Thursday, August 17, 2006

carbon sequestration as agricultural fertilizer

The journal Nature has a couple of interesting essays in last week's issue about carbon sequestration as a solution to global warming. In general, I'm somewhat skeptical about carbon sequestration, where excess carbon dioxide is pumped underground or down to the sea floor or something like that. It's treating the symptom, not the problem, it doesn't deal with the problem of peak oil, it might not work very well, and some failure modes could result in huge amounts of carbon dioxide being released into the atmosphere all at once. Imagine 50 years of sub-ocean sequestered carbon dioxide bubbling to the surface all at once, and what a potential climate shock could result!

This article (Nature subscription required, so probably only readable from a University network) proposes a different solution. Instead of storing carbon dioxide as a gas, with the risk of an accidental release, take carbon out of the agricultural cycle by turning waste products (like corn stalks) into a charcoal-like product. The mostly-carbon char, made by smoldering it in an oxygen-poor environment can then be used as fertilizer for fields. This way, the carbon that would otherwise have been released when the corn stalks are burned (as ethanol, say) or decomposed (if tilled into the earth) stays in the ground.
The particles of char produced this way are somehow able to gather up nutrients and water that might otherwise be washed down below the reach of roots. They become homes for populations of microorganisms that turn the soil into that spongy, fragrant, dark material that gardeners everywhere love to plunge their hands into.
Even better, the fertilized soil with the additional structure from the char keeps the carbon rather than releasing it into the air. Studies show that perhaps 2 1/2 times as much carbon can be stored in a field fertilized with char than without.
...[T]urning unimproved soil into terra preta [soil enhanced with char] can store away more carbon than growing a tropical forest from scratch on the same piece of land, before you even start to make use of its enhanced fertility. Johannes Lehmann of Cornell University in Ithaca, New York... estimates that by the end of this century terra preta schemes, in combination with biofuel programmes, could store up to 9.5 billion tonnes of carbon a year — more than is emitted by all today's fossil-fuel use...
It's even possible to generate fuels from the gaseous products of making charcoal, fuels that are actually "carbon-negative" once the beneficial effects of the char fertilizer are taken into account. It's quite a remarkable process and I'm very much impressed with the research. Perhaps carbon sequestration has promise after all?

One issue pointed out in the article is worth a mention here. There's a conflict in agriculture between growing organic and growing sustainably that doesn't always get talked about. In recent years, growers have started to use "no-till" approaches to growing various crops. Instead of plowing the earth periodically to kill weeds and loosen the soil, herbicides (relatively benign ones like Roundup) are used to kill weeds and eliminate the need for plowing. Corn stalks and so forth are left in the fields to eventually decompose and aid soil development. A major benefit of no-till farming is that topsoil doesn't get lost in runoff, and in fact plowing causes a lot of the carbon stored in soil to be released or washed away. No-till farming generally can't be used with organic crops, since the weeds are too much of a problem if you can't spray and you can't till up the earth. Arguably, non-organic no-till farming is better for the soil (in some ways) than at least some approaches to organic farming with a plow. (Organic no-till would likely be best, but it seems to be very hard to make work, and I don't think it's really being used yet.) The issue with char fertilizer is that the powdered charcoal really needs to be mixed into the soil by tilling; it's not effective if it's just dumped on top where it can easily blow away. In order to get the benefits of char, adding carbon to the soil and sequestering it from the atmosphere, you actually have to stop using one of the more environmentally-friendly agricultural practices of recent years, a practice that by itself significantly helps retail carbon in the soil! Still, on balance it sounds very promising...

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