## 3 Results
### Carbon Dioxide Sequestration Value  
Table 4 shows the carbon dioxide sequestration value starting from the annual removed biomass (epigean) of poplar and willow and the average price of carbon dioxide per year [57]. For the analysis, as argued by [PERSON] and [PERSON] [58], we have considered that carbon stocked is about 50% of the total tree biomass. Figure 3: Temporal evolution pattern of total and DTPA-extractable trace element concentrations in the soil at the experimental site for each treatment. Comparisons are made between initial (2015) and final (2018) values for Cd (**A**), Cu (**B**), Ni (**C**), Pb (**D**) and Zn (**E**). The 2D maps were obtained by assigning the value observed from each parcel to the midpoint of the respective parcel. The points were spatially interpolated with a natural neighbor method over a 100-by-100 point grid. The average carbon price used for the analysis is related to the emission trading system (ETS) that \"sets a limit (\"cap\") on total direct GHG emissions from specific sectors and sets up a market where the rights to emit (in the form of carbon permits or allowances) are traded. This approach allows polluters to meet emission reduction targets flexibly and at the lowest cost. It provides certainty about emission reductions, but not the price for emitting, which fluctuates with the market\" [59]. Considering that the prices used refer to carbon dioxide, we have transformed the biomass carbon stocked into biomass carbon dioxide stocked using a conversion factor equal to 3.67 [60; 61]. The analysis considers four annual removed biomasses in four different periods (2015 to 2018). In order to compare (i.e., sum) these values, it is necessary to report them in a single time period, fixed at 2018 (end of our analysis). For this purpose, a linear model for postponement of capital was applied (Equation (4)), as suggested by [PERSON], [PERSON] and [PERSON], and [PERSON] et al. [62; 63; 64]. \[\mathrm{C_{n}=C_{0}\cdot q^{n}} \tag{4}\]  
where  
\(\mathrm{C_{n}=value}\) of carbon dioxide sequestration at the end (year n) of our analysis (2018) (EUR ha\({}^{-1}\));  
\(\mathrm{C_{0}=value}\) of carbon dioxide sequestration at different years (i.e., 2015, 2016, 2017, 2018) (EUR ha\({}^{-1}\));  
\(\mathrm{q=1+r}\) (r is the market discount rate);  
\(\mathrm{n=length}\) of the period (time between \(\mathrm{C_{0}}\) and \(\mathrm{C_{n}}\)). A problem associated to this type of analysis is the uncertainty related to the choice of discount rate [65; 66; 67]. In order to reduce this uncertainty, a sensitivity analysis was conducted [68]. An important assumption is to maintain the permanence of the conditions during the analysis; for this purpose, a sensitivity analysis for one factor at a time was used [69]. In detail, the discount rate was considered variable over time by applying values between 1% and 8% [29; 67; 70]. Results are shown in Table 5.  
The values reflect the biomass obtained annually from the two species; therefore, considerably higher values are obtained for poplar than for willow. This is due to an average biomass collected in the years considered of 15.68 Mg ha\({}^{-1}\) (poplar) and 13.58 Mg ha\({}^{-1}\) (willow). Considering the different discount rates, the value range (differences between species) is between EUR 110 and 150. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Carbon Dioxide Stock**} & \multicolumn{4}{c}{**Year**} \\ \cline{2-6}  & **2015** & **2016** & **2017** & **2018** \\ \hline Willow B (Mg ha\({}^{-1}\)) & 15.40 & 30.43 & 29.88 & 23.83 \\ Polar B (Mg ha\({}^{-1}\)) & 34.28 & 32.45 & 25.85 & 22.37 \\ Carbon dioxide price Cs (EUR Mg\({}^{-1}\)) & 7.68 & 5.35 & 5.83 & 15.88 \\ Willow Pc (EUR ha\({}^{-1}\)) & 118.27 & 162.82 & 174.22 & 378.47 \\ Polar Pc (EUR ha\({}^{-1}\)) & 263.29 & 173.60 & 150.70 & 355.18 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Carbon dioxide sequestration of willow and poplar.