Effect of hyperaccumulating plant cover composition and rhizosphere associated bacteria on the efficiency of nickel extraction from soil.

Few attempts related the association of different plants on the efficiency of metal extraction, which hypothesized that such vegetation covers promoted the development and activity of microorganisms. Such studies mainly concerned crop associations and showed that co-cropping with non-hyperaccumulator plants enhanced hyperaccumulators growth and increased metal phytoaccumulation.We studied the effect of species richness (mono-cropping or co-cropping) with only hyperaccumulator plant species (Alyssum murale, Noccaea tymphaea, Leptoplax emarginata and Bornmuellera tymphaea), on the efficiency of Nickel (Ni) extraction from an ultramafic soil. The effects on some soil physicochemical properties and on microbial communities colonizing the rhizosphere were also evaluated.After four months of culture in controlled conditions, the estimation of plant biomass and Ni concentrations in shoots and roots were evaluated. In the meanwhile, microbial biomass carbon, size of cultivable rhizosphere bacterial community, as long as the potential production of auxin compounds, were evaluated for each treatment (one mixed cover, four monspecies covers and unplanted soil). Bacterial communities were also characterized by genetic (SSCP) and metabolic structures. Moreover, different microbial enzymes were measured.The co-culture of four species did not seem to significantly improve the process of phytoextraction. However, B. tympheae biomass was in the same range as N. tymphaea and L. emarginata but B. tymphaea bioconcentration coefficient was the highest of the four species, with more than 1% Ni in its dried shoots. Therefore, B. tympheae, and to a lesser extent N. tymphaae, were the two species with the greatest potential of phytoextraction of Ni in co-cropping systems.