Geoscience Reference
In-Depth Information
seedlings). However, some specialized plants have adaptive mechanisms
because they are able to live with their roots submerged. The mechanism
may be a morphological structure serving to transmit atmospheric
oxygen to the roots (aerenchyma, pneumatophores—see § 12.6) or a
peculiar biochemical function. Also, strong reduction sometimes leads
to dissolution of toxic amounts of Fe
2+
and Mn
2+
.
The content of dissolved oxygen in the soil solution can be determined.
But the method poses theoretical problems. Firstly, it is necessary to
work protected from the air to avoid contamination by atmospheric
oxygen (Boivin 2000). Secondly, the oxygen sensors are perturbed by
sulphur compounds (F. Feder, pers. comm.). Lastly, when oxygen is
totally absent, the method is obviously not useful to characterize the
soil the status of which may continue to change.
It is also possible to analyse the composition of the soil atmosphere,
but this requires special equipment. Plant growth is affected when the
concentration of oxygen falls below 9 per cent (Renault
et al.
1997), except
if the plant has developed a specialized adaptation (§ 12.6 Mangrove
swamps).
There are at least three ways of approaching the oxidation-reduction
state of a solution (Bartlett and James 1993; McBride 1994; Lovley 1995).
We shall review them below.
Determination of the oxygen content of soil water
12.1.2 Concept of 'pe' (Bartlett and James 1993)
Let us start with the following reduction half reaction:
aOx
+
be
-
+
cH
Æ
dRed
+
eH
2
O
where a, b, c, d and e are the stoichiometric coefficients. It corresponds to
the gain of a electron e
-
to form the reduced form
Red
of a redox couple
(
Red/Ox
). In these conditions, the equilibrium constant
K
is given by:
K
= [(
Red
)
d
(H
2
O
)
e
]/[(
Ox
)
a
(e
-
)
b
(H
)
c
]
where values in parentheses represent activities and where (
H
2
O
) has
the value 1 by convention.
Taking logarithms, we get
log K
=
log
[(
Red
)
d
/
(
Ox
)
a
] +
log
[1/(
e
-
)
b
] +
log
[1/(
H
+
)
c
]
As
log
1/
e = -log e = pe
, and obviously,
log
1/
H = -log H = pH
this becomes
