Phenanthrene and pyrene were the model PAH compounds used through this trial. They were purchased from Sigma Chemical Com- pany with a purity of 98%. A clay loam soil (pH 4.8 and 3.75 g kg−1 organic carbon, Udic Ferrosols) with no detectable PAHs was used in this study. The soil was collected from an experimental farm near Yingtan City, Jiangxi province, China, from a depth between 0 and 20 cm. The soil samples were air-dried, and passed through a 2-mm sieve. Soils were then spiked with a mixture of highly pure
phenanthrene and pyrene in acetone (10% total quantity of soil to be spiked). The spiked soil was transferred in a wide rectangular pan and then left under the fume hood for 2 days to evaporate any traces of acetone. When acetone was evaporated, the spiked soil was thoroughly mixed with uncontaminated soil, and then the total contaminated soil was passed through 2-mm sieve again to ensure homogeneity of treatment. The final concentrations for phenanthrene and pyrene both in the treated soil were 98.1 and 99.2 mg kg−1 by HPLC, respectively.
The treated soils (2 kg dry weight soil per pot) were packed into ceramic pots (diameter 20 cm, and height 15 cm), and incubated in a glass greenhouse for 7 days with 70% of water holding capacity, and fertilized with an inorganic salt (0.66 g urea and 0.64 g KH2 PO4 per pot). Ryegrass (Lolium perenne) was selected for this research on the basis of previous performance in field studies [24,27]. Pregermi- nated seedlings were grown in each pot, and the seedlings were thinned after 10 days of growth to leave 20 plants per pot for rye- grass.
2.2. Experimental design
There were four treatments in this trial: sterile soil without plants (T1); nonsterile soil without plants (T2); nonsterile soil with- out plants but receiving root exudates (T3); nonsterile soil with plants (T4).
Two types of pot were prepared. One was undivided (no cen- tral net barrier) to be used in T1 and T2. T1 is a sterile treatment, in which 30 ml of 2% formaldehyde solution was applied to inhibit microbial activity every 10 d (formaldehyde solution is a kind of sterilants in medicine field and in our experiments PAHs can not react with formaldehyde [28]). The pots in T3 and T4 were equally divided into two chambers with a vertical nylon mesh (35