Reduction and Oxidation - The Biogeochemistry of Submerged Soils - page 123

Fe, intermediate in sandy loams high in organic C and low in active Fe, and least
in acid clays high in active Fe. The increases in soluble P are particularly linked
to the transformations of Fe and changes in pH. The main processes are:

•

reduction of Fe(III) compounds holding P on their surfaces and within their
crystal lattices;

•

dissolution of Ca-P compounds in alkaline soils as the pH decreases and des-
orption of P held on variable-charge surfaces in acid soils as the pH increases;

•

displacement of sorbed P by organic anions and chelation of metal ions that
would otherwise immobilize P; and

•

mineralization of organic P.

Subsequent decreases in solubility may be due to re-sorption or precipitation on
clays and oxides as soil conditions continue to change, and decomposition of
organic anions chelating P or chelating Al and Fe with which it would other-
wise react.
Following submergence soils often release more P to solutions low in P but
adsorb more P from solutions high in P. This apparent paradox can be explained
by the reduction of Fe(III) oxides to poorly ordered gel-like Fe(II) compounds
with large surface areas. Phosphorus solubilized in soil reduction is sorbed on
the amorphous surfaces and desorbed when P is removed from the soil solution;
but fresh P added to the soil is removed from solution by sorption onto the
Fe(II) surfaces. Consequently many soils do not show significant increases in P
solubility during flooding (Willett, 1991), and with prolonged flooding the P may
become re-immobilized in less soluble forms.
Gradual immobilization of P with prolonged anaerobicity is shown in
Figure 4.14, which gives change in labile P over 3 years of double rice cropping
of a perennially wet soil (B ucher, 2001). The labile P declines even in plots
that received sufficient P fertilizer to more than off-set crop removals. Periodic
drying of the soil during the fallow periods tended to increase labile P in the soil,
but not in years when the soil remained anaerobic during the fallow (following
the 1998 and 1999 wet season crops). The effect was greatest when tillage was
delayed until the end of the fallow, resulting in more-reducing conditions in the
soil, and it carried through to the succeeding rice crop. Supporting laboratory and
greenhouse studies showed that changes in soil Fe with reduction and oxidation
were responsible for the changes in P.
Rapid drying and oxidation of the soil can also result in the P becoming
very insoluble (Brandon and Mikkelsen, 1979; Willett, 1979; Sah et al ., 1989;
Huguenin-Elie et al ., 2003). Re-oxidized Fe(II) compounds may be precipitated
in poorly crystalline forms with large specific surface areas, on and in which P
becomes immobilized. Hence upland crops grown in rotation with rice frequently
suffer P deficiency even though crops on similar soils not used for rice grow
healthily. The problem is in part also due to disruption of mycorrhizal networks
during flooding (Ilag et al ., 1987; Ellis, 1998; Miller, 2000).