Abstract 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.