Soil Stockpile Seed Viability Declines With Depth and Is Impacted by Surface Vegetation

Stockpiled soil will be used to operationally reclaim approximately half of the area disturbed by one oil sands mine in northern Alberta, Canada over the next few decades. However, there are concerns regarding the viability of native seeds in stockpiles relative to directly placed reclamation soil. To test the germination of seeds from a four-year-old soil stockpile, we took samples at different depths up to 90 cm from three surface vegetation communities (sweet clover, sow-thistle, and wheatgrass dominated). These samples were placed in the greenhouse on top of potting soil to allow any seeds to germinate for nine weeks. The highest species richness (42% of species) and total plant abundance (68% of all plants) were found at the surface of the stockpile (0-10 cm). Most of the species found in these soil samples were native (63%) and the most abundant species was slender wheatgrass, a native grass species. Vegetation type did not affect the species richness, but there was a higher average seedling abundance in the sow-thistle treatment. The vegetation types did impact the seed-bank community at the surface of the stockpile, with the most abundant species on the surface having the most impact on the seed bank community for each respective vegetation type. However, below the surface the most abundant species in the seed bank did not always reflect the dominant surface vegetation. Using soil stockpiles for final land reclamation in the future may be an issue because of the low number of viable seeds below the surface.