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

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.