Agriculture Reference
In-Depth Information
fertilization. Phosphate tends to be solubilized by the electrochemical changes
following soil submergence, but with prolonged submergence it may become
re-immobilized as reduced phases are precipitated (Section 4.3). Some of this
P might be re-solubilized by root-induced oxidation of the rhizosphere soil. On
the other hand, precipitation of amorphous Fe(OH)
3
close to and on rice roots
might be expected to immobilize P from the soil solution, impeding its access to
the roots.
Saleque and Kirk (1995) measured concentration profiles of P and other root-
induced changes near planar layers of rice roots growing in a highly weathered
P-deficient soil to which different amounts of P had been added (Figure 6.21). In
both P-fertilized and -unfertilized soil, the quantity of readily plant-available
P was negligible, so it was necessary for the plants to solubilize P. Some
90% of the P taken up was drawn from acid-soluble pools, probably associ-
ated with Fe(II) carbonates and hydroxides. There were also narrow zones of P
accumulation in an alkali-soluble pool which coincided with zones of Fe(OH)
3
accumulation near the roots. The zone of P depletion coincided with a zone of
acidification, caused by the processes discussed in Section 6.4. Kirk and Saleque
(1995) showed with a model of this system that the acidification and the P-
solubilizing effect of acidity in the soil were sufficient to account for the P
mobilized and absorbed by the roots. Solubilization accounted for at least 80%
of the P taken up in both the P-unfertilized and -fertilized soil, though only
about half the P solubilized was absorbed because the rest diffused away from
the roots. The amount of P solubilized greatly exceeded the amount immobilized
on Fe(OH)
3
precipitated near the roots. This is an extreme example, involving
particularly large pH changes, but it indicates the magnitude of the effects that
are possible.
By contrast, when a submerged soil is
dried
and oxidized, immobilization of P
on the ferric oxide formed may be the dominant process and plants may become
severely P deficient (Section 4.3). Huguenin-Elie
et al
. (2003) investigated the
mechanisms by which rice growing in alternately submerged and drained soils
extract P by measuring uptake from moist, flooded or flooded then moist soils
and comparing the results with model calculations allowing for solubilization
by various means. In all three water regimes the plants relied on solubilization
for most of their P. The roots were not mycorrhizal, as they will often not be
in intermittently flooded soils. In the moist soil, the uptake was only a third
of that in the flooded soils and was consistent with solubilization by organic
anion excretion from the roots, which appears to be the mechanism by which
upland rice in aerobic soil extracts P (Kirk
et al
., 1999; Trolove, 2000). In the
submerged then moist soil, uptake declined sharply as the soil dried because
P became immobilized. The final uptake was similar to that in the continuously
moist soil, indicating that some of the immobilized P was re-solubilized by roots,
possibly by excretion of organic anions.
