## 4 Discussion  
Biostimulation is considered a promising method of supporting hydrocarbon biodegradation processes, by eliminating element deficiencies, mainly nitrogen and phosphorus [12, 14, 15, 16]. Biostimulation can be combined with bioaugmentation techniques, which use microorganisms characterized by a high biodegradation potential, in both single strain and consortia. These microorganisms are often isolated from permanently contaminated  
Figure 5: The linear discriminant analysis effect size (LEfSe) analysis of predicted microbial taxa in deficient-nitrogen-level (red) and sufficient-nitrogen-level (green) samples. Figure 6: The relative predicted abundance of genes participating in PAH degradation in microbial orders in analyzed experimental variants. environments, and they are well described in the literature [12; 17; 18]. Methods using the enzymatic potential of microbes are considered effective for the degradation of most hydrocarbon fractions. However, there are some limitations in the effective decomposition of aromatic compounds, which belong to the fraction that is extremely resistant to biological degradation [40; 41; 42]. Similar conclusions are noted in our research, where the aromatic and polyaromatic fractions are the group of compounds most difficult to decompose. The conducted experiment confirms the increase in the efficiency of biological decomposition of the most problematic fraction, PAH, under the influence of low and optimal supplementation with nitrogen compounds (NP1 and NP2 variants). Interestingly, excessive biostimulation contributes to the inhibition of biological decomposition of all analyzed fractions of hydrocarbon compounds. Similar trends are mentioned in other studies [43; 44]. Uncontrolled biostimulation can be counterproductive in the context of hydrocarbon remediation. The decrease in biodegradation efficiency due to the excessive supply of nitrogen compounds is associated with a decrease in metabolic activity, which is noted both after 24 h (by 3.2%) and after 168 h (by 17.6%) of the experiment. The toxic effect of excessive concentrations of nitrogen compounds on the population of microorganisms could be related to the progressive accumulation of their metabolism products, such as ammonia and ammonium ions, or its oxidation products--nitrates. The phenomenon of the toxicity of ammonia and ammonium ions is very well characterized in the context of the impact on higher organisms, while, in the field of microbiology, this topic still requires detailed research [45]. Moreover, the research by [PERSON] et al. (2000) shows that microorganisms can differ significantly in the level of tolerance to the action of ammonia compounds [46]. This fact may be related to the decrease in alpha-biodiversity observed in the NP3 variant, which suggests the presence of an important selection factor. It is believed that microbial populations have a relatively high metabolic flexibility and functional redundancy; however, they react relatively quickly, with a change in taxonomic structure, to the action of variable, selective environmental factors [47]. This phenomenon is confirmed in the conducted genetic analyzes. Changes in the dominant bacterial classes in variants with a moderate and high supply of nitrogen compounds (NP2 and NP3) are observed. There is a dominance of the Gammaproteobacteria and Betaproteobacteria classes, as well as the Xanthomonadales and Burkholderiales orders, which suggests that these classes and orders are particularly important during the population response to changes in the supply of nitrogen compounds. Similar trends are observed in an earlier study [48]. The researchers find an increase in the share of Gammaproteobacteria and Xanthomonadales in soils subjected to a long-term fertilization process with nitrogen compounds. Therefore, it can be assumed that these changes are not accidental.