Ammonia deposition from livestock near designated sites

Ammonia deposition is the reason a livestock building 8 kilometres from a bog attracts an objection when the odour assessment showed nothing beyond the farm gate. Ammonia gas emitted from housing, manure storage and land spreading disperses, converts and deposits as nitrogen, and the habitats that suffer are the nutrient-poor ones: raised bogs, heathland, acid grassland, lichen-rich woodland. Those habitats are usually the designated ones, which is how a farm diversification scheme ends up in an assessment against European site thresholds.

Start with the constraint map rather than the shed drawing. The designations within 10 kilometres decide how much assessment the scheme needs.

Two thresholds, and what each one measures

Critical levels describe ammonia concentration in air, as an annual mean in micrograms per cubic metre. The value applied where lichens and bryophytes are an important part of the ecosystem is 1 microgram per cubic metre, and 3 micrograms per cubic metre is applied to higher plants. Most designated sites in England that are sensitive to ammonia fall under the lower figure, because their interest features include lichen or bryophyte communities.

Critical loads describe nitrogen deposition on the ground, in kilograms of nitrogen per hectare per year. They are habitat specific and expressed as a range, and the relevant figure depends on which habitat the designated feature actually is rather than on what the surrounding countryside looks like.

The practical difference matters. A concentration exceedance is a direct toxicity question close to the source. A deposition exceedance is a cumulative enrichment question that persists, because nitrogen already deposited does not leave.

Which threshold governs depends on the interest feature. A lowland raised bog with a Sphagnum and lichen assemblage is assessed against the lower critical level and a low critical load, and both can be exceeded by a modest unit at a few kilometres. A species-rich neutral grassland tolerates more nitrogen and sits under a higher critical load. Two designated sites the same distance from the same shed can therefore produce completely different answers, which is why naming the interest features on the constraints plan matters more than counting designations.

Screening, and when modelling becomes necessary

Screening starts with distance. Regulator practice for intensive farming permits is to look for internationally designated sites, meaning special areas of conservation, special protection areas and Ramsar sites, within about 10 kilometres, and for sites of special scientific interest, ancient woodland and local wildlife sites within about 2 kilometres. Those distances sit in regulator guidance, which is revised, so confirm the current version before you scope anything.

A simple screening tool gives a first estimate of concentration and deposition at the nearest sensitive point. Where the predicted contribution falls below a small percentage of the relevant critical level or critical load, the effect is normally treated as insignificant and the matter ends. The conventional figure for internationally designated sites is 1 per cent, with a less stringent figure applied to SSSIs, and again these are guidance thresholds rather than statutory ones.

Above the screening threshold, detailed dispersion modelling follows, run by an air quality specialist using hourly meteorological data, emission factors for the housing and storage types proposed, and receptor grids across the designated habitat. This is where the Air Quality Assessment stops being a desk exercise and becomes specialist work.

Emission factors are the input that decides the answer, and they vary by species, housing system, bedding, ventilation and manure handling. A naturally ventilated deep litter broiler house and a mechanically ventilated house with belt removal emit at different rates from the same bird numbers. State the factor used and its source in the report, because a consultee who cannot see where the number came from will assume the least favourable one.

In-combination effects decide most cases

A single new unit rarely breaches a threshold by itself. What breaches it is the new unit added to everything already there.

The in-combination assessment adds the process contribution to the existing background deposition, and then to the contributions of other permitted or consented units within the screening distance that are not yet reflected in the background. In parts of the country with dense poultry or pig production, background deposition at the nearest designated habitat already exceeds the critical load before the application is made.

That is an uncomfortable position to be in, and it needs saying plainly at pre-application rather than discovered at consultation. Where the critical load is already exceeded, the argument shifts from whether there is an effect to whether the additional contribution is so small that it cannot be distinguished, and Natural England will scrutinise that carefully.

What Natural England expects to see

The submission that gets a clean response has five things in it. A plan showing every designated site within the screening distances with its interest features named. The emission inventory, with animal numbers, housing type, manure and slurry storage, and spreading assumptions stated. The model set-up, including meteorological data, building downwash and stack parameters. Results at named receptors against the relevant critical level and critical load. And a mitigation package tied to the modelled case.

The last point is the one applicants miss. Mitigation that is described but not built into the modelled emission rate does not reduce the predicted contribution, and a consultee will assess what was modelled.

Mitigation that carries weight

In rough order of reliability: housing design that reduces the emitting surface and removes manure quickly, including frequent scraping, belt removal in laying systems and floor design that separates urine from faeces. Then storage, meaning covered or crusted slurry stores and covered manure heaps, which cut a large emission source at modest cost. Then spreading, using trailing shoe or shallow injection rather than broadcast, with incorporation timescales specified.

Air scrubbers, whether acid or biotrickling, can achieve substantial removal on mechanically ventilated housing, though they need power, water, maintenance and a disposal route for the scrubbing liquid. Regulators want evidence of in-service performance rather than manufacturer claims.

Tree belts come last. They recapture part of the plume, the effect is real and the quantification is contested, so treat them as supporting rather than as the answer.

Permits, EIA and the habitats duty

Intensive pig and poultry installations above the thresholds in the Environmental Permitting Regulations 2016 need a permit as well as planning permission, and the permit application carries its own habitats screening. Larger installations fall within Schedule 1 of the EIA Regulations 2017, where an environmental statement is mandatory and must be prepared by competent experts.

Separately, where a European site could be affected, the local planning authority acts as competent authority under the Conservation of Habitats and Species Regulations 2017 and has to be satisfied there is no likely significant effect, or carry out an appropriate assessment. Applicants usually prepare a shadow assessment to give the authority something to adopt, and odour is assessed alongside it on a separate receptor basis.

Conditions after consent normally fix the mitigation, the animal numbers and the manure management plan, and sometimes require monitoring using passive diffusion tubes at the designated site boundary.

You can draft the supporting reports with PlanScope and enter a postcode on the home page to see which ones a holding is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required, and ammonia dispersion modelling is one of them. This is general guidance, not legal advice, so check the position with your local planning authority or a qualified professional before relying on it.