Aided phytostabilisation reduces metal toxicity, improves soil fertility and enhances microbial activity in Cu-rich mine tailings.

(Aided) phytostabilisation has been proposed as a suitable technique to decrease the environmental risks associated with metal(loid)-enriched mine tailings. Field scale evaluations are needed for demonstrating their effectiveness in the medium- to long-term. A field trial was implemented in spring 2011 in Cu-rich mine tailings in the NW of Spain. The tailings were amended with composted municipal solid wastes and planted with Salix spp., Populus nigra L. or Agrostis capillaris L. cv. Highland. Plant growth, nutritive status and metal accumulation, and soil physico- and bio-chemical properties, were monitored over three years (four years for plant growth). The total bacterial community, α- and ß-Proteobacteria, Actinobacteria and Streptomycetaceae were studied by DGGE of 16s rDNA fragments. Compost amendment improved soil properties such as pH, CEC and fertility, and decreased soil Cu availability, leading to the establishment of a healthy vegetation cover. Both compost-amendment and plant root activity stimulated soil enzyme activities and induced important shifts in the bacterial community structure over time. The woody plant, S. viminalis, and the grassy species, A. capillaris, showed the best results in terms of plant growth and biomass production. The beneficial effects of the phytostabilisation process were maintained at least three years after treatment.

Organic amendments for improving biomass production and metal yield of Ni-hyperaccumulating plants.

Ni phytomining is a promising technology for Ni recovery from low-grade ores such as ultramafic soils. Metal-hyperaccumulators are good candidates for phytomining due to their extraordinary capacity for Ni accumulation. However, many of these plants produce a low biomass, which makes the use of agronomic techniques for improving their growth necessary. In this study, the Ni hyperaccumulators Alyssum serpyllifolium ssp. lusitanicum, A. serpyllifolium ssp. malacitanum, Alyssum bertolonii and Noccaea goesingense were evaluated for their Ni phytoextraction efficiency from a Ni-rich serpentine soil. Effects of soil inorganic fertilisation (100:100:125kgNPKha(-1)) and soil organic amendment addition (2.5, 5 or 10% compost) on plant growth and Ni accumulation were determined. All soil treatments greatly improved plant growth, but the highest biomass production was generally found after addition of 2.5 or 5% compost (w/w). The most pronounced beneficial effects were observed for N. goesingense. Total Ni phytoextracted from soils was significantly improved using both soil treatments (inorganic and organic), despite the decrease observed in soil Ni availability and shoot Ni concentrations in compost-amended soils. The most promising results were found using intermediate amount of compost, indicating that these types of organic wastes can be incorporated into phytomining systems.

Inoculation methods using Rhodococcus erythropolis strain P30 affects bacterial assisted phytoextraction capacity of Nicotiana tabacum.

In this study different bacterial inoculation methods were tested for tobacco plants growing in a mine-soil contaminated with Pb, Zn, and Cd. The inoculation methods evaluated were: seed inoculation, soil inoculation, dual soil inoculation event, and seed+soil inoculation. Each inoculum was added at two bacterial densities (10(6) CFUs mL(-1) and 10(8) CFUs mL(-1)). The objectives were to evaluate whether or not the mode of inoculation or the number of applied microorganisms influences plant response. The most pronounced bacterial-induced effect was found for biomass production, and the soil inoculation treatment (using 10(6) CFUs mL(-1)) led to the highest increase in shoot dry weight yield (up to 45%). Bacterial-induced effects on shoot metal concentrations were less pronounced; although a positive effect was found on shoot Pb concentration when using 10(8) CFUs mL(-1) in the soil inoculation (29% increase) and in the seed+soil inoculation (34% increase). Also shoot Zn concentration increased by 24% after seed inoculation with 10(6) CFUs mL(-1). The best effects on the total metal yield were not correlated with an increasing number of inoculated bacteria. In fact the best results were found after a single soil inoculation using the lower cellular density of 10(6) CFUs mL(-1).

Nickel solubilizing capacity and characterization of rhizobacteria isolated from hyperaccumulating and non-hyperaccumulating subspecies of Alyssum serpyllifolium.

Bacterial strains were isolated from the rhizosphere of three populations of the Ni-hyperaccumulator Alyssum serpyllifolium subsp. lusitanicum (A. pintodasilvae; M, S, and L), one population of Ni-hyperaccumulator A. serpyllifolium subsp. malacitanum (A. malacitanum; SB), and one population of the non-hyperaccumulator A. serpyllifolium subsp. serpyllifolium (A. serpyllifolium; SN). Isolates were characterized genotypically by BOX-PCR genomic DNA fingerprinting and comparative sequence analysis of partial 16S rRNA gene, and phenotypically by their Ni tolerance (0-10 mM), presence of plant growth promoting traits (indoleacetic acid (IAA)-, siderophore-, or organic acid-production, and phosphate solubilization) or capacity to produce biosurfactants. Among the collection of rhizobacteria, 84 strains were selected (according to their BOX-PCR profiles and phenotypic characteristics) to assess their ability to modify Ni extractability from Ni-rich (serpentine) soils. Metabolites produced by 13 of the isolates mobilized soil Ni (originating from the rhizosphere of both Ni-hyperaccumulators and non-hyperaccumulator). In contrast, Ni extraction using culture medium filtrates which had supported the growth of 29 strains was significantly reduced. The remaining strains had no effect on Ni mobility. Bacterial induced Ni mobilization was not related to Ni resistance or the phenotypic traits tested. Isolates with potential use in phytoremediation techniques will be further studied in a plant-microorganism-soil system.