## 3 Results
### Phytoextraction and Soil Quality  
The recovery rates for trace elements in the standard reference material were between 90.3 and 95% (Table 2), thus validating the generalized high accuracy and precision of laboratory procedures. Figure 1: Willow and poplar woody (**A**) and leaf (**B**) biomass yields for the four years of the trial. Values are mean \(\pm\) SD (n = 20) for each species. A 5% significance level was adopted for identifying significant treatment effects according to [PERSON]’s HSD test. Different letters on bars denote a significant difference among species. The observation of the phytoextraction rates overtime highlighted species-specific differences for specific trace elements (Figure 2). Cadmium annual extraction rate was not detectable in the establishment year for both species, averaged around 8 g ha\({}^{-1}\) in the second year (2016) and was significantly higher in willow (15 g ha\({}^{-1}\)) than in poplar (11 g ha\({}^{-1}\)) in the following two years. The annual rate of copper phytoextraction changed over time. Whilst poplar showed significantly higher phytoextraction in the first year as compared to willow (i.e., 265 vs. 97 g ha\({}^{-1}\)), the difference tended to reverse over the course of time so that, by the end of 2018, willow showed significantly higher extraction rates compared to poplar for this element (i.e., 146 vs. 96 g ha\({}^{-1}\)). Nickel was not detected in the biomass of either species in the establishment year and although its extraction rates increased over time, both species showed similar values (average 17 g ha\({}^{-1}\) yr\({}^{-1}\)). Highly significant differences were observed for lead in the establishment year where poplar again was the better performing species as compared to willow. Such differences were also observed in 2016 and 2017, whereas in the last year, no differences were found between the species in terms of Pb phytoextraction. With regard to zinc, except for the first year where poplar extracted more element than willow (i.e., 6500 vs. 2600 g ha\({}^{-1}\)), the extraction rate was very high for both species, averaging 3800 g ha\({}^{-1}\) yr\({}^{-1}\) for the following three years. \begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline \multirow{2}{*}{**Elements**} & \multicolumn{2}{c}{**Standard Material (mg kg\({}^{-1}\))**} & \multicolumn{2}{c}{**Observed in the Current Study**} & \multirow{2}{*}{**Recovery (\%)**} \\ \cline{2-2} \cline{5-8}  & **Mean** & & **SD** & **Mean** & & **SD** & \\ \hline Cd & 0.12 & \(\pm\) & 0.01 & 0.11 & \(\pm\) & 0.02 & 91.7 \\ Cu & 10.2 & \(\pm\) & 0.5 & 9.2 & \(\pm\) & 0.6 & 90.3 \\ Ni & 15.2 & \(\pm\) & 0.6 & 13.9 & \(\pm\) & 0.5 & 91.2 \\ Pb & 1.67 & \(\pm\) & 0.11 & 1.53 & \(\pm\) & 0.13 & 91.6 \\ Zn & 30.5 & \(\pm\) & 1.1 & 29.1 & \(\pm\) & 0.9 & 95.5 \\ \hline \hline \end{tabular}  
* Recovery of elements from analysis of certified reference material (CRM) of ryegrass ERM®—CD281 RYE GRASS, European Commission, Joint Research Centre, Institute for Reference Materials and Measurements, Belgium (n = 2 \(\pm\) standard deviation SD).  
\end{table} 
Table 2: Recovery rates for the standard material used in the current study. Figure 2: _Cont._  
The variation in soil pollution over time is reported in Table 3 and visually in the 2D maps in Figure 3. Although both species proved capable of decreasing the pollution concentration on average to a higher extent (i.e., 38% poplar and 34% willow) than natural attenuation (i.e., 4.5% reduction due to natural attenuation), significant effects were observed only in the DTPA-extractable fraction. Over a period of four years, poplar was able to significantly decrease the initial Cd (62%), Ni (57%), Pb (55%) and Zn (71%) DTPA-extractable fractions and nearly the initial total Cu (\(p\) = 0.055) and Zn (\(p\) = 0.059) soil concentrations as well. The plots planted with willow showed a significant reduction in DTPA-extractable Cu (38%) and an almost-significant (\(p\) = 0.06) decrease in DTPA-extractable Zn (70%).