Aida Bani

Improving the Agronomy of Alyssum murale for Extensive Phytomining: A Five-Year Field Study

Large ultramafic areas exist in Albania, which could be suitable for phytomining with native Alyssum murale. We undertook a five-year field experiment on an ultramafic Vertisol, aimed at optimizing a low-cost Ni-phytoextraction crop of A. murale which is adapted to the Balkans. The following aspects were studied on 18-m2 plots in natural conditions: the effect of (i) plant phenology and element distribution, (ii) plant nutrition and fertilization, (iii) plant cover and weed control and (iv), planting technique (natural cover vs. sown crop). The optimal harvest time was set at the mid-flowering stage when Ni concentration and biomass yield were highest. The application of N, P, and K fertilizers, and especially a split 100-kg ha−1 N application, increased the density of A. murale against all other species. It significantly increased shoot yield, without reducing Ni concentration. In natural stands, the control of graminaceous weeds required the use of an anti-monocots herbicide. However, after the optimization of fertilization and harvest time, weed control procured little benefit. Finally, cropping sown A. murale was more efficient than enhancing native stands and gave higher biomass and phytoextraction yields; biomass yields progressively improved from 0.3 to 9.0 t ha−1 and phytoextracted Ni increased from 1.7 to 105 kg ha−1.

Nickel Hyperaccumulation by Brassicaceae in Serpentine Soils of Albania and Northwestern Greece

Abstract Ultramafic soils are widespread in the Balkans. Albania and Greece are the richest in the number of endemics, including several hyperaccumulator species, growing on serpentine. The objectives of this study were to understand the potential of Ni hyperaccumulation of these species in close relation with the characteristics of their native soil environments. Collection of both plant samples (analysis of element concentrations in aerial parts) and soil samples (analysis of total elements, DTPA-extractable Ni, Fe, and Ni distribution in mineral phases) allowed evaluation of phenotypic efficacy in hyperaccumulating Ni. Nickel availability in soils is controlled by soil weathering and mineral-bearing phases. Unsurprisingly, the highest levels of Ni availability were associated with amorphous Fe-oxides in moderately weathered Cambisols or with high-exchange capacity clays in well-evolved Vertisols. The highest Ni concentrations in leaves were found in Alyssum murale in Pojska (Albania; 2.0%), Alyssum markgrafii in Gjegjan (Albania; 1.9%), Bornmuellera baldacii subsp. markgrafii in Gramsh (Albania; 1.4%), and Leptoplax emarginata in Trigona (Greece; 1.4%). We identified a new member in the Albanian Ni-hyperaccumulator flora: Thlaspi ochroleucum in Pojska (Albania: 0.13% Ni) and in Pishkash (0.14% Ni). With regard to Ni availability in soils, A. markgrafii (Albania) is the most efficient Ni-hyperaccumulator among all species. Alyssum murale, which is widespread in the serpentines of the Balkans, accumulates Ni, with leaf concentration being negatively correlated to total Ca content of soils regardless of Ni availability (DTPA extractable Ni). If this relationship is confirmed, it would mean that genetic variability is not the main factor that explains the hyperaccumulation performance of this species.

Selection and Combustion of Ni-Hyperaccumulators for the Phytomining Process

Ni recovery from serpentine soils by phytomining has proved feasible. Phytomining involves the crop of hyperaccumulating plants with high Ni contents and the valorization of Ni by pyro or hydrometallurgical process. In order to evaluate the Ni content of different plants, we analyzed the organs of 14 hyperaccumulators from three genera: Alyssum, Leptoplax and Bornmuellera. The highest concentration was recorded in the leaves of Leptoplax (34.3 ± 0.7 mg g−1 DM). Additionally, we investigated biomass combustion which is the first step of the process we designed to obtain a nickel salt. We showed that temperature and duration were important parameters to ensure a good quality of ashes. At the bench scale, the best conditions were 550°C and 3 h. In this way, we obtained ashes in which Ni could reach 20 wt%. Biomass ashes can be considered as a bio-ore for recovering metal value.

The effect of plant density in nickel-phytomining field experiments with Alyssum murale in Albania

Ultramafic vertisols cover large areas in Albania and offer opportunities for phytomining. We undertook a field experiment with native Alyssum murale on two representative Vertisols at a distance of 20 km from each other (Pojske and Domosdove, Albania), to test the effect of planting density (transplanted seedlings) on a phytomining cropping system. Both areas were cleared in late summer 2012 and then ploughed and the soils were characterised. At Domosdove, an area of 0.5 ha was planted with local native seedlings at a density of six plants per square metre in September 2012. Spontaneous plants that had germinated in Spring 2012 were left to grow without any competition from other plants on a second 0.1-ha plot at Domosdove. All plots were weeded manually in the autumn of 2012 and spring of 2013. Individual plants occupied ~1 m2 at maturity. At Pojske, an area of 0.3 ha was also planted in September 2012 with local native seedlings of A. murale at a density of four plants per square metre. Plants grown at initial densities of four and six plants per square metre did not fully cover the ground; gaps were filled in naturally by a second spontaneous generation of A. murale seedlings (recruits) that had germinated in Autumn 2012. Other weeds were eliminated with herbicides. At Domosdove, at densities of one and six plants and at Pojske of four plants per square metre, the biomass yield was 10, 5 and 10 t ha–1, respectively. Concentration of phytoextracted nickel was 77, 41 and 112 kg ha–1. We suggest that a density of four plants per square metre is suitable for phytoextraction with native populations of A. murale. A. murale can be a weed itself and lower the nickel phytoextraction yield. Plants responded differently in their native environment than in previous field trials in North America.

Element Case Studies: Nickel

Initial experiments using Mediterranean Ni-hyperaccumulator plants for the purpose of phytomining were carried out in the 1990s. In order to meet commercial phytoextraction requirements, a technology has been developed using hyperaccumulator species with adapted intensive agronomic practices on natural Ni-rich soils. Ultramafic soils in the Balkans display a great variability in Ni concentrations and available Ni levels, both in Albania and the Pindus Mountains of Greece. In Albania, Vertisols are currently being used for low-productivity agriculture (pasture or arable land) on which phytomining could be included in cropping practices. Alyssum murale occurs widely on these ultramafic Vertisols and is a spontaneous weed that grows among other crops. This review chapter presents the different steps that were investigated during the study of soil suitability, and selection of plants up to optimization of agronomic practices, at field scale, as recently developed to reach the implementation stage of Ni agromining in Albania. During a 7-year study we addressed the following questions: (i) what are the optimal soils for Ni agromining in terms of fertility and Ni availability? (ii) what is the phytoextraction potential of local populations of Ni hyperaccumulator species? (iii) what should be the agronomical practices used to optimize the cropping of A. murale for extensive phytomining adapted to a Balkan agricultural setting?

Correction to: A global forum on ultramafic ecosystems: from ultramafic ecology to rehabilitation of degraded environments

The affiliations of the coauthors, Antony van der Ent and Shota Sakaguchi have been incorrectly published in the original publication of the article. The correct affiliations are provided in this correction.