# Native Plant Capacity for Gentle Remediation in Heavily Polluted Mines
## 4 Discussion  
The content of trace elements in the soils analyzed varied markedly due to the long history of mining in the area [2; 3; 28]. Meanwhile, plant species that apparently grew in areas outside the spoil heaps accumulated high contents of some trace elements, e.g., Mn in soils under _C. salviofilus_ (see Table 1), which accounts for the high concentrations of this element that were found, especially in its roots (Table 3). The Mn determined the distribution of the _C. salviofilus_ samples in contaminated and uncontaminated soil, as reflected in the PCA ordination diagrams for soils and roots (Figures 4 and 6). The element with highest soil concentrations of all those analyzed was Fe, which was found mainly in the form of oxides and hydroxides as small particles, or associated in amorphous form with the surfaces of other minerals [4; 28; 44]. Fe correlated with the rest of the elements studied in the soil (Table S2 in Supplementary Material), linking all the trace elements studied in the mining area. This element was also correlated with Pb, Cu, and Mn in roots and leaves whereas Fe and Zn did not correlate in these plant organs, as pointed out by previous authors [44]. After Fe, the next most concentrated trace elements in the soils were Zn and Mn. The concentrations found in these two elements were, in general, higher than those reported by [28] in soils from sites close to our study area. However, the standard deviation of the means proved similar to ours, confirming again the great heterogeneity of the area, which, on the other hand, made it ideal for this type of study, since it provided highly variable concentrations and gradients. The Zn concentration in all of the contaminated soils under _C. salviofilus_ as well as in uncontaminated soil exceeded 1500 mg Kg \({}^{-1}\), the permissible threshold value established for industrial soils in Italy, DLgs 152/06 [17]. Moreover, the Pb concentration in our study was higher than observed by other authors in Sardinia [23; 28], values falling within the range indicated by [17] and researchers examining sediments [26]. The concentration of Pb was higher in all the contaminated soils, and in the case of _C. salviofilus_ also in uncontaminated soils, than that established as the permissible threshold value for industrial soils in Italy, DLgs 152/06 [17]. Cu values were  
Figure 7: Redundancy Analysis (RDA) ordination diagram showing the spatial arrangement of samples (points) with regard to the leaf (blue arrows) and soil (red arrows) concentrations in trace elements. low, but similar to those reported by [21] in Naples, or those indicated by [28] in the least polluted places in the Emilia-Romagna region. The leaves and roots of _D. viscosa_ registered concentrations of Zn and Pb similar to those observed by [23], in plants growing in soils of Sardinia, although in the soils studied by these authors the concentrations were lower. In other studies [28] the concentrations were lower than those found in the present study. The bioaccumulation factors in leaves and roots (BCFl and BCFr) resembled those noted by other authors [23; 28] but were less than 1 in all cases. However, the BCFr values of Pb for _D. viscosa_ in non-contaminated soils were very close to 1. Moreover, in _E. pithyusa_, higher BCF values for Pb and Cu appeared in non-contaminated soils than in the rest of localities and plant species. This could be due to the high concentrations of trace elements in contaminated soils that saturate the plant's capacity to store these elements in its tissues. However, when the concentration in soil is lower, the relationship with the concentration in the plant increases and the bioaccumulation factor approaches 1. Although the use of these factors is widespread in the literature, some [17; 45] have pointed out the dependence of these accumulation factors with the soil concentrations since they can lead to a false interpretation of the analytical data, especially when the plants grow in highly contaminated soils such as spoil heaps presented in this manuscript.