Acid deposition :: Neutral Grassland

[For Acid Deposition processes see overview link]

Effects and implications

  • Acidifying deposition represents a moderate threat to these grasslands as it exhausts their acid neutralising capacity.This leads to a favouring of acid tolerant/resistant species, especially grasses and ones that tolerate higher NH4+ to NO3- ratios (Bobbink and Roelofs, 1995; De Graaf et al., 1998) at the expense of low growing forbs.
  • Disappearance of endangered acid sensitive species when pH falls outside the pH range 4.5 to 6.5.
  • Effects of acidification associated with nitrogen (N) will be associated with the amount of ammonium that is nitrified. The mechanisms include a threshold pH
  • effect beyond which aluminium is mobilised plus suppression of nitrifying bacteria leading to a build up of ammonium which further acidifies the soil (Stevens et al 2009)

Overview: evidence, processes and main impacts

Neutral grasslands are semi-natural swards dominated by grasses with associated dicotyledonous herbs without the calcifuge / calcicole element on lowland clays / loams. They aremesotrophic,  with a pH of around 5.5-7.These ecosystems are generally poor in nutrients because of long agricultural use with low levels of manure addition and removal of plant parts by grazing or hay-making.

Evidence from the UK Countryside Survey (Haines-Young et al., 2000) shows a decrease in the floristic diversity of hay meadows correlated with anthropogenic N deposition (see Nitrogen deposition :: Neutral Grassland). This may be due to acidification and eutrophication associated with N deposition. Acidification can damage through the potential to increase concentrations of potentially toxic NH4+ ions and increase the solubility of toxic cations, e.g. aluminium. Acidification may also lead to lower P availability. These systems are considered to be moderately sensitive to acidification (Bobbink, 1998).

Pollutant deposition type and risk

Type of acid deposition

Pollutant

Risk areas

Dry deposition

Gaseous

SO2

Significant reductions in sulphur emissions have successfully addressed by International control measures. Areas where exceedances could still occur are around industrial zones and port areas (due to shipping emissions).

 Dry deposition

Gaseous

NOx

Grasslands near urban conurbations / roads

Wet deposition

precipitation and occult

(cloud, mist)

H+, NO3- SO42-

Upland sites are more at risk than lowland sites due to occult deposition.

Indicators of Acid deposition

  • Fall in soil pH
  • Lower base cation concentrations
  • Increase in soil water NH4+ concentrations
  • Change in grass species composition,  and a reduction in forb species

Examples of species specific responses

None available

What factors modify acid deposition impacts?

None known

Evidence of recovery

  • Increases in soil pH have been recorded over the UK through the period of the 1970s to the current decade over a range of soil types and habitats (RoTAP, 2012)
  • Direct over-sowing of improved swards with seed mixtures offers a simple and cost-effective method for diversifying grassland provided that “gaps” for establishment are created by grazing or mechanical disturbance (Walker et al., 2004).

Critical Load/Level: 

Critical Load/ Level

No estimate available

References: 

Bobbink, R.; Roelofs, J.G.M. 1995 Nitrogen critical loads for natural and semi-natural ecosystems: the empirical approach Water, Air and Soil Pollution 85 2413-2418
Bobbink, R.; Hornung, M.; Roelofs, J.G.M. 1998 The effects of air-borne nitrogen pollutants on species diversity in natural and semi-natural European vegetation Journal of Ecology 86 717-738
De Graaf, M.C.C.; Bobbink, R.; Roelofs, J.G.M.; Verbank, P.J.M. 1998 Differential effects of ammonium nitrate on three heathland species Plant Ecology 135 185-196
Haines-Young, R.H.; Barr, C.J.; Black, H.I.G.; Briggs, D.J.; Bunce, R.G.H.; Clarke, R.T.; Cooper, A.; Dawson, F.H.; Firbanks, L.G.; Fuller, R.M.; Furse, M.T.; Gillespie, M.K.; Hill, R.; Hornung, M.; Howard, D.C.; McCann, T.; Morecroft, M.D.; Petit, S.; Sier, A.R.J.; Smart, S.M.; Stott, A.P.; Watkins, J.W. 2000 Accounting for nature: assessing habitats in the UK countryside Accounting for nature: assessing habitats in the UK countryside
Stevens, C.J.; Dise, N.B.; Gowing, D.J. 2009 Regional trends in soil acidification and metal mobilization related to acid deposition. Environmental Pollution 157 313-319
Walker, K.J. ; Stevens, P.A.; Stevens, D.A. ; Mountford, J.O.; Manchester, S.J. ; Pywell, R.F. 2004 The restoration and re-creation of species-rich lowland grassland on land formerly managed for intensive agriculture in the UK. Biological Conservation 119 1-18