Ammonia :: Calcareous grassland

Effects and implications

[Ammonia is one of the key pollutants that contribute to nitrogen deposition. Please read the Nitrogen deposition - Calcareous grassland  record to understand the full impacts effects of nitrogen deposition including ammonia.]

  • Reduced species richness and diversity
  • Change in species composition
  • Loss of rare or endangered species
  • Loss of characteristic calcicolous mosses and lichens at risk from shading and N accumulation, if over storey species are stimulated.
  • Increased risk of drough effects

Overview: evidence, processes and main impacts

There have been observations made along transects away from agricultural sources in the Netherlands (Bobbink & Hettelingh 2011).

Given the expected low rates of deposition direct damage from ammonia is not expected to be high, although sensitive lichens may be at risk, especially if their surfaces are acidic. Ammonia is a reactive water soluble alkaline gas that will deposit to acid surfaces and where acidic gases are also present in the atmosphere e.g. SO2 (through co-deposition). Co-deposition with SO2 is a much smaller than 10 years ago given the significant fall in emissions (RoTAP 2012).  Calcareous grasslands occur on alkaline soils so their susceptibility to ammonia N deposition will be much less than acidic systems. 

Pollutant deposition type and risk areas

Type of pollutant

Form of N

Risk areas

Dry deposition



Rural areas with elevated background concentrations close to point sources e.g. intensive livestock agriculture.

Indicators of NH3 enrichment

None available

Evidence of species specific responses

No data

What factors modify NH3 deposition impacts?

No data

Critical Load/Level: 

Habitat/ Ecosystem Type Critical Load/ Level Status Indication of exceedance Reference
Higher plants

3 µg NH3 m-3 annual mean (uncertainty of 2-4 µg NH3 m-3)

UNECE, 2007

Direct visible injury; species composition changes. Ecosystems where sensitive lichens and bryophytes are an important part of the ecosystem integrity, the critical level is set at 1 µg NH3 m-3.

Lichens and Bryophytes

1 µg NH3 m-3 annual mean

UNECE, 2007

Loss of sensitive mosses and lichens communities. Communities become dominated by nitrophiles at the expense and virtual loss of acidophytes as bark pH becomes less acidic.