Effects of petroleum contamination on soil microbial numbers, metabolic activity and urease activity

► We proposed calorimetry to study soil microbial activity with petroleum contamination. ► Calorimetry, plate counting and soil urease activity fitted well together. ► The combined method is useful for evaluation of soil response to petroleum contamination. The influence of petroleum contamination on soil microbial activities was investigated in 13 soil samples from sites around an injection water well (Iw-1, 2, 3, 4) (total petroleum hydrocarbons (TPH): 7.5–78mgkg−1), an oil production well (Op-1, 2, 3, 4, 5) (TPH: 149-1110mgkg−1), and an oil spill accident well (Os-1, 2, 3, 4) (TPH: 4500–34600mgkg−1). The growth rate constant (μ) of glucose stimulated organisms, determined by microcalorimetry, was higher in Iw soil samples than in Op and Os samples. Total cultivable bacteria and fungi and urease activity also decreased with increasing concentration of TPH. Total heat produced demonstrated that TPH at concentrations less than about 1gkg−1 soil stimulated anaerobic respiration. A positive correlation between TPH and soil organic matter (OM) and stimulation of fungi–bacteria–urease at low TPH doses suggested that TPH is bound to soil OM and slowly metabolized in Iw soils during OM consumption. These methods can be used to evaluate the potential of polluted soils to carry out self-bioremediation by metabolizing TPH.