Small-volume additions of forest topsoil improve root symbiont colonization and seedling growth in mine reclamation

Beneficial soil microbes, such as mycorrhizal fungi and nitrogen-fixing bacteria, can improve plant nutrient acquisition and increase plant resilience to stressors. Yet, the waste materials left behind following major disturbances, such as mining, have negligible biological activity and fresh topsoil availability for reclamation is often limited. We tested if small-volume additions of native forest topsoil can improve early seedling survival and 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 physical and chemical effects. Willow (Salix scouleriana) and spruce (Picea engelmannii x glauca) seedling survival and growth increased with proportion of forest soil, which corresponded with increased ectomycorrhizal fungal colonization. Forest soil additions benefited seedlings grown in both overburden and tailings, with similar to 200 mL (25 %) forest soil additions generally supporting initial seedling growth comparable to seedlings grown in reference forest soil. Alder (Alnus viridis) showed minimal benefit from forest soil additions, likely due to a lack of nitrogen-fixing bacteria in the forest soil, highlighting the importance of matching the plant and microbial communities of the soil donor site with the target plant species. Differential results among plant species in sterilized soil indicated that plants exhibiting greater dependence on microbial symbionts (spruce and alder), benefited from the biological component of the inoculum, while willow, an early successional species with low mycorrhizal dependence, benefited from the physical and chemical properties of the forest soil. This research showed that targeted additions of small volumes of topsoil from native ecosystems can improve initial seedling survival and growth, and promote recovery of limiting soil microbial communities, making it a promising approach for mine reclamation when topsoil availability is limited.