Geoscience Reference
In-Depth Information
Denitrification and anammox. The nitrogen cycle is closed by the
transformation of NO
3
-
into gaseous N
2
, primarily by the action of
specialized bacteria
that are found in
both water and soil. This process (also called nitrate respiration) only
occurs at low oxygen concentration, and is therefore found in OMZs
(section 4.4.2):
−
called denitrifying bacteria
−
4 NO
3
-
+ 5 CH
2
O + 4 H
+
→ 2 N
2
(gas) + 5 CO
2
+ 7 H
2
O [4.20]
where CH
2
O represents organic matter. In the ocean, an additional
process, called anaerobic ammonium oxidation (anammox) transforms
NO
2
-
and NH
4
+
into N
2
:
NO
2
-
+ NH
4
+
→ N
2
+ 2 H
2
O
[4.21]
The N
2
produced by these two processes is returned to the atmosphere
[
8
], where the nitrogen cycle starts again.
In addition to N
2
fixation by bacteria [
1
], other processes can fix
nitrogen. These include the electrical discharges that occur naturally in
the atmosphere (i.e. lightning), which combines atoms of N and O to
form nitrogen monoxide (NO) [
9
]:
N
2
+ O
2
→ 2 NO
[4.22]
Moreover, human activities that include the combustion of wood
[
10
] and fossil organic matter (coal and hydrocarbons) [
11
] produce
NO and other nitrogen oxides (NO
x
), which are very toxic gases that
contribute to the greenhouse effect. Coal is formed on land [
12
],
whereas hydrocarbons are formed at the bottom of the sea from ocean
sediments [
12
], from where they are transported onto land by tectonic
plate movements (orogenesis) [
13
].
The two opposite processes of N
2
fixation and denitrification are
probably in equilibrium at the global scale over time periods of
several centuries or millennia. It is not certain whether these processes
are in equilibrium locally or regionally over times periods of years or
