## 4 Discussion
### Phytoextraction  
To date, successful long-term phytoextraction projects are still rare in the literature. A medium-duration field study (18 months) of Cd phytoextraction by _Celosia argentea_ showed that this species in combination with soil amendments can greatly improve the quality of Cd-contaminated soils [79]. Other researchers have shown that _Sedum plumbizincicola_ was able to reduce Cd and Zn concentration in the soil in long-term studies [80]. In the current study, we observed a species-specific response regarding the phytoextraction potential for some elements. For example, willow showed higher extraction rates for Cd and Cu than poplar over time. Hydroponic screening studies comparing the two species revealed that willow was able to translocate and concentrate Cd in the aboveground organs in association with a greater metal tolerance when compared to poplar [81]. This was particularly true for _S. matsudana_, which ranked among the most efficient willow species for Cd uptake and translocation to aboveground biomass [82]. This would support our findings at the field scale. Although Cu phytoextraction was higher in poplar than in willow in the first year, the tendency reversed in the following year showing better performances for willow than poplar, a result which is in line with data reported by other scientists [83]. Both species performed poorly with regard to Ni and Pb phytoextraction. However it should be noted that Pb, and to a lesser extent Cu and Ni in Salicaceae, are generally accumulated at the root level and this makes them more suitable for phytostabilization rather than phytoextraction [83]. High phytoextraction rates were observed for zinc with both species averaging annual extraction rates of 4000 g ha\({}^{-1}\). These figures are in accordance with those observed for poplar and willow in previous studies [84; 85; 86]. Finally, paired t-test comparisons of DTPA-extractable TE concentrations in soil after 4 years (Table 3) showed a statistically significant decline in soil concentrations of extractable trace elements compared to natural attenuation (unplanted)plots. In particular, we observed a significant (\(p<0.05\)) Cd (62%), Ni (57%), Pb (55%) and Zn (71%) decrease under poplar and a decrease in Cu (38%) under willow culture. Although a generalized decrease in Total TE concentration in the soil was observed for both species compared with unplanted plots (Figure 3), these differences were not statistically supported and should be regarded only as a qualitative reference. Examples of effective reductions of the TE total pool are very rare. For instance, in one studied carried out in the UK, it was observed that TEs from dredged sediments (mostly present as insoluble sulphides) could decrease due to oxidation following dredging that increased the metal mobility in the soil [87]. In the current study, we did not evaluate the recharge rate of the extractable TEs from the total TE pool. Therefore, it is difficult to predict the actual success of these species in reducing the total TEs in the long-term. There is still uncertainty around the question of whether or not a decrease in available TE soil pool will result in a prompt decrease in total TE concentration and to what extent this process may affect phytoextraction [88]. Therefore, a more in-depth investigation is still needed in regard to this particular aspect of phytoremediation.