One of the reasons behind the interest in using biochar in various plant cultivation systems is the discovery of the black soil, or terra preta, in the Amazon.
The black soils differentiate themselves from other rainforest soils by having a high humus and nutrient content (up to eight times higher carbon content and 1,000 times more plant-available phosphorus compared to surrounding soils) and a significantly higher prevalence of active populations of microorganisms. During studies of the black soils, researchers found that one component contributed more than others to the ability of the black soils to retain nutrients and benefit important soil microorganisms; namely biochar. The biochar in the soil was formed when the indigenous people burned down forests to expose area for cultivation through low-temperature burning. This low-intensity, controlled carbonation (rather than combustion) of the organic material meant that the carbon in the organic mass was not released in the form of carbon dioxide, but was instead bound in stable compounds (biochar) which remained in the soil.
In the early 2000s, the Dutch scientist and soil specialist Wim Sombroek coined the term “terra preta nova” (the “new black soil”), which became a collective term for new, environmentally sustainable models for agriculture and plant production based on lessons learned from the properties of the black soils. Much has happened since then, and the potential for using today’s biochar, produced in high-tech pyrolysis plants, lies not only in soil improvement, but spans several different areas.
The good properties of biochar
Plants that photosynthesise absorb carbon dioxide from the atmosphere and convert it into carbohydrates that are bound to the plant’s biomass. When the plant later dies and decays or is burnt, the carbon returns to the atmosphere as carbon dioxide and methane gases. When plant material is instead converted to biochar via pyrolysis, a large proportion of the carbon that the plants have accumulated can be bound into a much more durable product – biochar. Biochar is very stable in the soil and is estimated to have a half-life of 150 - 5,000 years. Biochar is therefore considered a carbon sink from a climate perspective, and in 2018 it was classified as a Negative Emission Technology by IPCC.
The carbon in the organic residues used in this project has been recently bound, in most cases in the year before pyrolysis, which means that carbon can be bound and stabilised in the form of biochar very quickly.