Wood ash use in forestry - a review of the environmental impacts

The use of wood fuel for energy production in the UK is setto increase in the near future as part of a government commitmentto increase renewable sources to 10 per cent by 2010. The ashgenerated as a by-product of combustion, whether for heat orpower generation, has potential use as a fertilizer in forestsystems. This review assesses the available information on factorsaffecting the quality of the ash and environmental implicationsarising from its application. The key determinants of wood ashchemistry are the tree species combusted, the nature of theburn process and the conditions at the application site. Woodash from hardwood species produces higher levels of macronutrientsin their ash than conifers, and the silica content is frequentlylower. A furnace temperature between 500 and 900°C is criticalto the retention of nutrients, particularly potassium, and determinesthe concentrations of potentially toxic metals including aluminiumin the ash. Fly ash, the lightest component that accumulatesin the flue system, can contain high concentrations of cadmium,copper, chromium, lead and arsenic and this ash should not beused as fertilizer. The form of the ash at application is important,with loose ash releasing Ca, K and Na more rapidly than granulatedash. Heavy metal, radionuclide and dioxin contamination of woodash-based fertilizers is minimal and unlikely to affect ecosystemfunction. The effects of wood ash are primarily governed byapplication rate and soil type. The benefits are maximized atlow dose rates, with possible toxicity from applications inexcess of 10 t ha^–1. For most forest sites, a single woodash application per rotation could replace all the nutrientslost after whole-tree harvesting (excepting N). Long-lastingpositive effects on tree growth have been observed on shallowpeats, in which the humus is slowly mineralized in responseto elevated pH and increased nutrient availability. In contrast,wood ash application to podzols is only effective in enhancingtree growth when nitrogen availability is non-limiting. To date,published research of wood ash effects on trees growing in claysand loams is minimal. A lag time for positive tree responsesto wood ash application is often observed, and may be the resultof phosphorous limitation at higher soil pH. The greatest reportedadverse ecological effects are to acidophilic ecosystems, particularlythe constituent bryophyte, soil bacteria and ectomycorrhizalcommunities.