Biochar

Biochar (also known as agrichar, black carbon or BC) is a carbon-rich material produced by heating plant-derived organic matter in an environment with restricted oxygen. Biochar production from sustainable sources of organic waste is currently being advocated as a way to reduce greenhouse warming. Advocates say the biochar process is carbon-negative, removing more carbon dioxide from the atmosphere than it produces. They also say charcoal is a sustainable method of soil enrichment and has the potential to help clean up environmental pollution.

The technology is strongly advocated by some scientists, notably Johannes Lehmann, associate professor of soil fertility management and soil biogeochemistry at Cornell University. Biofuel and energy companies are lobbying governments to subsidise biochar production and to allow it into the carbon trading program, as part of measures to counter greenhouse warming.

Biochar has, however, come under criticism from environmental groups, who warn that its claimed benefits are largely unproven and that it could lead to huge destruction of biodiversity.

Claimed benefits

Biochar advocates say charcoal can be used as a soil conditioner which provides a stable carbon storage (sequestration) system, enhances soil fertility, and filters out pollutants.

These properties are said by biochar advocates to be able to address some of the most urgent environmental problems of our time:

• Soil degradation and food insecurity
• Water pollution by agrochemicals and other toxins
• Climate change due to an excess of carbon dioxide in the atmosphere.

Terra preta

Charcoal has been deliberately incorporated into cultivated soils for thousands of years. The best known charcoal-improved soil is the terra preta of the Brazilian Amazon. Its high level of fertility makes it popular for growing cash crops such as papaya and mango. These crops are said to grow three times faster than on surrounding unimproved land. Similar soils have been identified in Ecuador, Peru, West Africa (Benin, Liberia), and the savannas of South Africa.

Terra preta contains high levels of soil organic matter and nutrients such as nitrogen, phosphorus, potassium and calcium. This is attributed in part to a high charcoal content.

The soil scientist Bruno Glaser and co-researchers found that terra preta soils “not only contain higher concentrations of nutrients such as nitrogen, phosphorus, potassium and calcium, but also greater amounts of stable soil organic matter.” They believe that charcoal is a key factor in the persistence of organic matter in these soils. Their investigations showed that terra preta soils contained up to 70 times more charcoal than the surrounding soils.

Terra preta not the same as charcoal-enriched soils

Farmers practicing slash-and-burn (“swidden”) agriculture know that incorporating some charcoal into soil gives a temporary boost to fertility, because fresh charcoal retains nutrients important for plant growth. But critics say that the secrets of terra preta’s success are not understood. They say it is far more than soil with added charcoal and that attempts to recreate it have not succeeded.

A report for Biofuelwatch, “Biochar for Climate Change Mitigation: Fact or Fiction?”, cautions that there is no evidence that the beneficial qualities of terra preta can be replicated by industrial production of charcoal: “While it is true that Terra Preta was incredibly successful, the indigenous peoples in pre-colonial Amazonia developed their technique over a long period based on small-scale, biodiverse farming techniques and a knowledge base that is now largely lost. Charcoal was only part of their technique.”

An open letter from a wide variety of NGOs warns against equating industrially produced charcoal with terra preta: “The success of terra preta has not been replicated. Modern 'biochar' is highly variable and results vary greatly depending upon the type of soil, the type of material used for making charcoal, and other factors.”

Their concerns are backed by a study showing that soil that has recently had charcoal added to it has different qualities from terra preta.

Soil scientist Bruno Glaser of Bayreuth University, Germany said in 2007 that the key to the success of terra preta is still unknown: "The secret of the terra preta is not only applying charcoal and chicken manure – there must be something else. You would need 50 or 100 years to get a similar combination between the stable charcoal and the ingredients."

Stanley Buol, a professor emeritus from the Department of Soil Science at North Carolina State University, said of the principle of simply adding charcoal to soil, "I'm skeptical about adding just a pure carbon source. It will be black and look good, but will it contain enough inorganic ions, such as phosphorus and nitrogen, essential to plant growth?"

Skeptics worry that the astonishing qualities of terra preta are being hijacked to sell industrially produced biochar.

The biochar production process

There is a wide range of potential charcoal feedstocks: wood and wood waste, agricultural waste, manure, leaves, food waste, green waste, straw, distillers’ grain, etc. The production process (pyrolysis) produces combustible synthesis gas (syngas), and oil (bio-oil) that can be burnt to produce heat, power, or combined heat and power. Charcoal, the third combustible product produced in pyrolysis, is the solid charred and carbon-rich residue.

Johannes Lehmann says that combining pyrolysis for energy production with charcoal additions to soil takes advantage of charcoal’s longevity, its ability to actively draw carbon dioxide from the atmosphere, regenerate degraded lands, and reduce environmental pollution.

Different types of biochar

Charcoal varies in its properties, depending on what it is made from and how it is made. For example, charcoal made from manure has more nutrients than biochar made from wood cuttings. But charcoal made from wood cuttings is stable over a longer period of time. Charcoal made at 700ºC is especially porous, making it useful for soaking up toxins in contaminated environments.

Because of this variability, a CSIRO report of 2009 warns against applying research findings on one type of charcoal to another: “These [differing] properties affect the interactions biochar has within the environment of its application as well as its fate.”