where CH2O represents soil organic substrates. In the
absence of O2 Fe(III) is generally the main oxidant in
the soil, its concentration typically exceeding concentrations of
NO3-, MnO2 or
SO42- by at least an order of
magnitude. Between 1 and 20% and sometimes as much as 90% of the free
Fe(III) oxides in the soil is reduced to Fe(II) over one to two months
of submergence. Some of the structural Fe(III) in soil clays is also
reduced. The course of soil reduction and the changes in redox and pH
are therefore generally dominated by the reduction of Fe(III).
As reduction proceeds, H+ ions are consumed in
Reactions (2)–(5) and the pH tends to increase. Simultaneously
CO2 is produced, but escapes from the soil only very
slowly, and it therefore accumulates to high partial pressures. The
accumulation of CO2 lowers the pH of alkaline soils and
curbs the increase in pH of acid soils. As a result the pHs of most
soils tend to converge following submergence in the range 6.5–7. Large
concentrations of Fe2+ ions develop in the soil
solution in the weeks following flooding, often several mM or tens of
mM, balanced by HCO3- formed from
dissolved CO2. The ion activity products of pure ferrous
hydroxides, carbonates and other minerals are often exceeded 100-fold.
Evidently precipitation of these minerals is inhibited, probably as a
result of adsorption of dissolved organic matter and other solutes onto
nucleation sites. However, once a sufficient supersaturation has been
reached there is a rapid precipitation of amorphous solid phases
containing Fe(II), which may later re-order to more crystalline forms.
The exact nature of the Fe(II) compounds formed is uncertain. But there
is evidence that mixed Fe(II)–Fe(III) hydroxides are formed, which have
some of the observed properties of the solid phase Fe(II) found in
reduced soils, including the greyish-green colours characteristic of
reducing conditions.
However in highly-weathered iron toxic soils typical of inland valleys,
reduction tends to be slower and the rise in pH causing precipitation of
Fe(II) compounds is more gradual, so that the soil solution and exchange
complex continue to be dominated by Fe2+ ions (Kirk,
Solivas & Alberto, 2003; Narteh & Sahrawat, 1999; Ponnamperuma, 1972).
A major factor in this is that such soils generally have large reserves
of acidity, and so the soil pH remains below the neutral range in which
the solubility products of the Fe(II) compounds are exceeded. Also as a
result of the acidity and slow reduction, concentrations of dissolved
CO2 tend to be smaller.