## 4 Discussion
[43] also confirmed such a distribution of Zn because they determined most of the Zn in fractions III and II. On the other hand, [PERSON] et al. [46] reported the highest percentage of Zn in the mobile fractions of compost. In light of the reports on the high mobility of nickel and zinc in composts [50; 51], the low metal contents in fraction I of composts and the higher metal contents in stable chemical forms are interesting results. Such changes seem beneficial from the point of view of environmental protection but less so when we consider the fertilizer aspect of the composts. Despite their significant affinity for organic matter, nickel and zinc form organic ligands characterized by a weaker stability than those created through copper [52; 53]. In this respect, [PERSON] et al. [54] emphasized the greater affinity of zinc for fulvic acids. Mineralization occurring during composting could cause the breakdown of these organometallic complexes, and the released metal ions were subsequently adsorbed on iron and manganese oxides or created more durable organic compounds within the resulting compost humus. It is necessary to remember that the humification process occurs simultaneously with the mineralization process, as a result of which more stable structures of humus compounds are created with a high sorption capacity in relation to metals [54]. [PERSON] et al. [50] particularly emphasized that, during the composting process, there was a reduction in both metals in water-soluble connections (and zinc additionally in the residual fraction) and increase in organic connections. An explanation for this phenomenon was also provided as the release of nickel and zinc ions from easily soluble connections that were the result of the decomposition of organic matter, which were then bound to stable organic connections. This finding is also confirmed by the results of the research by [PERSON] et al. [55]. With respect to the fractional distribution of Cu, it should be emphasized that both the sewage sludge [12] and the tested composts had the same distribution. The amount of Cu, regardless of the composting phase, increased in the following order: Fr. I \(<\) Fr. IV \(<\) Fr. II \(<\) Fr. III (based on absolute values not directly presented here). The amounts of Cu increased everywhere except for Fr. II, where a decrease in the amount of metal was recorded. [PERSON] et al. [56] and [PERSON] [27] reported the highest Cu amounts for Cu bound with organic matter of the tested composts. On the other hand, [PERSON] et al. [46] reported the highest percentage of Cu in the mobile fractions of compost. The addition of bulking agents resulted in an increase in the amount of Cu labile connections and a decrease in the amount of the organic fraction [27; 57]. The authors also noted an unclear trend in Cu mobility due to the transfer of the organic fraction into the changeable and carbonate fractions. The explanation may be the biogeochemistry of Cu, which is considered a metal that does not easily create water-soluble forms; therefore, it is not easily activated and is characterized by complexes with organic matter with high durability [53; 58]. Moreover, as a result of the decomposition of organic matter during composting, copper is released from soluble organometallic ligands, and then, it may be subjected to precipitation in the form of carbonates, which are extracted via the BCR method with an acetic acid solution (a reagent in fraction I). The increased amount of copper in water- and acid-soluble connections can also be interpreted based on the opinion regarding the creation of stable copper complexes with fulvic acids [59]. Although the composting process stimulated an increase in the copper content in fraction I, the microelement amount in these combinations was lower than that in the other fractions.