Fen Management Handbook - Scottish Natural Heritage

4.2.1 Groundwater and surface water
Ground and surface water are generally the most important carriers of nutrients
to and from fens. Surface water can contain high levels of nitrate in solution, and
phosphates, usually attached to silt particles. During high rainfall or floods, large
quantities of soil particles containing adsorbed nutrients enter the fen, resulting in
nutrient enrichment.
Land management practices on adjacent land and in the wider catchment can have
a major impact on the nutrient status of a fen. Farming in the UK often imports
more nutrients into the farming system as fertilisers and animal foodstuffs than
are exported in the form of agricultural produce. Nitrogen not utilised for plant/
crop growth remains in the soil and can leach out, resulting in an autumn or winter
‘flush’ of N into fens. Catchments affected by soil erosion can release substantial
amounts of sediment-bound phosphorus that can enter a fen.
Depending on the nature of the soil and rock, the process of groundwater
transmission can act as a filter mechanism to reduce nitrogen and phosphorus
availability. Chalk and clay particles, for example, can bind ions such as nitrates or
phosphates and therefore have a natural buffering effect which can protect wetland
systems, whereas other rock types, such as sandstone, are much less effective in
this respect.
Physical entrapment and retention of some nutrients (notably soil-bound P)
from surface water flow is possible where the water flows through vegetated
buffer strips, or where the P-laden sediment accumulates in lakes. Waterlogged
conditions in fen soils can reduce nitrogen availability through denitrification.
Cultivation and resultant
exposure of soil in the
immediate catchment of
a poor fen basin mire on
Anglesey, North West
Wales. This practice
can lead to sediment
inwash and subsequent
enrichment (P.Jones).
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