Agriculture Reference
In-Depth Information
6.5.3 SOLUBILIZATION OF ZINC
Zinc is often highly insoluble in submerged soils and Zn deficiency is an impor-
tant constraint to rice production throughout Asia (Chapter 7). In similar experi-
ments to those in the last section, Kirk and Bajita (1995) measured changes in Zn
pools near rice roots in anaerobic soil (Figure 6.22) and simultaneous changes in
Fe(II), Fe(III) and pH (Figure 6.16). As for P in the soil in Figure 6.21, the con-
centration of easily extractable Zn in the soil was negligible following flooding
and it was necessary for the plants to solubilize Zn to meet their requirements.
Zinc was mobilized from highly insoluble forms in the soil and re-precipitated
with Fe(OH)
3
and organic matter within 4-5 mm of the roots. The accumulation
continued over time but simultaneously there was a substantial depletion of the
accumulated fractions within 2mm of the roots. The zones of accumulation and
depletion coincided with zones of Fe(III) accumulation and soil acidification. The
authors concluded that Fe oxidation released Zn from highly insoluble forms and
that this Zn was re-adsorbed on Fe(OH)
3
and on organic matter in forms that
were acid-soluble and therefore accessible to the plants.
An additional benefit of acidification of the soil close to the root may be to
lower the concentration of HCO
3
−
in solution. High HCO
3
−
impairs Zn absorp-
tion (Dogar and Hai, 1980), its translocation to the shoot (Forno
et al
., 1975),
root growth (Yang
et al
., 1994), or all three. The modest decrease in pH near the
roots in Figure 6.16 (from pH 7.35 to 7.1) is equivalent to a two-fold increase
in H
+
concentration and if the CO
2
pressure is constant a two-fold decrease in
HCO
3
−
concentration.
There may also be effects via the concentrations of competing cations at the
root surface. In studies of short-term uptake of
65
Zn by rice from nutrient solutions
containing realistic Zn
2
+
concentrations, Giordano and Mortvedt (1974) found
uptake was inhibited by various metabolic inhibitors and by Fe
2
+
,
Mn
2
+
,
Ca
2
+
and Mg
2
+
as Cl
−
salts at typical concentrations in flooded soil solutions. Translo-
cation of absorbed Zn was also inhibited by Fe
2
+
and Mn
2
+
but not by Ca
2
+
or
Mg
2
+
.Cayton
et al
. (1985) also found antagonistic effects of competing cations
on Zn uptake. Absorption involves preferential binding of Zn
2
+
at cation exchange
sites in the root cell walls prior to active uptake across the plasma membrane.
Preferential binding concentrates Zn
2
+
at the sites of active uptake, and would be
sensitive to the concentrations of competing cations. But it is not clear whether
competition for exchange sites or for transporters in the plasma membrane is the
more important (Reid
et al
., 1996).
6.5.4
IMMOBILIZATION OF CATIONS
Metal cations in the soil solution may be immobilized by sorption onto iron
'plaque' on root surfaces in submerged soils, in the same way that solubilized
Zn
2
+
was re-adsorbed on ferric oxide in the experiments in Figure 6.22. Seques-
tering of metals on the external surfaces of wetland roots in this way limits uptake
