Biochar is a black carbon material produced from the decomposition of plant-derived organic matter (biomass) in a low- or zero-oxygen environment in the process of pyrolysis or gasification to release energy-rich gases which are then used for producing liquid fuels or directly for power and/or heat generation. Biochar is of increasing interest because of concerns about climate change caused by emissions of carbon dioxide (CO2) and other greenhouse gases (GHG). Carbon dioxide capture also ties up large amounts of oxygen and requires energy for injection (as via carbon capture and storage), whereas the biochar process breaks into the carbon dioxide cycle, thus releasing oxygen as did coal formation hundreds of millions of years ago. Biochar is a way for carbon to be drawn from the atmosphere and is a solution to reduce the global impact of farming (and in reducing the impact from all agricultural waste). Since biochar can sequester carbon in the soil for hundreds to thousands of years, it has received considerable interest as a potential tool to slow global warming.

Importantly, the addition of biochar to soils can bring many potential co-benefits. Biochar incorporation can act as a soil enhancer, improving agricultural productivity, particularly in low-fertility and degraded soils. It can improve soil water and nutrient holding capacity, which means less dependence on artificial fertilizers. The production of biochar via pyrolysis or gasification also means the production of bioenergy as electricity and/or heat, which can be translated as further potential for carbon abatement from avoided fossil fuel use. This combination of factors means that biochar has been labelled as carbon negative mitigation technology. Studies have shown that addition of biochar to the soil can increase the crop yield by 10% but more applicable to poor depleted tropical soils. Application of biochar could therefore reduce the dependence on fertilizers. There is also increasing evidence that biochar could suppress the emissions of methane and nitrous oxides from soils in addition to reducing nitrate leaching.

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The carbon atoms in biochar molecules are strongly bound to one another, and this makes biochar resistant to attack and decomposition by micro-organisms. By contrast, the carbon in most organic matter is rapidly (between 1 and 5 years) returned to the atmosphere as CO2 through respiration. Consequently, biochar is a potentially highly valuable way of stabilising carbon and storing it in soils and is one of very ways of removing CO2 from the atmosphere. There are a very wide range of potential biochar feedstocks: e.g. wood waste, timber, agricultural wastes, manure, leaves, food wastes, straw, paper sludge, green waste, distillerís grain, bagasse and many others.