Environmental Engineering Reference
In-Depth Information
Figure 22.6
The nitrogen cycle.
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 waterlogged, 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 waterlogged 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, 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
).
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