Sulphur Dioxide :: Bogs

Impact Type: Direct exposure to pollutant

Key Concerns:

Peats are naturally highly acidic and act as sinks for S (Bareham, 1996). The impacts on these systems due to atmospheric S pollution are generally associated with long term deposition loads leading to acidification (see Bogs :: Acid deposition). However, exposure to high concentrations of SO2 can also lead to direct toxic effects (UKCLAG 1996). Key concerns are: Impacts on functioning of the peat ecosystem, e.g. decomposition, sulphate reduction, organic acid production (Wilson et al., 1995). 

Changes in vegetation composition - in the historically highly polluted southern Pennines in northern England, where S was mostly dry deposited, peat profiles show evidence of significant changes over the last 200 years (Ferguson et al., 1978; Ferguson and Lee, 1983). Bryophytes have declined over this period, as has species diversity of higher plant communities, while Eriophorum species have increased. Such impacts may also have occurred on raised bogs in the central belt of Scotland, which in the past was subject to high SO2 concentrations. In Wales, Bragg & Tallis (2001) suggest that air pollution is a possible cause for the break up of bog vegetation.

Risk Areas

The concentrations of SO2 in the UK are well below the critical level with the largest concetrations in the urban and suburban areas of the country, with values of a few ppb, and in the rural Midlands and home counties, with concentrations of 1 to 2 ppb (2.7 - 5.4 ug  m-3). Elsewhere, in rural areas and especially in the west and north of the UK, SO2 concentrations are low at below 0.5 ppb (<1.3 ug SO2 m-3). Emissions from UK shipping are highest close to major ports where elevated concentrations may occur locally (ROTAP, 2012).

Additional Comments:

There is a major shortage of suitable experimental and observational data to define accurate critical levels for semi-natural habitats (which are less certain than for crops), but few areas of the UK are expected to be at risk.

Pollutant effects on bog vegetation and the underlying peat are closely related and should always be considered together. The impacts of SO2 need to be considered in relation to other pollutant threats and management impacts such as extraction, drainage, afforestation and restoration.

Critical Load/Level: 
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).

Cyanobacterial lichens

10 µg SO2 m-3 annual mean

UNECE, 2004 quite reliable i.e. the results of some studies are comparable

SO2 dissolves in water to produce acidic ions which are readily absorbed through the lichen thalli disrupting photosynthesis. SO2 has also been shown to inhibit the activity of nitrogenase, which is used by cyanobacterial photobionts to fix atmospheric nitrogen.

Reference: Gries, C. (2008). Lichen sensitivity to air pollution, Chapter 13 in Nash, TH, III (ed.) Lichen Biology (2nd. ed.), Cambridge University Press, Cambridge.

Ashmore, M.R.; Wilson, [Eds.] R.B. 1994 Critical levels of air pollutants for Europe Report of the Egham workshop
Bareham, S.A. 1996 Acid deposition and soils: a perspective for nature conservation Soils, Sustainability and the Natural Heritage 105-120
Bragg, O.M.; Tallis, J.H. 2001 The sensitivity of peat-covered upland landscapes Catena 42 345-360
Ferguson, N.P.; Lee, J.A. 1983 The growth of Sphagnum species in the southern Pennines. Journal Bryology 12 579-586
Ferguson, N.P.; Lee, J.A.; Bell, J.N.B. 1978 Effects of sulphur pollutants on the growth of Sphagnum species. Environmental Pollution 16 151-162
Gries, C. 2008 Lichen sensitivity to air pollution Lichen Biology
Wilson, E.J.; Skeffington, R.A.; Maltby, E.; Immirzi, P.; Swanston, C.; Proctor, M. 1995 Towards a new method of setting a critical load of acidity for ombrotrophic peat Water, Air and Soil Pollution 85 2491-2496

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