In this study, phospholipid fatty acids (PLFA) and carbon isotopes were used to characterize the response of in situ microbial communities to a pilot-scale wetland reclamation project in the Alberta oil sands, and to investigate their role in carbon cycling at the reclamation site. The Sandhill Fen reclamation project in the Athabasca oil sands region (Fort McMurray, Alberta, Canada) has created an artificial freshwater fen typical of the boreal forest region in which the oil sands occur. At this site, composite tailings (CT) residue was overlain with a thick sand cap and a freshwater fen constructed on top. Biomass in the peat material of the fen was comparable to that found in natural fens, and a comparison of PLFA profiles in peat, CT from a nearby site, and undisturbed wetlands in the area showed that microbial communities in Sandhill fen were more similar to those in the CT than those in undisturbed wetlands. Bacteria dominated the biomass, including a small percentage of sulphate reducing bacteria that are of particular interest in the reclamation project. Fungi and other eukaryotes were also present. Analyses of radiocarbon in total organic carbon (TOC) and residue from solvent extraction suggest that there was petroleum present in the peat layer of the fen. A small amount of
young carbon from the fen surface has been transported into the CT layer in the form of dissolved organic carbon. Radiocarbon also showed that microbes preferentially metabolized more modern carbon within the carbon sources available to them. Biomass was more related to the age of carbon in the samples than to the TOC concentration, with younger carbon in the peat associated with higher PLFA concentration.