Wood ash as a forest soil amendment: The role of boiler and soil type on soil property response

Wood ash is produced in large quantities in Canada as a by-product of the pulp and paper, sawmill, and bioenergy industries and it is anticipated that its disposal in landfills will not be a viable option. An alternative option may be to use it to amend forest soils. Wood ash is a complex mixture and its composition depends on several variables, including the combustion parameters of the boiler in which it is generated. We present an analysis of the amendment of two Canadian forest soils (a Brunisol from the Great Lakes-St. Lawrence and a Luvisol from the Carolinian forest regions of Ontario) with six different wood ashes collected from different biomass boilers through short-term controlled incubations. We show that following an 8-wk incubation period, amendment of the soils with wood ash led to small to moderate increases in soil pH, but had little effect on soil microbial activity and biomass. The concentration of important base cations such as calcium, magnesium and potassium as well as phosphorus generally increased in both soil types following amendment with different ash. This practice can return nutrients lost from forest ecosystems during harvesting; however, effects were found to be boiler-specific. Lastly, we show that four ash types led to small increases in cadmium in either soil; the concentration of all other measured heavy metals was not significantly increased following amendment, and in certain cases decreased, particularly with lead. The only potentially negative aspect encountered was elevated sodium, particularly with ash from one boiler, but unacceptable Na-absorptivity ratios were not exceeded. These results demonstrate that with proper characterization and selection of wood ash type and application rates, amendment of Canadian forest soils with wood ash may benefit forest ecosystems and is unlikely to disrupt the chemical and biological processes in soil environments.