Sustainable Back to the Future? Recycling an ash disposal method from the pioneer days may be a bet- ter option than landfilling. By Evelyne Thiffault D uring the 17th century, there was a high level of commercial activ- ity around wood ash in Nouvelle- France. Settlers were clearing forest- land 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 potassi- um 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 ag- riculture. Nowadays, increasing amounts of wood ash are produced as a byproduct of bioenergy production from forest bio- mass. 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 harvest- ing. Let’s take a closer look. Intensifying forest harvesting puts add- ed 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 solu- tion, 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 al- ways 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 con- tains high concentrations of heavy metals such as cadmium, which may have nega- tive effects on plants and animals, accu- mulate 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 moder- ate doses and stabilized forms (i.e., crushed or pelletized). Also, ash is effective at counteracting acid- ity in soils and in surface waters. It has also proven ef- fective at correct- ing tree nutritional deficiencies of po- tassium and phos- phorus, which are common in forested peatlands. Some field trials in Nordic coun- tries 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 ni- trogen. 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, 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 op- tion to consider in Canada? With grow- ing 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 un- likely to be so in the near future in our “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.” 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 one of the main rationales for ash recy- cling to the forest is not for forest manage- ment, 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 cur- rently used mainly by small heating plants 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 ben- efits, 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. CanadianBIOMASS 11 Biomass
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