Fen Management Handbook - Scottish Natural Heritage

– Catchment land use – modification of the catchment (e.g. through agricultural
intensification) can change nutrient concentrations in soils and in sub-surface
and surface water entering a fen. Atmospheric inputs can be altered by changes
in land use that occur in nearby catchments or in the wider area, as airborne
pollutants like ammonia are able to travel long distances before being deposited.
4.3.1 Limiting nutrients
Plant growth depends on availability of the required nutrients; low concentrations or
limited availability of essential nutrients restrict plant growth. The utilisation of some
nutrients may be limited by the availability of others. For example, low nitrogen
concentrations would restrict utilisation of a relative surplus of phosphate, in which
case nitrogen would be the limiting nutrient, but an increase in nitrogen without a
change to the amount of phosphate could quickly result in enrichment.
Shortage of the nutrient that is ‘limiting’ is often the key to maintaining species-rich
fen vegetation, as the growth of the more nutrient-responsive and often aggressive
plants is kept in check. Any extra input of the limiting nutrient can release this
‘brake’ on plant growth and enable plants to exploit more of the available nutrients,
resulting in increased growth of some plants and the loss of others less able to
respond to the new source of nutrient. Fens can therefore be very vulnerable to
even relatively small increases in concentration of the limiting nutrient.
66
Studies carried out on poor fens indicate these habitats are strongly
nitrogen-limited and therefore at risk of enrichment from even marginal
increases in nitrogen. Experiments in northern Sweden have shown
considerable accumulation of nitrogen being associated with increasing
sedge cover. In a study comparing rich-fen nutrient cycling in areas
with very low and high nitrogen deposition, nitrogen mineralisation was
shown to be much higher in fens receiving high nitrogen inputs, despite
the fact that these fens were managed by mowing (Nohara et al., 2002).
Phosphorus limitation is particularly typical of late succession habitats. In addition,
where nitrogen inputs are high, and especially where there is regular biomass
removal, phosphorus will usually become the limiting nutrient over time.
In a nutrient limited system, excess of the non-limiting nutrient may not result in any
signs of enrichment in the vegetation, as the plants are unable to make use of one
nutrient without sufficient amounts of the other. This does not mean the site is not
enriched, but that enrichment is not manifested in changes in the vegetation. It is
likely to be detectable in soil/peat and water samples.
Localised processes can have a very important bearing on the nature and
significance of nutrient limitation, and the role of calcium and iron in ‘locking-up’ P in
certain situations has already been mentioned.
Despite the concept and nature of nutrient limitation, most researchers and
practitioners agree that high nitrogen inputs to a phosphorus limited system would
still be highly undesirable, not least because nutrient limitation varies between
species, and only very small increases in the availability of a key limiting nutrient
may be needed to result in a significant effect. Nitrogen enrichment even where
not accompanied by phosphorus could also be a contributory factor in the loss of
dissolved organic carbon (DOC) loss from fens.