"This is not good news because it suggests that, as the world warms, soils are going to give back some of their carbon to the atmosphere," said Reich, who is the director of the Institute for Global Change Biology at U-M.
"The big picture story is that losing more carbon is always going to be a bad thing for climate," added Guopeng Liang, the study's lead author. Liang conducted this research as a postdoctoral researcher at the University of Minnesota and is now with Yale University and an exchange fellow at the Institute for Global Change Biology. The study was recently published in 'Nature Geoscience'.
By examining the impact of rising temperatures on the balance of carbon entering and exiting forest soils, researchers aim to better predict future climate changes. Forests currently store about 40% of the Earth's soil carbon.
While numerous studies have explored how climate change influences carbon flux in forest soils, few have spanned more than three years. Most previous research focused either on soil or air temperature independently, but not both, Reich noted.
Reich's experiment, which is considered the first of its kind, managed both soil and above-ground temperatures in open air without any enclosures and was conducted over more than a dozen years.
"Our experiment is unique," said Reich, who is also a professor at the U-M School for Environment and Sustainability. "It's far and away the most realistic experiment like this in the world."
Despite the high costs associated with running such a complex experiment over an extended period, the research was made possible through funding from the National Science Foundation, the U.S. Department of Energy, and the University of Minnesota, where Reich holds the title of Distinguished McKnight University Professor.
Collaborating with Reich and Liang were researchers from the University of Minnesota, the University of Illinois, and the Smithsonian Environmental Research Center. The team conducted the study at two sites in northern Minnesota, working with a total of 72 plots to investigate two different warming scenarios in comparison to ambient conditions.
In one scenario, plots were maintained at 1.7 degrees Celsius above ambient temperature, while in the other, the difference was 3.3 degrees Celsius (roughly 3 and 6 degrees Fahrenheit, respectively). The study found that soil respiration-the process of carbon dioxide release-increased by 7% in the milder warming scenario and by 17% in the more extreme scenario.
The carbon dioxide released during respiration originates from plant roots and soil microbes that metabolize carbon-rich materials like sugars, starches, decaying plant matter, and other organic substances.
"The microbes are a lot like us. Some of what we eat is respired back to the atmosphere," explained Reich. "They use the same exact metabolic process we do to breathe CO2 back out into the air."
Although carbon dioxide release increased in the warmer plots, the rise was not as steep as it could have been. The researchers noted that warmer temperatures led to reduced soil moisture, as water was lost more quickly from plants and soils. Because microbes thrive in moist conditions, drier soils limited the amount of respiration.
"The take-home message here is that forests are going to lose more carbon than we would like," said Reich. "But maybe not as much as they would if this drying wasn't happening."
Research Report:Soil respiration response to decade-long warming modulated by soil moisture in a boreal forest
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