An optical analysis of the organic soil over an old petroleum tar deposit

We analysed by an optical method (the small volume method) the composition and the vertical distribution of an organic soil which accumulated over an old petroleum tar deposit. The study site was an oil refinery now colonised by woody vegetation since the time of abandonment (1964), located at Merkwiller-Pechelbronn (Alsace, France). Comparisons were done with a nearby unpolluted control plot under similar vegetation. The humus form over the tar deposit was described as an Hemimoder. It was characterised by fine fragmentation of litter, darkening of tree leaves with depth and a dense mycelial mat associated with an ectomycorrhizal root system. Faunal activity was dominated by enchytraeids. The Mesomull described at the control plot was characterized by fast recycling of litter and earthworm activity.