Natural attenuation of petroleum contamination in forest ecosystems: role of pedological and biological (microorganisms, flora and fauna) characteristics – the CONPET project

International audienceThe CONPET project focuses on natural attenuation of petroleum contamination in forest soil. In order to understand the attenuation process, petroleum-derived compounds were analyzed in parallel to abiotic physic-chemical parameters and to biological indicators. Scientists from complementary disciplines gathered around a unique site (forest of Haguenau, Pechelbronn, France) contaminated by petroleum seepages inherited from a tertiary source rock exhibiting spatial contamination gradients from the seepage to more distant areas. This studied site was one of the first petroleum exploitation in Europe and more than 5000 drillings were carried out since the 16th century, leading to numerous petroleum seepages for hundreds of years.The main objectives of the study are i) to evaluate hydrocarbon contamination and its natural attenuation, and ii) to investigate how soil organisms adapted to the contamination and their involvement in hydrocarbon transformation.Two seepages (GUN and ETV) presenting different pedological characteristics were studied. Soil (0-20 cm and 20-40 cm depths, vertical gradient) and litter (when present) were sampled at five points along a 15 m-section (lateral gradient) from the seepage point. Hydrocarbon contents and molecular fingerprints were determined (GC-FID and GC-MS, respectively). Microbial activities (aerobic and anaerobic carbon mineralization, enzymatic activities related to C, N and P cycles) were measured on surface samples. Metagenomic DNAs were extracted to estimate bacterial and fungal abundance (qPCR) and diversity (Illumina sequencing). Root and shoot were sampled from the plant Carex brizoides in order to evaluate the level of antioxidant defense and lipoperoxidation. Mesoinvertebrates (mainly 0.2-2 mm microarthropods) and macroinvertebrates (> 2 mm) were studied through the sampling of litter and on soil blocks.On both sites, results were roughly similar. Hydrocarbon indexes showed lateral and vertical gradients of contamination. Molecular fingerprints indicated that petroleum was unaltered at the seepage and that degradation products appeared with increasing distance from the seepage. Similarly, microbial mineralization activities and enzymatic activities related to C and N cycles were higher at the closest points to the petroleum source and decreased toward the less contaminated area. In Carex brizoides, petroleum contamination caused little to significant oxidative stress on the two sites. While oligochaete, litter invertebrate and oribatida density tended to increase with the distance from the seepage in ETV site, this tendency was not observed in GUN site, where only litter invertebrate diversity increased.Our results indicated that natural attenuation occurred at both sites as evidenced by hydrocarbon molecular analysis. The resulting contamination gradient affected microbial densities and activities, fauna density or diversity in surface soil and litter and induced stress response in plants. Statistical analysis will allow integrating the whole set of data in order to evidence links between chemical fingerprints and biological characteristics