Polycyclic aromatic hydrocarbons (PAHs) are widely prevalent harmful organic pollutants. Enzymatic activities (such as those of dehydrogenases, catalase, protease and urease), as well as the microbial community structure and assembly (through 16S and ITS amplicon sequencing), were evaluated 90 days after PAH contamination and compared to those in normal soils. Microbial activity, as indicated by soil urease, catalase, and protease activities, was inhibited under PAH stress. Furthermore, PAH stress exerted significant impacts on the soil microbial community structure. Notably, PAH stress reduced soil bacterial and fungal biomass and inhibited the abundance of microbial taxa involved in soil carbon and nitrogen cycling (e.g., Marmoricola, Pedobacter, and Streptomyces), along with the majority of predicted responsive metabolic functions, particularly those related to amino acid and carbohydrate metabolism. PAH stress enriched PAH-degrading microorganisms, including Pseudomonas, Mycobacterium, Bacillus, Cycloclasticus, and Flavobacterium. The niche breadth of bacterial and fungal communities decreased significantly under PAH stress (51.5 and 14.1, respectively) compared to that in normal soil (63.7 and 22.3), which was further supported by Beta Nearest Taxon Index and co-occurrence network analysis. PAH stress increased the contribution of heterogeneous selection to soil microbial assembly (100%) compared to that in normal soil (80%). Thus, the majority of microbial community responses to PAH stress were adversely affected. These results suggest that PAH contamination may profoundly affect the soil quality by restricting the survival space of bacteria and fungi.

Understanding how plants respond to polycyclic aromatic hydrocarbons (PAHs) stress is essential for evaluating ecological risks and improving phytoremediation strategies. PAHs are widespread and persistent environmental pollutants that exert toxic effects on plants at different developmental stages. Although Buchloe dactyloides (Nutt) Engelm shows potential for phytoremediation of PAHs contamination, its root defense mechanism against PAHs remains unclear. To this end, transcriptomics and non-targeted metabolomics were used to study the changes in gene expression and metabolite profiles in roots under PAHs stress. After 70 days of PAHs exposure, B. dactyloides roots exhibited increased activities of catalase (CAT) (from 1.955 to 6.436; ca. 3.29) and peroxidase (POD) (from 94.507 to 124.901; ca. 1.32), higher levels of ascorbate (AsA) (from 7976.69 to 18,950.09; ca. 2.38) and glutathione (GSH) (from 21.08 to 37.23; ca. 1.77), and accumulation of proline (from 40.585 to 66.671; ca. 1.64). Significant differences in genes and metabolites were observed between the treatment and control groups, with a total of 4083 differentially expressed genes (DEGs) and 100 differentially accumulated metabolites (DAMs). Further comprehensive analysis of transcriptomics and metabolomics revealed the potential role of multiple pathways in the defense response of B. dactyloides roots against PAHs stress, including amino acid synthesis, flavonoid biosynthesis, galactose metabolism, glycerophospholipid metabolism, and other pathways. These pathways may contribute to antioxidative defense under PAHs stress. In addition, increased trehalose and soluble sugar contents likely supplied energy and osmoprotective functions under stress. These findings provide insights into the mechanisms of root adaptation to PAHs and may support the long-term phytoremediation potential of B. dactyloides.