Environmental Engineering Reference
In-Depth Information
The nitrogen cycle
Outside living organisms, nitrogen is in an oxidized state as molecular nitrogen (N
2
) in the
atmosphere or nitrate ion (NO
3
−
) in soils and oceans (Fig. 2.4). Before nitrogen can be con-
verted into organic nitrogen and become part of amino acids and proteins, it needs to be trans-
formed into ammonium ion (NH
4
+
) by a process called
fixation
, when the precursor is nitrogen,
or
assimilation
, when the precursor is nitrate ion (Garret and Grisham, 1999). The conversion
of ammonium into organic nitrogen (in the form of proteins, nucleic acids, amino sugars,
and urea) is called
immobilization
, whereas the reverse reaction that takes place during
decomposition of organic mater containing nitrogen into ammonia and ammonium ion is called
mineralization
.
Fixation is performed by two natural processes, lightening and specialized bacteria, or
industrially by the Haber-Bosch process. Fixation can be described by the following reaction:
N3H
+↔
2 NH
[2.7]
2
2
3
When in contact with water, ammonia (NH
3
) gets transformed into ammonium ion and the
resulting acidic pH draws the equilibrium toward the right in Equation 2.7:
NH
+
H O
+
↔+
NH
+
H O
[2.8]
3
3
4
2
Assimilation
, the reduction of nitrate ion into ammonium ion, is performed by green plants
and specialized fungi and bacteria (Garret and Grisham, 1999). And the reverse process, the
oxidation of ammonium ion into nitrate ion, also takes place by nitrifying bacteria in a two-
step process called
nitrification
(Equations 2.9 and 2.10):
[2.9]
+
−
+
NH
+→
O
NO
+
3H
4
2
2
[2.10]
−
−
+
NO
+
H O
→ +
NO
2H
2
2
3
N
2
Denitrification
Fixation
NO
3
-
N
2
O
Nitrification
NH
4
+
“ +” Carbon
immobilization
“ -” Carbon
mineralization
Organic
nitrogen
Figure 2.4
The nitrogen cycle.
