An experiment conducted by the University of Chicago (UChicago) has found that soil warming stimulates periods of abundant carbon release from the soil to the atmosphere, alternating with periods of no detectable loss in soil carbon stores.

In short, in a warming world, a self-reinforcing and perhaps uncontrollable carbon feedback will occur between forest soils and the climate system, adding to the buildup of atmospheric carbon dioxide caused by burning fossil fuels and accelerating global warming.

Jerry Melillo, a distinguished scientist from the Marine Biological Laboratory at UChicago, and his colleagues began this pioneering experiment in 1991 in a deciduous forest stand at Harvard Forest in US state of Massachusetts. 

They buried electrical cables in a set of plots and heated the soil five degrees Celsius above the ambient temperature of control plots. Over the course of the 26-year experiment, which is still ongoing, the warmed plots lost 17 percent of the carbon that had been stored in organic matter in the top 60 centimeters of soil.

"To put this in context," Melillo said, "each year, mostly from fossil fuel burning, we are releasing about 10 billion metric tons of carbon into the atmosphere. That's what's causing the increase in atmospheric carbon dioxide concentration and global warming."

The world's soils contain about 3,500 billion metric tons of carbon. Due to microbial activity in warmer soils, a significant amount of that soil carbon may be added to the atmosphere. If that happens, according to Melillo, it will accelerate the global warming process. 

Over the course of the experiment, Melillo's team observed fluctuations in the rate of soil carbon emission from the heated plots.

Phase I, from 1991 to 2000, was a period of substantial soil carbon loss that was rapid at first, and then slowed to near zero. 

In phase II, from 2001 to 2007, there was no difference in carbon emissions between the warmed and the control plots and during that time, the soil microbial community in the warmed plots was undergoing the reorganization that led to changes in the community's structure and function.

In the third phase, from 2008 to 2013, carbon release from heated plots exceeded that from control plots again. 

During the last phase, from 2014 to current, carbon emissions from the heated plots have again dropped, suggesting that another reorganization of the soil microbial community could be underway.

"The future is a warmer future. How much warmer is the issue," Melillo said. "In terms of carbon emissions from fossil fuels, we could control that. We could shut down coal-fired power plants, for example." 

"But if the microbes in all landscapes respond to warming in the same way as we've observed in mid-latitude forest soils, this self-reinforcing feedback phenomenon will go on for a while and we are not going to be able to turn those microbes off. Of special concern is the big pool of easily decomposed carbon that is frozen in Arctic soils. As those soils thaw out, this feedback phenomenon would be an important component of the climate system, with climate change feeding itself in a warming world," Melillo added.

The study appeared in the Oct. 6 issue of Science.

(Top photo credit: University of Chicago.)