Agriculture Reference
In-Depth Information
amount and quality of organic matter present, and the concentrations of easily
reducible Fe(III) and other reductants.
I now summarize the changes in electrochemical conditions that occur in the
soil with time following submergence as they affect the profiles and dynamics
of redox species.
4.2.2
CHANGES WITH TIME
Reduction of a submerged soil proceeds roughly in the sequence predicted by
thermodynamics:
O
2
+
CH
2
O
−−−→
CO
2
+
H
2
O
(4.27)
4NO
3
−
+
5CH
2
O
+
4H
+
−−−→
2N
2
+
5CO
2
+
7H
2
O
(4.28)
4H
+
−−−→
2Mn
2
+
+
2MnO
2
+
CH
2
O
+
CO
2
+
3H
2
O
(4.29)
8H
+
−−−→
4Fe
2
+
+
4Fe(OH)
3
+
CH
2
O
+
CO
2
+
11H
2
O
(4.30)
SO
4
2
−
+
2H
+
−−−→
2CH
2
O
+
H
2
S
+
2CO
2
+
2H
2
O
(4.31)
and
2CH
2
O
−−−→
CH
4
+
CO
2
(
4
.
32
)
Typically O
2
becomes undetectable within a day of submergence and then NO
3
−
is reduced. Reduction of NO
3
−
will not occur until the O
2
concentration reaches
a very small value. Likewise, whilst NO
3
−
is being reduced, the pe is poised in
the range 3 - 6 and reduction of Mn and Fe are prevented. However NO
3
−
will be
exhausted within a matter of days and then reduction of Mn and Fe may proceed.
In the absence of O
2
Fe(III) is generally the main oxidant in the soil, its
concentration typically exceeding concentrations of NO
3
−
, Mn(III, IV) or SO
4
2
−
by at least an order of magnitude (Chapter 3). Between 1 and 20 % and sometimes
as much as 90 % of the free Fe(III) in the soil is reduced to Fe(II) over 1 - 2
months of submergence (Ponnamperuma, 1972; van Breemen, 1988). Some of
the structural Fe(III) in soil clays is also reduced (Stucki
et al
., 1997). The course
of soil reduction and the changes in pe and pH are therefore generally dominated
by the reduction of Fe(III).
Changes in pe, pH and Alkalinity
It is difficult to obtain reliable measurements of
E
H
and hence pe in soils. Strictly,
only measurements made with the electrodes in soil solution extracts rather than
directly in soil are thermodynamically meaningful, and these are also subject to
various errors, particularly due to the presence of mixed redox systems. Nonethe-
less it is a useful parameter and is the only single electrochemical property that
can distinguish submerged soils from well-drained ones.
Figure 4.5 shows changes in pe, pH and Fe
2
+
in the soil solution of four
representative soils following flooding (IRRI, 1964). The figure shows that in
all the soils there is a minimum in pe after a few days followed by an increase,
