Ammonia :: Fen, Marsh and Swamp

Effects and implications

[Ammonia is one of the key pollutants that contribute to nitrogen deposition. Please read the Nitrogen deposition - Fen, Marsh and Swamp  record to understand the full impacts effects of nitrogen deposition including ammonia.]

  • Fens: Sphagnum species will be vulnerable to direct effects of ammonia.   
  • Base rich calcicolous fens (BCF): Dominated by calcicolous sedges and brown moss (eg. Scorpidium scorpoides ( Hypnum scorpoides) , but also P-limited, could become negatively affected by ammonium (Paulissen (2004))

Overview: evidence, processes and main impacts

Fens and marshes are characterised by a variety of vegetation types that represent their underlying geology, soil type. Some occur on calcareous soils while others are found on on acid, base poor soils, typically peats (fens) or organo-mineral soils and also impoverished poorly draining mineral soils (purple moor grass and rush pastures).  Deposition of ammonia will generally be higher to these ecosystems on sites on acid soils by virtue of it being an alkaline gas.

All the ecosystems are permanently (marsh, swamps and reed beds), seasonally or periodically waterlogged and ground fed i.e. minerotrophic. Thus these systems are not oligotrophic, receiving potentially nutrient rich or polluted water from the surrounding area as surface runoff and precipitation. Because they are usually moist, deposition is potentially high as ammonia is very soluble.

These ecosystems have not been included in manipulation studies, nor are we aware of any transect studies which have described changes that might be attributable to ammonia.

Pollutant type and risk

Form of N

Risk areas


Sites in rural areas with elevated background concentrations.  Higher concentrations and dry deposition are found close to point sources e.g. intensive livestock units but also wild animal (e.g. seal and bird colonies).

Indicators of NH3 effects

  • Bleaching of sensitive species e.g. some lichens and bryophytes especially Sphagnum.

Evidence of species specific responses




Eriophorum vaginatum

+ve increased cover

Sheppard et al 2011

What factors modify N deposition impacts?

  • None described
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.

Paulissen, M.P.C.P.; Ven, P.J.M.; Dees, A.J.; Bobbink, R. 2004 Differential effects of nitrate and ammonium on three fen bryophyte species in relation to pollutant nitrogen input. New Phytologist 164 451-458
Sheppard, L.J.; Leith, I.D.; Mizunuma, T. ; Cape, J.N.; Crossley, A.; S., Leeson ; Sutton, M.A.; Fowler, D.; Dijk, N. 2011 Dry deposition of ammonia gas drives species change faster than wet deposition of ammonium ions: evidence from a long-term field manipulation Global Change Biology 17 (12) 3589-3607

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