Plant-microbe-soil interactions and their role in phytotechnologies applied to trace metal-rich soils

Resumo

Soil degradation is defined as a decrease in soil quality due to an inappropriate use, by agriculture, industrial or urban processes. The most common contaminants are mineral soils and trace metals. Since soil formation is an extremely slow process and supports vital functions, soil can be considered a non-renewable resource which requires protection. In the last few decades, environmental friendly soil remediation techniques which are less invasive than conventional techniques of soil remediation have been developed. These, are known as Gentle Remediation Options (GRO) and are focused on the use of plants in combination with their associated microorganisms for the remediation of contaminated sites and can be divided in two main strategies: (a) phytoextraction, uses plants which are able to concentrate metals in shoot tissues (e.g. hyperaccumulators) and (b) phytostabilisation, that aims to achieve the in situ inactivation of trace metals by using metal-tolerant plants in combination with soil amendments. When the phytoextracted metal can be recovered from the biomass of the hyperaccumulators and commercialised with an economical gain, the technique receives the name of phytomining. The main limitation of hyperaccumulators is their low biomass production and slow growth. As an alternative, high biomass crops (i.e. Helianthus annus or Nicotiana tabacum) or woody species (i.e. members of the Salicaceae family) that compensate for a moderate accumulation with a high biomass have been described also as good candidates. GRO efficiency can be improved by the use of conventional agronomic techniques (soil fertilisation, organic amendments applications, crop rotation or intercropping), or novel techniques such as bioaugmentation (which consists in the inoculation of microorganisms which assist the growth and tolerance of the remediating plants). This thesis aimed 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 soilplant- bacteria system of the two subspecies of the Ni-hyperaccumulator Alyssum serpyllifolium, endemic to the Iberian Peninsula (spps. lusitanicum and malacitanum) were studied with the aim to characterise its associated bacterial community and to obtain a collection of strains with a possible use as inoculants for improving Ni phytomining techniques. Rhizospheric processes involved in Ni solubilisation were also studied and experiments for the improvement of phytomining efficiency using soil fertilisation and organic amendment addition was also carried out. On the other hand, greenhouse and field experiments were carried out on Zn, Cd and Pb or Cucontaminated soils from areas that have been affected by mining acitivies; the objective being the evaluation of phytoremeditiation potential of metal-(hyper)accumulators or metal-excluding plant species (Populus sp., Agrostis sp., Salix sp., Nicotiana sp., Helianthus sp., Noccaea sp.) growing in metal contaminated soils (Cd, Zn and Pb or Cu-contaminated). Changes in physicochemical parameters, plant growth and plant metal accumulation were monitored. Moreover, in the same soils studies were carried out to improve phytoremediation efficiency through the use of soil fertilisation, organic amendments or bacterial inoculation.