Availability of caesium radionuclides to plants - Classification of soils and role of mycorrhiza

At different locations in spruce stands spread rather homogeneously over southern Baden-Wurttemberg, samples of soil and plants were taken and the vertical distribution of the caesium radionuclides in the soil was studied. As a direct measure of the bioavailability, the aggregated transfer factor, T-ag, was determined for fern, bilberry, raspberry, blackberry, and clover. The T-ag (in m(2) kg(-1)) is defined by the specific caesium activity (in Bq kg(-1)) of the dry mass of the plants, divided by the total inventory (in Bq m(-2)) of the soil. It varies between 0.5 and 0.001 m(2)kg(-1), being highest for fern and lowest for blackberry or clover at all sampling sites. Most decisive for the value of the T-ag are kind of humus deposit, thickness and pH value of the humus layers. Also important are the soil properties, whereas geology has only a minor influence on T-ag. At different sampling sites in spruce forests, the T-ag can vary by two orders of magnitude for one plant species. Caesium desorption experiments were performed. We could not find a dependence of the transfer of caesium to the plant on the desorbability of caesium from the soil, which implies a more complex transport mechanism than simple ion exchange in the soil solution. It is suggested that the transport of caesium is mediated by mycorrhiza fungi. Therefore, we studied the density of mycorrhiza hyphae in the O-f, O-h and A(h) soil horizons of two sites differing in T-ag by a factor of 10. The densities of mycorrhiza hyphae in the Oh and Ah soil horizons each differ by a factor of 2 for the two sites. Yet, the effect of the hyphae density on radiocaesium uptake has to be a subject of further investigation. (C) 1998 Elsevier Science Ltd. All rights reserved.