Plant roots may accelerate carbon loss from soils

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.”