Soil inoculation improves tree seedling growth in substrates containing bitumen, but the effect varies by species and inoculum source

In Canada, companies mining bitumen must restore landscapes to self‐sustaining boreal ecosystems. Residual hydrocarbons can be present in some reclaimed landforms, and their potential effect on vegetation is unclear. In this region, forests occur on natural surficial bitumen deposits, and vegetation persists on bitumen ore piles that have since been abandoned by industry. We investigated whether practitioners can leverage plant–soil interactions that enable vegetation to grow in bituminous soils to facilitate the establishment of tree seedlings on reclaimed sites containing residual hydrocarbons. We characterized soil fungal communities using Illumina Mi‐Seq sequencing across field sites with varying hydrocarbon levels, focusing on fungi due to their potential effects on seedling performance. We then used soil from these sites as inoculum in a growth chamber experiment, adding it to five substrates, in which we grew jack pine ( Pinus banksiana Lamb.) and trembling aspen ( Populus tremuloides Michx.). Three of those substrates were sympatric to the inoculum, and the other two were reclamation materials differing in hydrocarbon concentrations. The field sites differed in fungal community composition, and the abandoned ore pile had the most distinct fungal community. We found that substrate type affected aspen seedling growth independent of inoculum source. In contrast, the effect of substrate type on pine seedling growth depended on the source of inoculum. Specifically, inoculum from the abandoned ore pile increased pine growth by up to 128% in one of the reclamation substrates tested. However, this same inoculum decreased or had no effect on aspen seedling growth across the substrates. The low colonization of roots by mycorrhizal fungi suggests that other microbes may underlie seedling responses. Synthesis and applications . Soil inoculation improved tree seedling growth in some substrates containing bitumen, but the effect varied by species and inoculum source. Inoculum from extreme environments, such as the abandoned ore pile, may harbour microbes effective in supporting pine seedling growth in soils with residual hydrocarbons. Testing this inoculum source in field trials would be the next step in evaluating its use in promoting pine seedling establishment on reclaimed sites.