Soil microbes in the rhizosphere of wetland species Juncus subsecundus under combined toxicity of cadmium and polycyclic aromatic hydrocarbons

[Truncated abstract] Cadmium and polycyclic aromatic hydrocarbons (PAHs) are two ubiquitous pollutants that are difficult to remove from soil. Constructed wetlands are a promising approach for remediating polluted sites. Juncus subsecundus, an emergent wetland species, is not a hyper accumulator, but its growth was better when exposed to low concentration of cadmium and PAHs together than PAHs or cadmium alone. The mechanisms underlying this phenomenon are unknown. In this thesis, a potential role of stimulation of soil microorganisms by PAHs was studied. J. subsecundus seedlings were grown in pots in soil treated with cadmium (5 mg kg-1), PAHs (phenanthrene + pyrene 50+50 mg kg-1) or both, and harvested after 5, 10, 20and30 days of cultivation. Measurements included dry biomass; cadmium accumulation in plants and removal from soil; tissue concentration of pyrene, phenanthrene andindole-3-acetic acid (IAA);soil microbial activity(dehydrogenase assay)and abundance of PAH-degrading bacteria; and microbial community structure (Biolog Ecoplates). The growth of J. subsecundus was negatively affected by the toxicity of cadmium and PAHs 5 days after transplanting; the PAH-only treatment particularly inhibited the growth of shoot tissues. The combined toxicity of PAHs and Cd was less severe than the toxicity of PAHs or cadmium separately. The presence of PAHs significantly increased the Cd concentration in roots 10-20 days after transplanting. The concentrations of copper and phosphate in shoot sand zinc in roots were reduced after 30 days of exposure to cadmium, whereas potassium concentration in roots increased after 20 days of exposure to cadmium. The translocation of cadmium upwards was significantly affected only by PAHs. However, the accumulation of cadmium in plants and removal from soil were independent of the PAH treatment. The concentration of pyrenein root tissues after 30 days of exposure was reduced by the presence of cadmium in soil. The concentration of IAA in shoots was higher in the Cd-only than the PAH-only treatment, suggesting cadmium might play a role in IAA production or metabolism in plants...