| Polycyclic aromatic hydrocarbons (PAHs) are widespread persistent organic pollutants in environment, especially soils, which can be detrimental to human beings. The aim of this study was to evaluate the role of rhizosphere effect in long-term PAHs polluted soils by using sequential extraction approach and toxicity equivalency factors.This study concerned the potential impact of the rhizosphere on the soil microbial community and PAH-degrading bacterial community in PAH polluted soils due to coal combustion. Rhizosphere and bulk soils were used to enrich PAH-degrading bacterial consortia and isolate PAH-degrading bacterial strains. And phenathrene and pyrene were used as the sole carbon source for degrading characteristic research.a. The experimental results revealed that the concentration of PAHs in situ polluted soils was much higher than normal standard limits for PAHs of soils in USA and EU. Because this sampling site was contaminated for a long term by a smithy factory, the PAHs pollution was most potentially generated by the coal combustion of that factory.b. The result of the sequential extraction of PAHs polluted soils proved that the primary fractionation was DCM extractable fraction among three extractable fractions and was crude humin-bound PAHs among three nonextractable fractions. And the sum of six sequentially extracted fractionations in rhizosphere soils of all four plant species was lower than that in bulk soils. For bulk soils, the profiles of sequentially extracted PAHs varied with plant species, especially butanol and DCM extractable fraction. However, the contribution of rhizosphere to PAHs dissipation is relatively small, because the profiles of sequentially extracted PAHs between bulk and rizosphere soils of the same plant species remain parallel.c. The discrepancy of toxicity equivalency concentrations between rhizosphere and bulk soils was much more significant than that of total PAHs concentrations, though the toxicity equivalency concentration of rizosphere soils was still lower than that of bulk soils. Furthermore, the proportion of toxicity equivalency concentrations of crude humin-bound PAHs was simultaneously decreased in both bulk and rhizosphere soil, illuminating that high molecular weight PAHs with big toxicity equivalency factors are difficult to enter crude humin fraction. After substituting the toxicity equivalency concentration for the concentration of individual PAHs, the profiles of toxicity equivalency concentrations were distinct from the profiles of total PAHs concentrations and varied with plant species.d. The concentration of PAHs in plants tissues was relatively low compared to that in soils, revealing that the enrichment of PAHs in soils by plants could be neglected. In partial least squares regression models, the concentration of PAHs in plant tissues was correlated with the fractions strongly associated with crude humin bound PAHs in soils.e. Total PAH concentrations in the rhizosphere were significantly lower than in bulk soils under all studied crops. Furthermore, the amount of DNA extracted from all rhizosphere soils was greater than that extracted from bulk soils. For the soil microbial community, rizosphere effect has more contribution than different plant species. However, the bacterial community structure of degrading consortia veried more significantly in different plant species than rhizosphere effect.f. Eleven strong bands were sequenced and identified as nine genera of Firmicutes, Bacteroidetes, Alphaproteobacteria, Betaproteobacteria, and Gammaproteobacteria. There were nine isolated strains belonging to five genera of the Firmicutes, Betaproteobacteria and Gammaproteobacteria.g. During the incubation period of 10-14 days, SR consortia can use phenathrene and pyrene, principle high molecular polycyclic aromatic hydrocarbons, as the sole carbon source for metabolism. The degrading rate reached 98% in the end.
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