## 3 Results and Discussion
### M. sylvestris Effect on TE Extractable Fraction and Mobility in the Sediment  
The pH of the sediment was measured to assess the potential impact of _M. sylvestris_ on its acidification by the root system [42]. pH variation has been reported as a factor that could significantly impact the solubility and mobility of TE in sediment [43]. All of the pH values measured varied between around 8 and 8.4 (Supplementary Materials Figure S1). In our sediment conditions, the cultivation of _M. sylvestris_ slightly increased the pH of the area 3 (0.16 unit) at the harvest time (\(p<0.05\)). The TE (Zn, Cd, Pb, Cu) sediment concentrations retrieved in the extract solution obtained by NH\({}_{4}\)NO\({}_{3}\) extraction are presented in Figure 2. These TE concentrations represent the most soluble fraction in the sediment solution [39] and therefore have a higher environmental risk because of their potential availability and leaching into the groundwater. The four TE extractable concentrations were measured on all of the sediment sampled at the beginning of the experiment (i.e., without the plant) and at the harvest. At both sampling periods, the extractable concentrations exceeded the extractable concentration frequently retrieved in soils [39]. Depending on the areas, the Zn and Cd boxplots (Figure 2) showed similar spreading data at the start of the growing process and at harvest. The spreading data were the highest in the area 3, especially at the time of harvest (Figure 2). Concerning the Pb boxplots (Figure 2), on all areas although the concentrations that were measured were low, deviations from the mean were high. For the Cu boxplots (Figure 2), the spreading data increased at the time of harvest in area 2 and area 3 compared to at the start. Figure 2: Sediment NH\({}_{4}\)NO\({}_{3}\) extractable concentration of Zn, Cd, Pb, and Cu in Area 1 (green), Area 2 (blue), and Area 3 (orange) at the start (\(n=5\)) and at harvest (\(n=10\)). The box plots indicate the median (black line), the 25 (lower bar), and the 75 (upper bar) percentiles; red cross = mean; the red line corresponds to the value references noticed in the ISO 19730:2008\(\copy\). Significant differences between conditions after Tukey HSD test are indicated by different letters at the level of \(\alpha=0.05\). In the beginning, the extractable concentrations of Zn in Area 3 were above values measured in Area 1 (\(p<0.05\), Figure 2), and this difference persisted at the end of the experiment. A moderate pH correlation could partially explain this slight area effect (\(p<0.05\), \(r=-0.34\)), as a lower pH could increase the soluble Zn fraction. _M. sylvestris_ cultivation did not change the Zn extractable concentration and the low sediment mobility (\(<\)0.1% of total Zn, Supplementary Materials Table S2). The extractable Cd concentrations (Figure 2) presented a similar pattern to Zn. The highest extractable Cd concentrations were measured in Area 3 at both sampling periods (\(p<0.05\)), indicating an area effect. The Cd extractable concentration correlation with the sediment pH could also explain this observation (\(p<0.05\), \(r=-0.43\)). The cultivation of _M. sylvestris_ also did not affect the extractable fraction (\(p>0.05\)) nor the mobility of Cd in the sediment (Supplementary Materials Table S2). Cd has been shown to be less mobile in alkaline soil [43], such as in the soil of our site. Nevertheless, compared to the three other TE, the Cd mobility could be considered high (\(<\)0.69% of the total Cd, Supplementary Materials Table S2). [PERSON] [43] suggested that the increase of soil alkalinity could reduce Cd\({}^{2+}\) adsorption due to the presence of other elements (Ca\({}^{2+}\) and Mg\({}^{2+}\)) and by the increase of CdOH\({}^{+}\), which could not easily interact with the cationic exchange capacity (CEC). It could be assumed that the high Cd mobility was due to the presence of ionic Cd in the sediment. Concerning Pb (Figure 2), the extractable concentration that was measured did not change depending on the area and the period (\(p>0.05\)).