Biochar Saves the Day

By Harry Short

Biochar (otherwise known as charcoal or black carbon) is a residue of the incomplete combustion of biomass. Scientists are excited by biochar as it could represent a way to significantly increase stable soil carbon in agricultural land, the land use type with the smallest carbon pool. Biochar breaks down to carbon dioxide slowly and when applied to cropland increases fertility and crop yields. The production of biochar also generates surplus bioenergy, which could be used to offset fossil fuels. Thus, biochar represents a synergistic solution to multiple problems: it improves agricultural output, while sequestering carbon and providing renewable energy.

How is biochar made?

Biochar is a naturally produced in wildfires, though the percent of carbon from the original biomass converted to biochar is quite modest (roughly 3%) (Lehman et al. 2006). However, in controlled burnings, where oxygen is limited or absent, roughly 50% of the carbon in the original biomass can be converted to biochar. This process is called pyrolysis, and by varying the temperature, pressure, and parent material, the amount of biochar produced can be altered. Though biochar is complex chemically, it possesses fewer oxygen-containing functional groups and is enriched for aromatic C residues compared to the parent material (Czimiczik et al. 2002).

Once produced, biochar is normally used as an agricultural amendment, but may also be used as a construction material or as a filter for exhaust streams (Okimori et al. 2003). Pyrolysis also produces bio-oil and syngas, which may be captured to generate energy. However, the ability of pyrolysis to do this, while also sequestering a net amount of carbon, is most intriguing from a global change perspective (Gaunt & Lehmann, 2008). Moreover, if the bioenergy produced during pyrolysis leads to displacement of fossil fuels, it could have an even greater impact on reducing greenhouse gas emissions.


(Lehmann, 2007)

Role in Carbon Mitigation

Biochar mineralizes at a very slow rate and represents a potentially significant sink for carbon. What is exciting for policy makers interested in arresting global climate change is that carbon sequestration via biochar seems relatively independent of human action once the biochar is applied to agricultural soils. Other forms of carbon storage depend on adherence to the original policy (reforestation, adding crop residues to land, etc), which may change as politics evolve in a particular jurisdiction. Since biochar is so chemically recalcitrant, once biochar is mixed with soil, its carbon is safely stored.

But for how long? The exact mineralization rate is difficult to determine for several reasons: (1) methodological constraints: decay constants determined in the laboratory fail to describe processes in the field and errors become significant when extrapolating from a small time period to a large time period, and (2) the diversity of parent materials for biochar production and the diversity of natural soils and climate patterns make the determination of a single rate impossible. For example, Glaser et al (2009) cite mean residence times of 2,000 years, whereas other work suggests means residence time of roughly a decade (Nguyen et al. 2008). Part of this discrepancy may result from differing soil types, climate, and the type of biochar formation. For example, biochar generated by humans is highly stable in soils from the Amazon basin (Liang et al. 2008), whereas black carbon generated from forest fires in African savannah is more labile (Nguyen et al. 2008).

The point is not that biochar fails to decay. Rather a fraction (98% to 30%) of the original biochar is incredibly resistant to mineralization (Preston & Schmidt, 2006; Nguyen et al.2008; Liang et al. 2008), so resistant that it may remain in the soil after centuries if not millennia. Thus, this portion represents a way to hedge the risk that a contemporary carbon sink might become a future carbon source.

It is important to note that most of the work on biochar has been with agricultural soils or soils that are already highly weathered. Other soil types and land uses may see different soil chemistry dynamics once biochar is added. For example, Wardle et al (2008) found that carbon release from boreal forest humus was increased with addition of large quantities of charcoal. They demonstrated that microbial activity was spurred by the addition of charcoal to humus. This research suggests that charcoal may enhance the carbon release in boreal forest soils, by increasing soil microbial biomass, thus enhancing the degradation of the labile carbon pool.

However, this study is limited in two respects: carbon measurements were taken of a mesh bag containing the original charcoal and humus, therefore soluble C fractions that leached into the soil were counted as ‘carbon release’, though the soluble C may have remained immobilized in the subsoil. Secondly, additions of 50% charcoal (as used in the study) are not realistic or recommended (this amount represents at least five times the maximum mass derived from agronomical experiments (Rondon, 2007).

More significant concerns about the use of biomass in forest soils deal with the ability of subsequent fires to mineralize biochar in the soil, thereby negating any sequestration (Preston & Schmidt, 2006). For this reason, it would be unwise to apply biochar in forests with recurring fires. Therefore, limiting biochar application to agricultural soils and biochar production feedstocks to crop residues seems justified until more research can be undertaken. However if concerns can be allayed, using forest residues from timber production along with biomass from abandoned agriculture lands could allow for the sequestration of 10% of US annual fossil fuel emissions (Lehmann 2007).

An Agriculture Amendment

Excitement around biochar as a soil amendment in tropical agriculture preceded our current focus on carbon storage and climate mitigation (Marris 2006). Wim Sombroek, a pioneer in the field, studied the famous terra preta soils of the Amazon basin. In contrast to surrounding soils, the terra preta anthrosols were noted for their dark color (due to having upwards of 10% carbon by mass) and drew attention for their high agricultural productivity. Due to high rainfall in the tropics, significant leaching of soils occurs, such that slash-and-burn agriculture requires years of fallowing to regenerate soil fertility. High rainfall also limits the productivity gains from exogenous application of fertilizer for these same reasons. However, terra preta soils resist leaching to an incredible degree (Lehmann et al. 2003). By adding charred matter to soils along with other refuse, the ancient residents of the Amazon, were able to produce soil that retained its fertility, allowing for long-term settlements to arise.

Initial interest was in transporting a slash-and-char model of agriculture to other tropical areas. Biochar improves soils, especially the weathered soils of the tropics in several important ways (Lehmann 2007b). Biochar has a high surface area for absorption and can form organometallic complexes, so it retains dissolved nutrients like ammonium and potassium well. It increases the pH, decreases Al saturation, and enhances the cation exchange capacity of soils (Glaser et al. 2002). Since biochar also contains a certain amount of ash, it also acts as a fertilizer in soils low in K, Ca, or Mg. Depending on the original texture of the soil, adding biochar may also improve the water retention of the soil, though this effect is mostly seen in very sandy soils.

Some caution must be offered. Though the availability of many inorganic ions is higher in soils with high biochar content, nitrogen availability is reduced in such soils, especially those in which the charcoal has aged (Lehmann et al. 2003). Though this may seem to suggest the need for higher fertilizer inputs, the complexing of nitrogen with fresh biochar and its slow release may actually require less fertilizer as plants will have a lower but continuous supply of nitrogen for growth. More research is needed here. Secondly, maximas exist in biochar application; depending on crop and original soil type, excess biochar application reduces biomass production and ultimately yields. As a result, caps on biochar applications should be erected.

Still, the literature is clear on the benefits to plant growth (above and below ground), crop yield, and foliar tissue nutrient concentrations for P, S, K, Ca, Mg, B, and Mo of moderate applications of biochar (~60g of bochar kg-1 of soil) to cropland (Rondon et al. 2007; Lehmann 2003). Many soil types and crop types could benefit from biochar.

Finally, charcoal production is a low-technology craft (prior to the discovery of coal, much of the pig iron in colonial America was forged using biomass-derived charcoal). Since the technology involved is relatively simple, advocates for slash-and-char agriculture argue that food production in tropical areas can be made more productive, while enhancing carbon sequestration without large investments of capital. Other than education about charcoaling, few barriers seem to exist. Mobilization of the world’s farmers seems more feasible with such a low-technology solution.

Can Biochar Be a Significant Carbon Sink?

Recognizing that an upper limit of application for biochar exists (approximately 60g kg-1 soil) and assuming the average density of soil is 2500lbs/ cubic yard (Hall, 1980), one can determine the amount of biochar that could safely be applied to a cubic yard of soil (approximately 68kg). Assuming the United States has approximately 350 million acres of cropland (excluding pasture and idled cropland) (Vesterby 2002), and that biochar will only be added to the top six inches of the soil, one estimates a total volume of 282 billion cubic yards of soil in the US. Assuming that 80% of the soil is enriched with biochar, we arrive at a maximum total biochar application amount of 15.4 GtC. If we take global cropland to be 1.5 billion hectares, this figure jumps to 163GtC, or approximately a fifth of the current carbon in the atmosphere or 8% of the soil carbon stored globally. This amount represents a ceiling of total carbon that could be stored in agricultural soils as biochar with no harm to those soils.

But how much crop residue is available to generate the biochar to be stored (assuming a massive mobilization)? Annual crop residues in the US total about 500 million tones, of which 203 million tonnes is carbon (Lal, 1998). Following recommendations that only 30% of the crop residue be removed to prevent erosion (SQNTDT, 2006), of which approximately half the carbon will remain as biochar after pyrolysis, one arrives at 30.5 MtC/yr in the US or approximately 322 MtC/yr for the world, assuming similar rates of crop residue production as in the United States. Since carbon is accumulating in the atmosphere at a rate of 3.3 GtC/yr (Bolin & Sukumar, 2000), global use of biochar could represent a 10% reduction in atmospheric carbon accumulation per year (assuming the accumulation rate does not increase significantly beyond that in 2000). Not a magic bullet, but certainly a wedge in the overall solution.

Bioenergy v. Biochar

Production of biochar involves deliberate inefficiency. Not all of the carbon is combusted and thus a significant amount of bioenergy is not released. Competing users of biomass might claim this to be a waste of crop residues. However, Gaunt and Lehmann (2008) show that a focus on efficiency, on oxidizing all the carbon in a unit of biomass, neglects to consider the total inputs to agricultural lands and the carbon mitigation benefits of returning biochar to the land. They calculate that total avoided GHG emissions from displacing natural gas for a bioenergy (complete combustion) approach to be roughly one third of the avoided emissions reaped from a biochar approach. Moreover, over 90% of the avoided omission for a bioenergy approach depends on displacement of fossil fuels, which may not actually occur. Whereas for a biochar strategy, only 14% of the avoided emissions depends on such displacement. The rest is derived from stabilization of carbon in soil, reduced nitrous oxide and methane emissions, and reduced fertilizer requirements. For example, if avoided nitrous oxide and methane release is converted to carbon dioxide equivalents, we see substantial emissions reductions (3867 kg CO2 ha-1 y-1 for wheat and 4126 kg CO2 ha-1 y-1 for corn) from application of biochar to croplands (Gaunt & Lehmann, 2008). Gaunt and Lehmann also estimate that between 90 to 100 kg CO2 ha-1 y-1 are avoided from reduced fertilizer use (and thus, manufacture). This suggests that policy makers and industry should take new interest in biochar.


Bolin, B., and R. Sukumar. 2000. Global Perspective. Pages 29-51 (Sections 1.1 through 1.4.4) in R. T. Watson, I. R. Noble, B. Bolin, N. H. Ravindranath, D. J. Verardo, and D. J. Dokken, editors. Land Use, Land-Use Change, and Forestry. A Special Report of the IPCC. Cambridge University Press, Cambridge, UK.

Czimiczik, C., Preston, C., Schmidt, M., & Schulze, E. (2002). Effects of charring on mass, organic carbon, and stable carbon isotope composition of wood. Organic Geochemistry 33:1207-1223.

Glaser, B., Parr, M., Braun, C., &Goodspeed, K. (2009). Biochar is carbon negative. Nature Geoscience 2: 2.

Gaunt, J., & Lehmann, J. (2008) Energy balance and emissions associated with biochar sequestration and pyrolysis bioenergy production. Environmental Science and Technology 42(11): 4152-4158.

Glaser, B., Lehmann., J., Zech, W. (2002). Ameliorating physical and chemical properties of highly weathered soils in the tropics with charcoal—a review. Biological Fertility in Soils 35:219-230.

Hall, Carl W. 1980. Drying and Storage of Agricultural Crops. AVI Publishing Company, Inc: Westport, CN.

Lal, R. (1998). The Potential of U.S. Cropland to Sequester Carbon and Mitigate the Greenhouse Effect. CRC Press: Boca Raton, Fl.

Lehmann, J. (2007). A handful of carbon. Nature 447: 143-144.

Lehmann, J. (2007b). Bio-energy in the black. Frontiers in Ecology 5(7): 381-387.

Lehmann, J. Gaunt, J., & Rondon, M. (2006). Bio-char sequestration in terrestrial ecosystems-a review. Mitigation and Adaptation Strategies for Global Change 11:403-427.

Lehmann, J., da Silva, J., Steiner, C., Nehls, T., Zech, W., & Glaser, B. (2003). Nutrient availability and leaching in an archaeological Anthrosol and a Ferralsol of the Cental Amazon basin: fertilizer, manure, and charcoal amendments. Plant Soil 249:343-357.

Marris, E. Black is the new green. Nature 442:624-626.

Nguyen, B., Lehmann, J., Kinyangi, J., Smernik, R., Riha, S., & Engelhard, M. (2008). Long-term black carbon dynamics in cultivated soil. Biogeochemistry 89:295-308.

Okimori, Y., Ogawa, M., & Takahashi, F. (2003). Potential of CO2 emission reductions by carbonizing biomass waste from industrial tree plantation in south Sumatra, Indonesia. Mitigation and Adaptation Strategies for Global Change 8:261-280.

Rondon, M., Lehmann, J., Ramirez, J., & Hurtado, M. (2007). Biological nitrogen fixation by common beans (Phaseolus vulgaris L.) increases with bio-char additions. Biological Fertility in Soils 43:699-708.

Soil Quality National Technology Development Team (August 2006). Crop residue removal
for biomass energy production: Effects on soils and recommendations. Soil Quality-Agronomy Technical Note 19. USDA:Washington, DC.

Vesterby, M., Krupa, K., & Lubowski, R. (November 2004). Estimating U.S. Cropland area. Amber Waves 2(5). Accessed at

Wardle, D., Nilsson, M., & Zackrisson, O. (2008). Fire-derived charcoal causes loss of forest humus. Science 320:629.


7 Responses so far »

  1. 1

    Biochar Soil Technology…..Husbandry of whole new orders of life

    Biotic Carbon, the carbon transformed by life, should never be combusted, oxidized and destroyed. It deserves more respect, reverence even, and understanding to use it back to the soil where 2/3 of excess atmospheric carbon originally came from.

    We all know we are carbon-centered life, we seldom think about the complex web of recycled bio-carbon which is the true center of life. A cradle to cradle, mutually co-evolved biosphere reaching into every crack and crevice on Earth.

    It’s hard for most to revere microbes and fungus, but from our toes to our gums (onward), their balanced ecology is our health. The greater earth and soils are just as dependent, at much longer time scales. Our farming for over 10,000 years has been responsible for 2/3rds of our excess greenhouse gases. This soil carbon, converted to carbon dioxide, Methane & Nitrous oxide began a slow stable warming that now accelerates with burning of fossil fuel.

    Wise Land management; Organic farming and afforestation can build back our soil carbon,

    Biochar allows the soil food web to build much more recalcitrant organic carbon, ( living biomass & Glomalins) in addition to the carbon in the biochar.

    Biochar, the modern version of an ancient Amazonian agricultural practice called Terra Preta (black earth, TP), is gaining widespread credibility as a way to address world hunger, climate change, rural poverty, deforestation, and energy shortages… SIMULTANEOUSLY!
    Modern Pyrolysis of biomass is a process for Carbon Negative Bio fuels, massive Carbon sequestration,10X Lower Methane & N2O soil emissions, and 3X Fertility Too.
    Every 1 ton of Biomass yields 1/3 ton Charcoal for soil Sequestration, Bio-Gas & Bio-oil fuels, so is a totally virtuous, carbon negative energy cycle.

    Biochar viewed as soil Infrastructure; The old saw, “Feed the Soil Not the Plants” becomes “Feed, Cloth and House the Soil, utilities included !”. Free Carbon Condominiums, build it and they will come.
    As one microbologist said on the TP list; “Microbes like to sit down when they eat”. By setting this table we expand husbandry to whole new orders of life.

    Senator / Secretary of Interior Ken Salazar has done the most to nurse this biofuels system in his Biochar provisions in the 07 & 08 farm bill,

    Charles Mann (“1491”) in the Sept. National Geographic has a wonderful soils article which places Terra Preta / Biochar soils center stage.

    Biochar data base; TP-REPP

    NASA’s Dr. James Hansen Global warming solutions paper and letter to the G-8 conference, placing Biochar / Land management the central technology for carbon negative energy systems.

    The many new university programs & field studies, in temperate soils; Cornell, ISU, U of H, U of GA, Virginia Tech, JMU, New Zealand and Australia.

    Glomalin’s role in soil tilth, fertility & basis for the soil food web in Terra Preta soils.

    UNCCD Submission to Climate Change/UNFCCC AWG-LCA 5
    “Account carbon contained in soils and the importance of biochar (charcoal) in replenishing soil carbon pools, restoring soil fertility and enhancing the sequestration of CO2.”

    This new Congressional Research Service report (by analyst Kelsi Bracmort) is the best short summary I have seen so far – both technical and policy oriented. .

    Given the current “Crisis” atmosphere concerning energy, soil sustainability, food vs. Biofuels, and Climate Change what other subject addresses them all?

    This is a Nano technology for the soil that represents the most comprehensive, low cost, and productive approach to long term stewardship and sustainability.

    Carbon to the Soil, the only ubiquitous and economic place to put it.
    Erich J. Knight
    Shenandoah Gardens
    540 289 9750

    Biochar Studies at ACS Huston meeting;

    Most all this work corroborates char soil dynamics we have seen so far . The soil GHG emissions work showing increased CO2 , also speculates that this CO2 has to get through the hungry plants above before becoming a GHG.
    The SOM, MYC& Microbes, N2O (soil structure), CH4 , nutrient holding , Nitrogen shock, humic compound conditioning, absorbing of herbicides all pretty much what we expected to hear.



    665 – III.


    Company News & EU Certification

    Below is an important hurtle that 3R AGROCARBON has overcome in certification in the EU. Given that their standards are set much higher than even organic certification in the US, this work should smooth any bureaucratic hurtles we may face.

    EU Permit Authority – 4 years tests
    Subject: Fwd: [biochar] Re: GOOD NEWS: EU Permit Authority – 4 years tests successfully completed

    Doses: 400 kg / ha – 1000 kg / ha at different horticultural cultivars

    Plant height Increase 141 % versus control
    Picking yield Increase 630 % versus control
    Picking fruit Increase 650 % versus control
    Total yield Increase 202 % versus control
    Total piece of fruit Increase 171 % versus control
    Fruit weight Increase 118 % versus control



    EcoTechnologies is planning for many collaborations ; NC State, U. of Leeds, Cardiff U. Rice U. ,JMU, U.of H. and at USDA with Dr.Jeffrey Novak who is coordinating ARS Biochar research. This Coordinated effort will speed implementation by avoiding unneeded repetition and building established work in a wide variety of soils and climates.

    Hopefully all the Biochar companies will coordinate with Dr. Jeff Novak’s soils work at ARS;

    I spoke with Jon Nilsson of the CarbonChar Group, in their third year of field trials ;
    An idea whose time has come | Carbon Char Group
    He said the 2008 trials at Virginia Tech showed a 46% increase in yield of tomato transplants grown with just 2 – 5 cups (2 – 5%) “Biochar+” per cubic foot of growing medium.

    Low Tech Clean Biochar;

  2. 2

    Jason MacDonald said,

    My first observation is on the carbon sequestration figure comparing biochar to the natural carbon cycle. It shows a 50% number on the natural cycle image that I assume to be the amount of carbon in the above-ground bio-mass that will be stored in the soil when the tree dies. My understanding was that only about 10% would naturally end up in the soil after heterotrophic respiration, or perhaps this is only referring to short term storage. But if that were the case, isn’t the upside to biochar even greater? You can have 50% of the carbon content of the biomass stored in biochar and then distributed to agriculture, where it will end up in the soil while offsetting fossil fuel use with the energy from biofuels and syngas.

    It is very interesting to think of carbon sequestration solutions in terms of their required maintenance. I imagine that that could make a large impact on the choices that policy makers make. One thing I don’t understand though is the idea that biochar would make agricultural lands more productive, but that the C would stay in the soil for a long period of time. Is it just that the fraction of Biochar that is not recalcitrant will be picked up by plant life looking to grow?

    My last question, At what scales is biochar being utilized outside of academic research? What mechanisms could promote its use?

    Really enjoyed the entry.

    Jason MacDonald

  3. 3

    Nina Shestakovich said,

    I was very interested in this article because it brought attention to a carbon sequestration technology that we did not discuss in class. I have heard of pyrolysis before but I was pleasantly surprised to find a decent review of biochar benefits to agricultural soil’s productivity. I agree with a previous comment that the figure with 50% of soil sequestration is not very clear and I skipped it all together. It also would be great if the author gave a small overview of pyrolysis process.
    I was curious why biochar is not making headlines for its great benefits to C sequester and improvement of productivity of agricultural soils? The blog article does not go in depth into disadvantages or obstacles of commercialization of biochar. If ancient Amazonians did it, what happened to it now? I bet there is some fertilizers lobbying involved…
    Great article!

  4. 4

    Drew Casey said,

    Very interesting and well-researched topic. The value of biochar for the agricultural industry is fascinating. One figure that I would like to know more about is the actual percentage of carbon that gets converted to biochar during controlled combustion. The article indicates up to 50%, but this seems extremely high compared to the estimated 3% of carbon retained after a forest fire (which doesn’t necessarily involve complete combustion of forest floor biomass). The production and use of biochar on a global scale is certainly not the only needed solution to increasing terrestrial carbon storage and reducing the atmospheric carbon accumulation rate. However, if the agricultural industry embraced the use of biochar on their lands, we could convert a sustantial volume of crop residue to biochar and add it back the same acreage to reap its benefits. The charcoal industry could see enormous global expansion in order to process the increased amounts of biomass.

  5. 5

    amrita said,

    Harry, I came across this article on black carbon that is generated during incomplete combustion in cook stoves used by many in the developing world. It portrays black carbon in a completely opposite light from your analysis and I thought it to be an interesting counter point. If black carbon is really the second largest contributor to global warming after carbon dioxide – we need to think carefully about how to use it to counter global warming and human health issues that it engenders.

    On a more cheery aside – I will be in Africa this summer working on energy efficient cook stoves that reduce black carbon outputs but I will take your thoughts on biochar with me 🙂


    • 6

      Harry Short said,

      Black carbon that has been aerosolized does seem to be a significant contributor to warming of the atmosphere. Though about half of the carbon in pyrolysis remains in a solid, non-aerosolized form, what about the other half? I don’t know what percentage of that carbon is released as black carbon, but I will try to find this figure out. Thanks for the comment, Amrita, and best of luck in Africa!

  6. 7

    One aspect of Biochar systems are Cheap, clean biomass stoves that produce biochar and no respiratory disease. At scale, the health benefits are greater than ending Malaria.
    A great example;

    Also , I would like the BioFuelWatch folks & George Medelblott to read the petition of 1500 Cameroon Farmers;

    The Biochar Fund

    The USDA-ARS have dozens of studies happening now to ferret out the reasons for char affinity with MYC fungi and microbes, but this synergy is solidly shown by the Japanese work, literally showing 1+1=4

    This is what I try to get across to Farmers, as to how I feel about the act of returning carbon to the soil. An act of pertinence and thankfulness for the civilization we have created. Farmers are the Soil Sink Bankers, once carbon has a price and the IPCC recognition of soil as a sink, they will be laughing all the way to it.

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