Can locally sourced inoculum and biochar synergistically improve the establishment of mycorrhizal fungi in mine tailings?

The severely degraded soils at abandoned mine lands present several environmental challenges, including extreme pH, nutrient deficiency, and compaction, which can limit plant growth. Amending soils with biochar and/or mycorrhizal fungi may attenuate these constraints. However, the efficacy of amendments for improving the establishment of native plant species in restorations can be context dependent; pairing the right plant species with the right amendment is crucial to the success of restoration goals. In a greenhouse experiment, we evaluated the growth responses of two native plant species in soils collected from the Formosa Mine Superfund Site (Riddle, OR, U.S.A.). Seedlings of Elymus glaucus and Pseudotsuga menziesii were grown in tailings amended with biochar, locally collected rhizosphere soil containing a suite of soil microorganisms including mycorrhizal fungi, a combination of biochar and rhizosphere soil, or were unamended. We hypothesized that (1) individual amendments would increase plant biomass compared to controls and (2) that the co-amended treatments would have a synergistic effect on plant growth. Instead, we detected no effect of rhizosphere soil or the co-amendment on the growth of either plant species and found that biochar reduced the shoot biomass and leaf chlorophyll content of E. glaucus. Despite the lack of growth response, we observed significant rates of mycorrhizal colonization in the plants amended with local rhizosphere soil, demonstrating that locally sourced fungi can survive and proliferate in the amended Formosa tailings. Thus, inoculated seedlings of native plant species could provide essential ground cover and long-term habitat for developing mycorrhizal communities in mine tailings.