Nitrogen deposition :: Acid grassland

Effects and implications

  • Nitrogen deposition provides a fertilization effect on acid grasslands which are generally N limited.
  • N deposition favours graminoids (grasses) at the expense of forbs and lower plants, especially where sites are surrounded by farmland.
  • Nitrophilous grasses tend to shade out slower growing species.
  • Nitrophilous grasses increase the amount of litter which falls on and shades out under-storey bryophytes (Berendse et al 1987).
  • Lower plants especially mosses at risk both from N accumulation (Carroll et al 2003).

Overview: evidence, processes and main impacts

Some components of acid grassland are able to flourish and increase their share of the canopy space under enhanced N deposition, leading to loss of species that are not able to increase their rates of C assimilation, through shading. UK surveys  (Maskell et al 2010; Stevens et al 2004, 2010) show clear, nitrogen  driven declines in species richness, predominantly amongst the forbs e.g. Cerastium arvense, Viola canina  and the N fixing forb, the annual Vicia lathyroides (Spring vetch) (Dupré et al 2010; Maskell et al 2010; Stevens et al 2006). Functional diversity is lost as grasslands became more grass dominated, showing increasing canopy height, leaf area index and productivity (Stevens et al 2011).  The loss of forbs will have implications for pollinators and the aesthetic qualities of the ecosystem. Vulnerability to N deposition is enhanced where grasslands are surrounded by seed sources for nitrophilic species: agricultural areas provide pools of propagules of N responsive species particularly graminoids.

Pollutant deposition type and risk areas

Type of N deposition

Form of N

Risk areas

Dry deposition

Gaseous

NH3

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

 

NOx

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

Wet deposition

precipitation and occult

(cloud, mist)

Ammonium, (NH4+)

Nitrate, (NO3-)

in varying proportions

Higher altitude grasslands will be affected by orographic enhancement (larger volumes but lower concentrations) and occult deposition (higher concentrations).

Indicators of N enrichment

  • Reduced forb cover (Stevens et al 2006) and increase in grass to forb ratio (Stevens et al 2009).
  • Absence of nitrophobic species that characterise the habitat.
  • greater presence of nitrophilous species and higher Ellenberg N.
  • Increase in canopy height
  • Reduced bryophyte cover (Carroll et al 2000).
  • Soil acidification
  • Increased rates of N mineralization, increasing NH4+ concentrations in soil solution and potential for toxicity.
  • Increased root surface phosphatase enzyme activity can be measured in response to nitrogen (Johnson et al 1999).

Examples of species specific responses

Species/group

Response

Reference

Potentilla erecta, Festuca ovina, Agrostis spp

decline

Carroll et al 2003

Nardus stricta

increase

Carroll et al 2003

Cerastium arvense, C. Semidecandrum, Trifolium arvense,

Vicia lathyroides, Viola canina

Peltigera didactyla, Cetraria aculeata Scapania gracilis, Racomitrium lanuginosum

decline

Stevens et al 2011, Emmett et al 2011, Table 2.2, p7

 

 

What factors modify N deposition impacts?

  • Grazing management: reduced grazing intensity will exacerbate effects of N stimulated nitrophilous grass growth.
  • High grazing intensity can exacerbate detrimental effects of N on ericoid components (Emmett et al 2001).
  • Availability of other nutrients through their effects on growth responses.

Critical Load/Level: 

Habitat/ Ecosystem Type Eunis Code Critical Load/ Level Status Reliability Indication of exceedance Reference
Alpine and subalpine grasslands E4.3; E4.4

5-10 Kg N ha-1 year-1

UNECE 2010 - Noordwijkerhout workshop expert judgement

Changes in species composition; increase in plant production.

472
Moist and wet oligotrophic grasslands: Heath (Juncus) meadows and humid (Nardus stricta) swards E3.52

10-20 kg N ha-1 year-1

UNECE 2010 - Noordwijkerhout workshop quite reliable

Increase in tall graminoids, decreased diversity, decrease of bryophytes.

472
Moist and wet oligotrophic grasslands: Molinia caerulea meadows E3.51

15-25 kg N ha-1 year-1

UNECE 2010 - Noordwijkerhout workshop expert judgement

Increase in tall graminoids, decreased diversity, decrease of bryophytes.

472
Non-mediterranean dry acid and neutral closed grassland E1.7

10-15 kg N ha-1 year-1

UNECE 2010 - Noordwijkerhout workshop reliable

Increase in graminoids, decline of typical species, decrease in total species richness.

472

References: 

Carroll, J.A.; Johnson, D.; Morecroft, M.D.; Taylor, A.G.; Caporn, S.J.M.; Lee, J.A. 2000 The effect of long-term nitrogen additions on the bryophyte cover of upland acidic grassland Journal Bryology 22 83-89
Dupre, C.; Stevens, C.J.; Ranke, T.; Bleeker, A.; Peppler-Lisbach, C. ; Gowing, D.J.G. ; Dise, N.B.; Dorland, E.; Bobbink, R.; Diekmann, M. 2010 Changes in species richness and composition in European acidic grasslands over the past 70 years: the contribution of cumulative atmospheric nitrogen deposition Global Change Biology 16 344-357
Emmett, B.A.; Rowe, E.C. ; Stevens, C.J.; Gowing, D.J.; Henrys, P.A. ; Maskell, L.C. ; Smart, S.M. 2011 Interpretation of evidence of nitrogen impacts on vegetation in relation to UK biodiversity objectives
Maskell, L.C. ; Smart, S.M.; Bullock, J.M. ; Thompson, K. ; Stevens, C.J. 2010 Nitrogen deposition causes widespread species loss in British Habitats Global Change Biology 16 671-679
Stevens, C.J.; Dise, N.B.; Gowing, D.J.; Mountford, J.O. 2006 Loss of forb diversity in relation to nitrogen deposition in the UK: regional trends and potential controls Global Change Biology 12 1823-1833
Stevens, C.J.; Smart, S.M.; Henrys, P. ; Maskell, L.C. ; Walker, K.J. ; Preston, C.D.; Crowe, A. ; Rowe, E. ; Gowing, D.J.; Emmett, B.A. 2011 Collation of evidence of nitrogen impacts on vegetation in relation to UK biodiversity objectives
Stevens, C.J.; Maskell, L.C. ; Smart, S.M.; Caporn, S.J.M. ; Dise, N.B.; Gowing, D.J. 2009 Identifying indicators of atmospheric nitrogen deposition impacts in acid grasslands Biological Conservation 142 2069-2075
Stevens, C.J.; Dise, N.B.; Mountford, J.O.; Gowing, D.J. 2004 Impact of nitrogen deposition on the species richness of grasslands Science 303 1876-1879