Restoring ecosystem function after oil sands surface mining involves reestablishing the biotic and abiotic ecosystem components that affect biogeochemical cycles and fluxes. In boreal forest ecosystems, pyrogenic carbon is a native soil component that affects a variety of biogeochemical parameters and biochar is its human-made analog. To evaluate the benefits of biochar amendment to reclamation cover soils, we compared characteristics and function of peat–mineral mix (PM) and forest floor–mineral mix (FFM) with and without biochar in an 18 wk greenhouse study. We assessed nutrient bioavailability (NO₃, NH₄, P, K, S, Mg, and Ca), foliar nutrient concentrations (N, P, K, S, Mg, Ca, Na, and Mo), soil respiration, rhizosphere polysaccharide concentration, soil organic matter stability, and Populus tremuloides Michx. seedling growth. Seedling growth increased significantly on PM cover soil with biochar. Biochar improved K nutritional status and potentially interacted with Na bioavailability in PM, affecting growth. Soil respiration significantly decreased in PM with biochar and increased in FFM. Soil organic matter stability was positively correlated with seedling growth and increased with biochar. Our findings suggest that biochar may have a significant positive effect on upland forest reclamation in the Athabasca oil sands region, especially on sites that are reclaimed with PM.