Phosphogypsum is a by-product of the phosphorus fertilizer production process and is typically stacked at the production sites. These stacks can potentially pose environmental hazards, which can be substantially reduced through reclamation by capping with soil and revegetation upon decommissioning. We conducted a study on a phosphogypsum stack using five soil capping depths (8, 15, 30, 46, 91 cm), an uncapped treatment, and five vegetation treatments (monocultures of four grass species Agrostis stolonifera L., Festuca ovina L., Deschampsia caespitosa (L.) Beauv., Agropyron trachycaulum (Link) Malte ex H.F. Lewis and one mix of the four species with Trifolium hybridum L.) to assess plant growth, health, rooting characteristics, and trace element uptake. Cobalt and nickel concentrations in plant tissue from plots with ≥ 15 cm soil capping were within ranges found at reference sites, whereas fluorine was three times elevated. Vegetation cover was significantly greater on capped than uncapped plots, being greatest for Agropyron trachycaulum (26%) and Festuca ovina (26%). Capping depths ≥ 15 cm had greater cover, biomass, and healthy plants than the 8 cm cover. Soil water content was similar in the 15–46 cm capping depth, with the lowest in the 91-cm caps. Fluorine, cobalt, and nickel were elevated in select plant tissue samples on the research plots relative to references, and cap depth affected tissue fluorine and cobalt concentrations but not nickel. Concentrations of these trace elements were lower than maximum tolerable levels for animal consumption. From this 5-year study, Agropyron trachycaulum and Festuca ovina and a soil cover depth of ≥ 15 cm are recommended for reclamation of phosphogypsum stacks.

Spoil heaps are the visible footprint of hydropower production, particularly in vulnerable alpine environments. Speeding up vegetation development by seeding commercial grass species has been a common restoration practice for the last 50 years, but we lack information on whether seeded species decline and allow native plant cover to develop. We visually estimated cover of native vascular plants and five seeded grass species (Agrostis capillaris, Festuca ovina, Festuca rubra, Schedonorus pratensis and Phleum pratense) on eight spoil heaps at different elevations (boreal–alpine zone) in western Norway. Spoil heap vegetation was censused twice (9–20 and 24–36 years after spoil heap construction); the undisturbed surrounding vegetation was also censused on the second occasion. Total cover on the spoil heaps showed some increase, but remained far below that in surrounding areas. Cover of seeded grass species in the surroundings was low (but not negligible), indicating suboptimal establishment ability. Seeded species usually covered less than 20 % of the spoil heaps, and only F. rubra, F. ovina and A. capillaris contributed substantially. Proportional cover indicated better initial establishment by seeded species, but their cover decreased between the censuses on all but the highest located spoil heap. The persistence of seeded grass species is problematic, and despite the decrease in proportional cover, they are likely to persist for decades on spoil heaps, posing a risk of invasion of surrounding areas. We therefore recommend replacing the practice of seeding with more appropriate restoration measures.