Influence mechanisms underlying the degradation of petroleum hydrocarbons in response to various nitrogen dosages supplementation through metatranscriptomics analysis

Exogenous nitrogen supplementation for the bioremediation of petroleum-contaminated soils is a widely adopted and effective environmentally friendly strategy. However, the mechanism by which varying nitrogen dosages affect hydrocarbon degradation pathways remains unclear. This study conducted bioremediation on soil with a total petroleum hydrocarbon (TPH) content of 17,090 mg/kg over 210 days. The results indicated that low-dosage nitrogen supplementation (136 mg N/kg, LN) achieved a 24.8 % TPH removal, significantly outperforming high-dosage nitrogen treatment (1360 mg N/kg, HN), which achieved only 12.8 % TPH removal. The LN treatment demonstrated nitrogen availability efficiency (NAVE) and nitrogen partial factor availability (NPFA) values of 6.03 mg/mg and 31.11 mg/mg, respectively, compared to -0.90 mg/mg and 1.60 mg/mg for the HN treatment. The metatranscriptomic data were employed to investigate differential gene expression across individual samples and for GO and KEGG functional annotation. The annotation results revealed a significant increase in the number of differentially expressed genes (DEGs) associated with each functional category as the nitrogen dose increased. Notably, the LN treatment upregulated genes such as bbsE, bbsF, golA, and badH, which are crucial for encoding aromatic hydrocarbon degradation. Additionally, pyruvate metabolism genes including aceE, acdA, and atoB were enriched due to the LN treatment. In contrast, the HN treatment promoted soil nitrogen metabolism through enhanced expression of nitrogen cycling-related genes such as narK, narG, narH, nirA, and nirK, contributing to competitive interactions with carbon metabolism and impeding hydrocarbon degradation by soil microorganisms. These results suggest that the regulation of nitrogen application is crucial for enhancing hydrocarbon biodegradation efficiency in petroleum-contaminated soils.