## 3 Results and Discussion
### Solubility of Trace Elements in Soils Driven by Applied Amendments and Earthworm Activity  
The extractability of metals in the 0.01M Ca(NO\({}_{3}\))\({}_{2}\) was dependent on the applied soil amendments and, in some cases, on the activity of the earthworms. Among all the analyzed TE, only the Zn and Cd were characterized by higher contents of the extractable fraction, while the other elements (Pb, Cu, Ni, and Cr) were below the detection limit (0.081 mg kg\({}^{-1}\), 0.050 mg kg\({}^{-1}\), 0.032 mg kg\({}^{-1}\), and 0.099 mg kg\({}^{-1}\), respectively for the Pb, Cu, Ni, and Cr). Generally, the content of extractable Cd (Figure 2) in the non-amended soil was about six times lower than that of the Zn (Figure 3), which was partly related to the magnitude of their total contents in the soil. On the other hand, the extractable Zn fraction accounted for 0.01% to 0.015% of their total content, while the Cd accounted for 0.03% to 0.38%. A higher proportion of soluble cadmium resulted from their constant hydrolysis, indicating the potential mobility and solubility of the metals in the soil. The value of this parameter was much higher for the Cd (pK = 10.1) than for the Zn (pK = 7.7) or the other analyzed TE. The content of the extractable Cd and Zn was strongly driven by the properties of the applied amendments. The organic amendments, such as the composts (ZC and GC), and biosolids (BB and GB) significantly increased both the Zn and the Cd extractability, while the calcium phosphate induced a slight increase in the soluble Zn. By contrast, the bentonite and iron oxide reduced the solubility of the Cd and the bentonite reduced the solubility of the Zn (Figures 2 and 3). The mobilization of metals, such as Zn and Cd, in contaminated soils treated with organic amendments was observed previously and can be attributed to pH shifts or the release of large amounts of humic acids from biosolids [42, 54, 55]. However, the observed increased Cd solubility in the compost-treated soils was not reflected by a raised bioaccumulation of Cd in the earthworm tissues. In fact, the compost reduced the BSAF value for the Cd. This discrepancy indicates that the 0.01M calcium nitrate extraction used might simulate Cd mobility, but not necessarily Cd bioaccumulation, in earthworms. The earthworms' activity also significantly influenced the availability of the metals; however, the observed effects were not the same for both metals and differed between the combinations. The earthworms reduced the Cd extractability only in certain cases--in the soils treated with the GC, ZC, RW, and BB, but also in the control without treatments (soil P),  
Figure 4: Content of extractable Cd in soils affected by soil amendments and earthworm activity. Different lower case letters within the same type of amendment application indicate significant differences between individual Cd concentration in soils with and without earthworms, while capital letters indicate significant differences between applied amendments (ZC-Zabrze compost, GC-GWDA compost, BB-Belchatow biosolids, GB-Grabów biosolids, CP-Calcium phosphate, IO-Iron oxide, BE-Bentonite, RW-Rock waste, CC-Limestone; ANOVA, Mann-Whitney test, \(p\) < 0.05). which suggests that this process was influenced by the interaction between the amendment properties and the activity of the earthworms. For the Zn, we observed the opposite processes--the earthworms slightly mobilized the Zn in the control and ZC-amended soil. When soil is ingested and passes through the earthworm gut, the pH value of the soil is adjusted to neutral pH via the excretion of calcium carbonate [32; 48]. The calciferous glands in the worms, including _Eisenia veneta_, secrete amorphous calcium carbonate, which crystallizes in the gut to calcite and aragonite [24; 40; 46; 47; 53]. The excretion of these minerals into the soil constitutes additional sorbent for metals and reduces their mobility in soils [18; 48]. This process could have contributed to the reduced Cd solubility in the compost-treated soils.