Processes in Roots and the Rhizosphere - The Biogeochemistry of Submerged Soils - page 200

of metal pollutants into the vegetation (Otte et al ., 1989; St-Cyr and Crowder,
1990; Ye et al ., 1997a, b; Hansel et al ., 2001).
In soils with small pH buffer powers, root-induced acidification may also
impede access of cations to roots, though it may diminish sorption onto oxides
as the surface negative charge decreases (Razafinjara, 1999). This is because the
overall concentration of the soil solution in a submerged soil depends largely
on the concentration of HCO 3 − , buffered by dissolved CO 2 . Therefore, if the
pH close to the root decreases below about 6.0, the concentration of anions in
solution also decreases and so the concentration of cations in solution and hence
their rate of diffusion to roots must decrease [Bouldin (1989) gives calculations
of this effect]. This may happen, for example, in 'iron toxic' soils which develop
large concentrations of Fe 2 + in solution. High rates of Fe 2 + oxidation and asso-
ciated H + generation result in a low pH in the rhizosphere, especially if the soil
is already acid or has a small pH buffer power. Hence the need to exclude toxic
Fe 2 + from the root by oxidizing it in the rhizosphere may impair the absorption
of nutrient cations by the root. Consistent with this the symptoms of iron toxicity
are often alleviated by applications of K salts.
A further complication is that the lowering of the rhizosphere pH and con-
sequent depression of HCO 3 − means that any Fe 2 + entering the root will be
accompanied by a proportion of Cl − or SO 4 2 − rather than HCO 3 − .WhenFe 2 +
enters with HCO 3 − , the acidity generated in Fe 2 + oxidation in the plant is neutral-
ized by conversion of HCO 3 − to CO 2 , which is assimilated or lost. However when
Fe 2 + enters with a non-volatile anion, Fe 2 + oxidation will produce the equiva-
lent amount of free H + in the plant, with damaging effects on plant tissues (van
Mensvoort et al ., 1985).
6.6 CONCLUSIONS
This chapter has shown the complexity of the chemical and biological processes
around wetland plant roots and the effects of the extreme electrochemical gradient
between the root surface and surrounding soil. Models of nutrient uptake by
plants in aerobic soil, which treat the root as a simple sink to which nutrients
are delivered by mass flow and diffusion but the root not otherwise influencing
the surrounding soil, work reasonably well for the more soluble nutrient ions.
However, the complexity of the wetland root environment is such that such
models are inadequate and more elaborate treatments are necessary. Many of the
mechanisms involved are still poorly defined and speculative, but their potential
significance is clear.