## 4 Conclusions  
A vast body of literature focuses on the short-term testing of the impact of soil amendment on plant growth or trace-metal solubility. The effect of amendments on the accumulation of potentially toxic trace elements in soil organisms is usually overlooked, despite its strong implications for the transfer of contaminants to the trophic chain. We tested a range of amendments commonly used in phytostabilization studies and practice.  
No earthworms survived the 4-week period after adding municipal biosolids to the soil, indicating the toxicity of fresh biosolids to these organisms. This fact reveals the limitations of testing biosolids in such short-term bioassays or/and real stress conditions shortly after the application of fresh biosolids to soils. This issue requires further studies involving the dynamics of soils' biological responses to the application of biosolids. The observed decrease in earthworm weight might indicate that no amendments fully alleviated the toxicity of the metals in this heavily contaminated soil from a post-industrial region.  
Cadmium easily transfers from soil and accumulates in 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. Cadmium accumulated in the earthworms at 112 mg per kg of earthworm tissue in a soil containing 146 mg of Cd per kg. For predators, eating earthworms from this type of soil results in the absorption of almost the same dose of cadmium as eating soil.  
The effectiveness of composts in the phytostabilization of metal-contaminated soils is well known. In our study, applying composts as single treatments reduced the pH and stimulated an increase in the solubility of cadmium and zinc. This study confirms that in contaminated ecosystems, compost must be accompanied by limiting to counteract soil pH shift and prevent the mobilization of metals.  
Despite the soil pH decrease, the significant potential of compost to reduce the accumulation of cadmium in earthworms was documented. A significant reduction in cadmium bioaccumulation was also observed after the addition of iron oxide and calcium phosphate. The potential increase in the accumulation of Zn after the compost amendment is not necessarily a negative development from the perspective of the trophic chain. In general, increases in the Zn supply reduce 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. Further studies shall involve testing combinations of amendments and longer-term experiments, also conducted under field conditions.  
Conceptualization G.S.; methodology, G.S. and M.P.; software, A.U.-J.; validation, A.U.-J. and M.P.; formal analysis, M.P.; investigation, A.U.-J. and M.P.; data curation, A.U.-J., M.P.; writing--original draft preparation, A.U.-J.; writing--review and editing, A.U.-J., G.S. and S.S.; visualization, A.U.-J.; supervision, G.S.; project administration, G.S. and M.P.; funding acquisition, G.S. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Science Centre, Poland, project no. 2015/17/N/ST10/03182 (The role of microorganisms in the colonization of smelter wastlands by plants and their impact on the bioavailability of trace elements).  
Not applicable.  
All data are available from the authors of the publication.  
We kindly acknowledge the GWDA Pila company and the Belchatow and Grabow Pilica river municipal sewage-sludge-treatment plants for sharing soil amendments for the testing.  
The authors declare no conflict of interest.