Biostimulant-treated Miscanthus × giganteus for phytomanagement: a two-layer rhizotron experiment on deep rooting, metal phytostabilization and uptakes

Background and aims: Phytomanagement of metal-contaminated soils with the perennial energy crop Miscanthus × giganteus is a cost-effective option combining valuable biomass production with metal phytostabilization. To revegetate degraded areas where adverse conditions (e.g., water scarcity) can hinder plant growth, the application of root biostimulants may enhance plant resilience and deep rooting development. This study evaluated the combined effects of mycorrhizal fungi and humic substances (MB treatment) on the establishment of miscanthus grown in a metal-contaminated soil, specifically: miscanthus deep rooting, root metal uptake and the resulting metal bioavailability changes within the soil profile. Methods: MB-treated and untreated plants were cultivated in greenhouse cylindrical rhizotrons (66L), enabling the study of deep root growth across two superimposed soil layers: L1 (0–0.45 m) and L2 (0.45–0.90 m). This soil from a former landfill was contaminated with zinc and copper, and to a lesser extent with nickel and lead. Results: The MB treatment significantly enhanced miscanthus rooting performance increasing root water uptake efficiency by + 2.5 g L⁻¹ and dry root density, which was nearly fourfold higher than in the control plants. Moreover, MB-treated plants exhibited greater root uptake of zinc, copper and lead. At the harvest, the MB treatment reduced Cu concentration in the soil pore water, while NH₄NO₃-extractable lead and zinc concentrations decreased in the L2. Conclusion: The MB treatment promoted miscanthus establishment and strengthened its phytostabilization capacity. Despite increased root metal uptake, unchanged shoot metal concentrations confirmed the miscanthus metal-excluder behavior, retaining shoot suitability for biofuel production.