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Abstract
To know microbial activity and diesel-removal efficiency influencing through plant roots, we examined the effect of the rhizosphere on phytoremediation of diesel-contaminated soils by alfalfa (Medicago sativa L.). Pots were treated with and without diesel and allowed to stabilize for 7 weeks, at which time four experimental/control groups were prepared: (1) planted diesel-contaminated soil, (2) unplanted diesel-contaminated soil, (3) planted uncontaminated soil, and (4) unplanted uncontaminated soil. Samples of rhizosphere and bulk soils were separately taken from all planted pots. After 7 weeks of alfalfa growth from seeds, the removal efficiencies in rhizosphere and bulk soil samples were 82.5% and 36.5∼ 59.4%, respectively. The total microbial activity was highest in diesel-contaminated rhizosphere soils. Significantly more culturable soil bacteria and hydrocarbon-degraders were found in diesel-contaminated rhizosphere soil versus unplanted and uncontaminated bulk soil, with a greater increase seen in hydrocarbon-degraders (172-fold) versus general soil bacteria (14-fold). DGGE (Denaturing Gel Gradient Electrophoresis) analysis revealed that the bacterial community structure was most highly influenced by the combined presence of diesel contamination and plant roots (39.13% similarity compared to the control), but that diesel contamination alone had a higher influence (42.31% similarity compared to the control) than the rhizosphere (50.00% similarity compared to the control). Sequence analysis and BLAST searches revealed that all samples were dominated by members of α -, γ -, δ - and ε -proteobacteria, and Chloroflexi. The rhizosphere samples additionally contained novel dominant members of α -proteobacteria and Cytophaga-Flexibacter-Bacteroides, while the diesel samples contained additional dominant α -proteobacteria and the rhizosphere plus diesel samples contained other ε -proteobacteria. Collectively, these findings indicate that the presence of plant roots (i.e., a rhizosphere) had a greater effect on bacterial activity in diesel contamination than did the absence of diesel contamination, whereas diesel contamination had a greater effect on bacterial community structure. These novel findings provide new insight into the mechanisms of phytoremediation.
ACKNOWLEDGMENTS
This work was supported by the Korea Research Foundation Grant by Korea Government (HOERD, Basic Research Promotion Fund) (KRF-2005-050-D00007).
