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A piece of biochar.

Biochar is charcoal used as a soil amendment. Biochar is a stable solid, rich in carbon, and can endure in soil for thousands of years.[1] Like most charcoal, biochar is made from biomass via pyrolysis. Biochar is under investigation as an approach to carbon sequestration.[1] Biochar thus has the potential to help mitigate climate change via carbon sequestration.[2][3] Independently, biochar can increase soil fertility of acidic soils (low pH soils), increase agricultural productivity, and provide protection against some foliar and soil-borne diseases.[4] Its potential benefits have at times been oversold.[5]


Pre-Columbian Amazonians are believed to have used biochar to enhance soil productivity. They seem to have produced it by smoldering agricultural waste (i.e., covering burning biomass with soil)[6] in pits or trenches.[7] European settlers called it terra preta de Indio.[8] Following observations and experiments, a research team working in French Guiana hypothesized that the Amazonian earthworm Pontoscolex corethrurus was the main agent of fine powdering and incorporation of charcoal debris to the mineral soil.[9]

The term “biochar” was coined by Peter Read to describe charcoal used as a soil improvement.[10]


Biochar is a high-carbon, fine-grained residue that today is produced through modern pyrolysis processes; it is the direct thermal decomposition of biomass in the absence of oxygen (preventing combustion), which produces a mixture of solids (the biochar proper), liquid (bio-oil), and gas (syngas) products. The specific yield from the pyrolysis is dependent on process conditions. such as temperature, and can be optimized to produce either energy or biochar.[11] Temperatures of 400–500 °C (752–932 °F) produce more char, while temperatures above 700 °C (1,292 °F) favor the yield of liquid and gas fuel components.[12] Pyrolysis occurs more quickly at the higher temperatures, typically requiring seconds instead of hours. High temperature pyrolysis is also known as gasification, and produces primarily syngas, which has been used as vehicle fuel in some times and places.[12] Typical yields are 60% bio-oil, 20% biochar, and 20% syngas. By comparison, slow pyrolysis can produce substantially more char (~50%); it is this which contributes to the observed soil fertility of terra preta. Once initialized, both processes produce net energy. For typical inputs, the energy required to run a “fast” pyrolyzer is approximately 15% of the energy that it outputs.[13] Modern pyrolysis plants can use the syngas created by the pyrolysis process and output 3–9 times the amount of energy required to run.[7]

The Amazonian pit/trench method[7] harvests neither bio-oil nor syngas, and releases a large amount of CO2, black carbon, and other greenhouse gases (GHG)s (and potentially, toxins) into the air. Commercial-scale systems process agricultural waste, paper byproducts, and even municipal waste and typically eliminate these side effects by capturing and using the liquid and gas products. The production of biochar as an output is not a priority in most cases.

Centralized, decentralized, and mobile systems[edit]

In a centralized system, all biomass in a region is brought to a central plant for processing. Alternatively, each farmer or group of farmers can operate a lower-tech kiln. Finally, a truck equipped with a pyrolyzer can move from place to place to pyrolyze biomass. Vehicle power comes from the syngas stream, while the biochar remains on the farm. The biofuel is sent to a refinery or storage site. Factors that influence the choice of system type include the cost of transportation of the liquid and solid byproducts, the amount of material to be processed, and the ability to feed directly into the power grid.

For crops that are not exclusively for biochar production, the residue-to-product ratio (RPR) and the collection factor (CF) the percent of the residue not used for other things, measure the approximate amount of feedstock that can be obtained for pyrolysis after harvesting the primary product. For instance, Brazil harvests approximately 460 million tons (MT) of sugarcane annually,[14] with an RPR of 0.30, and a CF of 0.70 for the sugarcane tops, which normally are burned in the field.[15] This translates into approximately 100 MT of residue annually, which could be pyrolyzed to create energy and soil additives. Adding in the bagasse (sugarcane waste) (RPR=0.29 CF=1.0), which is otherwise burned (inefficiently) in boilers, raises the total to 230 MT of pyrolysis feedstock. Some plant residue, however, must remain on the soil to avoid increased costs and emissions from nitrogen fertilizers.[16]

Pyrolysis technologies for processing loose and leafy biomass produce both biochar and syngas.[17]

Thermo-catalytic depolymerization[edit]

Alternatively, "thermo-catalytic depolymerization", which utilizes microwaves, has recently been used to efficiently convert organic matter to biochar on an industrial scale, producing ~50% char.[18][19]


Carbon sink[edit]

The burning and natural decomposition of biomass and in particular agricultural waste adds large amounts of CO
to the atmosphere. Biochar that is stable, fixed, and 'recalcitrant' carbon can store large amounts of greenhouse gases in the ground for centuries, potentially reducing or stalling the growth in atmospheric greenhouse gas levels; at the same time its presence in the earth can improve water quality, increase soil fertility, raise agricultural productivity, and reduce pressure on old-growth forests.[20]

Biochar can sequester carbon in the soil for hundreds to thousands of years, like coal.[21][22][23][24][25] Such a carbon-negative technology would lead to a net withdrawal of CO2 from the atmosphere, while producing and consuming energy. This technique is advocated by prominent scientists such as James Hansen, head of the NASA Goddard Institute for Space Studies,[26] and James Lovelock, creator of the Gaia hypothesis, for mitigation of global warming by greenhouse gas remediation.[27]

Researchers have estimated that sustainable use of biocharring could reduce the global net emissions of carbon dioxide (CO
), methane, and nitrous oxide by up to 1.8 Pg CO
-C equivalent
-Ce) per year (12% of current anthropogenic CO
-Ce emissions; 1 Pg=1 Gt), and total net emissions over the course of the next century by 130 Pg CO
-Ce, without endangering food security, habitat, or soil conservation.[28]

Soil amendment[edit]

Biochar is recognised as offering a number of benefits for soil health. Many benefits are related to the extremely porous nature of biochar. This structure is found to be very effective at retaining both water and water-soluble nutrients. Soil biologist Elaine Ingham indicates[29] the extreme suitability of biochar as a habitat for many beneficial soil micro organisms. She points out that when pre charged with these beneficial organisms biochar becomes an extremely effective soil amendment promoting good soil, and in turn plant, health.

Biochar has also been shown to reduce leaching of E-coli through sandy soils depending on application rate, feedstock, pyrolysis temperature, soil moisture content, soil texture, and surface properties of the bacteria.[30][31][32]

For plants that require high potash and elevated pH,[33] biochar can be used as a soil amendment to improve yield.

Biochar can improve water quality, reduce soil emissions of greenhouse gases, reduce nutrient leaching, reduce soil acidity, and reduce irrigation and fertilizer requirements.[34] Biochar was also found under certain circumstances to induce plant systemic responses to foliar fungal diseases and to improve plant responses to diseases caused by soilborne pathogens.[35][36][37]

The various impacts of biochar can be dependent on the properties of the biochar,[38] as well as the amount applied,[37] and there is still a lack of knowledge about the important mechanisms and properties.[39] Biochar impact may depend on regional conditions including soil type, soil condition (depleted or healthy), temperature, and humidity.[40] Modest additions of biochar to soil reduce nitrous oxide N
emissions by up to 80% and eliminate methane emissions, which are both more potent greenhouse gases than CO2.[41]

Studies have reported positive effects from biochar on crop production in degraded and nutrient–poor soils.[42] Biochar can be designed with specific qualities to target distinct properties of soils.[43] In an Columbian savanna soil, biochar reduced leaching of critical nutrients, created a higher crop uptake of nutrients, and provided greater soil availability of nutrients.[44] At 10% levels biochar reduced contaminant levels in plants by up to 80%, while reducing total chlordane and DDX content in the plants by 68 and 79%, respectively.[45] On the other hand, because of its high adsorption capacity, biochar may reduce the efficacy of soil applied pesticides that are needed for weed and pest control.[46][47] High-surface-area biochars may be particularly problematic in this regard; more research into the long-term effects of biochar addition to soil is needed.[46]


Switching from slash-and-burn to slash-and-char farming techniques in Brazil can decrease both deforestation of the Amazon basin and carbon dioxide emission, as well as increase crop yields. Slash-and-burn leaves only 3% of the carbon from the organic material in the soil.[48]

Slash-and-char can keep up to 50% of the carbon in a highly stable form.[49] Returning the biochar into the soil rather than removing it all for energy production reduces the need for nitrogen fertilizers, thereby reducing cost and emissions from fertilizer production and transport.[50] Additionally, by improving the soil's ability to be tilled, fertility, and productivity, biochar–enhanced soils can indefinitely sustain agricultural production, whereas non-enriched soils quickly become depleted of nutrients, forcing farmers to abandon the fields, producing a continuous slash and burn cycle and the continued loss of tropical rainforest. Using pyrolysis to produce bio-energy also has the added benefit of not requiring infrastructure changes the way processing biomass for cellulosic ethanol does. Additionally, the biochar produced can be applied by the currently used machinery for tilling the soil or equipment used to apply fertilizer.[51]

Water retention[edit]

Biochar is a desirable soil material in many locations due to its ability to attract and retain water. This is possible because of its porous structure and high surface area.[52] As a result, nutrients, phosphorus, and agrochemicals are retained for the plants benefit. Plants therefore, are healthier and fertilizers leach less into surface or groundwater.

Energy production: Bio-oil and Syngas[edit]

Main article: Biofuel

Mobile pyrolysis units can be used to lower the costs of transportation of the biomass if the biochar is returned to the soil and the syngas stream is used to power the process.[53][54] Bio-oil contains organic acids that are corrosive to steel containers, has a high water vapor content that is detrimental to ignition, and, unless carefully cleaned, contains some biochar particles which can block injectors.[55] Currently, it is less suitable for use as a kind of biodiesel than other sources.

If biochar is used for the production of energy rather than as a soil amendment, it can be directly substituted for any application that uses coal. Pyrolysis also may be the most cost-effective way of electricity generation from biomaterial.[56]

Direct and indirect benefits[edit]

  • The pyrolysis of forest- or agriculture-derived biomass residue generates a biofuel without competition with crop production.
  • Biochar is a pyrolysis byproduct that may be ploughed into soils in crop fields to enhance their fertility and stability, and for medium- to long-term carbon sequestration in these soils.
  • Biochar enhances the natural process: the biosphere captures CO
    , especially through plant production, but only a small portion is stably sequestered for a relatively long time (soil, wood, etc.).
  • Biomass production to obtain biofuels and biochar for carbon sequestration in the soil is a carbon-negative process, i.e. more CO
    is removed from the atmosphere than released, thus enabling long-term sequestration.[57]


Intensive research into manifold aspects involving the pyrolysis/biochar platform is underway around the world. From 2005 to 2012, there were 1,038 articles that included the word “biochar” or “bio-char” in the topic that had been indexed in the ISI Web of Science.[58] Further research is in progress by such diverse institutions around the world as Cornell University, the University of Edinburgh, which has a dedicated research unit.,[59] and the Agricultural Research Organization (ARO) of Israel, Volcani Center, where a network of researchers involved in biochar research (iBRN, Israel Biochar Researchers Network) was established as early as 2009.

Students at Stevens Institute of Technology in New Jersey are developing supercapacitors that use electrodes made of biochar.[60] A process developed by University of Florida researchers that removes phosphate from water, also yields methane gas usable as fuel and phosphate-laden carbon suitable for enriching soil.[61]

Possible commercial sector[edit]

If biomass is burned to biochar and put back into the soil, rather than being completely burned, this may reduce carbon emissions. Potentially, the bioenergy industry might even be made to sequester net carbon.[62] Pyrolysis might be cost-effective for a combination of sequestration and energy production when the cost of a CO
ton reaches $37.[62] As of mid-February 2010, CO
was trading
at $16.82/ton on the European Climate Exchange (ECX), so using pyrolysis for bioenergy production may be feasible even if it is more expensive than fossil fuel. This awaits the development of carbon trading credits for biochar, not available as of 2011.[63]

Current biochar projects make no significant impact on the overall global carbon budget, although expansion of this technique has been advocated as a geoengineering approach.[64] In May 2009, the Biochar Fund received a grant from the Congo Basin Forest Fund for a project in Central Africa to simultaneously slow down deforestation, increase the food security of rural communities, provide renewable energy and sequester carbon. Though some farmers did report better maize crops, the project ended early without significant results and with promises to the farmers not kept.[65]

Application rates of 2.5–20 tonnes per hectare (1.0–8.1 t/acre) appear to be required to produce significant improvements in plant yields. Biochar costs in developed countries vary from $300–7000/tonne, generally too high for the farmer/horticulturalist and prohibitive for low-input field crops. In developing countries, constraints on agricultural biochar relate more to biomass availability and production time. An alternative is to use small amounts of biochar in lower cost biochar-fertilizer complexes.[66]

Various companies in North America, Australia, and England sell biochar or biochar production units.[citation needed] In Sweden the 'Stockholm Solution' is an urban tree planting system that uses 30% biochar to support healthy growth of the urban forest. The Qatar Aspire Park now uses biochar to help trees cope with the intense heat of their summers.

At the 2009 International Biochar Conference, a mobile pyrolysis unit with a specified intake of 1,000 pounds (450 kg) was introduced for agricultural applications. The unit had a length of 12 feet and height of 7 feet (3.6 m by 2.1m).[67]

A production unit in Dunlap, Tennessee by Mantria Corporation opened in August 2009 after testing and an initial run, was later shut down as part of a Ponzi scheme investigation.[68]

See also[edit]


  1. ^ a b Lean, Geoffrey (7 December 2008). "Ancient skills 'could reverse global warming'". The Independent. Archived from the original on 13 September 2011. Retrieved 1 October 2011. 
  2. ^ "Geoengineering the climate: science, governance and uncertainty". The Royal Society. 2009. Retrieved 22 August 2010. 
  3. ^ Dominic Woolf; James E. Amonette; F. Alayne Street-Perrott; Johannes Lehmann; Stephen Joseph (August 2010). "Sustainable biochar to mitigate global climate change". Nature Communications. 1 (5): 1–9. Bibcode:2010NatCo...1E..56W. doi:10.1038/ncomms1053. ISSN 2041-1723. 
  4. ^ "Slash and Char". Retrieved 19 September 2014. 
  5. ^ Benoit Anthony Ndameu (November 2011). "Biochar Fund Trials in Cameroon: Hype and Unfulfilled Promises" (PDF). Biofuelwatch. Retrieved 19 October 2012. 
  6. ^ Solomon, Dawit, Johannes Lehmann, Janice Thies, Thorsten Schafer, Biqing Liang, James Kinyangi, Eduardo Neves, James Petersen, Flavio Luizao, and Jan Skjemstad, Molecular signature and sources of biochemical recalcitrance of organic carbon in Amazonian Dark Earths, 71 Geochemica et cosmochemica ACTA 2285, 2286 (2007) ("Amazonian Dark Earths (ADE) are a unique type of soils apparently developed between 500 and 9000 years B.P. through intense anthropogenic activities such as biomass-burning and high-intensity nutrient depositions on pre-Columbian Amerindian settlements that transformed the original soils into Fimic Anthrosols throughout the Brazilian Amazon Basin.") (internal citations omitted)
  7. ^ a b c Lehmann 2007a, pp. 381–387 Similar soils are found, more scarcely, elsewhere in the world. To date, scientists have been unable to completely reproduce the beneficial growth properties of terra preta. It is hypothesized that part of the alleged benefits of terra preta require the biochar to be aged so that it increases the cation exchange capacity of the soil, among other possible effects. In fact, there is no evidence natives made biochar for soil treatment, but rather for transportable fuel charcoal; there is little evidence for any hypothesis accounting for the frequency and location of terra preta patches in Amazonia. Abandoned or forgotten charcoal pits left for centuries were eventually reclaimed by the forest. In that time, the initially harsh negative effects of the char (high pH, extreme ash content, salinity) wore off and turned positive as the forest soil ecosystem saturated the charcoals with nutrients. supra note 2 at 386 ("Only aged biochar shows high cation retention, as in Amazonian Dark Earths. At high temperatures (30–70°C), cation retention occurs within a few months. The production method that would attain high CEC in soil in cold climates is not currently known.") (internal citations omitted).
  8. ^ Glaser, Lehmann & Zech 2002, pp. 219–220 "These so-called Terra Preta do Indio (Terra Preta) characterize the settlements of pre-Columbian Indios. In Terra Preta soils large amounts of black C indicate a high and prolonged input of carbonized organic matter probably due to the production of charcoal in hearths, whereas only low amounts of charcoal are added to soils as a result of forest fires and slash-and-burn techniques." (internal citations omitted)
  9. ^ Jean-François Ponge; Stéphanie Topoliantz; Sylvain Ballof; Jean-Pierre Rossi; Patrick Lavelle; Jean-Marie Betsch; Philippe Gaucher (2006). "Ingestion of charcoal by the Amazonian earthworm Pontoscolex corethrurus: a potential for tropical soil fertility" (PDF). Soil Biology and Biochemistry. 38 (7): 2008–2009. doi:10.1016/j.soilbio.2005.12.024. 
  10. ^ Read, Peter (27 March 2009). "This gift of nature is the best way to save us from climate catastrophe. Biochar schemes would remove carbon from the atmosphere and increase food supply, says Peter Read". Guardian. London. 
  11. ^ Gaunt & Lehmann 2008, pp. 4152, 4155 ("Assuming that the energy in syngas is converted to electricity with an efficiency of 35%, the recovery in the life cycle energy balance ranges from 92 to 274 kg (203 to 604 lb) CO2 MW-1 of electricity generated where the pyrolysis process is optimized for energy and 120 to 360 kilograms (790 lb) CO2MW-1 where biochar is applied to land. This compares to emissions of 600–900 kilograms (1,300–2,000 lb) CO
    MW-1 for fossil-fuel-based technologies.)
  12. ^ a b Winsley, Peter (2007). "Biochar and bioenergy production for climate change mitigation". New Zealand Science Review. 64.  (See Table 1 for differences in output for Fast, Intermediate, Slow, and Gasification).
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  23. ^ Lehmann 2007a, pp. 381, 385 "pyrolysis produces 3–9 times more energy than is invested in generating the energy. At the same time, about half of the carbon can be sequestered in soil. The total carbon stored in these soils can be one order of magnitude higher than adjacent soils.
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  40. ^ Dr. Wardle points out that improved plant growth has been observed in tropical (depleted) soils by referencing Lehmann, but that in the boreal (high native soil organic matter content) forest this experiment was run in, it accelerated the native soil organic matter loss. Wardle, supra note 18. ("Although several studies have recognized the potential of black C for enhancing ecosystem carbon sequestration, our results show that these effects can be partially offset by its capacity to stimulate loss of native soil C, at least for boreal forests.") (internal citations omitted) (emphasis added).
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