A sink: Carbon emissions from forest soils will increase with rising temperatures

Soils of boreal forests are important water reservoirs that trees use for photosynthesis without regenerating the carbon dioxide they inhale into the atmosphere.

But led by a personal test Peter Reich The University of Michigan has shown that on a warming planet, more carbon is leaving the soil than being added by plants.

„This is not good news, because as the world warms, soils are going to give some of their carbon back to the atmosphere,” said Reich, director of the Institute for Global Change Biology at UM.

„The big picture story is that losing more carbon is always a bad thing for the climate,” said Guopeng Liang, lead author of the study published in Nature Geoscience. Liang was a postdoctoral researcher at the University of Minnesota at the time of the study and is now a postdoctoral researcher at Yale University and an exchange fellow at the Institute for Global Change Biology.

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

Because of this, there are many research projects studying how climate change affects carbon fluxes from forest soils. But few persist for more than three years, and most see warming either in the soil or in the air above it, but not both, Reich said.

In what is believed to be the first experiment led by Reich, the researchers controlled both soil and underground temperatures in an open field without using any kind of enclosure. They also conducted studies for more than twelve years.

„Our experiment is unique,” said Reich, who is also a professor in the UM School of Environment and Sustainability. „This is the most realistic experiment in the world.”

The trade-off is that running such sophisticated experiments for so long is expensive. The research was supported by the National Science Foundation, the US Department of Energy and the University of Minnesota, where Reich is also a McKnight University Distinguished Professor.

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

The team worked on a total of 72 plots at two sites in northern Minnesota, investigating two different warming scenarios compared to ambient conditions.

In one, the layers were kept 1.7 degrees Celsius above ambient, and in the other, the difference was 3.3 degrees Celsius (or 3 and 6 degrees Fahrenheit, respectively). Soil respiration – the process by which carbon dioxide is released – increased by 7% in the moderate warming case and 17% in the extreme case.

Respired carbon comes from the metabolism of plant roots and soil microbes that feed on carbon-containing nutrients: sugars and starches released from roots, dead and decaying plant parts, soil organic matter, and other living and dead microbes.

„Microbes are like us. Some of what we eat is breathed into the atmosphere,” Reich said. „They use the same exact metabolic process that we do to breathe CO2 back into the air.”

Although the amount of carbon dioxide breathed in at higher temperatures increased, it did not jump as much as it could have, the researchers found.

Their test system also accounts for soil moisture, which decreases in warmer temperatures and causes rapid water loss from plants and soil. However, microbes prefer wet soil and dry soil to respire.

„The take-home message here is that forests are going to lose more carbon than we’d like,” Reich said. „But it’s not like they do if this drying doesn’t happen.”