## 1 Introduction
However, as was proven by [PERSON] [12; 21; 22], the total amounts of metals in SS do not necessarily mean that their use both as fertilizers and co-substrate for composting is a threat to soils. Using only a total content, it is assumed that all forms of metals have the same impact on the environment, which is contrary to the actual situation. Therefore, research on the chemical forms of metals in SS-derived composts requires great attention, with particular emphasis on the degree of their bioavailability to plants. Many authors [12; 14; 21; 22; 23] also claim that, compared to the total metal content, the chemical forms of metals in the solid phase are more important in determining their bioavailability for plants and leaching into groundwater. This approach is of great practical importance because it provides the opportunity to predict the possible behavior of the introduced metal along with the compost in the environment. During a proper process, metals are strongly complexed with stabilized organic matter. As a result, metals from composts enter the soil in more chemically stable combinations, and therefore, they become less available to plants [15]. As reported by [PERSON] et al. [19], this is noticeable in the case of composting with various additives that significantly reduce the bioavailability of metals, resulting in a marketable, safe material. The use of various additives (organic and inorganic) in the process of composting sewage sludge is also highly important because of its high moisture content, small particle size, and thick texture [19; 24]. According to [PERSON] et al. [24], bulking agents, especially those with a high level of recalcitrant carbon, reduce organic matter degradation, enhance the humification process, and consequently improve the quality of final compost products. Additionally, bulking agents are desirable because they significantly improve the structure of the mixture, ensuring proper gas exchange and reducing moisture. For this purpose, various types of straw, sawdust, and bark are most often recommended [15]. Considering that some metals, such as Cu, Zn, Mn, Ni, and Fe, are also micronutrients and necessary for plants, it is important to consider both their availability for plants and the undesirable nature of excess amounts. Therefore, the analysis of metal combinations becomes necessary for the soil application of compost, especially when the compost is made from SS with various additives as bulking agents. It should be noted that there are no specific methods for evaluating compost. The same techniques, as in the case of soils, are routinely used, and their usefulness and verifiability are satisfactory [12; 21; 22]. Considering the assessment of the mobility and bioavailability of metals in composts, two independent groups of analyses are proposed: sequential and single. Each of these methods has advantages and disadvantages. Sequential methods are time-consuming but provide precise and comprehensive information. The general idea of sequential extraction is based on subjecting the analyzed sample to subsequent sample \"attacks\" of solutions (reagents with different chemical properties: pH, redox potential, and complexing abilities) with increasing aggressiveness. In this way, the basic goal of sequential extraction is to achieve the successive washing of the component from various compounds and its various connections with the phase constant of the sample, with extraction taking place from the most mobile, easily soluble forms and ending with permanent, hard-to-dissolve forms. Such a goal can be achieved using the BCR sequential extraction method, which allows the separation of four fractions describing the combinations of elements with the solid phase of the tested sample with different degrees of solubility [25]. In turn, single extractions are quick and useful in routine monitoring, and although these methods can extract bioavailable amounts of metals, they allow only for general conclusions.