Immobilisation of metals in a contaminated soil with biochar-compost mixtures and inorganic additives: 2-year greenhouse and field experiments.

Besides carbon sequestration and improvement of soil properties, biochar (BC) has increasingly been studied as an amendment to immobilise heavy metals in contaminated soils. In a 2-year experiment, we analysed the effects of poplar BC (P-BC, mixed with compost) and gravel sludge with siderite-bearing material (GSFe) on a Cd-, Pb- and Zn-contaminated soil and on metal concentration in Miscanthus × giganteus shoots under greenhouse and field conditions. In the greenhouse, 1% (m/m) P-BC addition reduced NH4NO3-extractable Cd, Pb and Zn concentrations by 75, 86 and 92%, respectively, at the end of the study. In the leachates, P-BC (1%) could significantly reduce Cd and Zn in both years. In the field, P-BC (3%) induced a reduction of extractable Cd by 87% whereas a combination of P-BC + GSFe reduced Pb by 82% and Zn by 98% in the first year and by 83 and 96% in the second year. In contrast, the metal immobilisation in the soil was hardly reflected in the shoots of Miscanthus × giganteus which generally showed metal concentrations close to control. While Cd was not influenced in both years, Pb and Zn were slightly reduced. Our study confirmed that Miscanthus is an efficient metal excluder, corroborating its suitability for the production of renewable biomass on metal-contaminated soils.