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.”

Lehmann concurs, noting that the surface properties of charcoal “vary greatly depending on the organic matter used for making the charcoal and the charring environment such as temperature and O2 supply.”

Biochar for carbon sequestration?

The problem

Growing plants sequester carbon – that is, they remove it from the atmosphere and store it. This is why growing trees, crops, or other plants is recommended to counter the global warming effects of excess carbon dioxide in the atmosphere. The problem with this approach is that though plants act as a carbon sink (store) while they are growing, as soon as they begin to decay or are destroyed, they release their stored carbon into the atmosphere. The carbon sink becomes a carbon source.

Agricultural lands that have been converted to no-till, for example, may cease to capture additional carbon after 15-20 years, and forests eventually mature and start to release as much carbon dioxide as they take up. So growing plants, charcoal proponents point out, is an unstable method of sequestering carbon. They say that charcoal is a more stable method of storing carbon than the plants from which it was made. They say that carbon in this form can remain stable in soil for hundreds to thousands of years.

A solution?

Biochar is being put forward as a long-term sink for carbon. The idea is that some of the atmospheric carbon dioxide drawn down annually by plants could be incorporated into soils as charcoal and stored there in a form that is resistant to being returned to the atmosphere. Proponents of biochar want it to be included in carbon trading systems on the grounds of its ability to store carbon in a stable manner.

Charcoal is said by proponents to be risk-free in that it is not vulnerable to the kinds of changes that can make other types of carbon sink become carbon generators, such as converting no-till land to tillage, or forest fires.

How much?

Johannes Lehmann calculated greenhouse gas emissions reductions from three different biochar approaches:

• pyrolysis of forest residues from 200 million hectares of US forests used for timber production
• pyrolysis of fast-growing vegetation grown on 30 million hectares of idle US cropland for this purpose
• pyrolysis of crop residues for 120 million hectares of harvested US cropland.

Lehmann calculates that converting all US cropland to Conservation Reserve Programs – in which farmers are paid to plant their land with native grasses – or to no-tillage would sequester 3.6% of US emissions per year during the first few decades after conversion – just one third of what one of the above charcoal approaches can theoretically achieve.

There is currently political pressure to include charcoal production in government subsidy programs designed to combat global warming as well as to include it in carbon trading systems.

Lehmann writes of charcoal, “When combined with bioenergy production, it is a clean energy technology that reduces emissions as well as sequesters carbon. In my view, it is therefore an attractive target for energy subsidies and for inclusion in the global carbon market.”

A CSIRO report of 2009 endorses the idea that charcoal can act as a carbon sink, saying, “Current analyses suggest that there is global potential for annual sequestration of atmospheric CO2 at the billion-tonne scale (109 t yr-1) within 30 years.”

However, CSIRO also cautions that not enough studies have been done to know for sure how much carbon could be sequestered in charcoal-amended soils: “So far … the underlying published evidence arises mainly from small-scale studies that do not currently support generalization to all locations and all types of biochar.”

How long?

Research shows that in the short term, charcoal can reduce carbon loss from soils. A field study carried out in Brazil on highly weathered soil looked at how much carbon was lost from soil under different treatments. The losses of soil carbon were highest on chicken manure (27%) and compost (27%) treated plots, whereas the charcoal amended plots lost only 8 and 4% of their soil carbon content if mineral fertilized or not fertilized, respectively. This shows the resilience of soil carbon in charcoal amended plots. The authors suggest that a combination of charcoal and chicken manure might mimic the favorable properties of terra preta best.

If biochar is going to make a serious impact on the carbon cycle, it will need to store carbon over long periods of time. CSIRO land and water scientist Evelyn Krull says that charcoal “has a chemical structure that makes it very difficult to break down by physical, biological and chemical processes”. On this basis, biochar proponents are putting forward biochar as a reliable long-term carbon storage system.

However, a report for Biofuelwatch points out that even if biochar does store carbon successfully for a time, its carbon will at some point in the future be released back into the atmosphere – with unknown consequences. In this respect it is unlike fossil carbon, which in its natural form stays safely beneath the earth’s crust.

Many experts researching biochar do not focus on this aspect of the technology, restricting their thinking to the timescales used in the carbon trading schemes. CSIRO’s Evelyn Krull, for example, says, “We know that biochar is stable over the timescales of any [carbon] abatement scheme (100 years).”

But Krull concedes that timescales may vary depending on the charcoal feedstock used: “We don’t know how the different biochar products differ on scales of 100s to 1000s of years.”

CSIRO warns that no one knows how stable the carbon in charcoal-amended soils will prove to be: “it is difficult to establish the half-life of modern biochar products using short experiments … At the moment there is no established method to artificially-age biochar and assess likely long-term trajectories.”

Even Johannes Lehmann, one of the most vocal proponents of biochar, states that it is unclear as to how charcoal could affect the carbon cycle. An article in Environmental Health Perspectives quoting Lehmann as a source says that the numbers are “entirely theoretical” at this point, and “any effort to project the impact of biochar on the global carbon cycle is necessarily speculative” as proper tests have not been done.

Research carried out at the University of Amsterdam comes to a similar conclusion: “As to permanence, bio-char may do better than forests or landfilled biomass, but there are major uncertainties about net greenhouse gas emissions linked to the bio-char life cycle, which necessitate suspension of judgement about the adequacy of bio-char addition to soils as an offset for CO2 emissions from burning fossil fuels.”

The Rodale Institute, a research centre for organic farming, has begun researching biochar as a carbon sequestration method.

How does a carbon store become a carbon source?

One major problem with including biochar in a carbon sequestration program is that the processes through which stored soil carbon is released back into the atmosphere are not well understood.

Dr Mark Waldrop of US Geological Survey Soil Carbon Research is carrying out research to examine what kinds of events or processes could cause the carbon stored in charcoal-amended soils to be released back into the atmosphere. Possible means include wildfires or microbial species in soil.

Microorganisms and carbon stores

Dominic Woolf, in his report, “Biochar as a soil amendment: A review of the environmental implications”, points out a possible implication of changing the ecosystem through large applications of charcoal to soil: “If there are microorganisms that can utilise char as either an energy or carbon source, then the creation of large reserves of soil biochar may create an ecological niche that evolution can exploit.” Whether this ecological niche will be occupied in a way that is beneficial or harmful to other creatures and humans remains to be seen.

The microorganisms that colonize charcoal in soil may affect its ability to sequester carbon. One study found that some microorganisms are able to live with black carbon as their sole carbon source. When these microbes metabolize black carbon, carbon dioxide is released into the atmosphere. The authors of the study conclude that black carbon may promote growth of microorganisms and the decomposition of unstable carbon compounds. In other words, in certain circumstances, charcoal amended soils may become part of the greenhouse gas problem, not the solution.

Is biochar carbon-negative?

Bioenergy (the burning of plant material or biomass to produce energy) is often considered carbon-neutral since the carbon emitted in the use of the energy approximates to the amount removed from the atmosphere by the plants that make up the feedstock.

Using biomass to produce biochar and adding the resulting char to soil is claimed by advocates to be an improvement on bioenergy in that the entire process removes more atmospheric carbon than it gives off. Johannes Lehmann writes, “Biochar sequestration offers the chance to turn bioenergy into a carbon-negative industry.”

However, the question of whether biochar can truly be defined as carbon-negative depends on the assumptions used and which processes involved in its production and use are taken into account.

The CSIRO report of 2009 agrees that biochar technology can be defined as carbon-negative by factoring in the energy capture of gases produced during pyrolysis. In addition, any increased crop yield and any lessening of greenhouse gases from soils as a result of adding charcoal may enhance the carbon-negative effect.

However, critics point out that what is not factored into such calculations is the change in land use that will occur through the upscaling of the technology. A report for Biofuelwatch, “Biochar for Climate Change Mitigation: Fact or Fiction?”, challenges biochar’s claimed carbon-negative status on the grounds that it “completely ignores the numerous ecological and social impacts from land use changes that occur when massive demands for plant biomass are created, and is not supported by current scientific understanding of the fate of charcoal in soils.”

Resources

• Biochar: Effects on Soil