Current status and challenges in developing nickel phytomining: an agronomic perspective

Background Nickel (Ni) phytomining operationscultivate hyperaccumulator plants (‘metal crops’)on Ni-rich (ultramafic) soils, followed by harvestingand incineration of the biomass to produce ahigh-grade ‘bio-ore’ from which Ni metal or pureNi salts are recovered.Scope This review examines the current status,progress and challenges in the development ofNi phytomining agronomy since the first fieldtrial over two decades ago. To date, the agronomyof less than 10 species has been tested, whilemost research focussed on Alyssum murale andA. corsicum. Nickel phytomining trials have sofar been undertaken in Albania, Canada, France,Italy, New Zealand, Spain and USA using ultramaficor Ni-contaminated soils with 0.05–1 %total Ni.Conclusions N, P and K fertilisation significantly increasesthe biomass of Ni hyperaccumulator plants, andcauses negligible dilution in shoot Ni concentration.Organic matter additions have pronounced positive effectson the biomass of Ni hyperaccumulator plants, butmay reduce shoot Ni concentration. Soil pH adjustments,S additions, N fertilisation, and bacterial inoculationgenerally increase Ni phytoavailability, and consequently,Ni yield in ‘metal crops’. Calcium soilamendments are necessary because substantial amountsof Ca are removed through the harvesting of ‘bio-ore’.Organic amendments generally improve the physicalproperties of ultramafic soil, and soil moisture has apronounced positive effect on Ni yield. Repeated ‘metalcrop’ harvesting depletes soil phytoavailable Ni, butalso promotes transfer of non-labile soil Ni tophytoavailable forms. Traditional chemical soilextractants used to estimate phytoavailability of traceelement s are of limited use to predict Niphytoavailability to ‘metal crop’ species and henceNi uptake