A 150-day pot experiment was conducted with graminaceous plants grown in natural soil contaminated with petroleum. The relationships among microbial activity, dehydrogenase activity, catalase activity, soil moisture, and the petroleum degradation rate were analyzed. All three plants accelerated the degradation of petroleum compared with unplanted soil. Plant roots improved the soil moisture by about 5% (from 15% in unplanted soil to 20% in soil containing plant roots), and the number of microorganisms in the rhizosphere increased by more than three orders of magnitude. The induction of the rhizosphere environment and the intimidation of the petroleum changed the abundance and activity of the microorganisms. Dehydrogenase activity in the rhizosphere was 1.54 to 1.87 times the value in the unplanted soil, but catalase activity was 0.90 to 0.93 times the value in unplanted soil. The petroleum degradation rates in the rhizosphere were 2.33 to 3.19 times higher than in the unplanted soil. The effect of rhizosphere degradation clearly changed the hydrocarbon composition, increasing the degradation of alkane hydrocarbons with low and moderate carbon contents. The rhizosphere environment promoted degradation of the high-carbon-content hydrocarbons into low-carbon-content hydrocarbons. At the same time, the Pr/nC17, Ph/nC18, and Pr/Ph values increased by 0.99 and 2.69 units, and decreased by 1.25 units, respectively, compared with the undegraded oil. The plants also accelerated the isomerization of alkane hydrocarbons.

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