Back to the Future?

During the 17th century, there was a high level of commercial activity around wood ash in Nouvelle-France. Settlers were clearing forestland for crop cultivation, creating large amounts of residual wood that was not needed for fuel or construction material. The residual wood was burned to ash, which was leached to produce a potassium extract called potash, used in making glass, soap, and soil fertilizer. Nouvelle-France became the world’s largest potash exporter by the end of the 18th century. However, the industry faded as potash from mineral deposits were exploited in the following centuries, replacing the use of wood ash in agriculture.

There is an interesting parallel to be made: wood ash was long a byproduct of forest clearing for agriculture, provided a supplementary income to settlers, and was a necessary soil amendment for agriculture. Nowadays, increasing amounts of wood ash are produced as a byproduct of bioenergy production from forest biomass. It is often said that this ash should be recycled back to forest stands; this is the mitigation measure frequently offered to appease soil scientists concerned about the ecological effects of biomass harvesting. Let’s take a closer look.

Intensifying forest harvesting puts added pressure on forest soil fertility: it drains more nutrients from the ecosystem and reduces the soil’s capacity to neutralize acidity. Returning ash to forest sites thus seems like an obvious and sensible solution, as it returns some nutrients to the soil (calcium, magnesium, potassium, and phosphorus) and counteracts soil acidity by raising pH. In Nordic countries such as Sweden and Finland, the fertilizing value of ash in forest stands has been recognized since the early 20th century. However, research shows that wood ash may cause problems in forest systems, which are more complex than agricultural systems.

Ash recycling in forests does not always have the desired positive effects. Its pH-raising effect can negatively affect some plant communities, e.g., moss and lichen species. More worryingly, ash often contains high concentrations of heavy metals such as cadmium, which may have negative effects on plants and animals, accumulate in food webs, and leach into water. However, some good news is that scientific studies show that most negative effects are minimized when ash is applied in moderate doses and stabilized forms (i.e., crushed or pelletized). Also, ash is effective at counteracting acidity in soils and in surface waters. It has also proven effective at correcting tree nutritional deficiencies of potassium and phosphorus, which are common in forested peatlands. Some field trials in Nordic countries have shown up to 30-fold increases in tree growth following ash treatment in peatlands. However, this growth is only seen on fertile sites that have abundant nitrogen. On poor peatlands or mineral soil, ash may even impair growth, most likely by disrupting the nitrogen cycle. Thus, except at some specific sites, growth improvement is not a valid reason for ash recycling.

However, as my Nordic colleagues say, one of the main rationales for ash recycling to the forest is not for forest management, but to get rid of cumbersome waste. Otherwise, the ash goes to landfills, which is costly and not ecologically sound. Ash recycling for waste management is currently used mainly by small heating plants in Europe. A main bottleneck preventing it from becoming more widely used is the  current lack of efficient and economical ways to spread it on forest sites.

Is wood ash recycling in forests an option to consider in Canada? With growing interest in using forest biomass for bioenergy production, ash volumes will increase, along with questions about their management. The rationale in Europe is likely to apply here too, with ash recycling more a matter of waste disposal than forest management. If Nordics haven’t found a way yet to make it economical, it is unlikely to be so in the near future in our forests, as a large proportion of them are under extensive, rather than intensive, management and are not easily accessible.

I believe the best opportunity for wood ash recycling is the historical one: as a soil amendment for agriculture. The practice has been regaining momentum in Canada since the 1990s. It presents many ecological benefits, as it comes from a renewable source and its carbon footprint is leaner than that of other amendments such as agricultural lime. These benefits are similar to those associated with forest biomass for bioenergy.

Dr. Evelyne Thiffault is a research scientist in forest biomass at Natural Resources Canada and provides Canadian Biomass with her thoughts on sustainable biomass harvesting.