- The study looked at the release of carbon in temperate soils down to 100 centimeters (39.4 inches).
- A 4-degree Celsius rise in soil temperature led to an increase of 34-37 percent in the amount of carbon released.
- If all soils to react in the same way, that could unleash an amount of carbon into the atmosphere equivalent to about 30 percent of what human activities emit, according to the authors.
As the planet warms, the soil underfoot could release more carbon than previously thought, a new study has found.
“Previously, I think people thought that the surface is what matters,” said Caitlin Hicks Pries, an ecosystem ecologist Lawrence Berkeley National Laboratory in California. “They didn’t realize that there were these large carbon stores at depth as well.”
For a long time, we humans have been caught up with what’s going on in the top soil – down to about 20 centimeters (7.9 inches) or so – largely because that’s how deep the roots of our food crops usually extend. But that focus hasn’t given us an accurate picture of what happens to the carbon held below that depth when the temperature rises, it seems.
Hicks Pries led a team of researchers who dug deeper – to 100 centimeters (39.4 inches) to be exact – to figure out what we might be missing. Their research, published March 9 in the journal Science, showed that a 4-degree Celsius rise in temperatures would notch up the soil’s release of carbon dioxide by 34-37 percent – almost double what previous surface-level investigations had figured. If other types of soil behave similarly in response to a hotter planet, that could mean a big influx of climate-warming carbon into the atmosphere that “should not be ignored,” the authors write.
A spike of 4 degrees is about what many scientists expect for California by the year 2100, where the study took place, based on a model by the Intergovernmental Panel on Climate Change, Hicks Pries told Mongabay.
“It used to be the worst-case scenario,” she said, “but now it’s business as usual.”
To understand what happens to the carbon in the soils of a temperate forest, Hicks Pries and her colleagues headed east from Berkeley into the foothills of the Sierra Nevada Mountains. There, they setup a series of study plots.
The team used “post-hole drivers and sledge hammers” to pound 22 warming rods nearly 2 1/2 meters (2.7 yards) into the earth. “This is one of the reasons that people don’t do these studies,” she said. “It was a lot of work.”
Inside the heat-conducting rods, as well as at the surface, they placed heating cables – the kind that homeowners in cold climates wrap around pipes to make sure they don’t freeze.
Then, they turned up the heat and measured the release of carbon dioxide – the respiration – in the topsoil and down to a meter (39 inches) compared to areas that didn’t get the warm treatment.
A bump in temperatures kicks the metabolisms of the microbes living in the soil into a higher gear and makes their enzymes more potent. Carbon dioxide is a product of breaking down compounds for energy in many of these microbes, as it is in humans. Just as we huff more carbon dioxide into the air when we run, these tiny fungi and bacteria and protozoa expel more of the gas when they’re more active.
Until now, many scientists have figured that a lot of the carbon down deep was locked up with minerals, out of the reach of microbes and thus not a source of carbon we’d have to worry about much with climate change. But it turns out that 40 percent of the reaction to warming came from soil below the 15-centimeter (5.9-inch) mark, which caught Hicks Pries and her colleagues by surprise.
“We didn’t think that the deep soils would be as responsive to warming as surface soils,” she said.
But she was quick to point out that the carbon in the earth is “only half the story.”
“In order to close the carbon budget at these sites, we would have to know what the plants are doing as well,” she said, noting that plants too might ratchet up their intake of carbon with elevated temperatures in a way that could compensate for this previously unknown in the budget.
A separate study, published March 7 in the journal Scientific Reports, suggests that climate change could provoke soils to release more carbon than energized plants in a warmer climate are likely to take in.
What’s clear is that much of that deep-soil-carbon part of the story has been missing. Hicks Pries is eager to see her team’s research repeated elsewhere in different climates and in different types of soil to see if what they found in California’s temperate forests is typical.
Other studies have hinted that we haven’t been accounting for a lot of the carbon beneath our feet.
“What we’ve been finding is that worldwide over half of soil carbon is stored below 20 centimeters,” Hicks Pries added.
A similar response from other soil types could unleash an amount of carbon into the atmosphere equivalent to about 30 percent of what human activities emit, the authors write.
The takeaway, Hicks Pries said, is that scientists need to do more research. “This is something that we need to look at, because it is potentially a large source of carbon as the climate warms.”
- Pries, C. E. H., Castanha, C., Porras, R., & Torn, M. S. (2017). The whole-soil carbon flux in response to warming. Science, eaal1319.
- Reinsch, S., Koller, E., Sowerby, A., de Dato, G., Estiarte, M., Guidolotti, G., … & Liberati, D. (2017). Shrubland primary production and soil respiration diverge along European climate gradient. Scientific Reports, 7.
- Stocker, T. F., Qin, D., Plattner, G. K., Tignor, M., Allen, S. K., Boschung, J., … & Midgley, P. M. (2014). Climate change 2013: The physical science basis.
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