Successional stages and inferred functional profiles of bacterial communities under biocrusts in post-mining...

Mine tailings are inhospitable to plant establishment because of substrate instability, nutrient limitation, heavy metals, and temperature fluctuations at the soil surface. Biological soil crusts (biocrusts) and their associated microbial communities can initiate primary succession and facilitate plant-soil interactions, thereby supporting ecosystem recovery. Here, we characterized soil bacterial communities beneath biocrusts along a successional gradient in abandoned and rehabilitated molybdenum-bismuth mine tailings in Western Boreal Quebec. We collected 125 soil samples from bare soil, cyanobacterial-, chlorolichen-, and bryophyte-dominated biocrusts, as well as from a mixed bryophyte-lichen cover layer. Bacterial communities were assessed using amplicon sequencing (16S rRNA and nifH genes) and linked to soil physicochemical properties to infer functional potential. Soil pH, electrical-conductivity, and sulfur content were associated with bacterial diversity (distance-based redundancy analysis, R² = 0.20, p < 0.01). Rehabilitated sites exhibited moderate relative abundances of Proteobacteria (6.9%), whereas Actinobacteriota prevailed in nutrient-poor abandoned sites (17.1%), consistent with oligotrophic adaptation. Additionally, functional potential from chemoheterotrophy in later stages was associated with sulfur-oxidation (Spearman's ρ = 0.6, p < 0.05), with anoxygenic photoautotrophs potentially contributing to sulfur oxidation. Overall, our study indicates that bacterial communities may contribute to soil stabilization and could serve as key bioindicators of restoration success.