Geoscience Reference
In-Depth Information
Fe
3+
/Fe
2+
: in most soils with excess water, there is reduction of
iron;
SO
4
2-
/H
2
S
: in active mangrove swamps, the sulphates in seawater
are converted to sulphides including hydrogen sulphide; we shall
return to this later;
CO
2
/CH
4
: in some peatlands or rice paddies, when CO
2
is the
only electron acceptor, methane is produced; we shall return to
this too. It has been shown that addition of nitrates or sulphates
to these systems immediately stops emission of CH
4
; but the
mechanism involved is controversial, and it is not certain that
the phenomenon could be attributed to the fact that
SO
4
2-
and
NO
3
-
are reduced before CO
2
is (Roy and Conrad 1999).
In the absence of reducible entities, some bacteria have found a
procedure of replacement: they manufacture an acceptor from organic
matter. This is fermentation, the yield from which is very low so that the
bacteria responsible are forced to transform large amounts of material to
recover very little energy.
Field measurements of redox potential can be interpreted as given in
Table 12.4 (McBr ide 1994).
Corresponding redox potentials
Table 12.4
Relation of measured redox potentials to biochemical reactions
in the soil.
Redox potential, mV
Oxidation-reduction controlled by:
600
to
350
O
2
(aq) / H
2
O
Nitrates, nitrites /
NH
4
400
to
250
Four manganese species / Mn
2+
300
to
0
200
to
-40
Organic matter (
controversial
)
Fe
3+
/ Fe
2+
100
to
-100
SO
2-
/
S
2-
-70
to
-220
-150
to
-240
CO
2
/
CH
4
For example, as long as there are nitrates in the medium the potential
scarcely drops below 250 mV. In addition, clear water in contact with
the atmosphere has a potential of 500 to 300 mV. A submerged mud
registers between 300 and -300 mV.
However, the sequence of reactions greatly depends on the pH
(Fig. 12.1). In acid medium, green rusts could act by raising the redox
potential corresponding to iron species to 340 mV, that is above the
potential corresponding the NO
3
-
/NH
4
+
reaction (320 mV on average).
