Rhizodegradation is a promising cleanup technology where microorganisms degrade soil contaminants in the rhizosphere. A symbiotic relationship is expected to occur between plant roots and soil microorganisms in contaminated soils that enhance natural microbial degradation in soils. However, little is known about how this initial microbiota influences the rhizodegradation outcome in a context of different soil microbiotas. Recent studies have hinted that soil initial diversity has a determining effect on the outcome of contaminant degradation. To test this, we planted (P) or not (NP) balsam poplars (Populus balsamifera) in two soils of contrasting diversity (agricultural and forest) that were contaminated or not with 50 mg kg-1 of phenanthrene (PHE). The DNA from the rhizosphere of the P and the bulk soil of the NP pots was extracted and the bacterial genes encoding for the 16S rRNA, the PAH ring-hydroxylating dioxygenase alpha subunits (PAH-RHDα) of gram-positive and gram-negative bacteria, and the fungal ITS region were sequenced to characterize the microbial communities. The abundance of the PAH-RHDα genes were quantified by real-time quantitative PCR. Plant presence had a significant effect on PHE degradation only in the forest soil, whereas both NP and P agricultural soils degraded the same amount of PHE. Fungal communities were mainly affected by plant presence, whereas bacterial communities were principally affected by the soil type, and upon contamination the dominant PAH degrading community was similarly constrained by soil type. Our results highlight the crucial importance of soil microbial and physicochemical characteristics in the outcome of rhizoremediation. IMPORTANCE Polycyclic aromatic hydrocarbon (PAH) are a group of organic contaminants that pose a risk to ecosystems' health. Phytoremediation is a promising biotechnology with the potential to restore PAH contaminated soils. However, some limitations prevent it from becoming the remediation technology of reference, despite being environmentally friendlier than mainstream physicochemical alternatives. Recent reports suggest that the original soil microbial diversity is the key to harness the potential of phytoremediation. Therefore, this study focused on determining the effect of two different soil types in the fate of phenanthrene under balsam poplar remediation. Poplar increased the degradation of phenanthrene in forest, but not in agricultural soil. The fungi were affected by poplars, whereas bacteria and PAH degraders were constrained by soil type, leading to different degradation patterns between soils. These results highlight the importance of performing preliminary microbiological studies of contaminated soils to determine whether plant presence could improve remediation rates or not.

Abandoned unrestored mines are an important environmental concern as they typically remain unvegetated for decades, exposing vast amounts of mine waste to erosion. Several factors limit the revegetation of these sites, including extreme abiotic and unfavorable biotic conditions. However, some pioneer tree species having high levels of genetic diversity, such as balsam poplar (Populus balsamifera), can naturally colonize these sites and initiate plant succession. This suggests that some tree genotypes are likely more suited for acclimation to the conditions of mine wastes. In this study, we selected two contrasting mine waste storage facilities (waste rock from a gold mine and tailings from a molybdenum mine) from the Abitibi region of Quebec (Canada), on which poplars were found to have grown naturally. First, we assessed in situ the impact of vegetation presence on each mine waste type. The presence of balsam poplars improved soil health locally by modifying the physicochemical properties (e.g., higher nutrient content and pH) of the mine wastes and causing an important shift in their bacterial and fungal community compositions, going from lithotrophic communities that dominate mine waste environments to heterotrophic communities involved in nutrient cycling. Next, in a greenhouse experiment we assessed the impact of plant genotype when grown in these mine wastes. Ten genotypes of P. balsamifera were collected locally, found growing either at the mine sites or in the surrounding natural forest. Tree growth was monitored over two growing seasons, after which the effects of genotype-by-environment interactions were assessed by measuring the physicochemical properties of the substrates and the changes in microbial community assembly. Although substrate type was identified as the main driver of rhizosphere microbiome diversity and community structure, a significant effect due to 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 mine waste revegetation.

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14Abandoned unrestored mines are an important environmental issue since they typically remain 15 unvegetated for decades, exposing vast amounts of mine waste to erosion. Several factors limit the 16 revegetation of these sites, including extreme abiotic conditions and unfavorable biotic conditions. 17However, some pioneer tree species having high level of genetic diversity, such as balsam poplar 18 (Populus balsamifera), are able to naturally colonize these sites and initiate plant succession. This 19 suggests that some tree genotypes are likely more suited for acclimation to the conditions of mine 20wastes. In this study, two contrasting mine waste storage facilities (waste rock versus tailings) from 21the Abitibi region of Quebec (Canada), on which poplars have grown naturally, were selected. First, 22we assessed in situ the impact of vegetation presence on each type of mine wastes. The presence of 23 balsam poplars improved soil health locally by improving physicochemical properties (e.g. higher 24 nutrient content and pH) of the mine wastes and causing an important shift in their bacterial and 25 fungal community compositions, going from lithotrophic communities that dominate mine waste 26 environments to heterotrophic communities involved in nutrient cycling. Next, in a greenhouse 27 experiment, ten genotypes of P. balsamifera collected on both mine sites and from a natural forest 28 nearby were grown in these mine wastes. Tree growth was monitored during two growing seasons, 29after which the effect of genotype-by-environment interactions was assessed by measuring the 30 physicochemical properties of the substrates and the changes in microbial communities, using a 31 metabarcoding approach. Although substrate type was identified as the main driver of rhizosphere 32 microbiome diversity and community structure, a significant effect of tree genotype was also 33 detected, particularly for bacterial communities. Plant genotype also influenced aboveground tree 34 growth and the physicochemical properties of the substrates. These results highlight the influence of 35 Tree Genotype Impacts Rhizosphere Microbiomes 2 balsam poplar genotype on the soil environment and the potential importance of tree genotype 36 selection in the context of mine waste revegetation.