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."
But authors of another paper published today in Science find that char can trigger microbial activity when mixed with other soil components, enhancing carbon release from soil humus and offsetting some of char's potential benefits.
David Wardle and colleagues at the Swedish University of Agricultural Sciences in Umeå, Sweden put mesh bags filled with charcoal, humus, and with a half of each at three different sites in a Swedish forest and left them there for a decade.
They found that after 10 years, the amount of carbon and mass lost from the bags was greater in the mixture than expected based on the losses from the individual components.
About 23 percent of the carbon was lost from the mixture, Wardle said, compared with the predicted loss of 15 percent.
Charcoal has a large surface area and absorbs organic molecules well, he explained, creating an environment that boosts microbial degradation of the humus.
"I wouldn't dispute that charcoal may have a lot of potential," Wardle said, "But I think we have to acknowledge that it doesn't just sit there."
"It will be very important to look at the implications of that for natural black carbon cycles and for the approach of adding biochar to the soil as a means for carbon sequestration," Lehmann said.
"However, I don't see any evidence that would alarm me too much that the approach of putting biochar in soil will increase carbon losses from the soil."
Lehmann notes that the boreal forest environment found in Sweden has a rich layer of organic matter for bacteria to eat while agricultural soils, where char would likely be added, have far less.
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