Environmental Pollution Title: Metal Immobilization and Soil Amendment Efficiency at a Contaminated Sediment Landfill Site: a Field Study Focusing on Plants, Springtails, and Bacteria Author Names and Affiliations

28 Metal immobilization may contribute to the environmental management strategy of 29 dredged sediment landfill sites contaminated by metals. In a field experiment, 30 amendment effects and efficiency were investigated, focusing on plants, springtails and 31 bacteria colonisation, metal extractability and sediment ecotoxicity. Conversely to 32 hydroxylapatite (HA, 3 % DW), the addition of Thomas Basic Slag (TBS, 5 % DW) to a 33 5-yr deposited sediment contaminated with Zn, Cd, Cu, Pb and As resulted in a decrease 34 in the 0.01M Ca(NO3)2 extractable concentrations of Cd and Zn. Shoot Cd and Zn 35 concentration in Calamagrostis epigejos, the dominant plant species, also decreased in 36 the presence of TBS. The addition of TBS and HA reduced sediment ecotoxicity and 37 improved the growth of the total bacterial population. Hydroxylapatite improved plant 38 species richness and diversity and decreased antioxidant enzymes in C. Epigejos and 39 Urtica dïoica. Collembolan communities did not differ in abundance and diversity 40 between the different treatments. 41 42