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The construction of "sponge city" made the belts on both roadsides be built in the form of bioretention tanks, resulting in runoff pollutants were accumulated in the bioretention tanks. To identify the pollution level and source of polycyclic aromatic hydrocarbons (PAHs) in the bioretention tanks, the soil samples were collected on the eight typical roads of the sponge city. The total contents of ∑PAHs in the bioretention tanks were 10,524–20,462 μg/kg (mean 15,029 μg/kg), and the pollution level of PAHs in the non-flood season was significantly higher than that in the flood season. The PAHs in bioretention tanks were dominated by 4-ring PAHs, with a proportion of 39.36%. The characteristic ratio method was proposed to identify the chemical source of PAHs, and the results indicated that PAHs in the bioretention tanks were mainly supplied by the combustion source and petroleum source. The ecological risk quotient (RQ) values showed that the accumulation of PAHs in the bioretention tank had certain potential ecological risks, and the RQ values of pyrene (PYR) were the highest. Incremental lifetime cancer risks (ILCR) indicated that PAHs in soil posed negligible cancer risks by inhalation way, but much stronger risks by dermal and ingestion ways. The roads with the highest and lowest PAHs content were selected for microbial ecology analysis, and the microbial abundances showed significant decreasing trends from the flood season to the non-flood season. The PAHs accumulated in the bioretention tanks had significant impacts on the microbial community, which made the microorganisms tend to PAHs-degrading bacteria. • PAHs showed different accumulation levels in road green infrastructures of sponge city. • The combustion source was the main source of the PAHs in bioretention tanks. • Pyrene (PYR) in the bioretention tank had higher potential ecological risks. • There were potential carcinogenic risks caused by dermal and ingestion ways. • The accumulation of PAHs made microorganisms tend to PAHs-degrading bacteria. [ABSTRACT FROM AUTHOR]

As the bioretention facilities have been extensively built in the construction of "Sponge City" in China, urban non-point source pollutants could be effectively controlled. The influences on microbial community in three bioretention facilities with different underlying types/built times at field scale were studied. The results showed that nutrient loads in road runoff were 1.1–3 times higher than of roof. Compared with the other two rain gardens, the bioretention tank (BT) was relatively stable and showed >90% nutrient load reduction rates. The nutrients brought by the stormwater runoff changed the original habitat in the soil. Nutrient removal led to the deterioration of the living environment of microorganisms, and then the number of microorganisms and diversity changed. Ten dominant bacteria species were found in the facilities, and Proteobacteria were the most dominant bacteria phyla, with the relative abundances all >30%. Suspended solids were found to be the most important factor and had a high correlation to the most dominant microorganisms. With the increased operation period, the microorganism species tended to be concentrated on Proteobacteria , resulting in decreases of other functional microorganisms. The bioretention facility with the road underlying had the lowest microbial diversity, and the built time was the factor that had the greatest impact on microbial diversity. Based on these results, the future design and construction of bioretention facilities should consider the risk after nutrients entered the bioretention facility and the changes in microbial diversity. [Display omitted] • The nutrient removal effects of bioretention facilities with different underlying types/built time were monitored. • The changes of TOC and nutrients (N and P) contents were studied. • The evolutions of the microbial communities were explored. • The relationship between soil nutrient contents and microorganisms was built. • The influences of different design types on the microbial diversity were determined. [ABSTRACT FROM AUTHOR]