As discussed with Figure 30.6, the field management applied for the bioenergy production may not be efficient for phytoextraction with willows: for biomass production, Salix spp. are left 20 to 30 years on site and cut in winter every 3 to 4 years . From Figure 30.6 and data obtained at Dornach and Caslano, it was found that Cd and Zn concentrations were the highest in leaves. At Dornach, leaves accounted for approximately 20 and 15% of the biomass produced after 3 and 4 years, respectively, but concentrations were between twice (for Cd) and five times (Zn) larger than in stems . Thus, leaves must be harvested in order to reach the maximal extraction efficiency during phytoextraction and to prevent metal accumulation in the litter and the topsoil up to toxic levels. Because leaves must be harvested, the chipping process (chopping stems into small pieces) used in biomass production is not suitable; thus, other harvesting machines like adapted stick harvesters may need to be used. This problem may increase the price of the harvesting process and
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