Nickel phytomining constitutes a green strategy to recover valuable metals which has been demonstrated in agricultural soils developed over ultramafic rocks. In Galicia, ultramafic outcrops represent 5 % of the land surface and several quarrying activities are developed in these areas, generating large volumes of waste material which is accumulated in spoil heaps and tailings. The implementation of phytomining in quarrying-affected areas could provide an additional metal source to primary ores and, at the same time, reduce hazardous waste volumes and assist in site rehabilitation. Soils derived from the spoils – classified as Spolic Technosols – present physical, chemical and biological deficiencies which limit plant growth and soil functioning. In this context, we aimed to optimise the conditions for plant growth, by targeting several aspects: i) the selection of the most adequate hyperaccumulating plant species, ii) the use of soil amendments to improve physico-chemical and biological soil properties, iii) the design of optimal crop patterns and planting densities, and iv) the use of rhizosphere bacterial strains with plant growth promoting (PGP) traits and/or which influence Ni uptake by plants.

In September 2016, a phytomining field trial was established in a serpentine quarry (Bandeira, NW Spain) assessing the performance of four hyperaccumulating plant species (native populations of Noccaea caerulescens and Odontarrhena serpyllifolia and the Mediterranean spp. Bornmuellera emarginata and Odontarrhena muralis) under two regimes of fertilisation (inorganic NPK or composted sewage sludges (CSS)). After a 10-month growth period, the soil physico-chemical and biological properties were improved, especially in compost-amended plots. Nonetheless, further developments of the technique are necessary to fully optimise plant survival, growth, as well as the Ni yields. In May 2018, four new blocks were established, which were amended with different concentrations of CSS (2.5, 5 and 10 %) or distilled grape bagasse (at 2.5 % w/w). Inorganic NPK fertiliser was employed as control treatment. Also, the performance of the Mediterranean hyperaccumulator Bornmuellera tymphaea was evaluated along with those species employed in the first year of cultivation. Finally, the benefits of inoculating with the PGP bacterial strain LA44 (Arthrobacter nitroguajacolicus) on plant growth and Ni accumulation was assessed. Here we present the results obtained from the second growth season in the field.

We acknowledge the financial support received by the Spanish Ministerio de Economía, Industria y Competitividad (Mineco), the Fondo Europeo de Desarrollo Regional (FEDER) (CTM2015-66439-R), and the LIFE Environment and Resource Efficiency Programme (Life-Agromine; LIFE15 ENV/FR/000512).