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