The detoxification of iron cyanide in a soil-plant system was investigated to assess the total cyanide extracted from contaminated soil and allocated in the leaf tissue of willow trees (Salix caprea).They were grown in soil containing up to 1000 mg/kg dry weight (dw) of cyanide (CN),added as 15N-labeled potassium ferrocyanide and prepared with a new method for synthesis of labeled iron cyanides.CN content and 15N enrichment were monitored weekly over the exposure in leaf tissue of different age.The 15N enrichment in the young and old leaf tissue reached up to 15.197‰ and 9063‰,respectively;it increased significantly over the exposure and with increasing exposure concentrations (p ＜ 0.05).Although the CN accumulation in the old leaf tissue was higher,compared to the young leaf tissue (p ＜ 0.05),the 15N enrichment in the two tissue types did not differ statistically.This indicates a non-uniform CN accumulation but a uniform 15N allocation throughout the leaf mass.Significant differences were detected between the measured CN content and the C15N content,calculated from the 15N enrichment (p ＜ 0.05),revealing a significant CN fraction within the leaf tissue,which could not be detected as ionic CN.The application of labeled iron CN clearly shows that CN is detoxified during uptake by the willows.However,these results do not exclude other detoxification pathways,not related to the trees.Still,they are strongly indicative of the central role the trees played in CN removal and detoxification under the experimental conditions.