Acid deposition :: Improved Grassland

[For Acid Deposition processes see overview link]

Effects and implications

  • Increased removal of base cations with grassland fodder and fertiliser application leads to the excess of H+  ions in the soil and consequently lowering the pH value (Oenema, 1990; de Klein et al., 1997)
  • Lime (CaCO) is primarily applied to counteract the effects of acid deposition on agricultural grassland systems.
  • 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.
  • Increased risk of nutrient imbalance which will lead to stunted growth (P): phosphorus availability is likely to decline in soils with low pH and high Al concentrations through precipitation. Base cation availability and uptake will also be low in such mineral soils.

Overview: evidence, processes and main impacts

Grassland is improved to increase quantity and quality, i.e. nutritional status for grazers. The seed mixture may be chosen to reflect this. Less productive species and forbs would be excluded from the seed mix. The productive species may well be acid sensitive, though less likely to be affected by the nutrient inputs associated with acid deposition. In order to establish these more productive swards the soil is often limed. Where this is the case the soil will be relatively buffered against acid deposition effects: e.g. soil pH (falling) which in turn affects the solubility, mobility (increasing them) of toxic metal cations e.g. Al3+ and the availability and uptake of key nutrients, e.g. phosphorus and calcium. Acid deposition will make liming more necessary.

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

None expected unless near busy roads or combustion sources

Wet deposition

precipitation and occult

(cloud, mist)

H+, NO3- SO42-

Areas that are not limed

Indicators of acid deposition

  • Fall in soil pH
  • Increase in Al3+ concentrations once soil pH falls below ~ 4.4
  • Reduced P availability
  • Loss of clover

Examples of species specific responses

No examples available

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.

Critical Load/Level: 

Critical Load/ Level

No estimate available

References: 

de Klein, C.A.M ; Monaghan, R.M. ; Sinclair, A.G. 1997 Soil acidification: a provisional model for New Zealand pastoral systems. New Zealand Journal of Agricultural Research 40 541-557