Scattered throughout the Amazon rainforests are localized patches of exceptionally fertile soil referred to as the Amazonian Dark Earths. These soils were created by the people of this region more than 2,000 years ago by developing and adding biochar to soils year after year in the form of charcoal, a byproduct from cooking, mixed with broken pottery, animal bones and manure. As a result, these soils are some of the most fertile on the planet and home to more than 80,000 different plant species. Carbon is one of the key elements to healthy soils and flourishing plant life. The diversion of biomass from landfill or open burning in fields to biochar production offers an improvement in waste management. Also, the heat and the carriers of renewable energy co-produced during biochar production can be recovered to meet the energy needs of the local communities adopting a biochar system.
A recent research paper published online in the journal Biomass and Bioenergy argues that the battle against global warming may be better served by instead heating the biomass in an oxygen-starved process called pyrolysis, extracting methane, hydrogen, and other byproducts for combustion, and burying the resulting carbon-rich char. Plants absorb carbon dioxide from the air during photosynthesis, and transform it into carbohydrates and a variety carbon-based molecules to create their leaves, branches, stems and roots. Every year, plants absorb and retain about 60 gigatonnes of carbon, while decomposing biomass releases about the same amount of carbon to the atmosphere as CO2. If we prevent some of that biomass from decomposing, by converting it to biochar, and use it to improve soil fertility, we’re helping to address the issues of climate change and food security.
Even if this approach would mean burning more coal – which emits more carbon dioxide than other fossil-fuel sources – it would yield a net reduction in carbon emissions, according to the analysis by Malcolm Fowles, a professor of technology management at the Open University, in the United Kingdom. Burning one ton of wood pellets emits 357 kilograms less carbon than burning coal with the same energy content. But turning those wood pellets into char would save 372 kilograms of carbon emissions. That is because 300 kilograms of carbon could be buried as char, and the burning of byproducts would produce 72 kilograms less carbon emissions than burning an equivalent amount of coal. Such an approach could carry an extra benefit. Burying char – known as black-carbon sequestration–enhances soils, helping future crops and trees grow even faster, thus absorbing more carbon dioxide in the future.
A group of Canadian and French companies will build a $80 million plant in Quebec to turn forestry waste into biochar, so-called black carbon which can store carbon for hundreds of years and improve soil quality at the same time. Located about 850 kilometres northeast of Montreal, the plant will have an initial biochar production capacity of 10,000 tonnes per year. By 2026, annual plant production capacity will triple, making it the largest biochar facility in North America. When added to concrete or asphalt formulations, biochar brings new functionalities to the final material while helping to reduce its carbon footprint, a key issue for the construction sector. The production of biochar at high-temperature and with oxygen-free pyrolysis will generate surplus energy in the form of steam or pyrolysis oil, which can be directly reused on site. By transforming forest and agricultural residues into carbon sinks and fertilizers, creates value over the entire life cycle of the material.2
However, biochar has not been standardized. Different feed stocks and processing temperatures and times lead to products with different properties. Research is ongoing to find the combinations that make the best biochar for specific applications, and the best percentage of biochar in the soil to maximize production. Risk of contamination of biochar exists (PAHs, heavy metals, dioxins) when contaminated feedstocks are used and/or the process conditions used to make the biochar are such that temperatures are greater than 500 C are used. Also, extremely high rates of biochar application could have negative effects on earthworm survival rates – this would be in cases where application rates are greater than 67 kilograms per square meter of land (an impractical level of biochar application). More research is needed to fully map the life cycles of biochar’s effects on soil organic matter. The biggest downside so far, though, is the cost.
Biochars are generally found to increase soil water-holding capacity, which would enhance resilience of agricultural systems to drought, especially under climate change and may further explain the positive effects of biochars in sandy soils, especially in arid and semiarid areas. The properties of biochar and its effects within agricultural ecosystems largely depend on feedstock and pyrolysis conditions. Grass and straw biochars increase water-holding capacity to a greater extent than woody biochars. Heavy metals may be present in biochar produced from feedstocks such as sewage sludge and treated timber. Although the pyrolysis process concentrates most heavy metals, some metals such as Cd and Zn and can be partly volatilized during pyrolysis resulting in lower concentrations than the feedstock. Therefore, selecting the appropriate biochar type to address heavy metal contamination, suited to the soil properties, type of plant, and specific heavy metal, can result in effective remediation while safeguarding food quality.3
Biochar, an inert and highly porous material, can play a key role in helping soil retain water and nutrients, and in sustaining microorganisms that maintain soil fertility. Many soils have lost appreciable soil organic matter from overgrazing, cultivation, forest harvesting, and erosion. These soils could benefit from biochar additions during reforestation because it adds a highly recalcitrant form of carbon and promotes long-lasting effects: retention of cations, anions, and water. One way to increase the conversion of forest woody residues into biochar is to expand markets for using bioenergy and biochar. Biochar from woody biomass has been used to increase agricultural crop, grass, and urban tree growth. Because of these benefits, an obvious potential market for biochar is use in nurseries, especially if it could replace expensive, non-sustainable ingredients in growing substrates, such as Sphagnum peatmoss, perlite, or vermiculite.4
Biochar enthusiasts are a hopeful biochar could solve our energy, food, and climate woes. There are others calling out a need to beware of these messages. Turning soils into a commodity is profitable to industry but disastrous for the poor. Several patent applications have been made for charcoal use in soil and for pyrolysis with charcoal production. If granted, those will ensure that any future profits from the technology will go to companies, not communities. Those locally and culturally adapted methods depend on regional climate, soils, crops and biodiversity. Attempts to commodify soils and impose a “one-size-fits all” approach to soils and farming risks appropriating, undermining and destroying this knowledge and diversity just when it is most critically needed. There still needs to be field trials on various varieties of biochar to ensure biochar carbon really will store carbon reliably in soils. could solve our energy, food, and climate woes.5
“Bioenergy through pyrolysis in combination with biochar sequestration is a technology to obtain energy and improve the environment in multiple ways at the same time,” writes Lehmann in a research paper to be published soon in Frontiers in Ecology and the Environment. “The issue of how much you should burn and how much should go back to the land is partly an economic issue and partly a sustainability issue. The temperature used to heat the feedstock and length of time the plant material is exposed to that temperature strongly influence the biochar’s physical and chemical properties, so it is important to know about the production process for each biochar that you use. The production vessel plays a significant role in the particle size as well. The heating process can be fast or slow depending on the heating rate and exposure duration.
Biochar application is a relatively recent and attractive strategy for sustainable agriculture, but it is still in its infancy. Biochar, a promising soil amendment, is believed to help achieve sustainable agriculture by improving soil quality, reducing greenhouse gas emissions, and promoting crop production. It can become one of the key materials for a sustainable future green solution to improve soil fertility and productivity. Biochar companies can use wood cleared from forests at risk of wildfires, for example. And the biochar itself can also be used in other ways, including as an ingredient in concrete to help make the material stronger while it stores carbon. Biochar is considered a relatively permanent form of carbon storage, unlike planting trees that face the risk of later being cut down or burning in a forest fire. There is a broad consensus that biochar is a suitable tool for carbon sequestration and thus could help to mitigate climate change.6
1 https://www.technologyreview.com/2007/04/26/225876/the-case-for-burying-charcoal/
2 https://esemag.com/solid-waste/quebec-new-biochar-plant-largest-north-america/
3 https://onlinelibrary.wiley.com/doi/full/10.1111/gcbb.12885
4 https://www.fs.usda.gov/rm/pubs_journals/2020/rmrs_2020_dumroese_k001.pdf
5 https://www.rainforest-rescue.org/updates/1150/declaration-biochar-a-new-big-threat-to-people-land-and-ecosystems
6 https://www.weforum.org/agenda/2023/10/biochar-climate-change-mitigation-tool/