Plant and Soil 235: 11–20, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. 11 Genotypic effects in phytoavailability of radiocaesium are pronounced at low K intensities in soil N. Waegeneers 1,3,4 , M. Camps 2 , E. Smolders 3 & R. Merckx 3 1 SCK·CEN, Radioecology Laboratory, Boeretang 200, B-2400 Mol, Belgium. 2 Departament de Qu´ ımica Anal´ ıtica, Universitat de Barcelona, Diagonal 647, E-08028 Barcelona, Spain. 3 Laboratory of Soil Fertility and Soil Biology, Department of Land Management, K.U. Leuven, Kasteelpark Arenberg 20, B-3001 Heverlee, Belgium. 4 Corresponding author Received 22 August 2000. Accepted in revised form 9 April 2001 Key words: bioavailability, genotype, potassium, radiocaesium Abstract The differences in radiocaesium uptake between species were analysed in a series of solution culture and pot trials. Since radiocaesium uptake is very sensitive to the solution potassium (K) concentration, it was hypothesised that species depleting K in the rhizosphere to a larger extent, will have a higher radiocaesium uptake. Five species (bean, lettuce, winter barley, ryegrass and bentgrass) were grown for 18–21 days in nutrient solution spiked with 137 Cs and at 4 K concentrations between 0.025 and 1.0 mM. Shoot 137 Cs activities all decreased between 17- and 81-fold with increasing K supply. Shoot 137 Cs activities were 4-fold different between species at the lowest K supply and 3.4-fold different at high K supply. The same five species were grown in two 134 Cs spiked soils with contrasting exchangeable K but similar clay content. Shoot 134 Cs activities were up to 19-fold higher in the soil with lowest exchangeable K. Differences in shoot activity concentrations between the species were only 4.5-fold in the high K soil, but were 15-fold in the low K soil. Bulk soil solution 134 Cs and K concentration data were combined with radiocaesium uptake characteristics measured in solution culture to predict radiocaesium uptake from soil. Predictions were within 1.6-fold of observations in the high K soil but largely underestimated 134 Cs uptake in lettuce, ryegrass and barley in the low K soil. A solute transport model was used to estimate K and radiocaesium concentrations in the rhizosphere. These calculations confirmed the assumption that higher radiocaesium uptake is found for species that deplete K in the rhizosphere to a larger extent. Abbreviations: AAS – atomic absorption spectrophotometry; AC – activity concentration; CEC – cation exchange capacity; CF – concentration factor (m 3 g -1 ); DAS – days after sowing; K D – solid/liquid distribution coefficient; RIP – radiocaesium interception potential (mol c kg -1 ); TF – transfer factor (Bq g -1 /Bq g -1 ) Introduction The risk of foodchain contamination in radiocaesium contaminated land is traditionally expressed with soil– plant transfer factors (TF), the ratio of the activity concentration in the crop to that in the soil. Dif- ferent compilations of TF have been made (Nisbet and Woodman, 2000; UIR, 1989) and it appears that TF are highly variable, depending amongst others on ∗ FAX No: +32-1632 1997. E-mail: nadia.waegeneers@agr.kuleuven.ac.be crop and soil type. The variability of Cs availability between soils has been studied intensively in recent years (Gerzabek et al., 1998; Sauras et al., 1999; Smolders et al., 1997). It is now generally recognised that Cs uptake by plants is controlled by (i) the Cs in soil solution (= supply) and (ii) the K concentra- tion in soil solution which affects the radiocaesium uptake rate by the root (Roca et al., 1997; Sanc- hez et al., 1999; Smolders et al., 1997). As such, 137 Cs-availability increases with decreasing 137 Cs re- tention (by adsorption/desorption processes) and with