The effects of different levels of external nitrogen on the uptake, distribution and assimilation of iron cyanide complexes were investigated. Pre-rooted weeping willows (Salix babylonica L.) were grown in a hydroponic solution with or without nitrogen and amended with potassium ferrocyanide or potassium ferricyanide at 25.0 +/- 0.5 degrees C for 144 h. Faster uptake of ferrocyanide than ferricyanide was observed in willows gown in the deionized water. Negligible difference in the removal rate between the two chemicals was detected for willows grown in nutrient soultions with or without amendment of nitrogen. The volatilization of ferro- and ferricyanide due to transpiration through plant aerial tissues was below detection level. Less then 20% of the ferrocyanide or ferricyanide taken up from the N-free nutrient solution was recovered in the biomass and majority was accumulated in the roots. In contrast, less than 9.0% of both iron cyanide compelxes taken up was detected in the plant materials of willows grwon in the N-containing nutrient solution and roots were the major sites for accumulation of both chemicals. A large fraction of the ferro- and ferricyanide taken up from the hydroponic solution was assimilated during the transport within plant materials. Willows grown in the N-containing nutrient solution showed a higher assimilation potential for both chemical forms than those grown in the N-free nutrient solution in general. The information collectively suggests that uptake and assimilation mechanisms for ferro- and ferricyanide are largely different in willows; the strength of external nitrogen had a negligible effect on the uptake of both chemicals, while assimilation of ferro- and ferricyanide in plant materials was strongly related to the presence of easily available nitrogen in the hydroponic solution. (C) 2007 Elsevier B.V. All rights reserved.