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 20km from each other (Pojskë and Domosdovë, 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 Domosdovë, an area of 0.5ha 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 Domosdovë. All plots were weeded manually in the autumn of 2012 and spring of 2013. Individual plants occupied ~1m2 at maturity. At Pojskë, an area of 0.3ha 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 Domosdovë, at densities of one and six plants and at Pojskë of four plants per square metre, the biomass yield was 10, 5 and 10tha-1, respectively. Concentration of phytoextracted nickel was 77, 41 and 112kgha-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.