Acid deposition :: Calcareous grassland

[For Acid Deposition processes see overview link]

Effects and implications

  • Acidifying deposition probably represents a small threat to these grasslands, due to their inherent neutralising capacity.
  • The critical loads for calcareous grasslands are therefore large and generally not exceeded,  given the success in reducing S emissions.
  • Effects of acidification associated with nitrogen (N) will be associated with the amount of ammonium that is nitrified.
  • Where acidification is severe, detrimental effects on vegetation cover should be expected. There may be an increase in graminoids at the expense of forbs. .

Overview: Effects and implications

These low productivity grasslands occur on shallow, well drained, well buffered soils, above pH 6, (with a calcium carbonate content of ~10%, formed by weathering of chalk, base rich rock).

There are few studies of acid deposition effects on calcareous grassland as they should be well buffered. However, a study in the UK (Morecroft et al., 1994; Johnson et al., 1999) to look at N impacts used ammonium sulphate, albeit at large inputs 140 kg N ha-1 yr-1. This study showed that these ions do acidify. Over five years the pH fell from 6.8 to 5.2 (associated with nitrification) and vegetation cover likewise fell by 30%, although after three years no difference in species composition was found. Wilson et al. (1995) exposed artificially created species mixtures to acidified N spray treatments (NH4(SO4)2 +H2SO4 and NaNO3+HNO3) but only reported the results in terms of the N effect, rather than acidity.

The nutrient N enrichment associated with acid N deposition will favour those species that can tolerate and overcome low P availability under conditions of improved N supply and effectively scavenge and monopolise the additional N through storage, e.g. Brachypodium. Brachypodium is much less common in the UK compared to the Netherlands, where it flourishes on these N enriched calcareous sites (Wilson et al., 1995). Increased growth may cause biological drying of these thin shallow rooting soils as the potential for evapo-transpiration increases. Therefore, species of Carex (sedges) and Juncus (rushes) will decline as grasses, e.g. Lolium perenne and Holculs lanatus increase (Jeffrey and Piggott, 1973). Bryophytes (mosses) especially form an important component of these ecosystems and, like the vascular plants, are sensitive to desiccation stress, showing large inter-annual fluctuations in cover in response to rainfall. Acid deposition is likely to exacerbate desiccation stress especially in summer.

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 close to sources like roads and power stations

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
  • Change in grass species composition, loss of fine leaved species
  • Reduction in forb species

Examples of species specific responses

Species

Response

Reference

Thymus polytrichus 

-ve

Carrol et et al. 2003

Carex flacca 

-ve

Carrol et et al. 2003

Critical Load/Level: 

Critical Load/ Level

No estimate available

References: