Can bioenergy ash applications emulate the effects of wildfire on upland forest soil chemical properties?


Hannam, K.D., Fleming, R.L., Venier, L., Hazlett, P.W. (2019). Can bioenergy ash applications emulate the effects of wildfire on upland forest soil chemical properties?. Soil Science Society of America Journal, [online] 83 S201-S217.

Plain language summary

There is growing interest in producing less energy from fossil fuels and more energy from renewable resources. With its large forested area, most bioenergy in Canada is produced by the timber and paper industries, who burn leftover bits of wood and bark to generate heat and electricity. When these woody materials are burned, ash is left behind as waste. Because it is strongly alkaline, bioenergy ash is often treated as a waste material and disposed of in landfills; however, it also contains large quantities of critical plant nutrients, such as calcium and phosphorus. When woody materials are burned during wildfires, the resulting ash remains in the forest ecosystem, where it provides critical nutrients to the recovering forest vegetation. We hypothesized that applications of bioenergy ash to forest soils could be used to return critical nutrients to forest ecosystems while causing similar changes in soil properties to those caused by wildfires. To address this question, we conducted a meta-analysis (a statistical analysis of a large number of independent experiments) to compare the effects of bioenergy ash applications and wildfires on soil carbon, nitrogen and pH, as well as plant-available calcium and phosphorus. We found that both wildfires and bioenergy ash applications reduce soil carbon and nitrogen but wildfires have much stronger effects. Both wildfires and bioenergy ash applications also increase soil pH and available phosphorus and calcium, but bioenergy ash applications tend to have a stronger effect. We proposed that lower rates of bioenergy ash application or pre-treatment of bioenergy ash (to slow the rate at which nutrients become available for plant uptake) could help ensure that soil applications of bioenergy ash have similar effects on soil properties to those of wildfires.


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.

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