Geoscience Reference
In-Depth Information
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
. 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
and
Nitrosococcus
, which live in soil, fresh water and
the sea. The second group oxidizes NO
2
-
to NO
3
-
and
consists of the aerobic bacteria
Nitrobacter
. In addition
to the need for oxygen, the processes also require a
favourable pH (usually between 5 and 8) and a suitable
temperature. It follows that nitrification is much reduced
in water-logged, acid, alkaline or cold soils. Many micro-
organisms have the ability to chemically reduce nitrous
oxides (NO
3
-
,NO
2
-
, nitric oxide NO, nitrous oxide N
2
O)
under anaerobic conditions, when the compound is
used as a substitute for oxygen. This process is known
as nitrate reduction. When the reduction proceeds as
far as the gaseous products of nitrogen N
2
and nitrous
oxide N
2
O the process is called
denitrification
. This
extreme step is restricted to only a few genera of bacteria,
namely
Bacillus
,
Micrococcus
and
Pseudomonas
. In water-
logged soils as much as 15 per cent of inorganic nitrogen
may be lost to the atmosphere in this way. Even in well
drained soils denitrification occurs because there will
be anaerobic micro-environments where the diffusion of
O
2
is slow.
The loss of gaseous nitrogen from ecosystems by
denitrification is balanced by an approximately equal
process of
nitrogen fixation
which brings organic nitrogen
into plants and micro-organisms in the soil from gaseous
N
2
in the atmosphere. The list of organisms that are
capable of N
2
fixation has expanded enormously in recent
years. The basic classification is into
free-living fixation
,
carried out by aerobic bacteria (such as
Azotobacter
),
blue-green algae and anaerobic bacteria, and
symbiotic
fixation
, carried out by root-nodule bacteria, root-nodule
actinomycetes, and symbiotic associations with blue-green
algae. Unlike nitrification, nitrogen fixation can readily
occur in anaerobic soil conditions by either free-living
anaerobes (e.g.
Clostridium
) or symbiotic blue-green algae
(e.g.
Anabaena
).
Most N
2
fixation occurs through the symbiosis between
legumes and the
Rhizobium
genus of aerobic bacteria.
This is an example of
mutualism
, with both partici-
pants benefiting from the interaction. Plants receive NH
4
from bacteria, and bacteria receive in turn carbohydrate
and a home from the plant. It is estimated that legumes
in agriculture fix 40 Mt of N
2
every year, 5 Mt are fixed in
the rice crop alone, and 100 Mt are fixed in remaining
terrestrial ecosystems, especially on nitrogen-deficient
soils in the tropics. The importance of fixation by root
nodule associations between actinomycetes (especially
Frankia
) and a variety of perennial non-leguminous
plants is now recognized. Plant genera known to form
such nodules are
Casuarina
,
Hippophae
,
Myrica
,
Alnus
,
Dryas
and
Ceanothus
.
Other fluxes in the nitrogen cycle seem subsidiary,
but can have important effects at the local scale. Light-
ning produces nitrogen oxides in the atmosphere which
are brought to the soil surface by precipitation. Signifi-
cant quantities of nitrous oxides are also produced by
home heating and the internal combustion engine; such
pollution has been measured on trial plots at Rotham-
sted Experimental Station, Hertfordshire, to be 45 kg
N ha annually. Industrial fixation of nitrogen is quantita-
tively very important. Perhaps a quarter of all nitrogen
fixation is by Haber-Bosch fixation in the nitrogen
fertilizer industry. The fate of artificial nitrogen fertilizers
is a cause of concern, as the NO
3
-
anion is readily leached
from soils and causes 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
and prevent nitrogen leakage to rivers.
