Bioaugmentation-assisted phytostabilisation of abandoned mine sites in South West Sardinia

Phytostabilisation has been recognised as a cost-efficient and environmental friendly technology for in situ restoration of mining areas implying the creation of a vegetation cover for the long-term metal stabilisation. An improvement of the process can be obtained by exploiting the synergistic partnership plant–microbe, the so-called bioaugmentation-assisted phytoremediation. This implies the administration of selected plant growth promoting bacteria (PGPB), which significantly speed up the process by improving plant establishment, growth and health. The purpose of this study was to develop a bioaugmentation-assisted phytostabilisation technology based on autochthonous plant species and bacterial inocula from abandoned Sardinian mining areas (SW Sardinia, Italy), considered one of the most important mining districts for Pb and Zn extraction at global level during the last two centuries. In this work, bacterial strains associated with roots of P. lentiscus were: i) selected from plants spontaneously growing in abandoned Sardinian mining areas, ii) characterised for properties relevant for plant growth promotion and metal tolerance, and iii) tested for the ability to improve plant germination, survival and growth as well as metal immobilisation within root tissues at greenhouse-controlled conditions on soils from a tailing dump and a marshy area downstream from several mine sites,as representative of arid and humid habitats. A collection of 134 isolates was obtained from roots of shrubs spontaneously growing in both sites. Based on their phylogenetic position, 24 strains were analysed for the metabolic abilities relevant for promoting metal stabilisation and plant growth: metal tolerance, phosphate solubilisation, production of ACC deaminase, indol-acetic acid, and siderophores. Five different strains belonging to the genera Novosphingobium, Variovorax, Streptomyces, Amycolatopsis, Pseudomonas were finally selected based on their superior metabolic properties for the greenhouse phytoremediation tests. Among the tested inocula, the Variovorax strain, isolated from mine tailings and endowed with the ability to produce ACC deaminase and siderophore, was able to significantly increase germination and plant growth on marshy soil while no effect was found on mine tailings. Conversely, the bioaugmentation treatments with the other selected strains did not enhance germination and plant growth on both soils. Overall data demonstrated the bioaugmentation-assisted phytostabilisation with autochthonous selected strains is a valid technology for restoration of mine sites. Moreover, a high level of specificity was highlighted being the outcome of the treatment dependent on both the plant-microbe association and the properties of the habitat to be remediated.