Macrofauna and mesofauna from soil contaminated by oil extraction

Mexico is an oil producing country; the extraction of oil on land has left many sites with soil contaminated by oil spills. The aim of our study was to determine the impact of this contamination on the soil fauna; thus we compared the macro and mesofauna from a non contaminated soil to a moderately and highly polluted soil caused by oil extraction. Total petroleum hydrocarbons (TPH) and soil physicochemical characterization was determined. TPH results showed two areas: highly contaminated (8150mgTPH/kg) and moderately contaminated (1800mgTPH/kg). The macrofauna abundance was not significantly different between the sites. The Hymenoptera, Gastropoda, Isoptera and the earthworms were the most abundant groups. The Gastropoda population decreased with the increase of TPH concentration while other groups of macrofauna increased their density (ants, isopteran and earthworms). The mesofauna was significantly more abundant in the moderately contaminated area (50,500 Ind./m2). The main groups present were the Acari, Collembola and ants. The Acari orders were present in similar proportions both in the control soil and in the medium and highly contaminated area, while the Collembola families varied in their proportion in the three areas. The diversity index showed that the moderately contaminated site was the more diverse both in macro and mesofauna. Many groups of fauna (earthworms, ants and Isoptera) were positively correlated to some petroleum hydrocarbons (PH), such as naphthalene. The Gastropoda and Acari were the groups that were most negatively correlated to the different hydrocarbons. The PCA differentiated significantly three groups, both in the case of macrofauna as well as for mesofauna. Earthworms were clearly associated with TPH specially the native species Protozapotecia australis. These results indicate that oil spills could be a source of food for soil organisms after oxygenation and weathering.