Comparative analysis of

A quantitative analysis of 137Cs bioavailability in forest soils in the long term after the Chernobyl NPP accident based on a 3 years (1996-1998) investigation is presented. Five sites with different trees composition and properties of soil were studied to identify factors determining radiocaesium transfer to trees and different under story species. The following parameters were investigated: 137Cs activity concentrations and its speciation in various horizons of forest soil, accumulation of this radionuclide by different species of under story vegetation and distribution of their root biomass in the soil profile. It has been shown that one decade after the deposition maximum 137Cs activity in soil of the experimental sites considered is located in different soil layers dependent on moisture regime, characteristics of litter and soil properties. High level of heterogeneity of 137Cs specific activity in different parts of tree, which is related to physiological peculiarities of their functions, has been shown. The data obtained have demonstrated a non-uniform character for 137Cs distribution along trunks, which can be explained by radionuclide fixation by walls of xylem vessels and by changes in geometry along the tree trunk. It has been found that the radial distribution of 137Cs in the tree trunk is dependent on the availability of 137Cs in soil, which governs the transfer of this radionuclide via xylem sap, and on the properties of the xylem. The accumulation of 137Cs by both trees and under story species was influenced by 137Cs vertical distribution and availability in soil as well as by the root (mycelia) biomass distribution in different soil horizons. A "bioavailability factor", which takes into account the above factors, is proposed for comparative analyses of 137Cs transfer from soil to plants in different types of forest ecosystems