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Mine driven trace elements' pollution entails environmental risks and causes soil infertility. In the last decades, in situ techniques such as phytostabilization have become increasingly important as ways to tackle these negative impacts. The aim of this study was to test the individual and combined effects of different aided phytostabilization techniques using substrate from barren tailings of a Cu mine, characterized by extreme infertility (high acidity and deficiency of organic matter and nutrients). The experiment analyzed the growth of Populus nigra L. planted alone (P) or in co-cropping with Trifolium repens L. (PT), in pots containing mine soil amended with compost (1, 10, compost, soil, w/w) non inoculated (NI) or inoculated with plant growth promoting rhizobacteria (PGP), mycorrhizae (MYC) or a combination of bacterial and fungal inocula (PGPMYC). Non-amended, non-planted and non-inoculated reference ports were also prepared. Plants were harvested after 110 days of plant development and several biometric and phytopathological parameters (stem height, aerial biomass, root biomass, wilting, chlorosis, pest and death) and macro and micronutrient composition were determined. The growth substrate was analyzed for several physicochemical (pH, CECe, and exchangeable cations, total C and N, P Olsen and availability of trace elements) and microbiological (community level physiological profiles: activity, richness and diversity) parameters. The use of the amendment, P. nigra plantation, and inoculation with rhizobacteria were the best techniques to reduce toxicity and improve soil fertility, as well as to increase the plant survival and growth. Soil bacterial functional diversity was markedly influenced by the presence of plants and the inoculation with bacteria, which suggests that the presence of plant regulated the configuration of a microbial community in which the inoculated bacteria thrive comparatively better. The results of this study support the use of organ

The thesis focuses on the (1) characterisation and comparison of the bacterial communities associated with Ni-hyperaccumulating and metal-excluding plants growing in ultramafic soils naturally enriched in TE and (2) analysis of bacterial communities in soils contaminated with TE and treated with phytoremediation strategies. The work planned aimed to obtain information about bacteria potentially involved in the mechanisms of TE (hyper)accumulation and/or exclusion and to study the relationships between the bacterial communities and the changes induced in soil properties by phytomanagement.

This thesis aims to compare phytoextraction and phytostabilisation capacity of different plant species and to study the changes that they or their associated microorganisms produce in the bioavailability of metals in the rhizosphere. The soil-plant-bacteria system of the two subspecies of the Ni-hyperaccumulator Alyssum serpyllifolium, endemic to the Iberian Peninsula were studied with the aim to characterise its associated bacterial community. Rhizospheric processes involved in Ni solubilisation were also studied and experiments for the improvement of phytomining efficiency using soil organic amendment were carried out. On the other hand, greenhouse and field experiments were carried out on Zn, Cd and Pb or Cu-contaminated soils; the objective being the evaluation of phytoremeditiation potential of metal-(hyper)accumulators or metal-excluding plant species growing in metal contaminated soils. Moreover, in the same soils studies were carried out to improve phytoremediation efficiency through the use of soil fertilisation, organic amendments or bacterial inoculation.

The metal hyperaccumulating species play an important role in soil phytoremediation techniques. The Thesis Project focuses on the development of various methods to improve the ability of nickel phytoextraction in hyperaccumulating species. There are a variety of techniques used for this purpose. This study focuses primarily on the use of agricultural practices to increase the bioavailability of metals in soil, inoculation of plants with microorganisms involved in the process of phytoextraction and external application of growth promoting substances in the plant. Through the development of these techniques is possible to obtain plants with a greater potential for phytoextraction of heavy metals and therefore most likely to use in soil remediation. In this study is also included the study of five different populations of the nickel hyperaccumulator Alyssum serpyllifolium (ssp. lusitanicum and ssp. malacitanum) of the Iberian Peninsula (also known as A. pintodasilvae and A. malacitanum). Different populations included in this study represent the main serpentinitic areas of the Iberian Peninsula: the region of Tras-os-Montes (Morais and Samil) in the northeastern Portugal, Melide in the province of A Coruña, and Sierra de Aguas and Sierra Bermeja, located in the province of Malaga. The main conclusions of this Thesis can be summarised as: - The application of plant growth regulators (PGRs) or phytohormones may increase biomass production of Ni hyperaccumulating species such as Alyssum and Noccaea and, consequently, their metal phytoextraction capacity. The most effective PGR for enhancing Ni removal was the IAA-based product. - Bioagumentation using PGPR can be considered as a potential tool to optimised Ni phytoextraction (or phytomining) technologies, but it remains to be seen whether or not plant-associated microorganisms can further improve the shoot Ni removal on a field scale. This study has identified candidate strains which hence, could be useful for future fiel