Figure 2 Root-induced changes in the rhizosphere of rice in
iron toxic soil. (a) Leaf bronzing and stunted growth due to Fe
toxicity. (b) Leaf bronzing and oranging due to Fe toxicity with Mg
deficiency. (c) Root effects on the soil: (1) oxidation of ferrous iron
by O2 released from the roots; (2) venting of soil
CO2 into the root aerenchyma and associated changes in
soil carbonate equilibria; (3) excess intake by the root of nutrient
cations (particularly ammonium, NH4+)
over anions and associated release of H+; (4) in the
zone where there is no O2 release, re-reduction of
ferric iron fuelled by organic substrates (represented as
CH2O) diffusing across leaky root membranes. Note the
protons (H+ ions) consumed or produced in these
reactions will be buffered by proton-donating or -accepting groups in
the soil solid. (d) Calculated pH profiles in the rhizosphere for a
given flux of H+ due to the above root-induced changes
at different initial soil pH values (dotted lines) (Kirk, 2004). (e)
Sensitivity of pH changes at the root surface to important variables,
varied individually from standard values as in Panel (d).