Ann Arbor Times

The soils of northern forests serve as significant reservoirs, preventing carbon dioxide absorbed by trees during photosynthesis from re-entering the atmosphere. However, a study led by Peter Reich of the University of Michigan suggests that rising global temperatures may cause more carbon to escape from these soils than is being sequestered by plants.

“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, 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, lead author of the study published in Nature Geoscience. Liang conducted this research as a postdoctoral researcher at the University of Minnesota and currently holds positions at Yale University and the Institute for Global Change Biology.

Understanding how rising temperatures impact soil carbon dynamics is crucial for predicting climate changes. Forests store approximately 40% of Earth's soil carbon, making them vital in this context. Numerous studies have examined how climate change affects forest soil carbon fluxes; however, most are short-term and do not simultaneously consider both soil and air temperature changes.

Reich's experiment, considered pioneering in its field, controlled both soil and above-ground temperatures in an open-air setting without enclosures over more than a dozen years. “Our experiment is unique,” noted Reich. “It’s far and away the most realistic experiment like this in the world.”

This long-term project was financially supported by the National Science Foundation, U.S. Department of Energy, and University of Minnesota where Reich also serves as Distinguished McKnight University Professor.

The research team included colleagues from multiple institutions: University of Minnesota, University of Illinois, and Smithsonian Environmental Research Center. They worked on 72 plots across two sites in northern Minnesota to explore two warming scenarios compared with ambient conditions: one set at 1.7 degrees Celsius above ambient temperature and another at 3.3 degrees Celsius (approximately 3 and 6 degrees Fahrenheit).

Soil respiration—releasing carbon dioxide—increased by 7% under modest warming conditions and by 17% under more extreme warming conditions. This respired carbon originates from plant roots' metabolism and soil microbes feeding on various organic materials.

“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.”

Despite higher levels of respired carbon dioxide at increased temperatures, researchers found that decreased soil moisture due to faster water loss limited microbial activity since microbes prefer wetter conditions.

“The take-home message here is that forests are going to lose more carbon than we would like,” concluded Reich. “But maybe not as much as they would if this drying wasn’t happening.”