Improving nickel phytoextraction by co-cropping hyperaccumulator plants inoculated by plant growth promoting rhizobacteria

Phytomining is the conception of agro-metallurgical production chains based on cropping hyperaccumulator plants on contaminated or naturally rich (ultramafic) soils to produce high value metal compounds. This study aimed to evaluate the effect of enhancing multispecies hyperaccumulator covers with Plant Growth Promoting Rhizobacteria (PGPR) on Ni phytoextraction. Plant growth promoting rhizobacteria were isolated from the rhizosphere of two different plant associations (Bornmuellera tymphaea - Noccaea tymphaea (NB) and Bornmuellera tymphaea - Alyssum murale (AB)) collected from natural conditions. The screening of isolates from both plant associations for their PGPR traits revealed two PGPR strains (AB30 and NB24) affiliated to Variovorax paradoxus. Inoculation of mesocosms containing the same plant associations with these selected PGPR was performed after 5 months of culture. The biomass of the different plant parts and respective concentrations of nickel in the plants were recorded 1 month after inoculation. Root biomass of the inoculated plant associations was significantly higher than the respective non-inoculated ones. The total biomass of the inoculated plant association Bornmuellera tymphaea - Noccaea tymphaea was significantly higher than the total biomass of all others. Similarly, shoot biomass of the inoculated plant association B. tymphaea - N. tymphaea was significantly higher than that of all other covers. Results showed an increase in Ni uptake when plants were inoculated with PGPR strains, when compared with uninoculated plant associations. According to the plant cover, the inoculation increased Ni amounts in roots of 105.8 and 66.4 % respectively in ABi and NBi covers, and 39.9 and 79.6 % in the shoots. The combination of the hyperaccumulator plants N. tymphaea-B.tymphaea inoculated by one of the two PGPR strains (strain NB24), isolated from the rhizosphere of this mixed cover, seemed to be an interesting option for an efficient Ni phytoextraction to be tested in the field.