**5. Conclusions**

A properly optimized technology of biostimulation with nitrogen compounds may show a beneficial effect on the biological degradation of one of the most difficult to decompose fractions—PAH. However, excessive supplementation with nutrients may reduce the efficiency of the process, due to decreased metabolic activity of microorganisms and lower biodiversity. Changes in the C:N ratio result in a disturbance of the taxonomic structure of the bacteria population. It also affects the genetic potential of the metapopulation for the PAH degradation. Microorganisms belonging to the Xanthomonadales, Burkholderiales, Sphingomonadales, Flavobacteriales, and Sphingobacteriales orders have a high predicted potential for PAH biodegradation, and show a high quantitative fluctuation under the influence of nitrogen compounds. It can be suggested that the design of a molecular diagnostic test allowing the assessment of the genetic potential of environment by biostimulation, or bioaugmentation supported by biostimulation, should be based on a comprehensive analysis of many groups of microorganisms. Moreover, this analysis should take into account the observed possibility of fluctuations in the shares of the taxa mentioned above.

**Author Contributions:** Conceptualization, J.S.-P., P.C. and A.P.-C.; methodology, J.S.-P., J.C., W.J. and Ł.W.; validation, J.S.-P., J.C. and W.J.; formal analysis, J.S.-P. and A.P.-C.; investigation, J.S.-P., J.C., W.J., Ł.W., P.C. and A.P.-C.; data curation J.S.-P., J.C., W.J. and Ł.W.; writing—original draft preparation, J.S.-P.; writing—review and editing P.C. and A.P.-C.; visualization, J.S.-P.; supervision, P.C. and A.P.-C.; project administration, A.P.-C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by National Science Centre in Poland in the years 2014–2018, with the research project Opus no. 2013/11/B/NZ9/01908. Publication was financed within the framework of the Polish Ministry of Science and Higher Education's program: "Regional Excellence Initiative" in the years 2019–2022 (No. 005/RID/2018/19), financing amount 1,200,000,000 PLN.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Raw sequence data were deposited in Sequence Read Archive (SRA), as project no PRJNA831882.
