Nov. 17, 2008 -- As the climate warms, soils may not release as much carbon dioxide as predictions have suggested, according to a new study of Australian soils.
Dirt releases the greenhouse gas carbon dioxide when microbes in the soil digest organic matter from things like decaying plants. As temperatures warm, the microbes eat faster, releasing more CO2. This contribution is a big deal; the amount of CO2 released from soils worldwide each year is at least 10 times greater than the amount spewed by humans.
The new study, by Johannes Lehmann of Cornell University in Ithaca, NY, and colleagues points out that soils can't all be treated equally in models of climate change. Some of the carbon in soils is more readily digested than other carbon.
The team measured the amount of black carbon, tiny bits of charred material that remains after brush fires or forest fires, in soil across the Australian continent. They used information about the measurement sites combined with soil models to determine how fast the black carbon degraded. Black carbon resists degradation by microbes. Indeed, the team found that the black carbon in the sites tested sticks around for an average of at least 1300 to 2600 years.
The team also found that black carbon represented anywhere from 0 to 82 percent of the carbon in soil samples, depending on the site. They published their results in Nature Geoscience.
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Because black carbon degrades so slowly, samples with a higher percentage of black carbon will release less CO2 as the temperature warms than climate models predict, because climate models assume soil is more easily degraded than black carbon.
By including the contribution from black carbon, the team calculated that the CO2 emissions from soils will be about 20 percent less in two savannah regions of Australia than climate models predict over the next 100 years. That's approximately equivalent to Australia's CO2 emissions from aviation, Lehmann told Discovery News.
"A lot has been said about how much CO2 is in the atmosphere, and how much there might be in the oceans," Lehmann said. "Many people don't appreciate how much carbon is in soils. There is much more carbon in soils than there are trees and vegetation on Earth. There is more than double the amount of carbon in soil than there is CO2 in the atmosphere. A little more CO2 coming out of the soils makes a huge difference for the amount of CO2 in the atmosphere."
Because sampling of black carbon worldwide is so sparse, it is difficult to know how much of an effect black carbon could have on emissions estimates worldwide.
"[One] question is whether Australia is unique in its combination of old soils and importance of fire in the landscape, or whether stores of black carbon are in general greater than they have been thought," said Susan Trumbore of the University of California, Irvine. "As the authors point out, there are not enough data to say at this point."
Many parts of the world have landscapes that burn regularly, Lehmann noted. Such areas would be expected to have relatively higher levels of black carbon. But more measurements are necessary, he added. Black carbon has been sampled in only a few locations worldwide, and the amounts can vary widely.
Many scientists have expressed concern that as polar regions warm, the thawing soils may release large amounts of CO2 or methane. So, what is the potential for polar estimates to change based on Lehmann's findings?
The boreal forest of the far north burns often, according to soil scientist Jennifer Harden of the U.S. Geological Survey in Menlo Park, Calif., but black carbon does not appear to be as prevalent as in Australia. "Our studies have shown only small amounts of truly recalcitrant, stubborn carbon exist. We think it's because most of the protection from decomposition is because it's so cold."
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