Nitrogen Deposition Implications of the Local Plan

Durham County Council: Ecology Team

February 2012

1. Background

Nitrogen emissions to the atmosphere have increased substantially over the 20th century, mainly in the form of ammonia (NH3) from agriculture and nitrogen oxides (NOx) from industry. The main pollutants of concern for European sites are oxides of nitrogen, ammonia and sulphur dioxide (SO2).

Following atmospheric dispersion and chemical transformation, these chemicals are deposited across European landscapes. A large number of the habitats (and species, either directly or indirectly) listed under the Habitats Directive are sensitive and potentially vulnerable to atmospheric nitrogen deposition.

Nitrogen is the second most important plant nutrient behind carbon, and the productivity of terrestrial ecosystems is generally limited by nitrogen supply. However such communities exist in balance because their growth rates are contained by the level of available Nitrogen. Hence the increase in nitrogen deposition will be expected to exert a large impact on ecosystem biodiversity. Nitrogen deposition may cause changes to species composition, often including a reduction in species richness and a loss of sensitive ‘lower plants’; changes to soil microbial processes; changes to plant and soil biochemistry; increased susceptibility to abiotic stresses (such as winter injury) and biotic stresses (such as pests and pathogens); and it also contributes towards acidification.

Ammonia emissions are dominated by agriculture, with some chemical processes also making notable contributions. Nitrogen emissions are much more widely dispersed than ammonia, with the latter often deposited in high quantities to semi-natural vegetation in intensive agricultural areas. Reduced Ammonia is primarily emitted from intensive animal units and more recently vehicles with the introduction of catalytic converters. Thus effects of Ammonia are most common close to urban highway and roadside verges, and roughly within 500m of the point source depending on the size of the source. Aerosols of Ammonia, by comparison, are carried much further and contribute to wet deposition1. As such, it is unlikely that material increases in Ammonia emissions will be associated with Local Plans.

The loading 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 where the rainfall is much higher, the concentrations tend to be lower (www.apis.ac.uk). Nitrogen emissions, however, are dominated by the output of vehicle exhausts (more than half of all emissions). The EU has been tightening emission standards on new vehicles through various phased Euro standards. However, the “lab based” theoretical improvements have not translated into the real world situation in the UK.

Higher vehicle numbers on the UK roads and the level of congestion means that the cars are performing worse in terms of national emissions than had been calculated2. Within a ‘typical’ housing development, by far the largest contribution to Nitrogen (92%) will be made by the associated road traffic. Other sources, although relevant, are of minor importance (8%) in comparison. Emissions of Nitrogen could therefore be reasonably expected to increase as a result of greater vehicle use as an indirect effect of the Local Plan.

1 Wet deposition refers to acidic rain, fog, and snow. If the acid chemicals in the air are blown into areas where the weather is wet, the acids can fall to the ground in the form of rain, snow, fog, or mist. As this acidic water flows over and through the ground, it affects a variety of plants and animals 2 Challenges to reducing the threat of nitrogen deposition to the Natura 2000 network across the UK and Europe (Bareham) 2011.

2. The Habitats Directive

The Habitats Directive requires that all ‘plans and projects’ which are likely to have a significant effect on a Natura 2000 site have an appropriate assessment of the implications for the site. Subject to certain exemptions, the plans or projects can only be approved where they are shown to have no adverse effect on any Natura 2000 site. However, at present, there is no common approach for evaluating the effects of nitrogen deposition and concentrations on these sites.

For the conservation status of a habitat to be favourable, “the specific structure and functions which are necessary for its long-term maintenance exist and are likely to continue to exist for the foreseeable future”. Habitat structure and habitat function varies widely between different habitats, but it is clear that the various ecological processes essential for a habitat have to be present and functioning for the habitat to be considered to be at favourable conservation status (European Commission, 2006).

3. Critical Loads

The relationship between pollutant dose and the resulting environmental effect forms the basis for the critical load concept. The critical load is defined as:

‘A quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge.’

(http://www.unece.org/env/lrtap/WorkingGroups/wge/definitions.htm)

Critical loads of Nitrogen can be compared to past, present or future deposition rates in order to establish the amount of excess deposition, also called exceedance. In the UK the Air Pollution Information System (APIS) provides a comprehensive source of information on pollution and its impact on habitats and species. APIS has been developed by JNCC (Joint Nature Conservation Committee), the country conservation agencies, the UK environment agencies and the Centre for Ecology and Hydrology. It provides site specific information on deposition and critical loads.

4. The Current Situation in County Durham Despite not necessarily being listed under each sites vulnerabilities, in County Durham all of the designated N2K sites within the county boundary currently exceed the critical load for Nitrogen, with acid deposition also a problem at Castle Eden Dene, Moor House-Upper Teesdale, and the North Pennine Moors. Therefore as agreed by Natural England - any new source of pollution deposition is likely to exacerbate an existing adverse threat to these vulnerable sites. As detailed above, the majority of the likely source of Nitrogen is likely to come from an increase in traffic on the County main roads, due to increased housing and office/industrial development.

While it is recognised that the Local Plan also needs to tackle the issue of general diffuse air pollution (airports, power stations etc) this cannot be addressed at the local level. Durham County Council is only responsible for avoiding the individual contribution of the Local Plan to the “in combination” effect not for mitigating the “in combination” effect in its totality.

5. Intensive Livestock Industry Under the provisions of the IPPC Directive, large pig and poultry units are currently being authorised in the UK. Where these units are sited near to Natura 2000 sites and are judged to have a significant effect, they require an ‘appropriate assessment’ under the provisions of the Habitats Directive.

A number of studies from the early 1990’s have demonstrated that ammonia emissions from these units can be many times the critical level and critical load for the receiving habitat (Sutton et al, 2009).

There are currently however no application for any new large scale cattle units/pig farms/poultry farms in the county. The likely impacts caused by increased ammonia deposition have therefore not been considered further.

6. Is There Evidence to Suggest that Nitrogen Deposition is Impacting on the County’s Natura 2000 sites? Site condition assessments: The condition of SSSI (Site of Special Scientific Interest) land in England is assessed by Natural England, using categories agreed across England, Scotland, Wales, and Northern Ireland. The conservation agencies’ monitoring of SSSI condition is based on Common Standard Monitoring (CSM) guidance http://www.jncc.gov.uk/page-2219 . CSM provides a basic framework to ensure consistent monitoring in the UK. The main purpose of site assessment is to:

• determine whether the desired condition of the feature(s) of interest for which the site was designated is being achieved. This can enable judgements to be made about whether the management of the site is appropriate, or whether changes are necessary; and

• to enable managers and policy makers to determine whether the site series as a whole is achieving the required condition, and the degree to which current legal, administrative and incentive measures are proving effective.

There are six reportable condition categories: • favourable; • unfavourable recovering; • unfavourable no change; • unfavourable declining; • part destroyed; and • destroyed.

These site condition assessments currently provide the only regular means of assessing the state of the component SSSI’s within our European Sites. Utilising this as a means of assessing the current state of the County’s European Sites, the following is evident: • Moor House Upper Teesdale SAC – Majority unfavourable recovering • North Pennine Dales Meadows SAC – Majority favourable • North Pennine Moors SAC/SPA – Majority favourable • Castle Eden Dene – Majority unfavourable recovering • Teesmouth and Cleveland Coast SPA – Majority unfavourable recovering • Northumbria Coast SPA – Majority favourable • Durham Coast SAC – Majority favourable • Thrislington SAC – 100% favourable

However CSM is not sensitive enough, or designed to assess and attribute drivers of environmental change such as air pollution impacts. It is instead a very broad brush assessment of a sites designated features, which may not relate to the qualifying features of a European Site. In order to get a good understanding of cause and effect of air pollutants on site condition, a much more detailed assessment is required, along with a comparison between sites, than is possible within the CSM framework. As a result a site may be:

• reported as ‘favourable’, but air pollution is currently having an adverse impact (and monitoring is not ‘sensitive’ enough to detect); • site reported as ‘favourable’, but air pollution likely to adversely effect in future (time lag in response); or • site reported as ‘unfavourable’, and air pollution is a contributory cause, but it is not recorded as such. (JNCC Report no.426. January 2009)

It is therefore apparent that current monitoring assessments undertaken on component sites for our designated N2K sites are not conclusive enough in order to allow the Local Planning Authority to assess whether the nitrogen deposition is currently impacting on a European site’s integrity.

6.1 Case Study: Thrislington SAC In 2001 Natural England (then English Nature) undertook a detailed assessment of vegetation change at Thrislington Plantation NNR (SAC) (Report no.413. 2011). The assessment concluded that there was thought to be an increase in Bromus erecta and Brachypodium pinnatum (Tor grass) on the site. Possible causes were thought to be climate change and/or atmospheric deposition. It was recommended that further detailed monitoring should be undertaken to assess whether the current patches of the vegetation were increasing in size/spread, and whether management practices could influence this. It is understood that no further detailed surveys/monitoring assessments have been undertaken on the site (Natural England pers. Comm.). While the current management plan for the site indicates both species are still present on site, there is no indication whether this is thought to have increased in extent/frequency since 2001.

6.2 Other Studies It is well established empirically that tor grass reduces species diversity by competition and that its competitive ability is increased when nitrogen levels are increased but other nutrients are not (Bobbink et al. 1988; Hurst 1997). However, the probable source of the elevated nitrates is controversial.

Studies in the Netherlands suggest that increasing atmospheric nitrogen deposition has resulted in increased spread of tor-grass (Bobbink & Willems 1987). There is some evidence that elevated atmospheric nitrogen deposition levels reduce sward diversity in Britain (e.g. Stevens et al. 2005 for species diverse acidic U4 Festuca-Agrostis grassland) but the Dutch work is apparently the only published empirical study regarding tor-grass and nitrogen load. However, some authors argue that the Dutch studies have been misinterpreted and only propose that nitrogen deposition induces tor grass dominance (as tor-grass tolerate nitrogen addition) rather than maintaining it (Bobbink & Willems 1991).

In a study carried out in 19953 Nitrogen was applied to an artificial chalk grassland to stimulate recent increases in atmospheric Nitrogen deposition. The results of the study indicated that while inputs of Nitrogen above ambient levels stimulated the growth of the sward, there was no further increase in growth when the levels were increased from 20kg N ha-1 year -1 to over 80kgN ha-1 year-1. The study therefore concluded that there was no evidence that increases in atmospheric nitrogen deposition will result in grass dominance and a loss of species diversity. It was instead thought that the growth of both grasses and forbs were limited by the availability of phosphorus. Indeed a study of 11 chalk grassland sites in south-east England, showed that there was no correlation between higher nitrate soil levels and greater tor-grass dominance and concludes, “management is the overriding factor in determining [tor-grass] abundance” (ibid: 13-14; Baxter 1994).

Grazing has been introduced as a means of suitable sensitive management on parts of Thrislington, and in particular Exmoor ponies are currently being used due to their ability to graze less palatable grass types. This may be containing the spread of the species, but without a detailed monitoring programme this cannot be ascertained. The botanical assessment of Thrislington noted that the level of nitrogen deposited on Thrislington at the time (2001) was around 19-20kg/Nitrogen/ha/year. The current figures provided by APIS indicate a current level of 20.3kg/Nitrogen/ha/year on the site. This shows only a very slight change in the amount of deposition occurring on the site.

7. Possible Mitigation Measures

1. Introduction of buffers onto both source and receptors?

Guidance which has recently come out of the 2011 Nitrogen Deposition workshop4 states: In the case of nitrogen emissions to air, such buffer zones could be appropriate both for nitrogen oxides emissions from roads and for ammonia emissions from agriculture. Three aspects to such buffer zones should be considered:

3 Wilson, E.J et al. 1995. Are calcareous grassland in the UK under threat from nitrogen deposition? An experimental determination of a critical load. Journal of Ecology. 4 Nitrogen Deposition and Natura 2000 - Science and Practice in Determining Environmental Impacts (2011) increasing the distance from the source, allowing greater dispersion before the air reaches the sensitive area, such as an SAC; • increasing the dispersion between source and receptor, such as by planting tall rough vegetation, further diluting the pollutant before it reaches the sensitive area; and • encouraging deposition between the source and receptor, such as provided by planting tall vegetation as a buffer zone.

While current development patterns, main roads and the proposed housing allocations reduce the amount of flexibility with respect to the distance of the source, there is potential to increase dispersion and encouraging deposition of nitrogen between the source and the sensitive receptor (i.e. points 2 and 3). Interestingly current research on concentrations of ammonia and nitrogen deposition at roadside verges carried out in Scotland concluded that while the concentrations of the two gases along road verges have a strong dependence on traffic flows, the rate of dispersion close to roads is relatively rapid over the first 10m (15m for Nitrogen), so that any direct influence of the road is likely only within the roadside verge. (Cape, J.N., Tang, Y.S. et al. 2004. Concentrations of ammonia and nitrogen dioxide at roadside verges, and their contribution to nitrogen deposition)

A more recent study (2009) however suggests that NO2 concentrations are elevated above background levels up to approximately 250m away from local roads. The study also noted that Nitrogen concentrations were on overage 140% higher 12km downwind of a major urban area in the West Midlands (UK), compared to similar distances upwind. Therefore the location of sensitive sites in relation to prevailing wind direction and major sources is likely to affect the balance between local and background contributions to pollutant concentrations. (Gadsdon, S.R, Power, A.A. 2009. Quantifying local traffic contributions to NO2 and NH3 concentrations in natural habitats) It is noted that the prevailing wind in the North East of England is predominantly west, south westerly. Overall research has indicated that tree belts are the most successful means of decreasing dry deposition of NH3 (it is assumed that the effect on Nitrogen would be similar). These ‘sacrificial’ tree belts should be planted next to the sensitive receptors/sinks in order to be most effective. Another aspect to take into consideration is the size of the sensitive site, with smaller nature reserves benefitting the most, due to the impact of higher edge effect5. The outer perimeter of larger reserves effectively act as their own buffer zones. The design and species used also remains critical to the success of the buffer. (Dragosits, U et al. 2009)

2. Site management

Decreasing N deposition would, of course, be the preferred way to protect Natura 2000 sites from N induced ecosystem changes, if indeed N is proved to have an adverse impact on a sites integrity. However, management methods that remove N from a habitat can be useful in mitigating N deposition effects on ecosystems. From semi-natural habitats, such as grasslands and heathlands, which require an active management regime for their maintenance, intensified use of methods causing biomass removal by mowing or prescribed

5 The edge effect in ecology is the effect of the juxtaposition or placing side by side of contrasting environments on an ecosystem. This term is commonly used in conjunction with the boundary between natural habitats, and disturbed or developed land. Edge effects are especially pronounced in small habitat fragments where they may extend throughout the patch, thereby reducing the amount of ‘core habitat’. burning may at least partly mitigate N induced alterations (Mountford et al. 1996, Barker et al. 2004). Therefore Ensuring adequate and appropriate management of the sites, particularly for our grassland and heathland sites, may help reduce nitrogen loads within the habitat type.

Indeed concerns have been raised that the current discovery of possible impacts caused by atmospheric deposition, have been used as a scape-goat to attribute negative/adverse changes in botanical composition on sensitive sites. Where causes could be more than likely attributed to a lack of or inappropriate management.

For Dutch calcareous grasslands Willems (2001) suggests that N deposition effects can be decreased by mowing in early August. The mowing was thought to suppress the N favoured grass Brachypodium pinnatum (Tor grass) and promotes the original species-rich grassland vegetation. Cattle grazing is also thought to be an effective management method to control and reduce tor-grass, followed by annual herbicide application combined with grazing (Baxter & Farmer 1990, 1998; Croft & Jefferson 1994). Tor-grass is extremely unpalatable, and therefore effective grazing regimes are complex and require use of certain breeds. For heathlands, originally dominated by Calluna vulgaris, active management involving prescribed burning and mowing can mitigate effects of N deposition (Barker et al. 2004).

Active management may be a promising alternative for many semi-natural habitats, and often the N management can be incorporated in the management that is already imposed to maintain the conservational value of the habitat. For other habitats there is no, or very little, available information on management strategies mitigating effects of N enrichment.

8. Conclusions and Recommendations

The A19, due to prevailing wind direction, is likely to have a direct influence on the amount of nitrogen deposition affecting Castle Eden Dene SAC. However it is noted that the verges of this main road, are sufficiently ‘planted up’ with scrub and trees, both adjacent and to the north and south of the reserve. There is therefore little opportunity to alleviate any increases in traffic levels by putting buffers in place. In respect of Thirslington SAC, it is considered that there is potential for further tree planting at the southern end of the site adjacent to the unclassified road. However, Natural England advised in February 2012 that they would not be happy with any increased planting and the potential for planting was unlikely to be effective in reducing nitrogen deposition given the required width of buffers. The majority of the N2K sites in the county would be considered ‘large’ – apart from Thrislington. Therefore the introduction of a buffer may therefore have little positive impact on the site as a whole. The combination of inappropriate/insufficient management, and climate change is likely to cause the most impact on the qualifying features of our N2K sites.

Further detailed monitoring of sites should be undertaken. CSM is currently not sufficiently sensitive for detecting or attributing N deposition impacts on individual sites. Consequently sites may currently be recorded as in favourable/recovering condition and yet show signs of adverse N deposition impact. Indeed many CSM indicator species were found not to be sensitive to N deposition6. It is therefore recommended that a new site based monitoring system for N deposition impacts should be developed. This should incorporate complete

6 JNCC (2011) Evidence of nitrogen deposition impacts on vegetation: implications for country strategies and UK biodiversity commitments. summary of JNCC reports no. 447 and no. 449. floristic monitoring of replicate permanent quadrats located at random within fixed areas (e.g. a habitat area as initially mapped) over a number of years. This assessment should also incorporate species cover estimates, and a measure of biomass productivity, this would enable an even more sensitive indication of N deposition impacts, and by taking simple soil measurements (eg. pH and total C/N ratio) this would be useful to produce niche models to generate site-specific lists of species at risk. The development of such a monitoring system is not seen as appropriate at a County level, but rather something which needs to be developed Nationally and adopted by each region to ensure a uniform means of assessment.

In order to address possible impacts at a County level, there is a need for an over-arching policy within the County Durham Plan to reduce car use/encourage more sustainable means of transport.

The Local Plan should also include a policy that states that any potential new sources of nitrogen/ammonia should be sited more than 2km away from a receptor site (this should include any new sewerage works, poultry farms, intensive cattle farms, and large scale industrial units/biomass burners etc.), unless an HRA concludes no likely adverse impact.

References

Baxter B & Farmer A. 1993. The control of Brachypodium pinnatum in chalk grasslands: influence of management and nutrient status. English Nature Research Report R100. English Nature, Peterborough.

Bobbink R & Willems JH. 1991. Impact of differing cutting regimes on the performance of Brachypodium pinnatum in Dutch chalk grassland. Biological Conservation 56: 1-21.

Bobbink R & Willems JH. 1993. Increasing dominance of Brachypodium pinnatum (L.) Beauv. on chalk grasslands: threat to a species-rich ecosystem. Biological Conservation 40: 301-314.

Cape, J.N, Tang, Y.S, van Dijk, N, Love, L, Sutton, M.A, and Palmer, S.C.F (2004) Concentrations of ammonia and nitrogen dioxide at roadside verges, and their contribution to nitrogen deposition. Environmental Pollution pg. 469 – 478.

Dragosits, U, Theobald, M.R, Place, C.J, ApSimon, H.M, and Sutton, M.A (2006) The potential for spatial planning at the landscape level to mitigate the effects of atmospheric ammonia deposition. Environmental Science & Policy pg.626 – 638.

Gadsdon, S.R, and Power, S.A (2009) Quantifying local traffic concentrations to NO2 and NH3 concentrations in natural habitats. Environmental Pollution pg 2845 – 2852.

Hicks, W.K, Whitfield, C.P, Bealey, W.L, and Sutton, M.A. (2011) Nitrogen Deposition and Natura 2000 -Science and Practice in Determining Environmental Impacts, Findings of a European workshop linking scientists, environmental managers and policy makers.

Hurst A. 1997. Community dominance: an investigation into the competitive mechanisms in Brachypodium pinnatum, and possible methods for reducing its dominance on ancient chalk grassland. Unpublished DPhil thesis, University of Sussex.

Robertson, H.J, and Gibson, C.W.D (2001) Assessment of vegetation change at Thrislington Plantation National Nature Reserve, County Durham. English Nature Research Reports, Number 413.

Stevens, C.J, Coparn, S.J.M, Maskell, L.C, Smart, S.M, Dise, N.B, and Gowing, D.J (2009) Detecting and attributing air pollution impacts during SSSI condition assessment. JNCC Report No. 426.

Whitfield, C, Hettelingh, J-P, and Hall, J. Critical loads based nitrogen deposition assessment for Habitats Directive Article 17 reporting (http://jncc.defra.gov.uk/pdf/ap_NassessmentarticleforNFCs210611.pdf)

Wilson, E.J, Wells, T.C.E, and Sparks, T.H (1995) Are calcareous grasslands in the UK under threat from nitrogen deposition? An experimental determination of a critical load. Journal of Ecology.