Bacterial and fungal communities in soil and rhizosphere samples of balsam poplars growing in mining residues.

Abandoned unreclaimed mines are an important environmental issue since they typically remain unvegetated for decades, letting vast amounts of mine wastes exposed to erosion. Several factors limit the revegetation of these sites, including extreme abiotic conditions, unfavorable biotic conditions and low availability of nutrients. However, some pioneer trees are able to colonize these sites and could be used to initiate plant succession. These trees may have particular genotypes that increase their acclimation to conditions of mine wastes. In this study, two contrasting mining sites from the Abitibi-Temiscamingue region, Quebec, Canada, where poplars have naturally grown, were selected. First, we assessed the impact of vegetation on mining residues of both sites during a field study. The presence of balsam poplars improved soil health locally by improving physicochemical properties (e.g. higher nutrient content and pH) of mining residues and causing an important shift in bacterial and fungal community composition, going from lithotrophic communities that dominate mine waste environments to heterotrophic communities involved in nutrient cycling. Then, in a greenhouse experiment, ten genotypes of Populus balsamifera harvested on both mining sites and from a natural forest nearby were grown in these mining residues. Tree growth was monitored during two growing seasons, after which the effect of genotype-by-environment interactions on the physicochemical properties of the substrates and the microbial communities were investigated using a metabarcoding approach. Although the substrate type was identified as the main driver of the rhizosphere microbiome diversity and community structure, a significant effect of tree genotype was also detected, particularly for bacterial communities. Plant genotype also influenced aboveground tree growth and the physicochemical properties of the substrates. These results highlight the influence of balsam poplar genotype on the soil environment and the potential importance of tree genotype selection in the context of mining site revegetation.