Fen Management Handbook - Scottish Natural Heritage

Buffer zones can remove over 90% of the N and over 35% of the P from water
entering an alluvial floodplain from agricultural land, while losses of N and P from
surface water using a riparian forest buffer were measured as 83% and 81%,
respectively (Peterjohn & Correll, 1984). Similar high rates of N and P removal
have been documented in buffer zones comprising alder wood, poplar wood and
grassland vegetation types.
The vegetation type established in the buffer zone does not appear to be critical as
long as it is not subject to intensive agricultural management. Trees, shrubs and
permanent grassland are all equally suitable. The physical properties of the buffer
zone are important, including the structure of the vegetation, size, flow rate of water
and gradient. The aim is to reduce the rate of flow of surface water through the
buffer zone to enhance potential for nutrient removal before the water reaches the
fen. Diverting small streams and runnels across the buffer zone will help increase
residency time, as will removal/disruption of any agricultural land drains. Because
of the different processes associated with nitrogen and phosphorus uptake, dry
buffer zones are better for P removal while wet buffer zones with a high carbon
content (such as wet woodland, where leaf fall provides the carbon input) are better
for N removal.
The width of the buffer zone is important but optimum width depends on the
size of the catchment, the slope adjacent to the fen, soil type and the degree
of enrichment. The first 5 to 10 m of the buffer zone is often reported as being
the most important for nutrient removal, so the width need not necessarily be
great in order to provide benefits. A summary of the complex chemical and
biological processes that occur in buffer zones, depending on factors such as
carbon/nitrogen ratios, temperature and oxygen availability, together with further
information on practical use of buffer zones, is provided in Hawley et al (2004).
Infilling drainage features leading into a fen from adjacent improved grassland offers
another means of slowing down the passage of water into fens and thus increases
the chances of in-situ nutrient removal by marginal vegetation at the edge of the
fen. In addition, breaking field drains and creating Horseshoe wetlands is an option.
The impacts of any proposals on drainage or other aspects of neighbouring land
should be carefully assessed, and neighbours consulted as required.
Buffer zones can be supplemented with low earth banks (0.2-0.5 m high by 1-2 m
wide), sometimes supporting planted hedgerows, to provide an additional physical
impediment to surface water flow and thus increased residency time for inflowing
water. The use of biomass crops (including elephant grass and willow) as a means
of removing nutrients from wetland catchments is in its infancy, but deserves
consideration and further research.
8.3.3 Sediment removal from feeder ditches
Nutrient enriched sediments which have accumulated in water supply or feeder
ditches which may be negatively influencing fen habitat can be removed by suction
dredger, a technique known as ‘slubbing’. The sediment is then disposed of away
from the fen, usually in small bunds that re-vegetate naturally. This technique has
been used to some effect in the Broads, but slubbing is quite invasive. Work
should therefore be staggered, cleaning no more than 25% of the fen/ditch in
any one year over four years, repeating as necessary on a five to 25 year rotation,
depending on sediment accumulation rates. The expense of using the technique,
along with the potential for damage to ground and vegetation, makes it appropriate
for highly enriched sites only. Further information is provided in the Reedbed
Management Handbook (Hawke & José, 1996).
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