Geoscience Reference
In-Depth Information
Some living species catalyse only one part of the reaction sequence,
for example the transformation of nitrate NO
3
-
to nitrogen monoxide
(nitric oxide) NO. Others reduce only nitrite NO
2
-
. Contrarily, some
species catalyse all the reactions of the sequence of denitrification up
to gaseous nitrogen N
2
.
Table 12.5
Half-reactions corresponding to denitrification.
Reduction half reactions
Log
K
pe
at
pe
at
(= gain of electrons)
pH 5
pH 7
½ N
2
O + e
-
+ H
+
Æ
½ N
2
+ ½ H
2
O
29.8
22.9
20.9
NO + e
-
+ H
+
Æ
Æ
½ N
2
O + ½ H
2
O
26.8
19.8
17.8
½ NO
2
-
+ e
-
+ 3/2 H
+
Æ
¼ N
2
O + ¾ H
2
O
23.6
15.1
12.1
NO
2
-
+ e
-
+ 2 H
+
Æ
NO + H
2
O
19.8
9.8
5.8
½ NO
3
-
+ e
-
+ H
+
Æ
½ NO
2
-
+ ½ H
2
O
14.1
9.1
7.1
All these reactions are influenced by pH, and there could be an
optimum pH for each reaction (Simek and Cooper 2002). Overall,
denitrification works well at pH values that are neither too acid nor
too alkaline.
We say
nitric oxides
and write NO
x
for NO + N
2
O together.
Field measurements show that denitrification essentially results in
gaseous nitrogen. The oxides NO
x
are intermediate products made to be
destroyed. They are rarely emitted to the atmosphere in large quantity:
hardly more than 3 or 4 kg N ha
-1
y
-1
in highly reducing soils (Lamers
et al
. 2007). The oxide NO may not even appear.
The microorganisms involved are numerous and all of them are
not yet known although denitrification has been widely investigated
(Patureau 1995). They belong to the genera
Pseudomonas
,
Alkaligenes
and
Bacillus
. Among them are
P. a e r ug in o s a
,
B. brevis
,
B. licheniformis
and
B.
polymyxa
(Munch and Ottow 1986).
Denitrification in a water-saturated zone may amount to 500 g ha
-1
per day (Curmi
et al
. 1997). In a cultivated field, the emissions range
from 0 to 100 g ha
-1
of nitrogen per day (Garrido
et al
. 2000; Hénault
et al
. 2000). In all, the process represents a way of reducing the excess
nitrogen in our soils that have been greatly enriched through nitrogen
fertilization not always properly calibrated. But all wet lands do not
necessarily have a denitrifying role. See the case of rice fields discussed
later.