Phytoremediation is a promising technique for reclamating petroleum contaminated soils. Phytostimulation, is an important mechanism involved in degradation of petroleum hydrocarbons via stimulating microorganisms in the root zone. In this study, the effect of sewage sludge application on degradation of petroleum hydrocarbons contaminated soil was investigated. The experimental soil was collected from a petroleum-contaminated site around the Isfahan Oil Refinery Complex, (Bakhtiardasht - 32? 48' N, 51? 32' E). The soil had been contaminated by the leachates of the complex during several years of activity. The initial soil total petroleum hydrocarbon (TPH) concentration was approximately 3700 mg kg ?1 . Municipal sewage sludge was prepared from Northern Waste Water Refinery of Isfahan (Shahinshahr Township). The experiment was set up as completely randomized design in a factorial experiment. Three different levels of sewage sludge applied to the soil [0 (S0), 2 (S1) and 5 (S2) percent (w/w)] for 2 months in a controlled conditions greenhouse with maize ( Zea mays L .) inoculated with Piriformospora indica (P2) without P.indica (P1) and no plant (P0). Seeds were germination in an incubator and were inoculated in sterile conditions. Buds were planted in plastic pots having about 3 kg soil in three replicates and microbial respiration was tested in 3, 10, 17, 24, 31, 38, 45, 55, 65 and 75 days after the treatments. At the end of the experiment, TPH, microbial respiration and dry weight of root and shoot were measured. The results showed that the sewage sludge level (2 and 5%) had positive significant (P 0.01) effects on root and shoot dry matter compared to the no sludge. Application of sewage sludge improved microbial respiration and resulted in faster TPH dissipation especially in the presence of the plants. Over time, hydrocarbon decomposition rate increased but removed of T2 and T4 treatments occurred more rapidly. However, the rate of T4 and T6 treatments reduced so that the difference is not significant. Degradation of petroleum hydrocarbons in the second month was much higher than the other times and the maximum degradation occurred at T6S2 treatment.. Keywords: Oil pollution, maize (Zea mays L.), Piriformospora indica , sewage sludge
