Charred Earth Examined as Carbon-Storing Option

May 1, 2008 -- If only we could put all that climate-damaging carbon somewhere other than in the atmosphere. One idea is to make it into dirt.

According to new analysis, this approach could be a way to make energy and store carbon at the same time.

Plants sequester carbon in their tissues through photosynthesis, but when they die, microorganisms decompose them, releasing carbon dioxide, so the carbon removal is not permanent.

However, when plants burn in forest or brush fires, part of the carbon ends up as charcoal, or biochar, which is resistant to microbial attack and can stay in the soil for hundreds or thousands of years.

Biochar could be a useful long-term carbon storage option, especially because it can improve the fertility of the soil and enhance crop yields, according to Johannes Lehmann of Cornell University in Ithaca, NY. Lehmann proposes that plant residue or crops grown for bioenergy could deliberately be turned into biochar as a way to store carbon, while making energy in the process.

Biochar is made when it is pyrolyzed: heated with little to no oxygen. It is also possible to burn the resulting biochar for energy, which obviously produces more energy out of a given parcel of plants than putting the biochar back into soil, but it doesn't store any carbon.

Lehmann and colleague John Gaunt calculate that storing biochar produces 30 percent less energy, but avoids two to five times more CO2 emissions than burning biochar for energy.

The carbon saved by storing biochar is greater than the fossil fuel carbon that would be saved by using the equivalent amount of biochar to make energy, Lehmann adds. Their findings were published online in Environmental Science and Technology.

"A general consensus is that even if we invest and use an ever-larger proportion of the biomass produced on land for energy, we will never completely satisfy our energy needs," Lehmann says. "So, maybe what bioenergy can do with the biochar option is to reduce emissions and draw carbon out of the atmosphere."