# The Suitability of Short Rotation Coppice Crops for Phytoremediation of Urban Soils
## 3 Results and Discussion
### Phytoextraction Potential  
The performance of a phytoextraction arises from the combination of biomass production and efficacy of elemental uptake. The phytoextraction rate was calculated for Cu and Zn, the only metals signifacantly uptaken by plants (Figure 3). The sum of the the biomass produced from all the clones during the first year after the transplant was very low, leading to phytoextraction rates considerable lower than during the second year.  
Aboveground biomass accumulated in the majority of PTE in all the tested genotypes.  
Regarding Zn, during the first year the majority of PTE was present in leaves, while during the second year in stem. Biennial coppiced parcels of _Salix_ stored higher amounts of PTE in stem due both to the higher biomass produced and to the higher concentrations. Contrarily, _Poplar_ clones (in particular P1) presented similar phytoextraction rates regarding stem values and a significantly higher rate of extraction considering leaves.  
Belowground root accumulation of PTE is for interest shown in a phytostabilization experiment, to prevent the leaching of the contaminant to the aquifer or the runoff of it. In this experiment, Zn root uptake was constant over the two years for _Poplar_ and _Salix_ clones, between 6% and 17% of the total uptake of the plant (Table 4). _Robinia_ genotypes, instead, stored higher amounts of metals in roots using biennial coppicing (up to the 23% of the total), mostly due to the higher root biomass developed under this management.  
Figure 3: Estimated phytoextraction rates for the studied clones in the three plant tissues. For each clone, tissue uptakes are stacked.  
Root accumulation of Cu was more prominent than for Zn, in particular for annually coppiced plants during the second year, with up to the 44% of the Cu allocated in root biomass.  
Pooling all these data, we calculated the total uptake from plants, reported in Figure 3 and in Table S5 as average values for species. _Robinia_ clones were the most productive species for Cu phytoextraction (Figure 3), particularly with biennial coppicing, due to their greater biomass production.  
With this management, the more productive clone for Cu phytoextraction was R1, while using annual coppicing P1 yielded better results, although slightly lower than biennial coppiced R1.  
Zinc phytoextraction was more important, bearing the results in line with literature studies during field trials [13; 17]. Accumulation yields were higher with annual coppicing for poplar clones, with P1 having the highest phytoextraction rate during the second year (1077 g ha\({}^{-1}\)). _Salix_ and _Robinia_ clones performed better on biennial management, with clones S1 and S3 extracting, respectively, 919 and 997 g ha\({}^{-1}\) of zinc, not statistically different from P1.  
This total uptake did account also for the root system, containing between 9% and 11% of the Zn and not removed in phytoremediation trials. However, for the calculation of the translocation of PTEs in plant tissues it is of primary importance in the management of a trial and is evident from Figure 3 that leaves carry an important share of the PTE uptake. In particular, although the total uptake was not so large, the allocation in leaves represented 39% of the accumulation in clone P1, while in the case of willows 13% and the 8%, respectively, in S1 and S3 clone.  
In this case, therefore, a successful phytoremediation should include the collection of leaves, a practice little used in field trials, as it will increase costs. In an urban context the collection will be easier, as the city is already acquainted with the collection of leaves on the streets and is thus equipped.