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For assessment of clonal variability of willow plants seven clones of high biomass production willows (S. x smithiana S-218, S. x smithiana S-150, S. viminalis S-519, S. alba S-464, S. alba "Pyramidalis" S-141, S. dasyclados S-406, S. x rubens S-391) were planted in a pot experiment for three vegetation periods in two soils differing in total content of risk elements [39] as characterized in Table 7. Because of serious symptoms of Zn phytotoxicity at extremely contaminated Fluvisol the experiment was terminated after second vegetation period at this soil. Comparing the remediation factors, reasonable phytoextraction potential of willows was obtained for cadmium and zinc at moderately contaminated Cambisol where aboveground biomass removed about 30% Cd and 5% Zn of total element content, respectively. Clones showed different ability to remove Cd and Zn, depending on soil type and contamination level: S. x smithiana (S-150) and S.x.rubens (S-391) demonstrated the highest phytoextraction effect for Cd and Zn. The differences in accumulation between the clones are due to the property of clone and not to the soil element concentration or properties [24, 35, 40]. The mechanisms behind the accumulation, transport, and tolerance are specific for each of the different metals and the uptake property of the clone is stable [34]. In extremely contaminated Fluvisol production of willow biomass was limited by phytotoxicity of zinc resulting in phytoextraction efficiency not exceeding 1% for both Cd and Zn (Figure 4). Concerning arsenic and lead, poor ability of willows to translocate these elements from roots to aboveground biomass led to low removal of these elements from soil (less than 1%) and application of these plants for cleaning of As and Pb contaminated soil is not reasonable. Similarly Nissen and Lepp [22] found the evidence for the exclusion of Cu from shoot tissue indicating low potential for depletion of the plant available soil Cu pool as a consequence of repeated cropping and for concentration in combustion residues.

The cultivation of willows can decrease bioavailable portion of soil cadmium even in deeper horizons. The annual removal of 3 - 4% of plant-available portion of Cd was demonstrated in dependence on mobilization/immobilization processes of cadmium bound in less mobile soil fractions [19, 25, 41-42]. Moreover, potential of willow trees to reduce element leaching to groundwater as well as to stabilize soil characteristics was already described [41-42]. Difference between cadmium uptakes by roots within soil profile was not observed [25]. Perttu et al. [29] also described the uptake of Cd in Salix from deeper soil layers. The actual decrease in the topsoil after harvesting is therefore less than what may be theoretically calculated. However, the risk of increasing Cd concentrations, because of the relocation from deeper soil layers, is small as long as the percentage is less than about 60% of total uptake. Normally, more than 70% of the active roots are situated in the topsoil and there is no indication that the roots penetrating into the subsoil are more effective than roots in the topsoil.

Table 7. Basic characteristics and total element contents in experimental soils [39]

Soil

pH

Corg.

As

Cd

Pb

Zn

(%)

mg.kg-1

mg.kg-1

mg.kg-1

mg.kg-1

Cambisol

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