Exploring The Responses Of Phragmites Australis And Its Rhizosphere Microbes To Petroleum Pollution Using An Ecosystem Ecology Approach

Petroleum has become one of the world's largest sources of environmental pollution with industrial development. It is urgent to develpop effective techniques for remediation of petroleum pollution. Phytoremediation, an environmentally-friendly and cost-effective technology based on the removal of petroleum hydrocarbons from the polluted soil by plants and their associated microbes, is becoming an increasingly important field in environmental and ecological research. However, petroleum pollution has profound effects on plants, microbes, and their interactions. In this way, soil functions are degraded seriously under petroleum pollution. In this study, we used Phragmites australis as a phytoremediation species to examine the ecosystem-level responses of P. australis, microbes and their interactions to petroleum pollution, and emphasized the effects of petroleum pollution on the phytoremediation systems and the significance of biological adaptations toin phytoremediation in the Yellow River Delta. The aims of this thesis were to:1) examine the rhizosphere effects on microbial abundance and diversity in petroleum-polluted soils; 2) better understand plant-microbe interactions for phytoremediation of petroleum-polluted soil; 3) evaluate allocation and turnover of photosynthetically-fixed carbon as influenced by petroleum pollution; 4) analyze P. australis's use of different nitrogen forms and soil nitrogen transformation in the polluted environments. The major findings are summarized as follow:(1) Petroleum pollution had profound effects on P. australis, microbes, and their interactions. Plant size, biomass, and potentially photosynthetic capability were negatively related to petroleum concentration. Soil bacterial community structure and dynamics, colonization of arbuscular mycorrhizal fungi (AMF), extracellular enzyme activity, and nitrogen transformation driven by microbes were subject to changes after soil was polluted by petroleum. Higher rate of AMF colonization was found in P. australis roots in polluted soils compared to those in unpolluted soils at early stage of vegetative growth, suggesting AMF might play an important role in reducing initial nutrient deficiency of the plant in petroleum-polluted soil.(2) P. australis through rhizosphere effects could alleviate soil stresses and positively influenced soil microbes. P. australis roots could alleviate the stresses from soil salinization in the Yellow River Delta, and rhizosphere soils were mainly characterized by lower salinity and higher water content. For bacterial abundance, the numbers of total bacteria and hydroscarbon degraders were significantly higher in rhizosphere soils than those in bulk soils. Soil salinization was not the major determinant of total bacterial abundance. In addition, rhizosphere soils had higher bacterial diversity in comparison to that of bulk soil. High abundance and diversity of total bacteria with more hydrocarbon degraders in plant rhizospheres would potentially improve the roles of bacteria in maintaining ecosystem functioning and remediating polluted soils in the degraded ecosystems.(3) Early plant vegetative growth was the key period for phytoremediation. Petroleum pollution resulted in reduced P. australis performances across the growth stages, especially during vegetative growth. P. australis would be highly susceptible to petroleum pollution in the stage of plant vegetative growth. Petroleum had significantly positive effects on the rpoB, alkB and tol genes at plant vegetative growth stage, and greater abundance of these two genes was also detected at plant vegetative growth stage. These results showed that the effectiveness of phytoremediation was plant-dependent, and that the interactions between P. australis and petroleum-degrading bacteria appeared to be relatively stronger at plant vegetative growth stage.(4) P. australis increased biomass allocation to roots. Plant biomass significantly decreased under petroleum pollution, but root/shoot ratio both in plant biomass and 13C increased with increasing petroleum concentration, suggesting that the plant could increase biomass allocation to roots in petroleum-polluted soil. Furthermore, assimilated 13C was found to be higher in soil, microbial biomass, and soil respiration after the soil is polluted by petroleum. These results suggested that C released from roots rapidly turns over by soil microbes under petroleum pollution. The increased allocation of plant biomass to its below-ground parts might be adaptive in petroleum-polluted soil.(5) Plant assimilation of both inorganic and organic N was low in petroleum-polluted soil, but the percentage of organic N in total plant assimilated N increased with petroleum concentration, suggesting organic N made a great contribution to plant N availability. In addition, petroleum pollution promoted gross N transformations to some extent, but larger increases in gross N immobilization and nitrification rates relative to the increases in gross N mineralization rate might reduce inorganic-N availability to the plant. Therefore, the increased importance of organic N in plant N assimilation might be of great significance for plants growing in petroleum-polluted soils. Our results provide insights into the effects of pollutants on soil available N to the plant, and suggest that plants might regulate N capture under petroleum pollution.