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Combustion waste and fly ash disposal sites have unfavourable properties for revegetation. Owing to its phytoameliorative ability different alder species have long been used in the reclamation of degraded sites, as N-fixing species and forecrop for introducing more demanding tree species in reforestation. We present the growth parameters and effects of black alder (Alnus glutinosa), grey alder (A. incana), and green alder (A. viridis) on the physicochemical and biological properties of the technosols developed from the lignite combustion wastes disposal site in central Poland after 10 years of plant growth. The study plots were randomly arranged (72 m 2 of each plot, four replications for variant) at 3 species × 2 different soil treatments: CCW+L (combustion waste with lignite amendment in planting hole) and CCW (pure combustion waste). The obtained results indicate that soil treatment did not significantly influence the studied physicochemical soil parameters (pH, CEC, RESP, SOC, Ntot and macronutrients content) either in O horizon or in the mineral soil. The effect of species on pH, Ntot and macronutrients content was evident in the O horizon but not in the mineral soil. The better growth parameters were found for black alder than for grey alder. The mean height was 3.9 and 6.3 m for grey and black alder, respectively. Accumulation of litter layer (Ol) ranged from 2.9 to 3.6 Mg ha-1 (mega gram per hectare), and grey alder litter was characterized by the highest content of nutrients (N-P-K). Organic C content in the 0-5 cm mineral layer (A) increased to 38.06 and 47.80 g kg-1 (gram per kilogram dry soil), respectively for green and black alder. The highest N content in A layer was measured under black alder. Microbial respiration and biomass were significantly lower in the Ol layer under green alder than under two other alder species. However, in the mineral soil all three alder species stimulated microbialbiomass and activity similarly. Because of the highest growth parameters of black alder and the highest litter production under canopy and N content in the soil we recommend this species for revegetation of combustion waste disposal site.

Combustion of fossil fuels including coal is one of the sources of mercury pollution. Combustion waste from fly ash disposal sites poses a problem for the environment and constitutes a potential source of Hg, thus phytostabilisation is a crucial goal in the mitigation of fossil fuel impact. The paper presents mercury (Hg) concentration in technosols from combustion waste and in individual biomass components (fine roots, bark, stem wood and leaves) of alder species (black, gray and green alder) introduced as part of a long-term experiment to develop a method of phytostabilisation and afforestation of a lignite combustion disposal site. Mercury content in the combustion waste was elevated compared to the data for natural soils from uncontaminated forest areas, however, it did not exceed the amounts considered to be toxic. Hg content in technosols was related to clay and silt fraction content and phosphorus content. Mercury in the alder biomass accumulated mainly in the underground part, especially in the fine roots and displayed a positive correlation with acid and alkaline phosphatase and sulfur content, with no differences in the accumulation of Hg between the alder species. The obtained results indicate that the fine roots are the frontier of Hg biosorption in developed alder systems on combustion waste disposal sites.