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

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).