Environmental Fate Of Polycyclic Aromatic Hydrocarbons In Rhizosphere Interface And Microbial Responses

Phytoremediation is considered as the most promising remediation approach for the remediation of soil organic pollutants. The mechanism for PAHs phytoremediation in rhizosphere is one of the research frontiers of soil, plant, environment, and microbiology. Hence it is important to study the dissipation processes of organic pollutants in the rhizosphere of plant and microbial mechanism involved in the dissipation to support the theory for estimating efficient phytoremediation technology. The polycyclic aromatic hydrocarbons (PAHs), which are one kind of persistent organic pollutants (POPs), were selected as the target compounds in the present study. PAHs partition regulation, dissipation dynamic, and response of microbes in rhizosphere micro-region were studied, by using meta-analysis, quantitative structure activity relationship (QSAR), cosolvent models, sequential extraction, toxicity equivalence factors, DGGE, and qPCR technology, to clarify the environmental fate of PAHs and microbial responses in rhizosphere. The main results were as follows:(1) Meta-analysis was used to explore the interaction between plant root growth and dissipation of PAHs in soil based on the large body of literature published before 2008 in order to clarify integral influence of plant rhizosphere activities on PAHs dissipation. PAH dissipation was stimulated in plant rhizosphere. Plant has a positive effect on the number of microbial populations which were capable of degrading PAHs in the rhizosphere as compared to bulk soil. Furthermore, PAHs significantly suppressed the plant growth. Effect sizes in meta-analysis were employed as activity dataset for building quantitative structure-activity relationship (QSAR) models, and accumulative effect sizes of 16 PAHs were used for validation of these models. The QSAR models based on the molecular structure indices and effect size can be used to effectively predict the rhizosphere effect of PAHs dissipation. The results of meta-analysis and QSAR models provided the theoretical foundation for developing the rhizoremediation technology for PAHs.(2) The precise partition coefficients based on cosolvent models were employed for exploring the partition mechanism of PAHs in rhizosphere. Analysis of the robustness and predicting capability of various cosolvent models showed that Bayesian cosolvent model was the most robust among all tested cosolvent models. The order of adsorption capability of rhizosphere components for PAHs followed fungal cell walls>plant roots>soils. However, the contribution of plant roots and fungal cell walls on PAHs adsorption was less than 1%, because plant root and fungal biomass was extremely low. Iron plaque on root surface inhibited the adsorption of PAHs on rice root. The inhibition effect increased with the PAHs hydrophobicity. The PAHs partition coefficients on root were significantly increased after iron plaque was removed. These results are important for further research on PAHs rhizoremediation mechanism.(3) The 1-5 mm spatial gradients in rhizosphere were constructed with multi-layer rhizobox, and temporal gradients were built with sampling at 0,15,30, and 45 days after planting. Sequential extraction results showed that butanol-extracted fraction, indicating bioavailability of PAHs in soils, was consistent with PAHs dissipation in rhizosphere. Under the influences of oxygen releases from rice roots, bioavailability of PAHs, degrading microbial activities, and mineral nutrient in rhizosphere, the spatial distribution of PAHs dissipation were different at different levels of PAHs. The equivalent toxicity concentrations were decreased more than PAHs concentrations in rhizosphere. Evaluation of phytoremediation with the equivalent toxicity concentrations could integrat pollutant remediation with the health risk, and is significant for realizing efficient remediation in rhizosphere.(4) DGGE and qPCR techniques were used to study the responses of microbial community to PAHs pollution including PAHs degrading bacteria. The diversity and richness of microbes were stimulated at the 3 mm to 4 mm layer from root surface in rice rhizosphere. The promotion of microbial activities was the balance between influences of root exudates and oxygen. PAH degrading bacteria were promoted at 1-2 mm from root surface, where oxygen was released from rice root and nutrients were depleted due to root absorption. The niche of degrading bacteria was different from other bacteria. As a result, the PAH degrading bacteria avoid competing with other bacteria through dislocation competition strategy. The combined results of PAHs dissipation and microbial responses in rhizosphere would be helpful in clarifying PAHs remediation processes and microbial mechanism in rhizosphere.