The Effect of Soil Amendments on Trace Elements’ Bioavailability and Toxicity to Earthworms in Contaminated Soils

The aim of this study was to assess the impact of soil amendments, characterized by different sorption properties, on the effectiveness of trace elements’ (Cu, Zn, Pb, Cd, Ni, and Cr) stabilization and bioavailability to earthworms. The study was conducted as a microcosm experiment using soil derived from a heavily contaminated post-industrial area. The Eisenia veneta earthworm was cultured for 4 weeks in soils amended with materials characterized by different properties, origins, and potential effects on limiting the availability of metals in soils: two type of compost (Zabrze compost-ZC; GWDA compost-GC), two types of biosolid (Bełchatów biosolids-BB, Grabów biosolids-GB), calcium phosphate (CP), iron oxide (IO), bentonite (BE), rock waste (RW), and limestone (CC). After the incubation, the biomass and survival numbers of the earthworm species decreased significantly (p < 0.05). The accumulation of metals in the earthworm tissues expressed by the bioaccumulation factor value (BSAF) were dependent on the type of amendment applied to the soil. The highest decrease in the earthworms’ weight and survival rate was caused by compost (72%) and bentonite (33%), while the lowest was caused by the rock waste (10%) and iron oxide (11%). The biosolids exhibited the greatest toxicity, causing the mortality of all the earthworms. The accumulation of metals in earthworm tissues and the BSAF value were dependent on the type of amendment applied to the soil. The BSAF for the contaminated soil by Cd decreased to the greatest extent after the addition of ZC (by 57%), GC (55%), CP (41%), and IO (37%). A similarly positive effect was noted for Pb after IO addition (45% decrease). The Zn, Cr, and Ni concentration in earthworms, contrary to other elements, increased, regardless of the amendment. The results showed that the applied soil amendments were characterized by varying potential for the reduction in the metal bioavailability in the soil, depending on their composition and physicochemical properties. Moreover, earthworms may exhibit a diversified response to soil amendments as a result of the impact of amendment on the metal forms in soils and their direct impact on organisms. Generally, the Cd was easily transferred from the soil into and accumulated in the earthworm tissues. Our study confirms that this element creates the highest risk for the trophic chain in soils affected by the Zn and Pb smelting industry. Moreover, greater Zn supply reduces the accumulation of Cd in animal bodies. This study provides valuable practical knowledge on the short-term biological effects of a range of soil amendments in metal-contaminated soils