Environmental Engineering Reference
In-Depth Information
quarter of all nitrogen fixation is by chemical industrial fixation for the nitrogenous
fertilizer industry. The fate of these nitrogenous fertilizers is a cause of concern, as the
NO
3
−
anion is readily leached from soils and has the capacity to cause eutrophication of
streams and lakes. It is also potentially toxic to humans; the disease
methaemoglobinaemia ('blue baby syndrome') is due to high NO
3
−
levels in drinking
water which becomes reduced to NO
2
−
in the human body, causing problems with oxygen
uptake, particularly in infants. This has led to the designation of
Nitrate
-
vulnerable
Zones
(NVZs) in the United Kingdom to limit the use of nitrogen fertilizers.
THE PHOSPHORUS CYCLE
Phosphorus (P) is a major plant nutrient which is absorbed as the anion orthophosphate
(PO
4
3−
). This is the chemical form which dominates its environmental cycle. As an anion
it is denied an easily available and exchangeable reservoir on soil clays; being a solid, it
is denied a large reservoir in the atmosphere like nitrogen and sulphur (Table 22.7).
Phosphorus thus faces some unique problems. Most soils contain much phosphorus, but it
is often a limiting nutrient because most is unavailable to plants. The main features of the
phosphorus cycle are shown in Figure 22.7. One phosphorus store is in phosphorus-
bearing minerals in rocks such as apatite. Phosphorus is released from these minerals by
chemical and microbiological weathering. However, the major part of the phosphorus in
soils is in the organic matter, largely as inositol phosphates, and the PO
4
3−
will be
released as decomposition and mineralization processes take place. The fate of PO
4
3−
released by both mineral weathering and organic decomposition is crucial for the uptake
and recycling of this nutrient. Phosphorus availability depends mainly on the pH of the
soil. Under acid, low pH conditions, phosphorus is quickly precipitated as iron and
aluminium phosphates; both of these cations are more soluble at low pH. Under alkaline,
high pH conditions, phosphorus is precipitated as calcium phosphates in the presence of
the high calcium concentrations usually present at high pHs. In all three precipitates -
iron, aluminium, calcium - phosphorus is held tightly in a chemical form which is not
available to plants. It is around neutrality (approximately pH 7) that phosphorus is most
available. Some adsorption of PO
4
3−
will occur on to clay surfaces at neutral pH. Also
microbial activity will usually be at a maximum at neutrality, which results in increased
microbial mineralization, and the conversion of phosphorus from an organic to an
inorganic form. The 'phosphorus problem' is that soluble and plant-available PO
4
3−
is
present only in low concentrations in most soils, and is quickly converted to unavailable
forms. Micro-organisms are known to play an important role in solubilizing PO
4
3−
from
unavailable organic and inorganic stores. Large numbers of soil and marine micro-
organisms are able to solubilize apatites and possess the enzyme phosphatase which will
release PO
4
3−
from organic phosphorus. Fungi can also form mycorrhizae which help
phosphorus uptake by plants
