Serpentine soils, developed from ultramafic parent material, are typically rich in nickel (Ni). Moreover, high chromium (Cr) and cobalt (Co) concentrations, a Ca/Mg ratio < 1 and a low content of essential nutrients make these soils unattractive for agriculture. On those Ni-rich soils, most plant species exclude Ni from their shoots, while a small group of plants is accumulating extensive Ni concentrations in their aerial parts without showing any toxicity symptoms. Phytomining is a novel technology based on the cultivation of metal hyperaccumulator plants for metal recovery and economic profit. Given the extension of serpentine areas around the globe and the fact that most known hyperaccumulators are Ni hyperaccumulators, phytomining research is focused on the recovery of Ni. However, field studies that demonstrate the feasibility of phytomining are scarce. In this study the phytomining efficiency of two Ni hyperaccumulators was tested on field-scale on an Austrian serpentine soil.
The field experiment has been established near Bernstein, Austria (47,406397 N 16,260334 O) in autumn 2016, consisting of 24 plots of 10 m2 size. The soil was characterized by a pH of 6.1, a DTPA-extractable Ni fraction of 38.4 mg kg-1 and a Sr(NO3)2-extractable Ni fraction of 0.53 mg kg-1. Two Ni-hyperaccumulators were tested: the native hyperaccumulator Noccaea goesingensis and the Albanian species Odontarrhena chalcidica. Three months old seedlings of N. goesingensis were planted in October 2016 and three treatments were established: control (with 20 cm distance between plants), a high density treatment (10 cm distance between plants) and an intercropping treatment with Lotus corniculatus. O. chalcidica was planted in April 2017 after raising the seedlings 4 months in the greenhouse. A planting density of 4 plants m-2 was established for all treatments. The three treatments consisted of a control, intercropping with L. corniculatus and the application of elemental sulphur (0.46 g S kg-1 soil). All treatments were harvested in September 2017.
Biomass production (3.65 t ha-1) and Ni accumulation (13.4 g kg-1) was highest for O. chalcidica, resulting in a Ni yield of on average 49.2 kg ha-1 on the control plots. In comparison, N. goesingensis produced a biomass of 2.47 t ha-1 and accumulated 7.62 g kg-1 Ni, which resulted in a Ni yield of on average 23.8 kg ha-1 on the control plots. Intercropping with L. corniculatus affected the biomass production of both species negatively and the shoot biomass was significantly lower on the intercropping plots of N. goesingensis compared to the control. High density planting of N. goesingensis and sulphur application to O. chalcidica plots had no influence on the Ni yield. Hyperaccumulator growth led to a decrease of DTPA-extractable Ni and to an increase of soil pH, with the exception of sulphur-amended plots.
On the tested site, Ni phytomining was most successful with O. chalcidica. In future experiments different fertilisation regimes and other promising plant species will be tested in order to optimise plant growth and Ni yield.