## 4 Discussion
It can be also postulated that those dominant taxa, which are also the most differentially abundant between deficient and sufficient nitrogen variants, such as Xanthomonadales and Burkholderiales, are more competitive than the other bacteria groups in the use of nitrogen compounds. In an earlier study, [PERSON] et al. (2021) emphasize that long-term exposure to nitrogen compounds results in the dominance of microbial groups associated with ureolysis, and the nitrification and denitrification processes. It is found that the microbial groups that harbor the same or similar functional genes involved in nitrogen use can coexist together. This functional redundancy is well described in the literature, and it is believed that it may increase the stability of microbial metapopulations in the long term [49]. The LEFSe analysis identifies the most significant taxonomic differences between two classes of nitrogen supply (deficient and sufficient). The Xanthomonadales, Burkholderiales, and Sphingomonadales orders may be of key importance in the biological degradation of hydrocarbon fractions in environments with a periodically variable supply of nitrogen compounds, as well as in intensive biostimulation technologies. However, due to their high sensitivity to the variability of environmental parameters, this groups of microorganisms should not play a bioindicative role. PICRUSt analysis reveals that the Sphingobacterialies and Rhizolates orders have a high genetic potential for biodegradation of the PAH fraction. The nitrogen-fixingRhizobiales order seems to be particularly important. The decrease in its share in the NP3 variant, found after 168 h of biodegradation, can be related to the reduced degradation efficiency of PAH. The biodegradation potential of PAH by microorganisms belonging to the Rhizobiales order is described in the literature. It is believed that Rhizobiales effectively support the process of PAH bioremediation in environments with a deficiency of nitrogen compounds, due to the activity of genes responsible for nitrogen binding [50]. It can be suggested that the quantitative analysis of the Rhizobiales order is helpful in estimating the effectiveness of PAH natural attenuation in areas with a deficiency of nitrogen compounds, and the validity of bioremediation techniques supported by biostimulation. However, it should be mentioned that the process of biological decomposition of polycyclic aromatic hydrocarbons is a complex process. Many studies indicate the importance of cometabolism and metapopulation interactions in the degradation of this group of xenobiotics [51; 52; 53]. Therefore, it is not possible to indicate only one group of microorganisms that clearly determines the effectiveness of biological decomposition of PAH. Based on the current knowledge, it is difficult to make any hypotheses regarding the mechanisms that drive the observed population changes of microorganisms and modifications of the genetic pool in the context of the biological decomposition of hydrocarbon compounds, including PAH. However, it should be emphasized that properly optimized supplementation with nitrogen compounds has a positive effect on the processes of hydrocarbon remediation, if the dose of the biostimulator is optimally selected. Careless application of nitrogenous substances above optimal levels may contribute to the inhibition of biodegradation processes.