Nitrogen Deposition :: Fen, Marsh and Swamp

Effects and implications

  • Sphagnum species will be vulnerable to N both through direct N induced toxicity and indirectly from shading by increased vascular plants growth. The extent to which shading occurs will reflect the capacity of the vascular plants to use the addition N to increase C assimilation. Molinia caerulea , despite having a relatively low Ellenberg N score  appears to be a nitrophile in the presence of high NH4+ concentrations (van den Berg pers comm.).
  • Sphagnum mosses are the ecosystem engineering so that their loss will destabilise the system, through effects on acidity and knock on implications for carbon cycling, decomposition and most importantly C sequestration.

Base rich calcicolous fens:

  • Dominated by calcicolous sedges and brown moss (eg. Scorpidium scorpoides ( Hypnum scorpoides)). Brown mosses serve the same function as Sphagnum in more acidic situations.
  • Increase in tall graminoids and decrease in diversity of subordinate species ( Vermeer 1986)
  • Brown moss species are likely to decline while NH4+ tolerant S. contortum increases.
  • Reduced numbers of bryophyte species and biomass, thinning of the moss carpet.
  • Increase in vascular plant biomass, especially in response to NH4+

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). This latter pasture type consists of a mosaic of plant communities, reflecting the differences in water table and N is likely to differentially impact such communities and potentially disrupt the balance between them. This wide ranging variation in vegetation types means responses to N deposition have to be considered separately for the different ecosystem types and even within types.

However, all the ecosystems are permanently (marsh, swamps and reed beds), seasonally or periodically waterlogged and ground fed ie. minerotrophic, receiving potentially nutrient rich or polluted water from the surrounding area as surface runoff and precipitation. Thus atmospheric N deposition may not be the only source of N eutrophication in these systems. Nor will it necessarily provide the most N, making it difficult to predict likely effects of N deposition.  Atmospheric N deposition will, however, represent the only source of N deposition direct to the canopy, and especially to the epiphytic flora and terricolous mosses, lichens and liverworts. A study in central France showed that 55-65 % of N addition could be found in the Sphagnum layer (Francez & Loiseau 1999). Effects of atmospheric N deposition can take a considerable time to occur, ie. there can be a lag time of often 5 years or more (Wiedermann et al 2007).

Some species found in fens are also common to lowland raised bog and blanket mire where there are many more studies on N responses. Based on N addition experiments showing positive growth responses to N, these systems are considered to be N limited (see pp 74-79; Bobbink and Hettelingh, 2011). Concentrations of inorganic N, and especially P tend to be low in surface waters where bicarbonate ions dominate and low P availability is likely to restrict the potential for N eutrophication.

Rich fens despite containing rare and specialised species are understudied with few relevant N addition studies to inform on N impacts.

Pollutant type and risk

Form of N

Risk areas

NH3

Fens in rural areas with elevated background concentration.  Higher dry deposition is found close to point sources e.g. intensive livestock units.

NOx

Fens close to combustion plants, and major roads and urban areas.

Ammonium, (NH4+)

Nitrate, (NO3-)

in varying proportions

High, wet N deposition areas

Indicators of N enrichment

  • Increase in tall species e.g. Molinia

Base rich calcicolous fens:

  • Reduction in the thickness of the moss carpet
  • Reduction in cover of sensitive mosses
  • Increase in the invasive S. contortum cover.

Example evidence of species specific responses

Some examples of specific responses are given in the table below. This does not represent a comprehensive review of all species impacts.

Species

Response

Reference

S. magellanicum

-ve effects on nutrient status

Jauhaianen et al 1998

S. fimbriatum

-ve via shading, competition

Hogg et al 1995

S. palustre

-ve via shading, competition

Hogg et al 1995

S. contortum

None after 2 or 4y

Prefers NH4+ relatively N tolerant

Scorpidium revolvens

None after 2 y, after 4y decreased biomass with  NH4+

Prefers NO3-

Sensitive to NH4+

Eriophorum vaginatum

+ve increased cover

Wiedermann et al 2007

Molinia

+ve increased cover

Tomassen et al 2003, Hogg et al 1995.

Andromeda polifolia

+ve increased cover

Wiedermann et al 2007

Vaccinium oxycoccus

+ve increased cover

Wiedermann et al 2007

Critical Load/Level: 

Habitat/ Ecosystem Type Eunis Code Critical Load/ Level Status Reliability Indication of exceedance Reference
Valley mires, poor fens and transition mires D2

10-15 kg N ha-1 year-1

UNECE 2010 - Noordwijkerhout workshop quite reliable

Increase sedges and vascular plants, negative effects on bryophytes.

472
Rich fens D4.1

15-30 kg N ha-1 year-1

UNECE 2010 - Noordwijkerhout workshop expert judgement

Increase in tall graminoids, decrease in bryophytes.

472

References: 

Hogg, P.; Squires, P.; Fitter, A.H. 1995 Acidification, Nitrogen Deposition and Rapid Vegetational Change in a Small Valley Mire in Yorkshire. Biological Conservation 71
Jauhiainen, J.; Wallen, B.; Malmer, N. 1998 Potential NH4+ & NO3- uptake in seven Sphagnum species New Phytol 138 287-293
Tomassen, H.B.M. ; Smolders, A.J.P. ; Lamers, L.P.M. ; Roelofs, J.G.M. 2003 Stimulated growth of Betula pubescens and Molinia caerulea on ombrotrophic bogs: role of high levels of atmospheric nitrogen deposition Journal of Ecology 91 357-370
Wiedermann, M.M. ; Nordin, A.; Gunnarsson, U.; Nilsson, M.B. ; Ericson, L. 2007 Global change shifts vegetation and plant-parasite interactions in a boreal mire Ecology 88 454-464