Impact Type: Direct exposure to pollutant
While very large concentrations of SO2 may negatively effect grass growth, studies have shown that grasses may evolve a tolerance to high SO2 levels (Ayazloo and Bell 1981, UKCLAG 1996). The latter would imply that differential effects could be expected on permanent improved grassland than on resown swards and temporary grassland.
SO2 emissions have been reduced dramatically over the last decade so that currently no direct negative impacts of SO2 would be expected on improved grassland in the UK (RoTAP, 2012). By contrast, in this agricultural situation, where crop removal depletes soil sulphur levels grasslands and croplands may be short of sulphur. In this case S may need to be added as part of the fertiliser supply.
The present shortage of 'sulphur fertilisation' of agricultural land has sometimes led to the suggestion that the presence of S in the atmosphere was a good thing. In the past S was free, now it must be added as part of the mineral fertiliser supply. By contrast, it can also be argued that the present situation of having to add mineral fertiliser, focuses the S where it is needed, and avoids unwanted exposure and damage to semi-natural vegetation and rivers.
|Habitat/ Ecosystem Type||Critical Load/ Level||Status||Reliability||Indication of exceedance||Reference|
|Forests and semi-natural vegetation||
20 µg SO2 m-3 annual mean and half-year(Oct-March) mean
|UNECE, 2004||quite reliable i.e. the results of some studies are comparable||
Low temperature appears to enhance the negative effects of SO2, and the lower critical level of 15 µg SO2 m-3 is used where the effective temperature sum (ETS) (i.e. the sum of temperatures) above 5oC is below 1000 degree days (d.d) (Ashmore et al., 1994).