Co-contamination with PAHs and TEs reduces the metal uptake of Salix aquatica grandis and increases dissipation

The joint presence of various contaminant categories in soil reduces the remediation potential of numerous plant species. However, the mechanisms remain poorly understood, and particularly for tree species. To achieve this goal, we performed a co-contamination experiment with Salix aquatica grandis cuttings cultivated for two months under controlled conditions in two soils with two levels (low and high) of trace element (TE) contamination, spiked or not spiked with 600 mg·kg⁻¹ polycyclic aromatic hydrocarbons (PAHs). Our results revealed a significant negative impact of PAH spiking on the mobility of Zn, Cd, and Ni, with average reductions of approximately 35 %. These changes in mobility were associated with lower concentrations in Pb and Cd in plant shoots and a decrease in the total amount of Zn extracted by the plants. These findings emphasize the influence of co-contamination on the phytoremediation potential of high-biomass species. The physiological responses of plants were also affected. Malondialdehyde (MDA) concentrations in leaves increased in response to PAH contamination, indicating oxidative stress, whereas the total antioxidant content varied in relation to TE concentrations in the soil. Furthermore, analysis of the root exudates revealed that peroxidase activity-specifically when the diamonifluorene substrate was used-responded positively to the presence of PAHs, suggesting the activation of enzymatic pathways. Overall, the combination of PAHs and TEs induced diverse and complex effects on both contaminant dynamics and plant physiological health. These results highlight the need to consider co-contamination scenarios in the evaluation and design of phytoremediation strategies particularly for long time protocols.