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A rhizospheric biotest, consisting of a thin layer of substratum in close contact with roots of Lolium multiflorum, was used on two contrasting contaminated soils (Cabezo and Brunita) issued from a former mining area in La Union (Spain). On top of this biotest, soil characterisation, including CaCl 2 selective extractions, was performed. Total heavy metal concentrations were the highest in the soil from Cabezo, but CaCl 2 extractions indicated higher heavy metal mobilities in Brunita soil. On the base of heavy metal concentrations and biomass production in L. multiflorum seedlings, availability assessed by the rhizospheric biotest was higher than the values obtained from CaCl 2 extraction, except for Mn and Pb. Rhizospheric biotest also revealed higher heavy metal bioavailability for Cabezo. The low pH of Brunita (3.47) could explain the high CaCl 2 -extractable heavy metal concentrations as well as the high transfer factor found for Cu, Mn and Zn in this substrate. Cu, Mn and Zn toxicities were also detected for shoot tissues. Transpiration rates were clearly lower for seedlings exposed to Brunita than for those exposed to Cabezo, while water use efficiency was higher for the former (4.8 mgDWml −1 ) than for the latter (3.8 mgDWml −1 ). Iron nutrition was found to interfere with heavy metal root absorption, mainly through negative interactions during root absorption. It is concluded that rhizospheric test offers the advantage to consider the root-soil interactions in a dynamic perspective and constitutes a useful tool for the assessment of heavy metal availability on contaminated soils. Heavy metal bioavailability assessment should not be based on only one measure alone, but on different and complementary approaches.

31 32Backgrounds and Aims Plant phytostabilization potential is a direct function of root 33systems. An experimental design was developed to investigate the impact of Cd and 34Zn on the root distribution and morphology of Lolium perenne and Trifolium repens. 35Methods Seedlings were transplanted into columns filled with washed quartz and 36 irrigated daily with Cd-or Zn-containing nutrient solutions during one month. Roots 37were then harvested at different depth and scanned to determine root length density 38 (RLD) and diameter. Pot experiments were also performed to quantify metal, lignin 39 and structural polysaccharides concentrations as well as cell viability.

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