## 2 Materials and Methods
### Determination of Soil Enzyme Activity  
To reflect the microbial activity of diesel-polluted soil, we measured the dehydrogenase activity, fluorescein diacetate (FDA) hydrolase activity, urease activity, and phosphatase activity in the soil, which reflect the soil microbial activity, pollutant degradation function, and nutrient cycling capacity, respectively.  
Dehydrogenase activity was measured using the 2,3,5-triphenyltetrazolzolium chloride (TTC) reduction color method [48]. Briefly, 1.0 g soil sample was mixed with 2.0 mL glucose solution (0.1 M, stored at 4.0 \({}^{\circ}\)C) and 2.0 mL TTC (0.5% solution, sealed storage), In a shaking incubator at 30 \({}^{\circ}\)C for 12 h, 1.0 mL sulfuric acid was added to stop the enzyme reaction. After being stood still, the supernatant was extracted with 5.0 mL of acetone. After shaking and mixing, it was centrifuged at 5000 rpm for 8.0 min. The absorbance at 486 nm was determined, and the dehydrogenase activity was expressed as ug triphenylformanan (TPF) per g dry soil. All samples were repeated in triplicate.  
FDA hydrolase activity was measured as follows. Briefly, 2.0 g soil (wet weight) was mixed with 15 mL potassium phosphate buffer (pH 7.6, 60 mM) in a 50 mL centrifuge tube. Then 200 \(\mu\)L fluorescein diethyl ester was added (1 mg/mL, dissolved in acetone). The reaction system was incubated in a shaking incubator at 30 \({}^{\circ}\)C and 175 rpm. After 20 min incubation, 15 mL chloroform/methanol mixture solution (vol/vol 2:1) was added to stop the reaction. Centrifuge at 8000 rpm for 12 min, take the supernatant and measure it with a spectrophotometer at 490 nm. The reaction system without adding fluorescein diethyl ester was used as a blank control.  
Urease activity was measured using a modified literature method [49]. Briefly, 2.0 g of air-dried soil in a 50 mL centrifuge tube was mixed with 600 \(\mu\)L of toluene to completely wet the soil for 15 min. Then 2.0 mL of urea (10% solution) and 4.0 mL of citrate buffer (pH 6.7) were added. After reacting at 37 \({}^{\circ}\)C and 145 rpm in a shaking incubator for 24 h, 20 mL of 37 \({}^{\circ}\)C water was added. Then, shake and filter through a 0.45 \(\mu\)m filter. 1.0 mL of filtrate was transferred to the 25 mL graduated constant volume tub with the addition of a little deionized water. 4.0 mL of sodium phenolate (prepared for sodium phenolate solution) and 3 mL of sodium hypochlorite solution were added and shaken. After standing for 20 min, the solution turned into indophenol blue. Then add deionized water to reach the marked line. Measure the OD\({}_{578}\) (Optical Density) value after mixing, and the urease activity was expressed as mg NH4\({}^{+}\)-N per g dry soil per hour. All samples were repeated in triplicate.  
Phosphatase activity was measured as follows. Briefly, 2.0 g of air-dried soil was added into a 50 mL centrifuge tube, mixed with 0.5 mL of toluene. 8.0 mL of phenyl disodium phosphate solution (0.5% concentration) was added, and the reaction was shaken in the incubator at 37 \({}^{\circ}\)C and 175 rpm for 2.0 h. The suspension was filtered through the dense filter paper and 2.0 mL of the filtrate was transferred to the 25 mL graduated constant volume tub with the addition for 10 mL of deionized water. 0.15 mL of ammonium chloride-ammonium hydroxide buffer (pH = 9.8) and 0.25 mL of 4-aminoaminolipine (concentration 2%) and 0.25 mL of potassium ferricyanide were added in sequence, respectively. Finally, the OD510 value was measured by the spectrophotometer within 15 min after the addition of deionized water to the mark. All samples were repeated in triplicate.