Root uptake and distribution of radiocaesium from contaminated soils and the enhancement of Cs adsorption in the rhizosphere

Uptake by roots from contaminated soil is one of the key steps in the entry of radiocaesium into the food chain. We have measured the uptake by roots of radiocaesium and its transfer to shoots of a heathland grass, sheep fesc ue (Festuca ovina L.) from two contrasting agricultural soils, a sandy podz ol and a clayey calcareous soil. A culture device which keeps the roots sep arate from the soil was used thus allowing rhizosphere soil to be obtained easily and enhancing the effect of root action. Biomass production and Cs-1 37 in shoots and roots were recorded. Cs adsorption was studied on both the initial, nonrhizosphere soil and on rhizosphere soil in dilute soil suspen sion. Cs desorption was measured by resuspending subsamples of contaminated soil in solutions containing various concentrations of stable Cs. The prop ortion of Cs fixed, i.e. not readily desorbable, was calculated by comparis on of the adsorption and desorption isotherms. Uptake by roots varied consi derably between soils and did not appear to be diffusion limited. Root-to-s hoot transfer did not differ for the two soils studied. Root action conside rably enhanced Cs adsorption on the soils, particularly the in sandy podzol with a low Cs affinity. The value of K-d was increased by up to an order o f magnitude. A large proportion of adsorbed Cs was found to be fixed, the K -d was up to seven times greater on desorption than adsorption, indicating that up to 80% of adsorbed Cs was not readily exchangeable. Root action had little effect on the fixed fraction.