Acid deposition :: Acid grassland

[For Acid Deposition processes see overview link]

Effects and implications

• Root damage especially from Al³⁺ toxicity resembles short stubby, sometimes blacked tips. This is generally found where acidification of the soil has released soluble Al³⁺ into the soil solution. This can lead to changes in species composition.
• Increased risk of nutrient imbalance which will lead to stunted growth : phosphorus (P) availability is likely to decline in soils with low pH and high Al concentrations in rainfall. Base cation availability and uptake will also be low in such mineral soils.
• Enhanced leaching, loss of base cations and buffering capacity.

Overview

Acid grassland is characterised by a range of plant species such as heath bedstraw Galium saxatile, sheep`s fescue Festuca ovina, common bent Agrostis capillaris, sheep`s sorrel Rumex acetosella, sand sedge Carex arenaria, wavy hair-grass Deschampsia flexuosa, bristle bent Agrostis curtisii and tormentil Potentilla erecta, with presence and abundance depending on community type and locality.  Acid grassland types found on coastal dune and shingle habitats are included in the Dunes, shingle and machair record.

Acid grassland is often associated with lowland heath, parklands or coastal cliffs. Soils are nutrient-poor, free-draining, pH 4 to 5.5 and overlie acid rocks (sandstone and granites) or deposits such as sands, gravels and acid clays. Lowland acid grassland occurs below 300 metres and is normally managed for pasture.

Acid grasslands can have a high cover of bryophytes and parched acid grassland can be rich in lichens.  

Stevens et al (2010) showed a clear relationship with soil pH and N deposition along the gradient used in the study. This was reflected in the community composition, which showed an increase in the proportion of species with a preference for acid conditions. Looking at total acid deposition instead of N deposition against index of ‘soil acidity preference scores’ showed that acidification from sulphur deposition and its historic impact may still be apparent.  The study concluded that such impacts of increasing acidity are limiting the pool of species able to survive with soil acidification being the dominant process responsible for a decline in species richness with increasing N deposition. However the study also reported that it is likely that there is an element of eutrophication and other soil chemical changes involved too.

Similarly given strong evidence for mobilization of Al and heavy metals it is likely that the concentration of metal ions could play an important role in reducing species richness. Roem, Klees & Berendse (2002) also conclude that aluminium toxicity is the main driver of species reductions in heathland and acid grasslands in the Netherlands.

Nitrogen deposition treatments at Warldow Hay Cop study site (Derbyshire) (Horswill et al., 2008) caused the grassland soils to lose 23–35% of their total available bases (Ca, Mg, K, and Na) resulting in acidification. Aluminium, iron and manganese were mobilised and mineral nitrogen availability increased in the acid grasslands and many species showed foliar N enrichment. The resulting acidification, metal mobilisation and eutrophication are implicated in driving floristic changes. (see also Nitrogen deposition :: Acid grassland).

In the uplands, acid grassland usually exists as a consequence of overgrazing of heath.  Threats to lowland acid grasslandinclude nutrient-enrichment through fertilizer application (and nitrogen deposition), use of herbicides and pesticides, liming, reseeding or ploughing for arable crops. Poor management including overgrazing and neglect can lead to rank vegetation and scrub encroachment. Activities like afforestation, mineral extraction, landfill and development have led to large losses andrecreational use can damage the ecosystem via compaction and soil erosion.

Pollutant deposition type and risk

Indicators of Acid deposition

• Fall in soil pH
• Reduced P availability
• Lower base cation concentrations
• Change in grass species composition, loss of fine leaved species
• Reduction in forb species

Examples of species specific responses

None found

What factors modify acid deposition impacts?

High rainfall, which promotes leaching, can exacerbate effects associated with base cation loss.

Evidence of recovery

Increases in soil pH have been recorded over the UK for this habitat. Evidence of the possible rate of recovery from any future reduction in N deposition has been seen at experimental sites at Warldow and Thursley Common (Surrey) where soil pH has shown signs of recovery (UKREATE, 2010). There is little or only weak evidence of floristic recovery in these systems, though lower plants have shown the clearest signs of recovery at these two acid grassland sites (Arróniz-Crespo et al., 2008)