Implications of Topsoil Placement Depths for Microbial Communities in Oil Sand Mine Reclamation.

Open pit mining in the Athabasca Oil Sands Region of Alberta, Canada is one of the largest localized industrial disturbances requiring reclamation – now exceeding 750km². Government regulations require reclamation to an equivalent capability of the pre-disturbance ecosystem by returning major ecosystem processes once mining activities have ceased. Expensive handling costs and scarcity of topsoil resources necessitates judicious management and application of capping materials. Soil microbial ecology plays an integral role in reclamation by mediating nutrient availability and organic matter decomposition. This study compares soil reclamation materials by evaluating placement depths within the context of microbial community structure and function, as determined by phospholipid fatty acid (PLFA) analysis and community level physiological profiles (CLPP). Seven treatments encompassing two topsoil depths of forest floor mix (FFM) and peat mineral mix (PMM), two subsoil types and a natural (NAT) benchmark site were measured on Syncrude Canada’s Aurora Capping Study. Preliminary analysis suggests greater soil respiration rates in FFM and NAT than in PMM treatments, with no difference attributable to subsoil or placement depth. Nutrient profiles from 5 cm depth revealed greater total inorganic N availability in PMM, while FFM followed similar trends to NAT when N P K S availability was compared. Non-metric multidimensional scaling indicated profound discrepancies in plant available macro and micro nutrients between topsoil substrates and placement depths (p<0.001), with shallow FFM application most closely resembling NAT. We hypothesize that under these conditions, shallow FFM applications create an adolescent terrestrial ecosystem capable of supporting endogenous microbial communities. Nutrient and microbial community profiles could contribute to future work assessing restoration trajectory in newly reclaimed sites and provide an indicator of reclamation success when filing for government certification.