Scientists Search for Carbon Solutions in Amazonia’s ‘Black Earth’

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Imagine if in a poverty-stricken sector of the equatorial band, littered with acidic soils barely fit for farming, there were jet-black patches of dirt, seeded with charcoal and so fertile that they could be planted continuously for over 40 years without applying fertilizer.

Then imagine that those patches were so loaded with carbon that they had six to seven times the amount of carbon per pound of the surrounding soils, that Western scientists could partially replicate the process through which the black earth was made, and that by burying carbon in earth they could augment soil fertility and, perhaps, leach carbon out of the atmosphere and reverse global warming.

Perhaps the jig is already up—too much detail. What we’re talking about is terra preta, or more colloquially, biochar, the Amazonian miracle soil.

NASA Climatologist James Hansen has endorsed terra preta—literally black earth—as a carbon “draw-down strategy.” British chemist James Lovelock, originator of the Gaia hypothesis, says it is the “one way we could save ourselves.”

So let’s check it out. But first, what is it?

Terra preta was first documented in the Brazilian Amazon by Dutch soil scientist Wim Sombroek in the 1950’s. Similar deposits exist in Ecuador, Peru, Benin, Liberia and South Africa. They are thousands of years old.

In Brazil, they were created by throwing fecal matter, organic refuse, charcoal, broken pottery, and other detritus into the Amazonian soil. Whether this was done intentionally or not is unclear. But the enormously fertile terra preta, usually running in patches along riverbanks, supported massive pre-Colombian civilizations, numbering in the hundreds of thousands to millions.

Its main ingredient is biochar—the charcoal. It was and is biologically active. Micro-organisms and fungi nest in its pores, helping to keep the soil abnormally fertile and nutrient-rich. And the carbon, created by burning organic plant waste, secures it safely underground where it can stay for decades or centuries.

What’s perhaps most interesting about biochar is that it appears to be replicable using human technology.

To create biochar – and produce energy at the same time – biomass, such as degrading trees, shrubs, grasses, or organic wastes, can be heated to 400 or 500 degrees with the complete or partial exclusion of oxygen, a technique called pyrolysis. As Cornell ecologist Christopher Lemann explains:

At these temperatures, biomass undergoes exothermic processes and releases a multitude of gaseous components in addition to heat. … Both heat and gases can be captured to produce energy carriers such as electricity, bio-oil, or hydrogen for household use or powering cars.

Such a system produces between three and nine times more energy than is input. Depending on the configuration of the heating system, 50 percent of the biomass can be converted into biofuel and 50 percent stored in carbon—the biochar. Compare to carbon sequestration after simply burning off biomass: under 3 percent of the total carbon in the biomass, or letting it naturally decompose, which sequesters between 10 and 20 percent of the carbon. The biochar can then be placed into carbon-starved soils.

A study published in 2006 in Mitigation and Adaptation Strategies for Global Change suggests that tropical zones are excellent places to use biochar for another reason: the frequent use of slash-and-burn agriculture.

If instead of slashing and burning covering vegetation, farmers in such regions were to slash-and-char, using small kilns, 12 percent of annual emissions resulting from land-use chance could be offset. The biochar resists conversion to CO2 for a long time, but it is unclear how long, because of the time-scales involved. The biochar may also reduce methane and nitrous oxide emissions from soil, two extremely potent greenhouse gases.

In that same study, the authors note:

The projected amount of renewable fuels would potentially yield an amount of sequestered bio-char of 5.5–9.5 PgCyr−1, if pyrolysis were to be used. … The maximum potential sequestration of 9.5 PgCyr−1 would exceed today’s anthropogenic emissions from fossil fuels of 5.4 PgCyr−1 even if no fossil fuels are substituted by renewable fuels in the future.

The numbers are astounding but immediately raise a question: Is biochar viable?

Certainly it is technologically and politically viable. The UN Convention to Combat Desertification submitted a proposal at Poznan to make it a Clean Development Mechanism, and the proposal will be under consideration at UN Framework Convention on Climate Change negotiations in Copenhagen later this year.

Economic viability is a separate issue. One company working to produce small-scale biochar production units is Ecovolve. Its founder, Jason Aramburu, observes, “most gasification systems require expensive gas cleaning equipment, which is only economical at a size of several megawatts or more.” But bringing biomass to concentrated plants in order to process it, turn it into biochar and biogas, and then redistribute it is an energy intensive endeavor.

Ecovolve’s units—still in testing stages—allow for biomass to be turned into biochar at the same location that the organic waste is produced. The units also produce energy, making them a carbon-negative energy source.

So, while biochar has not quite made it, it may be close. A bill to provide funding to biochar initiatives in the United States was proposed in 2007 by former Sen. Ken Salazar, now the secretary of the Interior. The proposal is still languishing in committee. One wonders what they’re waiting for.