Agriculture Reference
In-Depth Information
Concentration
O
2
NO
3
−
NH
4
+
Mn
2
+
Fe
2
+
CH
4
Figure 5.1
Indicative concentration profiles of
redox species with depth in sub-
merged soil
5.1.1 PROCESSES INVOLVED IN SEQUENTIAL REDUCTION
The sequence of reactions by which organic matter is oxidized following sub-
mergence loosely follows the predictions of thermodynamics—i.e. in the order
of decreasing free energy change—as described in Chapter 4. However, rates
of reduction vary greatly between soils and there are complicated interactions
between the microbial processes involved. Hence it is difficult to predict
apriori
,
for example, how long after submergence a given soil will become methanogenic
and what the rate of methane production will be.
The free energy change for a particular redox reaction varies with pe, pH, and
the concentrations of reductants and oxidants according to Equation (4.26):
2
.
303
RT
n(
pe
1
−
log
(
Ox
2
)(
Red
1
)
(
Ox
1
)(
Red
2
)
pe
2
)
−
G
=−
In this equation, the value of (Red) is a function of the nature of the reductant,
its solubility, the crystallinity of solid phases containing it, effects of solubilizing
agents, transport limitations, and other factors. Likewise the value of (Ox) is a
function of various factors. As discussed in the previous chapter, most redox
reactions are very slow and the prevailing conditions are therefore sensitive to
catalysis. Three types of catalysis are involved:
•
Abiotic, for example by adsorption of reactants onto mineral surfaces, dis-
tinguished from biotic catalysis by the absence of a temperature optimum.
