by Kathleen Scalise
As global warming heats the Earth's surface, soils worldwide will release carbon dioxide into the air and intensify environmental problems, report researchers at Berkeley, UC Irvine and UC Santa Barbara in the April 19 issue of Science.
If the Earth's temperature rises even half a degree, forest soils alone will release as much carbon dioxide as 25 percent of that emitted annually from burning fossil fuels in cars, factories and the like, according to the paper. Carbon dioxide is a colorless gas which helps trap heat in the atmosphere.
"The amount of carbon that would be shed into the atmosphere from soils would be very significant," said Ronald Amundson, associate professor of ecosystem sciences here. His collaborators were Susan Trumbore of UC Irvine and Oliver Chadwick of UC Santa Barbara.
"Soils add carbon to the air as microbes decompose the humus they contain. This brown stuff in soil (is) a very dynamic part of the global carbon cycle," said Amundson.
How carbon cycles from soils into the atmosphere and back is important because of the so-called greenhouse effect, in which light from the sun bounces off the Earth as infrared radiation and is trapped by carbon dioxide molecules and atmospheric water vapor. Without this interaction, the Earth would be colder than 50 degrees below zero.
But too much of a good thing could be a problem. Because carbon dioxide molecules are spewed into the air as pollutants and blanket the Earth, the result could be global climate change with temperatures climbing as much as 5 degrees Celsius by the middle of next century.
Previously there was not enough data to factor soils into global warming predictions, though they were considered potentially important.
"There's a constant cycle of carbon dioxide from photosynthesis into plants that die and then decompose into the soil," said Amundson. "The carbon is finally released from the soil back into the atmosphere as carbon dioxide by microbes."
Heating of soil through global warming will stimulate the microbes and force the soil to give up more carbon into the air, researchers found. Also, the warmer the site is, the faster biological processes go on and the faster the carbon cycle works.
Oddly, the research depended on an unanticipated result of nuclear testing. From above-ground nuclear testing in the '50s and '60s, carbon-14 in atmospheric carbon dioxide shot up to nearly double what it was and has been slowly dropping toward pre-testing levels. This in effect pulsed soils worldwide with a radioactive tracer for carbon.
Carbon-14 provides a chemical signature to see how fast carbon is cycled in soils, said Amundson. To compare carbon-14 in soils before the pulse and after, the scientists needed old soil samples. Berkeley held the treasure: 15,000 jars of dirt collected to map California soil types sitting in a warehouse for 40 years.
"Every once in a while the department had to make a decision whether it should throw them out," Amundson said. Keeping them paid off. The dirt is one of two such extensive and well-documented soil collections in the U.S.