Long-term revegetation on iron mine tailings in northern Québec and Labrador and its effect on arbuscular mycorrhizal fungi

The ecological restoration of mine tailings requires the seeding of suitable plant species and the presence of root microbial symbionts to successfully establish vegetation communities. Here we characterized 29 years of revegetation on four iron mine tailings located within the boreal-taïga transition zone of Québec and Labrador, Canada, by measuring the diversity and relative cover of plant species and the root colonization and diversity of arbuscular mycorrhizal fungi (AMF).In the first year after revegetation, over 75% of the plant cover was composed of seeded annuals. During the second year, the seeded biennial Secale cereale covered about 20% of the plots, whereas the seeded and non-seeded perennials represented respectively 50% and 25% of the plant cover. Three to four years after revegetation, the plant cover was composed of about 75% seeded and 25% non-seeded perennials. Five to nine years after revegetation, plant coverage shifted to about 50% seeded and 50% non-seeded perennials, and the first woody plant to colonize these tailings, Salix sp., covered 5%. After ten years of revegetation, non-seeded perennials represented 65% of the plant cover.Root colonization by AMF increased simultaneously with the coverage of perennial species. Percentage of root mycorrhization increased from 0 to 20% during the first two years to 50-55% during years 5 to 9 and up to a maximum of 75% after ten years. We identified Claroideoglomus (mostly C. claroideum) and Rhizophagus (mostly R. intraradices) as the most frequent AMF genera. Nonetheless, Rhizophagus was only detected in plots monitored over five years post-revegetation, suggesting that this genus is best adapted to the mid-term survival of herbaceous perennials on these areas. Two years after revegetation, 50-75% AMF hyphae, 25-30% arbuscules and only 1-12% vesicles were detected in roots. From three to twenty-nine years after revegetation, 40% hyphae, 30% arbuscules and up to 30% vesicles were detected in roots sampled in late summer. 30% of vesicles were also detected in roots sampled in early summer, suggesting that the intraradical phase of AMF life cycle in this northern region can span a two-year period.Our results demonstrate the efficacy of the agronomic method that has been used for over 25 years by Irrigation NORCO for the ecological restoration of iron mine tailings in this northern region of Canada.