Open AccessCommunication Characteristics, Ecological Risks, and the Impacts on Soil Carbon Cycling of PAH Pollution in the Soil of a Retired Coking Plant in Zaozhuang, Northern China Nanjing Institute of Environmental Sciences, Ministry of Ecology and Environment, Nanjing 210042, China * Author to whom correspondence should be addressed. Toxics 2026, 14(6), 503; https://doi.org/10.3390/toxics14060503 (registering DOI) Submission received: 20 April 2026 / Revised: 5 June 2026 / Accepted: 8 June 2026 / Published: 9 June 2026 Abstract During the industrial restructuring in China, numerous outdated coking enterprises were phased out. Despite the cessation of production for several years, the soil in the production area of the retired coking plant remains heavily contaminated with polycyclic aromatic hydrocarbons (PAHs), which continue to adversely affect soil health. However, research on the pollution characteristics of soil PAHs under prolonged PAH exposure and the associated changes in functional genes related to soil carbon cycling is still inadequate. This study aims to identify the pollution characteristics and ecological risks of PAHs in the coking plant and to investigate the effects of long-term PAH contamination from abandoned coking plants on the functional genes involved in soil carbon cycling. It was found that PAHs in the soil were predominantly composed of high-molecular-weight PAHs (HMW-PAHs), which constituted 65.7% to 83.4% of the total PAH content. The total concentration of PAHs in the surface soil ranged from 3.79 to 554 mg·kg −1, with an average concentration of 147.6 mg·kg −1. Source analysis based on isomer ratios indicated that PAHs primarily originated from the combustion of coal and biomass. Utilizing the toxicity equivalent factor (TEF) method, we found that the PAH levels in the CA group exceeded the Serious Risk Concentration, indicating that PAH pollution poses a potential threat to the ecological environment. Metagenomic analysis revealed that the gene abundance of alpha-amylase in the CA group was significantly higher than that in the OLA group ( p 3.0). The formula for calculating PN is as follows: P N = p i a v e 2 + p i m a x 2 2 (1) Pi represents the single-factor pollution index for contaminant i in the soil; pi = Ci/Si; Ci is the measured concentration of contaminant i; Si is the standard value for contaminant i, using the screening values for PAHs specified in GB 36600-2018 [ 16]; Piave is the average of all pollution indices in the soil; and Pimax is the maximum of all pollution indices in the soil. Due to the significant differences in toxicity levels among PAHs, the concentration of a single PAH cannot fully represent the overall risk of soil pollution. Therefore, the Nemerow index method (PN) is used to assess the comprehensive pollution degree of soil PAHs. The limitation of the Nemero index method lies in its tendency to amplify the maximum value, lack of weight differences, strong dependence on standards, and the resulting single comprehensive value. It has a weak ability to identify the spatial heterogeneity of regional pollution and is prone to evaluation errors. This evaluation method can be combined with single-factor analysis, such as the toxic equivalency quantity (TEQ) method, to enhance accuracy. The BaP Toxic Equivalency Factor (TEF) was proposed by Nisbet and LaGoy in 1992 [ 17]. It is used to multiply the concentration of individual PAHs by a toxic equivalency coefficient to convert it to a BaP equivalent concentration, thereby assessing the ecological risk posed by PAHs. The formula for calculating the TEQ is as follows: T E Q b a p = ∑ C i ୍ଠ T E F i (2) Ci represents the detected concentration of PAH component i, and TEFi represents the TEF value corresponding to component i. TEQbap represents the total toxicity equivalent of the 16 PAHs relative to BaP (mg·kg −1). Subsequently, the ecological risk of each group of soil samples was assessed based on the Serious Risk Concentration (SRC) for BaP of 7 mg·kg −1, as specified in the Dutch ecotoxicological models for environmental policy [ 18]. The toxicity equivalent method converts the mixture of multiple PAHs into a unified quantitative toxicity standard, effectively addressing the problem of significant differences in toxicity among different PAHs and the coexistence of various concentrations of PAHs in the soil for risk assessment. It is applicable to the ecological risk assessment of PAHs. The TEQ method assumes that the toxic effects of each component are additive, ignoring the synergistic or antagonistic interactions between chemical substances. This results in the actual risks being underestimated or overestimated. This method can only be used as a preliminary screening tool for specific pollution groups (such as polycyclic aromatic hydrocarbons and dioxin-like substances). When dealing with complex cross-category pollution, the TEF method cannot provide an accurate assessment of cumulative health risks. 2.5. Source Analysis 2.6. Data Analysis Origin 2021 (OriginLab, Northampton, MA, USA) and Excel 2020 (Microsoft, Redmond, DC, USA) were used to process the data. The R software (version 3.2.2) was used to carry out PCoA, Wilcoxon analyses, and Stacked bar chart. The R language packages are in the Supplementary Materials. 4. Conclusions In this study, we utilized a combined approach of risk assessment and metagenomics to investigate the ecological risks and impacts on soil carbon cycling associated with the retired coking plant’s soil. The soil contamination at the coking plant primarily consisted of high-molecular-weight polycyclic aromatic hydrocarbons (HMW-PAHs), which accounted for 65.7% to 83.4% of the total pollutants. The Nemerow index revealed that the soils in the CA group were severely polluted. The principal sources of soil PAHs were identified as coal and biomass combustion. Notably, the alpha-amylase gene in the CA group exhibited a significant increase ( p < 0.05), suggesting that PAHs may enhance the starch-hydrolysis metabolism of soil microorganisms. This integrated methodology of ecological risk assessment and metagenomics provides a more comprehensive understanding of the environmental risks posed by the coking plant’s soil. It is imperative that retired coking plants implement remediation measures and effective environmental management strategies to mitigate the impact of PAHs on the environment. The limitations of this study include a restricted sampling quantity and background control; therefore, future research should aim to increase the sampling quantity to facilitate a more thorough investigation of various types of coking sites. Supplementary Materials The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/toxics14060503/s1. Table S1: Concentration of 16 PAHs in each soil sample; Table S2: TEQ bap of 16 PAHs in each soil sample. Author Contributions Writing—original draft preparation: L.Z. (Liping Zheng); writing—review and editing: L.Z. (Liping Zheng), Y.H., Y.Y., Q.L., L.Z. (Lei Zhang), Z.X., and X.L.; conceptualization, resources, and funding acquisition: Y.H. All authors have read and agreed to the published version of the manuscript. Funding The work was funded by the National Key Research and Development Program of China (Nos. 2018YFC1801100 and 2018YFC1801103). Institutional Review Board Statement Not applicable. Informed Consent Statement Not applicable. Data Availability Statement The original contributions presented in this study are included in the article/ Supplementary Materials. Further inquiries can be directed to the corresponding author. Conflicts of Interest The authors declare no conflicts of interest. References Figure 1. Remote sensing image of the study area. Figure 1. Remote sensing image of the study area. Figure 2. Proportion of different ring counts of PAHs in soil samples. Figure 2. Proportion of different ring counts of PAHs in soil samples. Figure 3. Source analysis of PAHs isomer ratio. ( a) The ratio of PAH isomers based on Flua, Pyr, Ant and Phe; ( b) The ratio of PAH isomers based on Flua, Pyr, BaA and Chr. Figure 3. Source analysis of PAHs isomer ratio. ( a) The ratio of PAH isomers based on Flua, Pyr, Ant and Phe; ( b) The ratio of PAH isomers based on Flua, Pyr, BaA and Chr. Figure 4. PCoA analysis diagram of genes of the carbon cycle. Figure 4. PCoA analysis diagram of genes of the carbon cycle. Figure 5. Significantly different functional genes. Figure 5. Significantly different functional genes. Table 1. Statistics of the contents of 16 PAHs in surface soils of the coking plant (mg·kg −1). Table 1. Statistics of the contents of 16 PAHs in surface soils of the coking plant (mg·kg −1). PAHs Min Max Mean Std. Deviation CV% Sample Sizes NaP 0.153 7.09 2.37 2.49 105.38 8 Acy 0.12 20.1 5.15 6.60 128.17 8 Ace ND 7.7 2.13 2.59 121.77 8 Flu 0.12 11 4.05 4.79 118.31 8 Phe 0.433 59.8 17.84 21.62 121.21 8 Ant 0.1 27.4 7.05 9.36 132.79 8 Flua 0.6 68.4 19.65 23.62 120.16 8 Pyr 0.5 62.3 17.61 21.13 120.02 8 BaA 0.233 46 11.22 14.94 133.17 8 Chr 0.3 43.3 10.39 13.86 133.45 8 BbF 0.467 75.4 18.33 24.22 132.10 8 BkF 0.2 28.7 6.99 9.24 132.19 8 BaP 0.267 51.1 12.62 16.58 131.38 8 IcdP 0.15 24.3 5.97 7.79 130.51 8 DahA ND 6.47 1.75 2.11 120.85 8 BghiP 0.15 19.5 4.99 6.22 124.87 8 Σ16PAHs 3.79 554 147.6 Min: Minimum value; Max: Maximum value; Std. Deviation: Standard Deviation; CV%: coefficient of variation, CV (%) = standard deviation/mean × 100; ND: Not detected. Table 2. Nemerow index (PN) of PAHs in soil samples. Table 2. Nemerow index (PN) of PAHs in soil samples. S1 S2 S3 S4 S5 S6 S7 S8 PN 17.37 32.88 66.72 8.62 2.26 2.31 0.35 1.31 Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content. © 2026 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license. Share and Cite MDPI and ACS Style Zheng, L.; He, Y.; Yan, Y.; Li, Q.; Zhang, L.; Xing, Z.; Lu, X. Characteristics, Ecological Risks, and the Impacts on Soil Carbon Cycling of PAH Pollution in the Soil of a Retired Coking Plant in Zaozhuang, Northern China. Toxics 2026, 14, 503. https://doi.org/10.3390/toxics14060503 AMA Style Zheng L, He Y, Yan Y, Li Q, Zhang L, Xing Z, Lu X. Characteristics, Ecological Risks, and the Impacts on Soil Carbon Cycling of PAH Pollution in the Soil of a Retired Coking Plant in Zaozhuang, Northern China. Toxics. 2026; 14(6):503. https://doi.org/10.3390/toxics14060503 Chicago/Turabian Style Zheng, Liping, Yue He, Yifan Yan, Qun Li, Lei Zhang, Zhe Xing, and Xiaosong Lu. 2026. "Characteristics, Ecological Risks, and the Impacts on Soil Carbon Cycling of PAH Pollution in the Soil of a Retired Coking Plant in Zaozhuang, Northern China" Toxics 14, no. 6: 503. https://doi.org/10.3390/toxics14060503 APA Style Zheng, L., He, Y., Yan, Y., Li, Q., Zhang, L., Xing, Z., & Lu, X. (2026). Characteristics, Ecological Risks, and the Impacts on Soil Carbon Cycling of PAH Pollution in the Soil of a Retired Coking Plant in Zaozhuang, Northern China. 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