The Why Files The Why Files --

Buried charcoal: global warming star?

Bury charcoal, save the earth?

This week, the World Meteorological Organization reported that the past decade was "very likely" the warmest decade on record. So let’s turn our attention to one of the oddest -- but also most available -- solutions to global warming: biochar, AKA charcoal. Biochar is made by heating plant material -- branches, farm wastes, even food scraps -- without oxygen, making charcoal, and smoke that can be converted into biofuel.

Biochar is not as sexy as windmills, ethanol from cellulose, or storing carbon dioxide in deep rocks, but it has virtues:

Islands of rich, black soil in the Amazon basin contain charcoal -- the original biochar. Many scientists believe this "terra preta" (black earth in Portuguese) supported large human populations before the Spanish arrived in the 1500s.

Biochar is an extraordinary additive for poor soils. Tests show that biochar plus fertilizer greatly outstrips the productivity increase due to fertilizer alone. With charcoal mixed in the soil, water and nutrients stay put, beneficial microbes prosper, and plants grow better.

Biochar retains roughly 25 to 50 percent of the carbon contained in the biomass it was made from. Because all of that carbon came from the atmosphere, burying biochar could be a "carbon-negative" technology that reduces atmospheric concentrations of the greenhouse gas carbon dioxide.

The oil condensed from a biochar-maker’s smoke can make a liquid biofuel. Because the fuel’s carbon originates in the atmosphere, burning this fuel does not, unlike fossil fuel, raise the concentration of greenhouse gases.

For controlling greenhouse gases, biochar is getting face time at the climate confab in Copenhagen. A scientific document prepared for the meeting (see #1 in the bibliography, p. 50) observed that biochar could store up to 400 billion tons of carbon over the twenty-first century, and that "even the most conservative estimates of 20 billion tons of carbon sequestered by 2030 could have a significant impact on atmospheric greenhouse gas concentrations. Biochar could be an essential component of systems management necessary for meeting the climate change challenge."

Biochar production process

Illustration showing adding char to soil stores carbon dioxide, high farm yields, no deforestation
From original image by Biochar Fund
Biochar removes carbon dioxide from the air and stores it in the soil while making poor soil more productive.

Ancient roots

The use of charcoal to improve soil is rooted in history, geography and archeology. When the Spanish conquistador Francisco de Orellana explored the Amazon basin in 1542, he reported enormous cities. Because later explorers did not see these cities, and the poor, weathered Amazon soils did not support good crops, his account was discounted.

Perhaps he’d been eating hallucinogenic vines in the Amazon...

A metal statue of mans face with helmet, beard, frown and a eyepatch over the left eye
Conquistador Francisco de Orellana traveled the Amazon River in 1542 and noted large Indian populations, who were apparently planting their crops in soil containing charcoal.

Much credit for the discovery of terra preta goes to geographer Wim Sombroek, who had survived the food-scarce World War II years in Holland while eating from a vegetable garden that his father fertilized with ash and charcoal. After the war, Sombroek took an interest in the Amazonian dark soil, correctly surmising that it contained charcoal, and then realizing that the dark soil was not just a matter of history. Modern people still preferred to plant gardens in terra preta.

 Man standing in field smiling, right hand has plant with strong roots, left hand plant with weak roots
Photo: Malende, Cameroon Biochar Fund
A farmer in Cameroon shows the difference between the roots of a maize plant grown in biochar (left) and in bare soil. Biochar boosts production by providing support for essential soil microbes.

Hundreds of years after the charcoal entered the soil, the ground was still fertile.

Archeologists attribute terra preta to people. According to Anna Roosevelt, professor of anthropology at the University of Illinois, "All the [terra pretas] so far investigated are full of intact human cultural features such as house foundations, hearths, cemeteries and platforms." She adds that the normal activities of daily life in tropical forests produce a huge amount of charcoal and refuse, including butchered remains, feces and plant remains, but "there is no evidence that [terra pretas] were created by charcoal production specifically."

However, Clark Erickson, a professor of anthropology at the University of Pennsylvania, who studies how people change landscapes, says that "to accumulate this depth of organic matter means that lots of people were probably living there, pretty densely, [adding charcoal] intensely over long periods of time." All fires create ash, he notes, but "ash doesn’t help the soil very much. When wood is scarce, you burn it so you get the most heat out of it, usually burn it down to ash."

Somehow, he says, the ancients "were short-cutting the process, creating charcoal and burying it."

Black is beautiful

Black earth may extend far beyond the Amazon, says biochar researcher Christope Steiner of the University of Georgia. "In some of the richest soils, up to 35 percent of soil organic carbon is charcoal. The traditional grassland soils in the north-central United States are among the most productive soils in the world, because the grassland was burning every year, and accumulating a lot of charcoal."

Farmers bent over tilling the soil with black biochar over the brown dirt
Farmers introduce biochar into the soil in Cameroon.

As interest in biochar has grown, trials in Australia, Costa Rica, Japan, Africa and elsewhere are showing that the greatest crop benefit exists in the degraded, low-productivity soils that are widespread in Australia, the Amazon and Africa.

A greenhouse study, for example, showed that applying charred chicken manure increased production of cowpea, a major African grain, by 146 percent (see #2 in the bibliography). When conventional potassium fertilizer was added to the chicken-poo biochar, the increase spiked to 185 percent.

A non-profit group called Biochar Fund is supporting biochar projects in Africa, and says the results of the 1500-farmer Cameroon project, are "remarkable, in that they demonstrate how biochar consistently helps to boost crop productivity in tropical soils, sometimes in a spectacular manner." We could not find published results, however.

Although these studies continue, a long tradition of planting gardens in the "terra preta" (black soil) in the Amazon demonstrates that charcoal has long-lasting benefits for crops.

But how much carbon could biochar store in the soil?

Terry Devitt, editor; Steve Furay, project assistant; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

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