Complementary approaches to integrate microbial diversity into revegetation and phytoremediation on abandoned mine tailings

Mine tailings are the principal byproduct of ore processing for metal extraction. They represent an important source of contamination for the surrounding environment due to wind-driven dust dispersion, water erosion, and leaching. Composed mainly of silt- or sand-sized grains, tailings are low in nutrients (N, P, K) and contain minimal organic matter. Two main factors that contribute to toxicity in mining residues are the high concentrations of heavy metals and their extreme pH. Metal contamination in mine residues is characterized by heterogeneous metal levels and site-specific multi-elemental contamination depending on the geological nature of the mine from which they originated. Most tailings disposal sites are characterized by a reduced plant colonization and a compromised heterotrophic microbial community. Ecological restoration has emerged as a primary objective for the development of stable and productive ecosystems in mining areas. In this context, microorganisms are pivotal for maintaining ecosystem functionality; specifically, metal-microbe interactions can influence metal mobility and bioavailability offering substantial potential for bioremediation. Furthermore, mining environments harbour specific plant taxa capable of colonizing these substrates and sequestering metals from the soil through uptake and accumulation. The synergistic interaction between plants and microorganisms is fundamental to maintain soil structure and nutrient cycling. Indeed, phytoremediation is recognized as an ecologically sustainable technique for the revegetation of degraded mining sites. Among phytoremediation approaches, phytostabilization occurs at the root-substrate interface, where excluder-type metallophytes and associated microorganisms mitigate metal mobility and bioavailability. Phytostabilization appears to be a promising strategy owing to the use of autochthonous plant species and their associated microorganisms naturally adapted to the site-specific harsh conditions. To elucidate the factors governing phytostabilization under field conditions, a multifactorial approach is essential. Investigating plant-microbiome dynamics is crucial for formulating robust restoration strategies. High-throughput sequencing technologies can now provide a high-resolution analysis of microbial diversity, thereby enhancing our understanding of the complex dynamics of communities within mine tailings. The integration of microbiological insights into phytoremediation approach is therefore imperative. The investigated case study site was the abandoned Campo Pisano tailing dump in the Iglesiente district, which was one of the most important Zn-Pb mining areas in Europe for centuries. In the disposal site, mine residues are characterized by high and heterogeneous metal levels in a complex environmental context, thus representing an ideal site for exploring the impacts of metal contaminations on microbial communities.