Carbon feedback from forest soils to accelerate global warming

After 26 years, the world's longest-running experiment to discover how warming temperatures affect forest soils has revealed a surprising, cyclical response: 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. Overall, the results indicate that in a warming world, a self-reinforcing and perhaps uncontrollable carbon feedback will occur between forest soils and the climate system, adding to the build-up of atmospheric carbon dioxide caused by burning fossil fuels and accelerating global warming.

Melillo and colleagues began this pioneering experiment in 1991 in a deciduous forest stand at the Harvard Forest in Massachusetts. They buried electrical cables in a set of plots and heated the soil 5° C above the ambient temperature of control plots. Over the course of the 26-year experiment (which still continues), 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 says, "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. If a significant amount of that soil carbon is added to the atmosphere, due to microbial activity in warmer soils, that will accelerate the global warming process. And once this self-reinforcing feedback begins, there is no easy way to turn it off. There is no switch to flip."

Over the course of the experiment, Melillo's team observed fluctuations in the rate of soil carbon emission from the heated plots, indicating cycles in the capacity of soil microbes to degrade organic matter and release carbon. Phase I (1991 to 2000) was a period of substantial soil carbon loss that was rapid at first, then slowed to near zero. In Phase II (2001-2007), there was no difference in carbon emissions between the warmed and the control plots. During that time, the soil microbial community in the warmed plots was undergoing reorganization that led to changes in the community's structure and function. In Phase III (2008-2013), carbon release from heated plots again exceeded that from control plots. This coincided with a continued shift in the soil microbial community. Microbes that can degrade more recalcitrant soil organic matter, such as lignin, became more dominant, as shown by genomic and extracellular enzyme analyses. In Phase IV (2014 to current), carbon emissions from the heated plots have again dropped, suggesting that another reorganization of the soil microbial community could be underway. If the cyclical pattern continues, Phase IV will eventually transition to another phase of higher carbon loss from the heated plots.