Biodegradation of petroleum hydrocarbons in contaminated clayey soils from a sub-arctic site: The role of aggregate size and microstructure

► Clayey, oil-contaminated soils formed aggregates larger than 0.25 mm in biopiles. ► Nutrient addition enhanced oil biodegradation only in aggregates larger than 2 mm. ► Aggregates larger than 2 mm had more pores that were accessible to bacteria. ► The number of aggregate pores accessible to bacteria influenced biostimulation.