[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 |
|---|
|
No estimate available |