Topsoil additions improve root symbiont colonization and seedling performance in mine reclamation

Beneficial soil microbes, such as mycorrhizal fungi and nitrogen-fixing bacteria, can improve plant nutrient acquisition and increase resilience to stressors. Yet, after major disturbances such as mining, reclamation materials contain negligible biological activity and fresh topsoil availability is often limited. We tested if small-volume additions of native forest soil can improve early seedling growth and promote colonization of beneficial root symbionts. In a greenhouse experiment, we grew seedlings representing different functional groups in tailings and glacial-till overburden from the Mount Polley Mine, Canada. We applied 5% (38 mL) and 25% (188 mL) forest soil additions for comparison with tailings/overburden controls and reference forest soil. The experiment was replicated with sterilized soil to isolate the biological effects of the forest soil from the physiochemical effects. Willow (Salix scouleriana) and spruce (Picea engelmannii x glauca) seedling growth increased with proportion of forest soil, which corresponded with increased ectomycorrhizal fungal colonization.