## 3 Results and Discussion
### Influence of Amendments and Earthworm Activity on Soil pH  
It is well established that soil pH is a key factor affecting adsorption-desorption behaviors and, hence, the bioavailability of TE to earthworms [10; 13]. In our study, this parameter was also analyzed to estimate the dependence of soil pH on the earthworms' activity and the soil amendments (Figure 2). The earthworms' activity did not significantly affect the soil pH. In contrast to earthworms, the additives caused a significant pH shift over the course of the experiment. The organic amendments lowered the soil pH\({}_{\text{H2O}}\) drastically from 8.0 to 6.9-7.1, in the biosold (BB, GB)- and GC-treated soils, while the ZC caused only a small pH decrease (0.2 unit). This difference can be explained by the pH of the amendments: the pH of ZC was alkaline (7.4), whereas the other organic amendments were slightly acidic (5.3 and 6.1 for the GC and GB, respectively) or neutral (6.7 for BB) (Table 1). The observed pH decrease explains the greater Cd and Zn solubility in the GC-, GB-, and BB-amended soils (Figures 3 and 4). The bentonite induced a small increase in the soil pH, since its initial pH was 9.0. The calcium carbonate did not have a strong effect on the soil pH, which was related to the initially alkaline soil pH.  
The pH of the contaminated soils is usually the most important chemical property governing trace-metal solubility through a range of sorption, precipitation, or occlusion processes [52]. The metal sorption related to particular soil constituents is also pH-dependent--organic soil constituents can bind some metals strongly, even at acidic pH, by forming chelates with humic acids, whereas the sorption capability of iron oxides becomes stronger at a pH of around 7 [53].  
[PERSON] et al. [52] observed that earthworm activity increased soil pH due to the excretion of calcium compounds into the environment by calciferous glands. It must be pointed out that all types of earthworm, with or without calciferous glands, may increase soil pH due to their alkaline urine [47]. In our study, metal toxicity might have affected these processes.  
Figure 3: Content of extractable Zn in soils affected by soil amendments and earthworms activity. Different lower case letters within the same type of amendment application indicate significant differences between individual Zn concentration in soils with and without earthworms, while capital letters indicate significant differences between applied amendments (ZC-Zabrze compost, GC-GWDA compost, BB-Belchatów biosolids, GB-Grabów biosolids, CP-Calcium phosphate, IO- Iron oxide, BE-Bentonite, RW-Rock waste, CC-Limestone; ANOVA, Mann-Whitney test, \(p<0.05\)).  
Figure 2: Influence of amendment application and earthworm activity on soil pH. Different lower case letters within the same type of amendment application indicate significant differences between pH in soils with and without earthworms, while capital letters indicate significant differences between applied amendments (ZC-Zabrze compost, GC-GWDA compost, BB-Belchatów biosolids, GP-Grabów biosolids, CP-Calcium phosphate, IO-Iron oxide, BE-Bentonite, RW-Rock waste, CC-Limestone; ANOVA, Mann-Whitney test, \(p<0.05\)).  
However, the lack of effect of the earthworms on the soil pH was mainly related to the fact that the initial soil pH was alkaline.