ABSTRACT
Total nickel concentrations in soil normally range below 50 mg kg-1 but can exceed 2500 mg kg-1 in soils formed on ultramafic bedrock or polluted soils. Nickel is an essential micronutrient and while its concentrations in plants commonly vary between 0.05 and 5 mg kg-1 they can exceed 1.000 mg kg-1 in so called hyperaccumulator species. While the latter exhibit tolerance even at such high Ni concentrations, most plants start to show toxicity symptoms at tissue concentrations between 10 to 50 mg kg-1. Root activities including Ni uptake and release of Ni-solubilising compounds (H+, organic ligands) either create gradients of depletion of accumulation of labile Ni in the rhizosphere, depending on the balance between the rates of uptake and resupply by convection, diffusion and desorption from the soil solid phase. While resupply of Ni is generally not limited near roots of non-accumulator species, hyperaccumulator species tend to deplete labile Ni in their rhizospheres but typically release organic ligands that presumably contribute to sustain Ni concentrations in soil solution by formation of Ni-DOM complexes. Recent advances in chemical imaging along individual roots at microscale reveal that pattern of depletion and accumulation of Ni are more complex than known from rhizobox studies, e.g. with bimodal distribution perpendicular to the root plane and variation along the root axis. Organic Ni complexes and proton excretion can also enhance mineral weathering and release of Ni in the rhizosphere. Root exudates such as sugars and organic acids promote specific microbial communities in the rhizosphere, including those with plant growth-promoting (PGP) traits, which in turn can modify Ni solubility and the plant´s tolerance to adverse Ni concentrations in soil. The diversity of Ni uptake strategies in plants and related root activities offers tools for the management of Ni-polluted soils, including phytostabilisation using excluders and phytoextraction by hyperaccumlator species. Management of the rhizosphere by inoculation with PGP bacterial strains holds promise for further enhancement of phytoremediation and phytomining technologies.
