# The Suitability of Short Rotation Coppice Crops for Phytoremediation of Urban Soils
## 3 Results and Discussion
### PTE Concentrations in Plant Tissues  
During the field experiment, the metal uptake from species was evaluated each year in plants coppiced every year or biennially, to find which method would better balance metal uptake and biomass production.  
In Figure 2 the results for Cu and Zn are reported as the most representative contaminants, although Cu concentrations were under the legislative limits. Nickel and Pb were detected in roots and in leaves of some clones, while in the stems their concentration was not detectable (data reported in SM, Table S4). Cadmium and Co values in plants were below the detection limit in most of the samples, in line with the low bioavailable concentrations in soils.  
Concentrations of metals in the different tissues are in line with previous studies in moderately contaminated soils [17; 39] where it is reported that specific plants concentrate PTE in different parts.  
Copper concentrations in leaves rose moderately during the second year in _Poplar_ clones and in S2, with higher results in the leaves of the biennial rotation plants. This higher translocation was reflected in the stem of both species, where most of the genotypes have metal concentrations significantly higher in biennial coppiced plants than in annual. This is probably due to the major efficiency of the uptake from roots in undisturbed plants, partially confirmed from the metal concentrations in roots, that are significantly higher in both samples taken at the end of the second year than after the first year.  
With regard to Zn, the accumulated concentrations during the second year were significantly higher than during the first year for all clones of _Salix_ and _Poplar_. In the leaves, the concentrations in coppiced blocks of the second year were significantly higher than in non coppiced ones. Clones P1, P3, and S2 presented the highest concentrations in leaves, while in stem and roots S2 and S3 genotypes accumulated the highest levels of Zn. In these last tissues, annual and biennial coppicing did not determine statistically different concentrations.  
From a management perspective, as stem is the tissue mostly removed in phytoremediation applications (as only few plantations remove leaves from the field), _Salix_ clones with biennial coppicing would be the most suitables in terms of relative translocation.  
On the contrary, _Robinia_ was not an accumulator of Zn, as tissue concentrations were 10 fold lower than _Poplar_ and _Salix_ clones and did not increase in the second year.  
Observed concentrations for _Poplar_ and _Salix_ species were similar to published studies in mildly contaminated soils [13; 40], with a clear increase in PTE uptake during the second year after transplant.  
A suitable method to evaluate the ability of plants to accumulate PTE in tissues is the calculation of the bio-concentration factor (BCF) [31]. The only tissue where a concentration (with respect to soil) occurred was leaves, were BCF for both annual and biennial coppicing blocks during the second year were higher than in the first. In order to have the potential to be used as a phytoextractor, a plant should have leaves and stem concentrations of the contaminant higher than the concentration in the soil. In this field trial, concentrations in stem and roots were similar to the concentration of Zn in the soil, thus with a BCF around or lower than 1, probably because of the short duration of the experiment.