Terra preta is the very fertile black soil found mostly in parts of the Amazon basin, and believed to have been created by people mixing fine particles of charcoal and other stuff into the soil. A whole lot of voodoo has grown up around the subject, with unscrupulous charlatans, head in the sand naysayers and all manner of other life forms clustering around the idea. Over at The Other Place I dumped on biofuels from a great height because in essence they are mining the soil. Doesn't matter how slowly; at some point, the fun will have to stop. In the comments on that post, Karl and Anastasia weighed in by saying that bio-char, a potential residue after extracting bioenergy, could be returned to the land to close the loop. I dumped on that idea too.
Now I’m not so sure. Not about bioenergy. I still think that’s a false god, methadone for the oil-addicted masses. But terra preta could just be a solution to excess carbon dioxide in the atmosphere.
Why the change of heart? Because today I went to a very interesting presentation by a very smart, very rich and very tenacious man who has a history of wild and crazy ideas that turn out to be correct, and he talked about terra preta not as a by-product of bioenergy production but as a direct solution to removing carbon dioxide from the atmosphere.1
He had some interesting figures to share, although in my excitement, I didn’t write all of them down. As I remember, to get atmospheric carbon down from 380 parts per million or thereabouts to 280 ppm we would “only” have to spread a 1 mm layer of bio-char each year on, I think, just the cultivated part of agricultural land for 50 years. That amounts to 5 cm of bio-char, and gets us back to pre-industrial levels of atmospheric carbon dioxide.
It didn’t seem all that much.
What struck me, listening, was that the argument was not couched in terms of bioenergy at all. I think reference may have been made to using some of the energy derived from the pyrolysis of plant matter that creates bio-char in order to power the plant.2 It was all about learning to make use of bio-char to create terra preta -- which almost certainly needs to be tailored specifically to different growing areas -- in order to sequester carbon. The oil addicts will have to take care of themselves. This was about saving the planet for the rest of us.
I still have lots of questions, which under the circumstances I wasn’t able to ask. Like:
- If the pyrolysis plants have to be large, how will the bio-char be returned to the soils from which it came?
- If the plants can be small, can they be made simple enough and cheap enough for individual communities to use them?
- Would it pay? Really?
- What about the other soil nutrients? Should the bio-char go back whence it came? Or is this a good way of transferring nutrients from reasonably self-sustaining systems (prairie?) to land currently unfit for cultivation? Maybe that just adds costs to transport and incorporate the bio-char.
I just wanted to note that maybe bio-char is a useful solution to carbon sequestration, although sustainable bioenergy remains a pipe dream.
I'm not going to name names or give details because I'm honestly not sure how public the idea is; I Googled the guy, obviously, and the idea. He's there, but he doesn't seem to be associated with the idea, so I'm keeping mum, for now. ↩
Pyrolysis requires energy, so the process certainly is not going to be self-sustaining. ↩