| Polycyclic aromatic hydrocarbons (PAHs), composed of two or more fused aromatic rings, are ubiquitous, bioaccumulative and persistent pollutants. With rapid population growth, concurrent urban expansion and industrial development, contamination of PAHs in the environment has been a serious public health problem that greatly affects human beings health, and ecosystems functions. Urban forests as an important component of urban ecosystems, it plant leaves could absorb pollutants such as PAHs from the air, and plant also could affect soil prepertis including microbial community and enzymes that metabolize PAHs. Hence, urban forests play an critical role in improving the environment and achieve harmonious development of man and nature.In this study, four widely used for urban green plant species(Cinnamomum camphor a,Opsmanthus fragrans, Magnolia grandiflora and Redrlowered loropetalum) were chosen to determine PAHs content in leaves and investigated the effects on PAHs on soil microbes and enzymes activities. The results could provide scientific basis for plant species selection and arrangement. Mian results showed as follows:1. Charactersistics of PAHs content in four plant species leavesPAHs contents in leaf differed significantly with plant species (p<0.05) and were 7.58mg·kg-1,4.34 mg·kg-1,3.66 mg·kg-1 and 11.13mg·kg-1 for C. camphora, O. fragrans, M. grandiflora and R. loropetalum, respectively. PHE content in leaves was the highest of PAHs for C. camphora, O. fragrans and R. loropetalum, and FLA and PYR was the second and the third one. Howerver, for leaf of M. grandiflora, the individual categories of PAHs contents in leaf were ranked in order as PHE> FLA> NAP. The 2-4 ring PAHs were the main components of the 16 PAHs and accounted for more than 90%, the 5-6 ring PAHs comprised a small percentage of total PAHs and was lower than 10%.2. Seasonal and daily changes in PAHs contents in four plant species leavesPAHs contents in four plant leaves showed significant differences among four seasons (P<0.05). The highest PAHs contents in leaves of C. camphora occurred in April and July, while that of R. loropetalum occured in January and October. In January, diurnal variations of PAHs contents in C. camphora, O. fragrans, M. grandiflora were similar, with the highest content appeared in 14 p.m. and the lowest content appeared in 20 p.m.. But the PAHs content in R. loropetalum was highest in 14 p.m and was lowest in 8 a.m. In April, the PAHs content in C. camphora, O. fragrans, R. loropetalum was highest in 15 p.m, but the PAHs content in C. camphora was smaller from 9 am to 3 am the following day, and that in O. fragrans, R. loropetalum were the lowest at 9 pm. The PAHs contents in M. grandiflora changed little in April. In July, PAHs contents of leaves in four plants showed a similar change pattern that was the single curve, the highest PAHs contents appeared at 20 pm and the smallest contents appeared at 14 pm.3. Relationship of leave structures with PAHs contentsThere were closely relationships between leaf anatomical structures and PAHs contents for plant species. Palisade tissue thickness, spongy tissue thickness and the total leaf thickness were negatively correlated with PAHs contents. The thinner were the palisade tissue thickness, spongy tissue thickness and the total leaf thickness, the more did the aired and particle PAHs enrich. Other targets of leave structures showed no significant difference with PAHs contents (p>0.05), such as stratum corneum was hard to be penetrated and stomata in different trees would selected to scavenge the different particle sizes of PAHs.4. Effect of PAHs on soil microbial communityUsing pot experiment in greenhouse, the soils planted with C. camphora, O. fragrans, M. grandiflora and R. loropetalum seedlings were treated without diesel oil addition (control) and at three diesel contents (2g·kg-1, L1; 10 g·kg-1, L2 and 50g·kg-1, L3) to stimulate different levels of PAHs pollution. Soil microbes were investigated for one year. The numbers of bacteria, fungi, actinomyces and total microbes in PAHs polluted soils of C. camphora, M. grandiflora and R. loropetalum were lower than that in control soils. The numbers of bacteria, total microbes in PAHs polluted soils of O. fragrans were higher than that in control soils. The number of fungi in polluted soils of O. fragrans and R. loropetalum were higher than that in control soils. The numbers of actinomyces followed the order of L1>CK>L2>L3.5. Effects of PAHs on soil enzymesCatalase activities in polluted soils of M. grandiflora, O. fragrans and R. loropetalum were stronger than those in control soils. Catalase activities of C. camphora were in order of L3=CK>L1>L2. Phosphatase activities in polluted soils of four tree species were stronger than those in control soils. Polyphenol oxidase activities in polluted soils of M. grandiflora, O. fragrans were weaker than those in control soils, and the order of C. camphor a was ranked as L1>CK>L2>L3, and that of R. loropetalum was followed the order of L2>L1>CK>L3.
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