## 4 Discussion  
In our study, increasing doses of diesel oil were found to significantly increase the content of most of the analysed elements in the soil, such as Pb, Cu, Mn, Zn, Cd, Ni and Fe. However, the highest dose (10 cm\({}^{3}\) per kg) was found to reduce chromium and cobalt content. An increase in the content of selected trace elements in soil following contamination by petroleum products was demonstrated by the studies of [PERSON] et al. [70] and [PERSON] et al. [71]. [PERSON] et al. [71] reported the greatest changes for Fe (568% increase), Mn (280% increase) and Cr (276% increase). [PERSON] et al. [72] conducted a study to characterise the content of trace elements in the soil surrounding an oil field in Fahud, Oman. Soil analysis has revealed an elevated presence of zinc, copper, lead and vanadium, compared to uncontaminated soil. Soil contamination with petroleum products affects its physical and chemical properties, including lowering the pH level of the soil [71] and reducing the total amount of base-exchangeable cations, the soil's base saturation and its cation exchange capacity [73]. These changes determine the mobility of trace elements and their ability to move through the soil profile [74]. As reported by [PERSON] et al. [75], soil contaminated with 10% petroleum had a lower pH (6.54 versus 6.95) and increased electrical conductivity than uncontaminated soil. This can lead to disruption of the environment's ionic balance and a reduction in soil permeability. Trace element mobility increases as soil pH decreases [76]. At low pH, trace elements exist in more bioavailable free ionic forms, primarily due to reduced adsorption onto mineral or organic surfaces and reduced formation of poorly soluble phosphates and carbonates [77]. Petroleum derivatives also affect the oxidoreductase potential of the soil, leading to its reduction [78]. Under these conditions, the solubility of Fe-Mn hydroxides, to which TEs are bound, increases, resulting in the remobilisation of biologically available TEs [79; 80]. Due to their chemical composition, petroleum pollutants contribute to the co-contamination of soil with TEs [81], which could explain the elevated levels of cadmium, nickel and iron observed in the samples containing the highest dosage of diesel oil in the present study. Figure 8: Relative effect of factors on trace elements in soil (in percent). Of the soil materials compared in this study, bentonite and calcium oxide were found to be the most effective at reducing the negative effects of diesel oil on soil. At the site with the highest level of petroleum contamination (10 cm\({}^{3}\) per kg), the application of bentonite reduced the soil content of Pb, Zn and Mn. The favourable effect of bentonite in reducing the content of TEs under conditions of soil contamination by petroleum substances can be explained by its strong sorptive properties with respect to petroleum products (0.20-0.50 g of petroleum products per g of sorbent) [82]. Thanks to its significant sorptive surface area (both external and internal), bentonite plays a major role in soil chemical processes, affecting the mobility and availability of trace elements [83]. Additionally, the high content of silicon dioxide, magnesium oxide and calcium oxide reduces the acidity of the soil upon introduction of bentonite [84], which may also explain the limited accumulation of Ni, Cr and Fe in the series with the addition of this material in this study. Similar observations were made by [PERSON] et al. [85], who evaluated the suitability of bentonite as an immobilising agent in the remediation of trace element-contaminated soil. Following the application of 2.5% bentonite to the soil, reduced accumulation of zinc (by 15.8%), nickel (by 31.6%) and copper (by 18.6%) was recorded relative to the control sample (without bentonite addition).