Terra Preta, Biochar, Black Gold: a Climate Change Solution

It's no silver bullet, but Terra Preta de Indio, a centuries-old agricultural-waste management and fertilization practice, may provide part of the solution to global warming - and to the gathering world food shortage.

Terra Preta is a literal description of the “dark earth” that European explorers first discovered in the Amazon basin, earth that researchers now believe was enriched with charred agricultural waste. Preparing and mixing this biochar into the earth is a great way to sequester carbon AND to fertilize crops.

There are a host of challenges - a large number of hurdles to clear before biochar can be guaranteed as a useful solution to climate change - but when asked if it's a possible goal, Cornell University Assoc. Professor Johannes Lehmann, one of the world's leading experts on biochar, said: “Absolutely!”

The Wiki definition of biochar is just “charcoal produced from biomass.” It comes most commonly from heating organic material in the absence of oxygen - a process called pyrolysis.

But words like pyrolysis make the process seem more complicated than it needs to be. In the Amazon basin, it is believed that farmers discovered Terra Preta when they were burning off agricultural waste - the stalks and stems of crops they had already picked. Instead of just setting this material afire, the farmers would stoke the blaze until it was good and hot and then bank up the “biomass” so that it smoldered in great heat but with little or no oxygen. As a result, instead of mixing with oxygen and gassing off as carbon dioxide, as much as 50 per cent of the carbon was preserved in a solid state - as charcoal.

Plowed back into the soil, this material is rich in available nutrients and incredibly persistent. It doesn't break down and gas off as other biomass will and it isn't easily washed away, like most petrochemical fertilizers.

It is possible to recreate this process today in an industrial context, where you can also capture the off-gases created in pyrolysis and use them as an additional energy source.

But, according to Prof. Lehmann, there are more than a few challenges. The first is that it is difficult to scale up the process without compromising its efficiency. That is, if you are having to transport huge amounts of biomass to a central facility, you run the risk of using up as much energy as you are preserving - especially if you then have to truck the biochar to fertilize distant fields.

The “price of carbon” is also too low to make the operation profitable if the goal is purely sequestration. The carbon trading schemes currently in place set a price of carbon between $2 and $4 a tonne. Prof. Lehmann estimates that it would have to be at least $10 a tonne before industrial scale biochar sequestration operations became economically feasible.

But Lehmann says there are already practical applications. For example, a one innovator is using pyrolysis to dispose of “chicken litter” - the relatively toxic effluent that comes in large quantities from the backsides of industrially farmed chickens. This particular farmer gathers the litter and heats it in a gasification chamber, creating a syngas that he then uses to heat the chicken barns. What's left over is safe for use as fertilizer. The beauty in this design is that the feedstock (tonnes of chickenshit) is close to the gasification chamber, which is equally close to the barns that need heat. Virtually no extra energy is used up in transportation.

In our telephone interview on Monday, I had started out by asking whether this process might also be useful in dealing with the 10s of 1,000s of acres of standing dead timber left behind by the mountain pine beetle kill in British Columbia. That dead forest has become a huge and dangerous source of additional CO2 in the atmosphere. I wondered: could the branches and needles be mulched, charred and shipped to the prairies, replacing many tonnes of petrochemical fertilizer?

Lehmann pointed out the obvious energy costs of harvesting, processing and transporting material, but it seems to come back to the question of pricing carbon “correctly.” If there was an economic instrument that put real value on keeping CO2 out of the atmosphere, this would seem like a good place to start. 


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