2.1 Soil sampling and site description
Soil samples were collected from two oil production plants (Site Ⅰ:
37°59′20″N, 118°36′57″E and Site Ⅱ: 37°55′45″N, 118°35′53″E) located in
Shengli Oilfield, Hekou District, Dongying City, Shandong Province
(Fig. S1a ). In each site, three groups of soil samples were
collected (0~20 cm): short-term contaminated soil (PS)
within a one-meter radius near the working oil pumping machine
(Fig. S1b ), long-term contaminated soil (OS) within a one-meter
radius near the abandoned oil pumping machine (Site Ⅰ for eight years,
site Ⅱ for six years), and uncontaminated control soil (Control) nearby.
There were five replicates for each sampling group, and ten soil cores
thoroughly homogenized each replicate. Thus 30 soil samples were
collected in total (two sites × three groups × five replicates). Soil
samples were transported to the laboratory in sterilized plastic bags on
drikold as soon as possible. After passing a 4-mm strainer, part of the
soil samples was air-dried for physicochemical properties determination.
The remaining soil samples were stored at -80℃ for DNA analyses.