In ancient Amazonia, the waste disposal method of choice was to bury and burn. Wastes were baked beneath a layer of soil. This process, known as pyrolysis, produced a charcoal soil amendment rich in carbon. The result was terra preta, literally “black earth” in Portuguese. Today, terra preta soils cover up to 10 percent of the Amazon basin, retaining extraordinary amounts of carbon.
These ancient roots of what is now called biochar have modern promise for agriculture and the atmosphere. Biochar is commonly made from waste material ranging from peanut shells to rice straw to wood scraps. During the slow baking of biomass in the near or total absence of oxygen, gas and oil separate from carbon-rich solids. The output is twofold: fuels that can be used for energy and biochar that can be used to enrich soil.
When biomass decomposes on the earth’s surface, carbon and methane escape into the atmosphere. Biochar retains most of the carbon present in biomass feedstock and buries it. Rendered stable, that carbon can be held for centuries in the soil—a much-delayed return to the atmosphere. Theoretically, experts argue, biochar could sequester billions of tons of carbon dioxide every year.
terra preta soils…10 percent of the Amazon: Mann, C. C. “The Real Dirt on Rainforest Fertility.” Science 297, no. 5583 (August 2002): 920–923.
Wim Sombroek uncovered…black earth: Sombroek, Wim. Amazon Soils: A Reconnaissance of the Soils of the Brazilian Amazon Region. Pudoc, Centre for Agricultural Publications and Documentation, 1966.; Woods, William I., Wenceslau G. Teixeira, Johannes Lehmann, Christoph Steiner, Antoinette WinklerPrins, and Lilian Rebellato, eds. Amazonian Dark Earths: Wim Sombroek’s Vision. Berlin: Springer, 2009.
one gram of biochar…surface area: Chia, Chee H., Adriana Downie, and Paul Munroe. “Characteristics of Biochar: Physical and Structural Properties.” In Biochar for Environmental Management: Science and Technology, 89-109. London: Earthscan, 2015.
crop yield increase of 15 percent: Jeffery, Simon, Diego Abalos, Kurt A. Spokas, and Frank G.A. Verheijen. “Biochar Effects on Crop Yield.” In Biochar for Environmental Management: Science, Technology and Implementation, 301-326. London: Earthscan, 2015; Jeffery, Simon, Frank G.A. Verheijen, Martijn van der Velde, and Ana Catarina Bastos. “A Quantitative Review of the Effects of Biochar Application to Soils on Crop Productivity Using Meta-Analysis.” Agriculture, Ecosystems and Environment 144, no. 1 (2011): 175-187.
[potential to] sequester…carbon dioxide: Kleiner, Kurt. “The Bright Prospect of Biochar.” Nature Reports Climate Change (2009): 72-74; Hertsgaard, Mark. “As Uses of Biochar Expand, Climate Benefits Still Uncertain.” Yale Environment 360. January 21, 2014.
[growth in] companies: IBI. State of the Biochar Industry 2015, International Biochar Initiative, 2015.
Revision: It was the hallmark of an agricultural system that differs dramatically from pervasive practices today: the wholesale conversion of Amazonian forest to annual crops, such as soybeans for livestock feed. When forest is cleared and vegetation burned, a residual layer of carbon remains, but only for a short period of time.