Soils have the ability to absorb carbon dioxide and influence its concentration in the atmosphere. Biochar can be used to increase the ability of soils to sequester carbon and simultaneously improve soil health.
Image source: http://katerva.org/tag/biochar/
What is biochar?
Biochar is just charcoal made from biomass—which is plant material and agricultural waste—hence the name ‘biochar’. It is a fine-grained charcoal produced from pyrolysis: the slow burning of organic matter in a low- or no-oxygen environment. What differentiates biochar from charcoal is its purpose; it is produced as an additive to soils, mainly to improve nutrient retention and carbon storage.  Although the history of biochar extends thousands of years, its science is still relatively poorly understood.
Image source: Odette V.
History of biochar
The term ‘biochar’ was coined in recent times, but the origins of the concept are ancient. Throughout the Amazon Basin there are regions—up to two metres in depth—of terra preta. This is a highly fertile dark-coloured soil that has for centuries supported the agricultural needs of the Amazonians.
(Image source: Google Images)
Analyses of the dark soils have revealed high concentrations of charcoal and organic matter, such as plant and animal remains (manure, bones and fish). Terra preta’s productivity is due to good nutrient retention and a neutral pH, in areas where soils are generally acidic. Interestingly, terra preta exists only in inhabited areas, suggesting that humans are responsible for its creation. What has not been confirmed is how terra preta was created so many years ago.
Many theories exist. A frontrunner is the suggestion that ancient techniques of slash-and-char are responsible for the dark earth. Similar to slash-and-burn techniques, slash-and-char involves clearing vegetation within a small plot and igniting it, but only allowing the refuse to smoulder (rather than burn).  Combined with other biomass and buried under a layer of dirt, the smouldering char eventually forms terra preta.  It is from these hypotheses of early slash-and-char practices that modern scientists have developed methods for producing biochar.
1. J Lehmann and S Joseph, eds, Biochar for environmental management, Earthscan publishing, London, 2009.
2. EG Neves, RN Bartone, JB Petersen and MJ Heckenberger, The timing of Terra Preta formation in the central Amazon: new data from three sites in the central Amazon, 2004, Springer: Berlin; London.
3. Terra preta means ‘dark earth’ in Portuguese.
4. pH stands for potential of Hydrogen and measures acidity. A neutral pH is neither acidic nor alkaline; J Lehmann and S Joseph, p. 67.
5. Slash-and-burn techniques are the cutting and burning of vegetation to make way for agricultural activities.
6. E Ring, ‘Amazonion Terra Preta’, www.ecoworld.com, 27 November 2007, viewed 30 June 2009, http://www.ecoworld.com/blog/2007/11/27/terra-preta/.