By Chris Schnepf, University of Idaho, and Darren McAvoy, Utah State University
Biochar has many possible agricultural benefits. Given the large role that fire plays in western forests, biochar has likely also already played a significant role in Northwest forests, as evidenced by the charcoal commonly found on top of or buried in our forest soils. Biochar shows promise in providing additional benefits in restoring heavily disturbed forest sites, such as forest roads, skid trails, and landings. For more information, see a chapter in a recent biochar book detailing the current state of North American forest biochar research.
Most of the enthusiasm around biochar in the forestry community, however, is related to using forest management residues to create biochar and useable fuels, such as bio-oil and syngas. Many of our forests are overstocked. Reducing stocking can reduce fire risk, strengthen forest resilience to insects and diseases, and provide many other benefits. Some of the trees cut in these efforts can be taken to a mill and turned into wood products. However, there will always be leftover material that is too small to be used for wood products, particularly when many of the cut trees are small (e.g., pre-commercially thinning sapling trees).
Needles, branches, and stem wood left in a forest after a management activity (“slash”) are not necessarily wasted. Some of that material benefits forest soil health, depending on the soils and the nature of the material. But after meeting forest soil needs, some slash must usually be treated or removed to keep fire risk (“slash hazard”) within acceptable limits.
For a stand regeneration cut, such as a clearcut, some of this might be accomplished with a prescribed broadcast burn across the entire unit. Otherwise, excess slash is typically piled and burned, costing time and money, and generating greenhouse gases. However, if this material can be used to generate revenue from fuel or biochar, slash treatment costs could be off-set, and a portion of the carbon released as greenhouse gases from slash burning would also be sequestered in biochar, or substitute for non-renewable energy sources. Biochar is relatively stable carbon and can persist in soil for hundreds or even thousands of years. Carbon from non-charred decaying wood does not last nearly as long. Therefore, biochar could reduce the amount of carbon going into the atmosphere.
People have been talking about ways to use slash rather than burning it for many years. The biggest challenge has been the cost of collecting the material and hauling it to a processing site. Utilities that use biomass to produce energy often frame feasibility by how much biomass is within a certain distance to the facility. For example, they might only be able to afford to bring in feed stock 30-40 miles to the plant.
One way out of this conundrum is to bring the processing to the material. This idea is not new. When horses were the chief means of hauling logs to the mill, many small sawmills were scattered all over Idaho. Recently, there has been more discussion of technologies that process biomass in the woods. Utah State University Forestry Extension has been developing biochar production capabilities since 2010, and recently introduced simple flame-cap biochar kilns to Utah. Also called Oregon Kilns, each kiln is a metal box, five feet square and two feet high, with handles so four people can easily move it around the woods. Dry branches and logs up to 7 inches diameter are stacked into the kiln with a topping of smaller material. The kiln is lit from the top so that a cap of flames soon covers the surface of the kiln; this flame-cap limits the oxygen introduced into the kiln and helps to burn up the smoke as it rises through the flames. More material is added slowly until approximately five times the volume of the kiln has been added. When the kiln is full of coals, but not yet turned to ash, the fire is quenched with 80 gallons of water and stirred until the fire is extinguished. Any container that is available can be used, including old bath tubs, cattle troughs, or even an open pit. The biochar can be added to compost piles or spread in the forest to improve soil fertility and water holding capacity.
Biochar seems to hold great promise to benefit farms, gardens, and landscapes; make forest treatments more affordable; and sequester more carbon. For more information, you may want to check other articles on this blog, or links available through the Pacific Northwest Biochar Atlas.
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