CADMIUM AND ZINC IN PLANTS AND SOIL SOLUTIONS FROM CONTAMINATED SOILS

In an experiment using ten heavy metal-contaminated soils from six Eur opean countries, soil solution was sampled by water displacement befor e and after the growth of radish. Concentrations of Cd, Zn and other e lements in solution (K, Ca, Mg, Mn) generally decreased during plant g rowth, probably because of uptake by plants and the subsequent redistr ibution of ions onto soil exchange sites at lower ionic strength. Spec iation analysis by a resin exchange method showed that most Cd and Zn in non-rhizospbere solutions was present as Cd2+ and Zn2+; respectivel y. The proportion of free ions was slightly lower in rhizosphere solut ions, mainly due to an increase in dissolved organic carbon during pla nt growth. Solution pH increased during plant growth, although the bul k soil pH generally remained constant. Cd concentrations in leaves and tubers were more closely correlated with their total or free ionic co ncentrations in rhizosphere solutions (adjusted R-2 greater than or eq ual to 0.90) than with their concentrations in soils (adj. R-2 greater than or equal to 0.79). Cd concentrations in non-rhizosphere solution s were only poorly correlated with Cd concentrations in leaves and tub ers. In contrast to Cd, there were no soil parameters that individuall y predicted Zn concentrations in leaves and tubers closely. However, m ultiple correlation analysis (including Zn concentrations in rhizosphe re solutions and in bulk soils) closely predicted Zn concentrations in leaves and tubers (adj. R-2 = 0.85 and 0.70, respectively). This sugg ests that the great variability among soils in the solubility of Zn af fected the rate of release of Zn into solution, and thus Zn uptake. Th ere was no such effect for Cd, for which solubility varied much less. Furthermore, the plants may have partly controlled Zn uptake, as they took up relatively less at high solution concentrations of Zn. Free io nic concentrations in soil solution did not predict concentrations of Cd or Zn in plants better than their total concentrations in solution. This suggests that with these soils, analysis of Cd and Zn speciation is of little practical importance when their bioavailability is asses sed.