Abstract: Five oil-degradation bacterial strains were cultured, isolated and screened from soil at Xifeng Oilfield of Gansu Province, northwestern China. On-site experiments by adding different levels of urea and ammonium dibasic phosphate to oil-contaminated soil lasted 63 d, the effects of nitrogen and phosphorus contents on the oil degradation rate by mixed bacterial agent from 5 strains were investigated. The results reveal that nitrogen and phosphorus contents in oil-contaminated soil of Xifeng Oilfield are in low level, and it is not conducive for bioremediation；however, artificially adding a certain amount of nitrogen and phosphorus for bioremediation of oil-contaminated soil with mixed bacterial agent have significant promoting effects. The changes of nitrogen and phosphorus show two stages in the 63 d microbial remediation process for 1. 5% and 3% oil-contaminated soil：the contents of nitrogen and phosphorus decrease rapidly in the first stage(0-28 d)；while they fluctuate after 35 d. The microbial remediation effect for the 3% oil-contaminated soil is obvious, and the maximum oil-degradation rate reaches 52. 5%. GS-MC was used to measure and analyze the degradation rate and evolution pattern of mixed bacterial agent for hopane- the major constituent of oil. The results show that, in the optimal ratio of nitrogen to phosphorus, the degradation rate of mixed bacterial agent for hopane-like compounds in the oil-contaminated soil is over 80%, the highest degradation rate(86. 3%)for onocerane.

Key words: microbial remediation, oil-contaminated soil, nitrogen and phosphorus content, hopane