Carbon emissions from forest soils likely to increase with rising temperatures

Northern forest soils are critical reservoirs that keep the carbon dioxide that trees inhale and use for photosynthesis from returning to the atmosphere. But a unique experiment led by Peter Reich of the University of Michigan shows that on a warming planet, more carbon is being lost from the soil than is being added by vegetation.

“This is not good news because it suggests that as the planet warms, soils will return some of their carbon to the atmosphere,” said Reich, director of the Institute for Global Change Biology at UM.

“The general finding is that losing more carbon will always be a bad thing for the climate,” said Guopeng Liang, lead author of the study published in Geosciences of natureLiang was a postdoctoral researcher at the University of Minnesota during the study and is now a postdoctoral researcher at Yale University and an exchange scholar at the Institute for Global Change Biology.

By understanding how rising temperatures affect the flow of carbon in and out of soils, scientists can better understand and predict changes in our planet’s climate. Forests, meanwhile, store about 40 percent of Earth’s soil carbon.

That’s why there have been many research projects to study how climate change affects carbon flux in forest soils. But few have lasted more than three years, and most have looked at warming either the soil or the air above it, but not both, Reich said.

In this experiment, considered the first of its kind, conducted by Reich, the researchers controlled both the temperature of the ground and that of the surface in the open air, without using any type of enclosure. They also continued the study for more than a dozen years.

“Our experiment is unique,” ​​said Reich, who is also a professor at the University of Michigan’s School of Environment and Sustainability. “It’s by far the most realistic experiment of its kind in the world.”

The price to pay for running such a sophisticated experiment for so long is high. The research was funded by the National Science Foundation, the U.S. Department of Energy and the University of Minnesota, where Reich is also a professor emeritus at McKnight University.

Reich and Liang were joined in this study by colleagues from the University of Minnesota, the University of Illinois and the Smithsonian Environmental Research Center.

The team worked at two sites in northern Minnesota, with a total of 72 plots, studying two different warming scenarios relative to ambient conditions.

In one plot, the temperature was kept 1.7°C above ambient temperature, and in the other, the difference was 3.3°C (about 3 and 6°F, respectively). Soil respiration, the process that releases carbon dioxide, increased by 7% in the most modest warming case and by 17% in the most extreme case.

Respired carbon comes from the metabolism of plant roots and soil microbes that feed on carbon-containing snacks available to them: sugars and starches leached from roots, dead and decaying plant parts, soil organic matter, and other living and dead microorganisms.

“Microbes are very similar to us. Some of what we eat is breathed into the atmosphere,” Reich says. “They use the exact same metabolic process that we do to breathe in CO₂ “Get back in the air.”

Although the amount of carbon dioxide respired increased in plots at higher temperatures, it probably didn’t increase as much as it could have, the researchers found.

Their experimental design also took into account soil moisture, which decreases at higher temperatures, leading to faster water loss from plants and soils. Microbes, however, prefer wetter soils, and drier soils limit respiration.

“The takeaway message is that forests are going to lose more carbon than we would like,” Reich said. “But maybe not as much as they would if this drying didn’t happen.”