Rhizoremediation of a dioxin-like PCB polluted soil by alfalfa: Dynamic characterization at temporal and spatial scale

- Rhizoboxes and molecular techniques were used to study the temporal and spatial dissipation of PCB77 in alfalfa rhizosphere.
- PCB dissipation rate in the rhizosphere was closely related to both time course and the distance from alfalfa root surface.
- Bacterial gene copies in different rhizosphere zones followed the order: rhizosphere > near-rhizosphere > far-rhizosphere.
- Microbial diversity was greatly improved in rhizo- and near-rhizosphere, but not in far-rhizosphere after rhizoremediation.

This study investigates the temporal and spacial dissipation dynamics of a dioxin-like polychlorinated biphenyl (PCB 77) in the rhizosphere of alfalfa. A three-chamber rhizobox was designed to compare the PCB 77 dissipation efficiency in the rhizosphere, near-rhizosphere, and far-rhizosphere zones. Culture-independent techniques, including quantitative PCR (qPCR), Biolog-ECO plate, and denatured gradient gel electrophoresis (DGGE) were employed to investigate the variation of bacterial quantity, metabolic diversity and community structure in the alfalfa-rhizobium symbiosis rhizosphere at different rhizoremediation stages. PCB dissipation rates in different rhizosphere zones were in the order: rhizosphere (90.9%) > near-rhizosphere (80.5%) > far-rhizosphere (31.7%). The number of the bacterial 16S rRNA gene copies in the rhizosphere zone in the polluted treatment reached the highest value of all the treatments. Microbial metabolic diversity, as indicated by average well color development (AWCD) in both rhizosphere and near-rhizosphere zones, had recovered from the PCB 77 pollution. The soil bacterial community diversity improved greatly in the rhizosphere of alfalfa, with some new species appeared in the rhizosphere and near-rhizosphere zones. In conclusion, the dissipation of PCB 77, the quantity of total soil bacteria, soil microbial metabolic diversity, and soil microbial community structure were significantly improved in rhizosphere and near-rhizosphere zones of alfalfa.

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