Recycling of ash - for the good of the environment?

Abstract The increasing use of wood fuels to replace fossil fuels in energy and heat production results in increasing amounts of waste in the form of ash. Since wood ash contains nutrients that trees need in the right proportions, except for N, it is a potentially excellent forest fertiliser. However, any harmful elements, e.g., heavy metals are also concentrated in the ash, which has raised concern about possible adverse effects that ash fertilisation could induce in the environment. A considerable body of new results has been published on ash fertilisation impacts on, e.g., heavy metal concentrations in berries and mushrooms, ground vegetation, soil microbial processes, greenhouse gas emissions and watercourses. In this review, we synthesise this information to map the environmental benefits and risks related to ash fertilisation. We pay special attention to peatland forests, N-rich ecosystems where ash may induce considerable increases in timber production, but for which a thorough evaluation of environmental impacts has been lacking. The longest monitoring periods currently span more than five decades. In well-targeted sites, ash increases tree production and/or reduces soil acidity for decades. No enrichment of heavy metals in the food webs or leaching of heavy metals to watercourses has been reported. CO 2 emissions increase in the longer term (10–50 years), especially from N-rich peat soils. Also, changes in plant community may be so extensive that ash application cannot be recommended where conservation of the original vegetation is required. Immobilisation of heavy metals in soil depends on the neutralising effect of ash on soil acidity. The most crucial question that remains to be answered is how long this effect lasts, and what happens thereafter. Future research should investigate further whether heavy metals may accumulate in plant roots, even if above-ground parts remain unaffected. Finally, the duration of the impact of ash fertilisation on the nutrition of peatland trees, as well as optimal schedules of repeated fertilisations for different rotations, still need to be verified.