Can Bioenergy Ash Applications Emulate the Effects of Wildfire on Upland Forest Soil Chemical Properties?

Core Ideas Bioenergy is growing; the ash generated as a by‐product is often treated as waste. Ash applications could be used to emulate wildfire effects on forest soil chemistry. Wildfires and ash applications increase soil phosphorus and calcium and raise pH. Guidance on ash dosage rates and pre‐treatment for emulating wildfire is required. As efforts to combat climate change intensify in Canada and around the world, the use of forest biomass to produce energy is expanding rapidly. At the same time, there is an urgent need for environmentally sustainable methods of handling the ash generated during biomass combustion. Currently, bioenergy ash is often landfilled, placing significant pressure on Canada's waste disposal infrastructure. In some countries, however, the use of bioenergy ash as a nutrient‐rich forest soil amendment is strongly encouraged. Given that forest management in Canada is often driven by the ‘emulation of natural disturbance’ paradigm, bioenergy ash could have potential as a management tool for improving wildfire emulation in harvested stands. We compared published values of wildfire ash chemistry with those for Canadian and European bioenergy ash and found that they are similar. We used meta‐analysis to examine changes in soil carbon and nitrogen pools, extractable phosphorus, exchangeable calcium and soil pH following wildfires and applications of bioenergy ash on upland forested sites. Both wildfires and bioenergy ash can reduce forest floor C and N pools: wildfires by direct combustion of organic matter, and ash applications by an apparent increase in organic matter decay. Both wildfires and bioenergy ash applications increase extractable P, exchangeable Ca and pH in surface mineral soils. Although bioenergy ash applications can trigger larger increases in available P and pH in surface mineral soils than wildfires, controlling ash dosage rates or pre‐treating the ash to slow the rate of nutrient release could attenuate some of these effects.