## Materials and Methods
### Sediment Sampling and Analysis  
As described in [PERSON] et al. [18], two sediment sampling moments were performed during the experiment, one just before sowing and the second at harvest. The samples were collected with a hand auger, which permitted the sampling of the first 20 cm of the sediment depth. The sampling before sowing (middle of May) was conducted at five random collection points per plot, while the sampling during the harvest (end of July)  
Figure 1: Aerial pictures of the site experiment and the plot device with the different areas (VSRC: very short rotation coppice; Area 1; Area 2; and Area 3), _Malwa sylvestris_ L. sowing design, and pictures of the different actions realized on the field.  
consisted of ten random collection points per plot. For each sample that was collected, around 250 g of fresh sediment was dried until constant weight was achieved in a forced-air oven (40 \({}^{\circ}\)C), and the dry sediment (DS) was mechanically ground (Retsch BB51) and sieved to <2 mm. After this preparation, the pH was measured, and the extraction of TE by ammonium nitrate (1 M, NH\({}_{4}\)NO\({}_{3}\)) was performed. The pH measurements were based on NF ISO 10390 (2005), in which a mixture of 5 g of DS and 25 mL of distilled water was shaken for 2 h, followed by 1 h of rest before the pH was measured (pH meter Hanna edge). The TE was extracted from the DS following the NF ISO 19730:2008 (E) [39], in which 25 mL NH\({}_{4}\)NO\({}_{3}\) (1 M) was mixed with 10 g of DS. To obtain the leachate containing the extractable TE fraction, after 2 h of shaking, the mixture was filtered (0.45 \(\upmu\)m, Millipore), and nitric acid (HNO\({}_{3}\), 69%) was added until a pH of 2 was reached.  
A sediment composite was created for each area by mixing 50 g of fresh sediment per sample collected before sowing. On these three composites, the analysis of the agronomic sediment parameters was subcontracted (Laboratoire Departemental d'Analyses de la Mayenne, La Mayenne, France, Supplementary Materials Table S1). Moreover, total digestion of the three sediment composites was performed in triplicates after a preparation step (drying at 40 \({}^{\circ}\)C; ball mill grinding at 250 \(\upmu\)m, Lavallab Pulverisette 6). In a microwave digester (Mars Xpress CEM), a mixture of 0.2 g of sediment and 6 mL of hydrofluoric acid (HF, 48%) was digested. Then, boric acid (5.5%) was added to neutralize the HF. The mixture was completed at 50 mL with Milli-Q water and was filtered at 0.45 \(\upmu\)m (hydrophilic Teflon). The TE concentrations in digestates were analyzed either by inductive-coupled plasma optical emission spectrometry (ICP-OES, Agilent 5100, Agilent Technologies Inc., 5301 Stevens Creek Blvd Santa Clara, CA 95051, USA) or inductive-coupled plasma mass spectrometry (ICP-MS, Agilent 7500) depending on the concentration of the sediment sample concentration. One standard reference material was used for analytical quality control (NIST SRM 2710, Montana soil, National Institute of Standards & Technology, 100 Bureau drive Gaithersburg, MD 20899, USA). For all of the digestion sets, recoveries were between 0.80 and 0.93 for Cu, 0.95 and 0.98 for Pb, and 0.96 and 1.04 for Zn in the reference sediment. For Cd, no recoveries could be calculated, as the results obtained from ICP-OES analysis were below the limit of quantification (LoQ).  
The extractable and total TE concentrations in the sediment were used to calculate the % TE mobility, with % mobility = [TE] sediment extractable/[TE] sediment total * 100.