Study On The Rhizosphere Effects And Mechanisms In Phytoremediation For PAHs Contaminated Soils

Polycyclic aromatic hydrocarbons (PAHs) are typical persistant organic pollutans (POPs). They are formed during anthropogenic industrial activities and severely threaten the ecological function of soil, the agricultural food quality and human health due to their widespread occurrence, persistence in soil ecosystems, toxic, carcinogenic, mutagenic and recalcitrant to degradation properties. Remediation for contaminated soil with PAHs has been focused in the world. Phytoremediation is a promising alternative approach to soil remediation due to its convenience, cost-effectiveness and environmental acceptability.In this dissertation, the rhizospere effects in phytoremediation, mechanisms of enhanced PAHs degradation and affecting factors were investigated based on the review of the present research status on phytoremediation of soil PAHs pollution. The secreting characteristics of plants root exudates under the press of PAHs were studied. Moreover, the microbial ecological mechanism in rhizospere effects during phytoremediation were revealed and interpreted. The main results are shown as follows:(1) Ryegrass (Lolium perenne L.) could appearently enhanced dissipation of phenanthrene (Phe) and pyrene (Pyr) in soil. At the end of a 50 d experiment, the dissipation rates of Phe and Pyr in rhizospheric soils were ranged from 83.5%～95.0% and 37.4%-57.3% respectively, which were significantly higher than in the bulk soils. Phe was easier to be remediated than Pyr and ryegrass had good tolerance to low and moderate concentration (about 5 mg/kg and 50 mg/kg) of PAHs, while high concentration (about 500 mg/kg) might restrain the growth of rygrass. Plant uptake or accumulation was not the main mechanism of PAHs degradation enhanced by plants. The enhancement of PAHs degradation was mainly caused by rhizosphere effects.(2) Low molecular weight organic acids, carbohydrate and amino acids are considered to be the most common components in root exudates. At the tested extent (pyrene concentration of 3 mg/L,6 mg/L and 9 mg/L), low molecular weight organic acids in root exudates of ryegrass increased significantly in content with the increasing concentration of pyrene whereas no significant difference was observed in the composition of organic acids. Total carbohydrate content responded nonlinearly as the pyrene concentration increased:it initially increased and then decreased, showing no statistically significant difference. The secreting of total amino acids was significantly enhanced and the amounts were measured to be 1.37,2.02 and 2.65 times higher than that of controls under the treatments of 3 mg/L,6 mg/L and 9 mg/L pyrene respectively but no significant difference in the composition was observed. Furthermore, the increase in content of 19 kinds of common amino acids components differed from one another with 11 kinds of them increasing statistically at the level of P<0.05 in the tested concentrations.(3) Compared with the unplanted treatments, the residual concentrations of pyrene in the rhizobox planted with ryegrass decreased apparently, showing strong rhizosphere effects. Between treatments without ryegrass, no difference was detected in pyrene residual concentration. The general trend in the degradation of pyrene was typically near-rhizosphere> rootcompartment> far-rhizosphere. The degradation of pyrene in different soil compartment was as following order: 2 mm> 3 mm> 1 mm> 4 mm> 5 mm> root compartment> far-rhizosphere (>5 mm) while soil microbial biomass carbon was observed to have an opposite trend in variation. Negative correlations were significant (P<0.05) between the residual concentrations and soil microbial biomass carbon, dehydrogenase activity and polyphenol oxidase activity in various sampling zones of soils with growth of ryegrass plants. The above results indicated that the variation trend of soil microorganism under the gradient effect with respect to distance from the root surface was the main ecological mechanism of pyrene residual concentration trend in variation, and the rhizosphere effects on pyrene degradation were restricted in some extent around plant root.(4) The pyrene residue responded nonlinearly as the added root-exudates dose increased:it decreased initially and then increased with the increasing added root-exudates dose at the tested extent. Changes of microbial biomass carbon and microbial quotient were opposite to the trend of pyrene residue in contaminated soils, which indicated that the change of soil microbial biomass might account for the dynamic changes of pyrene residue in contaminated soils. The variation trend of bacteria number was in accordance with that of pyrene degradation, indicating that the pyrene degradation was mainly conducted by bacteria and the effects of root exudates on pyrene degradation were carried out by impacting the population of bacteria. The change of endoenzyme-dehydrogenase activity was the same with soil microbial biomass and major microbial communities, which further elucidated that the changes of soil microbe and microbial biochemistry characteristics were the ecological mechanisms that influences pyrene degradation as the added root-exudates dose increases in the pyrene contaminated soil.