Fen Management Handbook - Scottish Natural Heritage

Many of the floristic changes which are most likely to be associated
with enrichment can also result from other factors, particularly grazing
at lower stocking levels or for shorter periods than would be required
to maintain the vegetation without significant change in structure or
species composition. Disentangling such effects can be extremely
difficult but important clues can be provided by observing where change
is occurring, particularly with respect to locations adjacent to key water
inputs or boundaries with intensively managed farmland.
4.6.3
What can the soil, water and catchment land use tell you about nutrient enrichment?
Detailed soil and water chemical testing can be very useful for more in-depth
monitoring of a fen’s nutrient status; use of phytometric techniques (where the
fertility of the soil is bio-assayed by measuring the growth of a test species,
generally either great hairy willowherb (Epilobium hirsutum) or reed canary-grass
(Phalaris arundinacea) should be considered in preference to soil testing in wet fen
soils – see Wheeler et al, (1992). However, very basic but nonetheless valuable
assessments can be made from observation which can provide a basis from which
to develop more detailed monitoring programmes (see Section 10: Monitoring to
Inform Fen Management).
Peat soils that dry out on a regular basis undergo a greater degree of mineralisation,
which releases stored nutrients into the fen system. Even if the drying affects
only part of a site, the signs of enrichment may be seen across the entire site as
nutrients are ‘flushed’ across the fen as it rewets. A constant wetting and drying
cycle is considered highly detrimental as it continually releases and flushes nutrients
into the fen. In addition, peat that has a ‘gritty’ feel has mineral soil sediments
washed onto it and these sediments may hold phosphorus stores that could be
released into the system. Soil erosion may also be noticeable in the catchment and
is a likely sediment and nutrient source.
The water source is likely to be the most important factor to
consider in an initial assessment of nutrient enrichment on a site.
– Groundwater, sub-surface and surface flow from an intensively farmed
catchment is more likely to be enriched with inorganic nitrogen.
– Fens that fall within Nitrate Vulnerable Zones are at risk of enrichment
from high nitrate concentrations.
– Fens in the vicinity of aerial pollution sources such as major roads,
airports and intensive animal and poultry rearing units, or fed by water
from enriched rivers/lakes/canals, are also at risk of enrichment.
– Look out for obvious sources of nutrient inputs such as field drains
entering a fen or regular fertiliser application or waste spreading on
adjacent fields, particularly where this does not comply with good
practice guidelines or statutory requirements.
Simple pH and EC monitoring of water (including inputs such as streams, water
within the fen and water outputs) with relatively inexpensive field meters can provide
some insight into the fen’s chemistry and nutrient status. Redox can provide an
indication of whether soils are chemically reducing, and thus broadly suitable for
a wide range of obligate wetland plants.
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