, which is threatening much of our precious wildlife here in Sussex and elsewhere. I have collated it in order to bring the spotlight of attention to this pressing problem and to try and hopefully explaining it in a little more detail. (The following paragraph is more technical, so if you wish, skip to the following).

The term nitrogen deposition, is used to describe the input of nitrogen from the atmosphere into the landscape in which we live. Of most concern are the impacts of nitrogen deposition to our terrestrial ecosystems, including the marine environment and water supply. 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, while NH3 is transformed to ammonium (NH4+). Both the primary and secondary pollutants may be removed by wet deposition (by rainfall) and by dry deposition (direct deposition of gases and aerosols). However, it is clear that NOX emissions are much more widely dispersed than NH3, the latter often deposited in high quantities to semi-natural vegetation adjacent to intensive agricultural areas. Reduced N (NHx) is primarily emitted from intensive animal units and more recently, vehicles with the introduction of catalytic converters.

The effects of NH3 are most common close to roadside verges, and within 1-500m of the traffic 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 amount of nitrogen in wet deposition will depend on the amount of precipitation and the amount of nitrogen. In the east, nitrogen concentrations can be quite high due to the low rainfall, whereas in the west the rainfall is much higher but the concentrations here tend to be lower.

Because the availability of nitrogen is often the main growth limitation in semi-natural ecosystems the response of the majority of plants is positive initially, they growing better. Such communities evolved in balance because their growth rates are checked by this low level of nitrogen. When the availability of nitrogen increases this balance is upset and some, with many native plants losing out due to competition from a few species that can take greater advantage of the increased nutrients. Management intervention can help maintain the balance by conservation grazing (such as that by Sussex Pony Grazing), cutting, etc, removing the faster-growing, aggressive species.

Habitats most at risk from nitrogen are those rich in mosses and lichens and where species richness is comprised of slow growing species. These include the species-rich heathland and chalk grassland habitats as represented in Sussex. Competition from invasive species, often grasses such as tor grass on the South Downs and purple moor grass on the heathlands, pose a serious threat. Nitrogen will often increase leaf litter, reducing the amount of light passing through to ground dwelling species. Nitrogen deposition can also increase the risk of damage from weather events, e.g. drought (summer and winter) and frost. Where deposition leads to enhanced foliar nitrogen concentrations there is increased risk of damage from pests and pathogens both above and below ground. Detrimental impacts of nitrogen below-ground include loss of species diversity with respect to fungi and reductions in the decomposer invertebrate populations.

In 2011, a Habitat Regulations Assessment was undertaken for the Wealden District Councilís (WDC) Core Strategy. This identified future residential and commercial development as a driver of increasing atmospheric pollution through its impact on traffic growth. Further work on potential deposition impacts suggested that residential or commercial development within 7km of the Ashdown Forest could have detrimental effects on this designated area through increasing nitrogen deposition. A 7km development ‘Buffer Zone’ was therefore introduced following consultation. WDC considers that limiting overall levels of housing and other growth within a 7km protection zone around Ashdown Forest, is necessary to meet the Council’s responsibility under international Habitat Directives to protect this site. This policy is due to be challenged in the High Court in early 2014.

To conclude, heathland habitats including those at Ashdown Forest and Chailey Common and, the chalk grassland areas within the South Downs National Park, are very sensitive to increased nitrogen deposition, most of which is related to traffic emissions and, agricultural inputs. So motorists, please respect the speed limits across Ashdown Forest. Farmers, be more diligent with fertilizer applications near to hedgerows and water bodies!


Material abstracted from the following documents:†† (the UK Air Pollution Information System).

‘The Local Plan 2013 ñ 2027 Protecting the Ashdown Forest.’ Wealden District Council.

‘Guidance Note on Reducing Nitrogen Deposition at the Ashdown Forest Special Area of Conservation and Special Protection Area.’ Wealden District Council.


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