## 2 Materials and Methods  
### Experimental Design  
The aged PAH-contaminated soil used in this research was obtained at Shengli Oilfield, Dongying City, Shandong Province (sampling depth of 250 mm) with the approval of the Ministry of Safety and Environmental Protection of Shengli Oilfield. Soil analysis was performed by the Key Laboratory of Terrestrial Ecological Processes of the Institute of Applied Ecology, Chinese Academy of Sciences. The physicochemical properties of contaminated soil were: pH, 7.66; C, 45.77 g/kg; \(p\), 0.65 g/kg; \(n\), 0.73 g/kg; and available phosphorus, 0.002 g/kg, respectively. Uncontaminated soil samples were collected in Shenyang Wanliutang Park for dilution. The physicochemical properties of uncontaminated soil were: pH, 6.7; C, 12.82 g/kg; \(p\), 0.44 g/kg; \(n\), 0.8 g/kg; and available phosphorus, 0.011 g/kg. The concentration of PAHs in the contaminated soil ranged between 228 mg/kg and 398 mg/kg. The soil samples were sieved through a 4.00 mm sieve to ensure homogeneity. According to the previous results, none of the plants tested was able to grow in the aged PAH-contaminated soil. We added uncontaminated reference soil at a ratio of approximately 1:1, and diluted the contaminated soil to 149.17-187.54 mg/kg. The tested plant known as _E. purpurea_ was screened from 14 plants after four-year-long experiments and had a short life cycle and grew logarithmically from 100 to 120 days. Therefore, we choose to sample at 60, 120, and 150 days to observe the overall changes in the microbial community.  
The experiment was divided into two treatments--an experimental treatment (_E. purpurnea_ planted in PAH-contaminated soil, labeled as S) and a control treatment (no crop planted in PAH-contaminated soil, labeled as C). Each treatment has 24 pots (\(\Phi\) = 20 cm, h = 18 cm, and soil = 2.5 kg), including 12 blank control pots. A disc of filter paper was placed at the bottom of each 20 cm pot to prevent the dry soil from escaping through the drainage holes, and the pots were placed on saucers. Each pot was initially filled with 2.3 kg of PAH-contaminated soil, then a layer of uncontaminated soil (0.3 kg) was placed on the surface of the contaminated soil. Purple coneflower seeds (20 seeds per pot) were sown on the surface of the uncontaminated soil, and finally the seeds were covered by 0.2 kg of PAH-contaminated soil. After the germination of the plants, 15 healthy seedlings were left in each pot. Corresponding control treatments were in PAH-contaminated soil without planting of plant seeds. They were placed in a growth chamber (41\({}^{\circ}\)48\({}^{\prime}\) N, 123\({}^{\circ}\)26\({}^{\prime}\) E). After 60, 120, or 150 days of cultivation, _E. purpurea_ grown in the experimental soil (149.17-187.54 mg/kg) was harvested. The soil in the C group was watered in the same way, and all treatments were processed within 150 days. The cycle was 16 h/25 \({}^{\circ}\)C during the day and 8 h/15 \({}^{\circ}\)C at night. These plants were watered every other day to maintain a moisture content of approximately 60% by weight [18]. The experiments were performed from 28 April 2009 to 29 September 2009 and lasted for 150 days.