Big storage solution for carbon dioxide?
Through photosynthesis, land plants remove an estimated 60 billion tons of carbon from the atmosphere each year, far outstripping the roughly 4 billion net tons added annually by human activity. Charring a significant fraction of this biomass could change the global warming equation, which is driven largely by the concentration of carbon dioxide in the atmosphere.
Record of temperatures, compared to averages, 1951-1980
That 60 billion-ton figure has attracted attention from a flotilla of biochar advocates, including representatives of industry, physical and social scientists, and activists working on hunger and poverty. According to them, putting a decent percentage of that carbon into long-term soil storage could counterbalance some of the pollution that is causing global warming, and simultaneously benefit farmers and hungry people in places with poor soil.
Furthermore, the smoke from making biochar can make biofuel, as we’ll see on the next page.
Terra preta in Amazonia shows that biochar can remain in the soil for hundreds of years, suggesting that it could provide a long-term solution to the problem of rising carbon dioxide in the atmosphere. In contrast, burying organic matter -- fresh or composted -- is a short-term fix, as most of that carbon is oxidized and returned to the atmosphere within a few years. Even growing trees -- a favorite "sink" of carbon, only stores carbon until the tree rots or burns.
How much carbon?
Although charcoal making machines can char many types of dry biomass, most advocates of biochar focus on waste material, and adamantly oppose clear-cutting forests, growing biochar plantations or removing enough crop residue to allow the soil to erode.
To assess the upper limit of biochar’s potential, James Amonette of Pacific Northwest National Laboratory, and David Laird, of the U.S. Department of Agriculture’s National Laboratory of Agriculture, assumed that 50 percent of crop residues and 67 percent of forestry wastes would be charred (see #3 in the bibliography).
If this gigantic amount of biomass was fed into pyrolysis machines and the charcoal buried, biochar could store between 0.4 and 0.9 billion tons of carbon per year. If the smoke is used to make biofuel, the net impact would equal removing 0.9 to 1.8 billion tons of carbon from the atmosphere each year, Amonette says.
That’s a huge part of the 4 billion tons of carbon that people add, net, to the atmosphere each year.
Laird and Amonette estimated that land could store a total 80 to 270 billion tons of biochar carbon, depending on how deep the biochar is buried in the soil. For comparison, since the industrial revolution people have raised the atmospheric burden of carbon by about 220 billion tons.
Biochar must be handled safely to prevent fire and dust problems, and reaching this huge storage potential would depend on a vast number of factors, including the quantity and quality of feedstocks, charcoal makers, transport equipment and soils, and how policies, incentives and costs impact biochar and competing technologies.
But let's be skeptical for a minute. Could this backfire? Could adding carbon to soil release carbon from soil?
As biochar advocates argue at the Copenhagen conference that charcoal should be deemed a permissible "offset" for carbon dioxide polluters, making it eligible for funding under greenhouse gas control mechanisms, skeptics ask if biochar is ready for prime time. Their qualms take direct aim at the heart of the biochar benefit: the fertilizing effect that comes from stimulating soil microbes.
Although this increases the transfer of nutrients to plant roots, it may not benefit the greenhouse gas equation, says David Wardle, a professor of ecology at the Swedish University of Agricultural Sciences. "Charcoal is not inert; it has physical properties that cause it to influence other components of the soil system. It has a charged surface, and there is a lot of evidence that it promotes microbial growth substantially. When microbes grow, they need to break down organic molecules [in the soil humus] to live, and in doing so they are respiring some carbon and releasing it as CO2."
In a brief (and contested) study in Science (see #4 in the bibliography), Wardle found such a loss of carbon in soil adjacent to biochar, but he does not deny that biochar might still help store carbon. "It would still be a net positive, but just not as much as it seems. If you add X tons of charcoal to the soil, the net storage is X - Y. We just don’t know what Y is. ... While the carbon in charcoal may be sequestered, this microbial growth causes decomposition and loss of carbon from the soil."
At the proving ground
The fate of carbon, both in biochar and the soil, needs further investigation, says Lucas Reijnders of the University of Amsterdam. The specific process used to make char, and the soil type, temperature and moisture, are all "important determinants of how fast it will degrade," he says, "but I have not seen good research regarding minimizing degradation of carbon stocks already present in soils and of carbon newly added to soils as biochar. Considerable manpower and quite a number of years will be needed for getting reasonably certain knowledge in these respects."
At present, Reijnders says, the greenhouse impact of a large-scale biochar project is "unknown."
However, Laird, of the U.S. Department of Agriculture, says that stimulating microbial activity in soil results in more nutrients cycling through the soil, "which in general is associated with enhanced plant growth and enhanced photosynthesis, and hence enhanced removal of carbon in CO2 from the atmosphere. This could be an offsetting mechanism for stabilizing soil organic carbon."
Such a phenomenon seems apparent in the large natural experiment in Amazonia, Laird adds. "We certainly know that terra preta soils in the Amazon have large amounts of charcoal and a large amount of ... natural organic matter."
A study published this year (see #5 in the bibliography) found a confusing picture on 16 different biochars in three soil types. In some cases, the charcoal stimulated removal of soil carbon, in other cases it was reduced or unchanged. "The take-home message is that it depends on the char type, and the soil type and climate," says Laird. "This is not a one-size-fits-all situation."
As a method for storing carbon, the simple technology of burying charcoal is in competition with high-tech, expensive and speculative proposals for carbon capture and storage from power plants. As a source of bioenergy, biochar competes with ethanol and biodiesel, which have their own problems in terms of feasibility, cost and environmental impact.
So why don’t we hear more about the ancient technology of charcoal? "Biochar for carbon sequestration has not had strong financial support, compared to carbon capture and storage," says Christophe Steiner of the University of Georgia. "Biochar is a much more realistic route for carbon capture, but nobody knows about it."
Today, the major land-based carbon sink is growing forests, "but they are only capturing carbon as long as they grow, and the sequestration depends very much on what happens afterward," says Steiner. "If the trees are used for toilet paper, the capture time is very short. Biochar has a lot of advantages, the risk of losing the carbon is very small. It cannot burn or be wiped out by disease."
A systems analysis
Before biochar can go global, studies would need to examine its impact on crops, soils, water and pollution, says Johannes Lehmann, associate professor of soil biogeochemistry at Cornell University. A full analysis must also look "at broader economic questions, such as sustainability in specific communities, the availability of feedstock, the crop responses, any competing technologies and the uses of the bioenergy. A wide range of parameters need to be looked at the scale of implementation, rather than at the prototype scale that is available now."
Still, Lehmann says, "The compelling argument to fully explore biochar is that this technique is at our fingertips. We have the technology to test it at the scale of relevance; it can be brought from the laboratory experiment to fully fledged commercial scale right now."
With its complex interactions with food, crops, soil, biomass, energy and climate, the "biochar system can look very different in different locations," Lehmann says. "The system can range from small household-scale cookstoves in a rural agricultural community to single-farm units to heat animal barns or manage organic wastes, to biorefineries in the commercial realm."
Assessing all of that may be difficult, but is that more difficult or expensive than testing methods to remove and compress billions of tons of carbon dioxide from smokestacks for underground storage? Does biochar seem more risky than spreading iron to fertilize the ocean so algae will soak up more carbon dioxide?
Biochar "goes against the conventional wisdom, people don’t think this way," says William Woods, a professor of geography at the University of Kansas, who has been studying terra preta since 1993. "People think, ‘Tropical soils are infertile, burning is bad, trees are good, composting is good,’ but that’s all short-term thinking. Tropical soils can be bad, but they also can be really good. Human manipulation of environment has not always been deleterious. It’s been wonderful in many cases, and this is one of them."
Terry Devitt, editor; Steve Furay, project assistant; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive