Biochar: 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

• Main page: Biochar
• The Biochar Lobby