## 4 Discussion  
Giant reed, considered for many years only as a weed, is now considered as an energy crop that can be used in marginal areas that cannot be used by conventional agriculture [2; 38]. Due to anthropic activities, marginal lands are increasing worldwide mainly because of salinization caused by irrigation [39] and heavy metal contamination caused by industrial activities [40]. This could mean more opportunities to cultivate energy crops which are environmentally friendly, such as giant reed crops, in the near future. The consideration of novel strategies for the sustainable use of marginal lands in Mediterranean areas was the aim of the European project, OPTIMA (Optimization of perennial grasses for Biomass Production in the Mediterranean Area) [41]. In this framework, considering four perennial species, cardoon, giant reed, miscanthus and switchgrass, the OPTIMA project highlighted giant reed when compared to the other energy crops [42; 43]. Furthermore, deep-rooted perennial grasses such as giant reed (_Arundo donax_ L.) and switchgrass (_Panicum virgatum_ L.) can provide a contribution to the reduction of greenhouse-gas emissions [44; 45], producing biomass to displace fossil fuels [46] and for the phytoremediation of polluted soils. In our study, _A. donax_, switchgrass and mixed meadow biomasses produced in a polluted area in northern Italy were compared for two harvest times (October and February) in a three year experiment, with the aim of assessing the capacity of phytoextraction of the heavy metals and the best outcome for energy production (mainly anaerobic digestion and combustion) (Figure 1). This research started from the results of project LUCAS's topsoil \begin{table}
\begin{tabular}{c|c c c c c c} \hline \hline \multirow{2}{*}{**Heavy Metal**} & \multicolumn{5}{c}{**Uptake in Third Year (kg ha\({}^{-1}\))**} \\  & **Ad Oct** & **Ad Feb** & **Pv Oct** & **Pv Feb** & **MM Oct** & **MM Feb** \\ \hline Zn & 3.87 & 0.99 & 0.84 & 0.80 & 0.91 & 1.14 \\ Fe & 2.37 & 2.39 & 1.57 & 1.96 & 1.68 & 1.96 \\ Cu & 2.09 & 1.7 & 0.12 & 0.11 & 0.13 & 0.08 \\ Mn & 0.69 & 1.04 & 0.31 & 0.30 & 0.41 & 0.31 \\ Al & 0.52 & 0.83 & 0.51 & 0.72 & 0.44 & 1.06 \\ Mo & 0.07 & 0.05 & 0.04 & 0.04 & 0.05 & 0.05 \\ Cr & 0.05 & 0.09 & 0.06 & 0.07 & 0.04 & 0.04 \\ Ni & 0.03 & 0.03 & 0.03 & 0.02 & 0.01 & 0.01 \\ Pb & 0.02 & 0.02 & 0.01 & 0.02 & 0.01 & 0.02 \\ Cd & 0.007 & 0.004 & 0.006 & 0.002 & 0.002 & 0.002 \\ Co & 0.005 & 0.004 & 0.003 & 0.002 & 0.002 & 0.003 \\ As & 0.004 & 0.002 & 0.002 & 0.001 & 0.002 & 0.001 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Heavy metal uptake in the Caffaro area considering the yield from the third year of the experiment. Values are listed for _A. donax_ L. (Ad), switchgrass (Pv) and mixed meadows (MM), for October (Oct) and February (Feb) harvests. Higher values are shown on a gray background. survey, which built a consistent spatial database of polluted soil across Europe. In this project it was estimated that 137,000 km\({}^{2}\) needs local assessment and eventual remediation action [47]. Giant reed biomass has found wide applications in energy production; it can be used in combustion, gasification and pyrolysis and to produce biogas [1]. In fact, when harvested in autumn its biomass can be used in anaerobic digestion in substitution/partial integration with the traditional energy crops [48; 49; 50], and the biomass harvested in winter can be used for the combustion process, due to its high heating value of about 18.7 MJ kg\({}^{-1}\)[1]. The estimated yields for _Arundo_ and switchgrass in the third year, obtained in this work, reported in Figure 2, (respectively, about 33 and 19 tons ha\({}^{-1}\)), substantially confirm the data present in the literature where _A. donax_ in the Mediterranean basin is the most productive energy crop [1; 51; 52; 53].