Fen Management Handbook - Scottish Natural Heritage

8.2.2 Livestock management
Phosphorus is a major component in poultry, pig and milking cattle feed, but is
frequently above livestock’s requirement. When excreted and eventually spread onto
fields, there is a risk of excessive phosphorus enrichment. N and P excretion from
pig units can be reduced significantly by changes in feedstuffs, which can also prove
cost effective for farmers. A synthetic form of P (phytase) can be added to the diet of
poultry and pigs, which decreases the P in manure.
Provision of cattle bridges and fencing to reduce poaching can also be significant in
reducing nutrient enrichment (see 6.2.9).
8.2.3 Other land management
A range of other techniques can be used to reduce the release of nutrients and other
pollutants within the catchment, for example:
– Installing phosphate-stripping plants on sewage treatment works that discharge
into a river that forms part of a fen’s water supply.
– Installing or repairing systems which separate clean (mainly rainfall) and dirty water
in farm-yards. Replacement of septic tanks with mains sewerage treatment See
Defra, UK - Environmental Protection - Water - Diffuse pollution of water
Further specialist advice is recommended to assess appropriateness of these options
to specific locations and design detail, particularly for options likely to incur high
capital costs (see Appendix V for further advice).
8.3 Managing nutrient pathways
8.3.1 Reedbed filtration
Passing the water which feeds a fen through a reedbed can help trap the majority of
the suspended solids and some of the dissolved nutrients before it enters the fen. The
reedbed might be a separate area to the fen, specially constructed, or a part of the fen
set aside for this purpose. The size of the treatment reedbed needs to be matched to
the nutrient loading (i.e. concentration of nutrients multiplied by estimated flow) of the
incoming water. Not surprisingly, larger wetlands are required for heavier loadings, but
small in-line wetlands can be effective where a fen is fed by seepage, small ditches or
streams.
The principles of constructed reedbed treatment systems are well known: water
draining vertically down through the rhizomes is used initially to convert any ammonium
present to nitrate. Another design, in which water passes laterally through the
rhizomes, denitrifies the nitrate to gaseous forms of nitrogen. There will be some
uptake of N by the reed, but this is eventually recycled through decomposition of
the leaf litter. A high rate of P removal can be achieved by using a highly alkaline or
calcareous substrate, such as limestone or crushed brick rubble, so that the phosphate
is adsorbed/precipitated to calcium carbonate and other calcareous minerals such as
apatite. Constraints on the design of any constructed reedbed may arise from available
space, topography and variability in the flows of water in the inflow stream. Natural
reedbeds are used for water treatment under certain circumstances, though there
may be negative impacts on their conservation value. Hawke and José (1996) and the
Constructed Wetlands Association can provide further details.
Drains at the edge of a fen can be used to help intercept nutrients – particularly if
vegetated with species which are responsive to nutrients. In addition routing drains
into constructed wetlands will improve the effectiveness of nutrient removal.
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