Environmental Engineering Reference
In-Depth Information
THE NITROGEN CYCLE
Nitrogen (N) is the plant nutrient needed in greatest quantities (after carbon, oxygen and
hydrogen), and forms an essential part of the structure of plant proteins. Its cycle is
complex, involving atmosphere, soil and organic material, and depends upon the
activities of a range of specialized micro-organisms. The main features of the cycle are
shown in Figure 22.6.
Organic materials are added to the soil surface upon the death of a plant or its organs.
Waste products are also added which contain significant quantities of nitrogen. In
whatever form, the conversion of this organically bound nitrogen into a form in which it
can again be absorbed by plants (e.g. nitrate, NO
3
−
) is referred to as mineralization. In
detail, mineralization comprises several distinct and separate steps which have their own
particular chemistry and microbiology. The first step is the breakdown of the organic
nitrogen molecules (largely proteins) into ammonia (NH
3
) or ammonium ion (NH
4
+
).
Under well drained, slightly acid conditions NH
3
is produced in large quantities; at
neutral or alkaline pH, NH
4
+
predominates. This stage is known as
ammonification
and
is carried out by a wide range of heterotrophic soil bacteria which gain their energy from
organic carbon. The NH
4
+
ion can be readily absorbed by plants and micro-organisms in
theory, but in reality most is used by a specialized group of nitrifying bacteria which
obtain their energy by oxidizing NH
4
+
or NH
3
. These chemautotrophic bacteria obtain
energy by carrying out a chemical reaction rather than from organic carbon already
assimilated by a plant or animal. The processes which convert NH
3
and NH
4
+
to NO
3
−
are
known collectively as
nitrification
.
Nitrification is a vital conversion for ecosystems and agricultural crops and has been
studied in considerable detail. Two separate groups of chemautotrophic bacteria are
involved. The first group converts NH
4
+
to nitrite (NO
2
−
) and consists of the aerobic
bacteria
Nitrosomonas
