Fen Management Handbook - Scottish Natural Heritage

4.2.2 Atmospheric inputs
Atmospheric inputs of nutrients can also be highly significant to fen ecosystems,
particularly for very nutrient-poor systems, such as base-poor fens. The burning
of fossil fuels (electricity generation and transport) is the main contributor to
atmospheric nitrogen enrichment of fens, although intensive farming practices like
poultry and pig farms can also emit aerial pollution.
Atmospheric emissions of nitrogen compounds have increased approximately
five-fold over the last 50 years. Gases can enter fens either as dry deposition
of nitrogen dioxide (NO 2 ) and ammonia (NH 3 ), usually close to the source of
emission, or dissolved in rain or fog as ammonium (NH 4+ ) and nitrate (NO 3- ) ions.
The deposition of these compounds is also associated with acidification because
other components of the nitrogenous bearing gases result in sulphuric acid.
This acidification is a particular problem on soils that do not contain calcium or
magnesium carbonate, which would otherwise neutralise the acids.
The term ‘critical load’ is used to identify the maximum deposition rate of nitrogen
(or other air-borne pollutants) to a habitat, above which adverse effects are likely to
occur. It is expressed as kilograms of nitrogen per hectare per year (kg N ha -1 yr -1 ).
Critical loads for fen ecosystems are shown below.
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Critical loads for poor and rich fens, as defined by Bobbink et al. (2002)
Ecosystem Type Critical Load
kg N ha -1 yr -1
Signs that critical load has been
exceeded i.e. observable impact in
the fen
Base-poor fens 5 – 10 Increased sedges and other vascular
plants. Negative effects on peat
mosses
Base-rich fens 15 – 35 Increased tall graminoids (grasses,
sedges)
Decreased species diversity
Fens demonstrate increased sensitivity to atmospheric nitrogen loading where:
– groundwater nutrient inputs are low;
– the system is N rather than P limited;
– there is no removal of N and P in biomass via management; and,
– the vegetation is oligotrophic.
Estimates of atmospheric nitrogen inputs (and other air-borne pollutants) to
individual sites in the UK can be obtained from the Air Pollution Information System
at www.apis.ac.uk, which provides modelled loads on a 5 km square basis in
response to user-supplied NGR’s. Where inputs approach or exceed the critical
loads given, then atmospheric inputs of nitrogen represent a significant risk of
enrichment to that habitat.