Fen Management Handbook - Scottish Natural Heritage

4.1 Key nutrients
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Nitrogen (N), phosphorus (P) and potassium (K), collectively referred to
as ‘plant macronutrients’, are the most significant agents of enrichment
as they are the major plant nutrients that typically limit plant growth
in a fen. Other chemical elements are also important, most notably
oxygen (O), carbon (C), calcium (Ca) and a range of elements including
those collectively termed micronutrients (e.g. magnesium, copper, iron,
selenium).
4.1.1 Nitrogen
Nitrogen (N) is an important nutrient which can limit plant growth in many
ecosystems. In peatlands, the majority of the soil’s nitrogen occurs as organic N
but this can be converted to ammonia and nitrate by micro-organisms via a process
known as mineralisation. Dry and wet atmospheric deposition of nitrate (NO 3 -) and
ammonia (NH 3 ) add nitrogen to the soil. Ammonia can also be converted to nitrate
via nitrification, a process that occurs mainly at neutral pH under aerobic conditions;
both ammonia and nitrate are then available for uptake by micro-organisms and
plants and excess amounts can move freely in solution. In addition, plants like alder
are able to fix atmospheric nitrogen.
4.1.2 Phosphorus
Phosphorus (P) is another major plant nutrient. Bio-available phosphorus is largely
in the soluble orthophosphate form, which can be taken up directly by plants.
In fens, dissolved phosphorus interacts with and becomes strongly bound to
sediments, and therefore unavailable to plants. This phosphorus adsorption
process, or chemical binding, can be modified by some situations, in particular
when the redox potential falls to a very low level. In these conditions, chemically
bound phosphorus can be released and become plant available, leading to a ‘flush’
of phosphorus into the fen by internal nutrient cycling, or released from the fen to
adjacent habitats. In addition, some plants are able to use root surface enzymes
(phosphatases) to release phosphate from organic stores.
Phosphorus availability is also strongly pH dependent. For example, under acidic
conditions (below pH 6.5) iron and aluminium oxides will adsorb plant available
orthophosphate, but the process is reversed when the acidity is reduced and pH
rises above 6.5. This releases phosphorus, aluminium and iron back into the system
and in some instances results in a potentially toxic ‘flush’ of aluminium and iron,
some of which might be exported from the fen. Conversely, at pH values above
7, high calcium levels can result in the formation of insoluble calcium phosphates
which increasingly immobilize phosphorus.
4.1.3 Other chemical ions that can influence nutrient status
Potassium is an important plant nutrient, but as it is generally available in soluble
ionic form (K + ), it is rarely a limiting nutrient or a factor in enrichment.
Calcium (Ca 2+ ) is an important ion, because of its status as a nutrient, and also
because of its ameliorating effect on the acidity of a wetland habitat via calcium
carbonate and bicarbonate ions, which in turn affects the bioavailability of other