Agriculture Reference
In-Depth Information
4
Reduction and Oxidation
The characteristics of submerged soils depend above all on the reduction and oxi-
dation (redox) reactions that take place as a result of oxygen being excluded from
the soil. With few exceptions, redox reactions on the Earth's surface are driven
by the redox disequilibrium caused by photosynthesis. In photosynthesis green
plants use solar energy to reduce inorganic carbon to strongly reduced organic
compounds, and simultaneously water is oxidized to O
2
. Non-photosynthetic
organisms tend to restore equilibrium by catalysing the oxidation of the organic
compounds back to inorganic compounds in energy-yielding reactions. The bulk
of this oxidation takes place in soil. In aerated soils, the preferred oxidizing agent
is O
2
itself. However where O
2
is not available, as for example in submerged
soils, alternative oxidants must be used. These may be organic, in which case the
process is fermentation, or inorganic, in which case it is anaerobic respiration,
though this term is often used to cover fermentation as well. In this chapter I
give an overview of the thermodynamics of redox reactions and their kinetics in
natural systems, and I then discuss the particular redox processes that occur in
submerged soils.
4.1 THERMODYNAMICS AND KINETICS OF REDOX REACTIONS
4.1.1
ELECTRON ACTIVITIES AND FREE ENERGY CHANGES
In the same way that acid - base reactions involve the transfer of protons between
proton donors and proton acceptors, redox reactions involve the transfer of elec-
trons between electron donors, called reducing agents or reductants, and electron
acceptors, called oxidizing agents or oxidants. Thus when a redox reaction takes
place, a reductant loses electrons and is oxidized to its conjugate oxidant:
Red
1
−−−→
Ox
1
+
n
e
−
(
4.1a
)
and simultaneously an oxidant gains electrons and is reduced to its conjugate
reductant:
Ox
2
+
n
e
−
−−−→
Red
2
(
4.1b
)
where e
−
represents the electron. Equations (4.1a) and (4.1b) are redox half
reactions or couples, and together they constitute a complete redox reaction.
