Terra preta means "black earth". More importantly, if less literally, it means fertile soil—created 1000s of years ago out of nutrient-starved rainforest dirt by the strange alchemy of charcoal.

No one knows exactly how Amazonian natives made terra preta, but that isn't stopping modern agriculture scientists from attempting to recreate, and build on, the successes of this ancient farming technique. Using biochar—charcoal created in an oxygen-free environment—they're hoping to improve soil quality and sequester carbon. But first, they have to deal with that pesky little thing called evidence.

Biochar really is a promising product, but we're only beginning to understand how promising it might be, where it would be most useful and, even, how it works.

The best biochar is made by pyrolysis, according to Kurt Spokas, Ph.D., a USDA-ARS soil scientist and adjunct professor at the University of Minnesota. Pyrolysis takes plants, animal manure or any other kind of organic biomass, traps it in an oxygen-free environment and heats it to around 550°C. At the end, you're left with biochar, and a mixture of hot gases and some liquids. Condense the vapors and collect the liquids and you get liquid fuel and enough combustible gas to fire up the next batch of biomass.

Advocates have long hoped that biochar—spread over farm fields—would improve soil quality and crop yields, while simultaneously trapping carbon in the soil.

The science on the second goal is a little more clear-cut than the first.

Biochar definitely does imprison carbon, and does it better than normal charcoal, said John Bonitz, a farm outreach and policy advocate with the Southern Alliance for Clean Energy. The charcoal left behind by a campfire, for instance, is chemically made up of carbon joined to lots and lots of oxygen molecules, but is primarily ash and has lost most of its carbon to burning. Like sorority girls in a slasher film, the oxygen is easily picked off by bacteria, which speeds up the process of decomposition, breaking the chemical bonds and leaving the carbon that does remain to drift back into the atmosphere.

Subtract the oxygen, however, and the carbon molecules get tough—forming ring structures that don't easily shatter and are more resistant to microbial attack, Spokas said. Lab research, done by him and others, suggests that these bonds have the potential to hold fast for anywhere between hundreds to hundreds of thousands of years. That means less carbon in the atmosphere. It's also good news for anyone who'd like to see carbon neutral, or even carbon negative, biofuel production. Of course, that's in a test tube.

"There's a whole suite of caveats that come along with those estimates because we can't mimic the natural environment in the laboratory," Spokas said.

In fact, most of what we know about biochar comes from the lab. Spokas' team is one of the first in the United States to start running tests in the (literal) field, as part of the USDA-ARS multi-location biochar and pyrolysis research initiative. However, they've only been at that for two years. Not long enough, he says, to make definitive statements, particularly when it comes to biochar's impact on soil quality. The key question—"Does biochar-infused soil lead to more crops and better soil fertility?"—is still wide open.

But there is some tantalizing data coming out of those lab tests. It seems that, by putting microbial life on slow-mo, biochar also works to trap nitrogen in the soil. Not only does that mean less nitrous oxide—another greenhouse gas&mash;in the atmosphere, it could also mean less nitrogen fertilizer applied to the ground, and less excess nitrogen leaching away into the water supply.

Spokas says field trials will make it clear what types of soil benefit the most from biochar—right now, it looks like the Midwest might not get that much of a boost, compared to, say, the sandy soils of the Southeast. Researchers also want to find out whether biochar alone will do the trick, or if a successful soil stew needs more ingredients.

"Terra preta research indicates that there was kitchen garbage discarded with the charcoal," said Bonitz. "And that would increase the bacteria and fungi activities in the final product."

Ultimately, the lack of information centers around the fact that terra preta is old and biochar, well, isn't.

"Currently all ongoing biochar research is on short time periods, maybe 10 years at most. Whereas, with terra preta, we're looking at the residual effects of 1000s of years," Spokas said. "It's a good inference that we could see some positive benefits from biochar, but we're still trying to figure it out. And if there are any short-term negative effects, we wouldn't see those in the terra preta research."

Image courtesy Flickr user Doug Beckers, via CC.