Soil, long thought to be a semi-permanent storehouse for ancient carbon, may be releasing carbon dioxide to the atmosphere faster than anyone thought, according to Oregon State University soil scientists.
In a study published in this week's online edition of the journal Nature Climate Change, the researchers showed that chemicals emitted by plant roots act on carbon that is bonded to minerals in the soil, breaking the bonds and exposing previously protected carbon to decomposition by microbes.
The carbon then passes into the atmosphere as carbon dioxide (CO2), said the study's coauthor, Markus Kleber, a soil scientist in OSU's College of Agricultural Sciences.
He said the study challenges the prevailing view that carbon bonded to minerals stays in the soil for thousands of years. "As these root compounds separate the carbon from its protective mineral phase," he said, "we may see a greater release of carbon from its storage sites in the soil."
It's likely that a warming climate is speeding this process up, he said. As warmer weather and more carbon dioxide in the air stimulate plants to grow, they produce more root compounds. This will likely release more stored carbon, which will enter the atmosphere as CO2—which could in turn accelerate the rate of climate warming.
"Our main concern is that this is an important mechanism, and we are not presently considering it in global models of carbon cycling," Kleber said.
CO2 is a major driver of the current warming of Earth's atmosphere. By failing to account for accelerated soil-carbon decomposition, the study suggests, current climate-change models may be underestimating carbon loss from soil by as much as 1 percent per year.
"There is more carbon stored in the soil, on a global scale, than in vegetation or even in the atmosphere," said Kleber. "Since this reservoir is so large, even small changes will have serious effects on carbon concentrations in the atmosphere, and by extension on climate."
One percent may not sound like much, he added. "But think of it this way: If you have money in the bank and you lose 1 percent per year, you would be down to two thirds of your starting capital after only 50 years."
Between 60 and 80 percent of organic matter entering the soil gets broken down within the first year in a chain of decomposition that ends with CO2, Kleber said. Most of the remaining carbon gets bound to the soil's minerals through a variety of physical and chemical mechanisms. When this happens, the carbon is protected because the microbes can't get at it to break it down.
For the past couple of decades, scientists have assumed that these carbon-mineral bonds amounted to a long-lasting "sink" for soil carbon—keeping it out of the atmosphere by storing it in a stable form over many centuries.
"But from the beginning, there was a question that made a lot of folks uneasy," said Kleber. "If carbon keeps going into the soil and staying there, then why aren't we drowning in carbon? Isn't there some process that takes it back into the cycle? That part was not very well researched, and it was what we were trying to find."
The researchers tested three model compounds for common "root exudates"—chemicals commonly excreted by plant roots—to see how strongly each one stimulated the microbes that drive organic-matter decomposition.
In the laboratory, using a syringe and pump, they applied oxalic acid, acetic acid and glucose to soil taken from a dry-climate agricultural area and a wet-climate forest, both in Oregon. They conducted the experiment over 35 days to simulate a flush of root growth in the spring.
Prevailing theory, said Kleber, would predict that the hungry microbes would respond most strongly to the nutritious glucose, which would give them the energy to tackle the rest of the organic matter, including the carbon.
"And this is likely happening to a certain extent," he said. "But our big surprise was that the energy-poor oxalic acid generated a much stronger response from the microbes than the energy-rich glucose."
When they analyzed the water stored in the oxalic acid-treated soil, the researchers saw there was eight times more dissolved carbon in it than there had been before. Additional laboratory tests confirmed the finding that the acids were breaking the carbon-mineral bonds.
"The significance of this research," Kleber said, "is that we have documented for the first time a mechanism by which long-stored soil carbon is cycled back into the system."
Oxalic acid is a good stand-in for a whole suite of root compounds that are excreted by plants in the root zone, Kleber said. "Roots excrete several compounds similar to oxalic acid. We can assume that many root exudates act in a similar way."
Explore further: Soil's large carbon stores could be freed by increased CO2, plant growth
Mineral protection of soil carbon counteracted by root exudates, www.nature.com/doifinder/10.1038/nclimate2580