Vast quantities of carbon are locked up in Earth’s soils. But as temperatures rise, soils are generally less able to hold on to their carbon, previous research has shown. And now researchers have found that soil texture also plays an important role in how readily carbon is sequestered underground: Coarse soils are substantially more vulnerable to releasing their carbon than previously believed. That’s bad news, the team suggests, because coarse soils are preferentially found in high-latitude regions like the Arctic, which are warming much faster than the rest of the world.
Not the Forests or the Atmosphere
Many people think of the world’s forests or atmosphere as containing the largest planetary stockpile of carbon, said Iain P. Hartley, an ecologist at the University of Exeter in the United Kingdom. But humble soil actually holds that honor. “It’s the biosphere’s most important carbon store,” said Hartley. “More carbon is stored in soils than [in] all the plant biomass and the atmosphere put together.”
Several trillion metric tons of carbon are sequestered in the soil, mostly in the form of organic matter derived from plants and animals. But it’s not locked up there forever: Soil is also home to microbes, which readily break down organic compounds and release their carbon into the atmosphere. And that process is apt to change the aboveground world that we inhabit, said Hartley. “Anything that changes in the amount of carbon stored in soils has the potential to impact atmospheric chemistry and therefore our climate.”
Previous research has shown that soils in warmer climates more readily release their carbon. That’s because carbon-containing organic compounds generally break down faster in warmer conditions, Hartley told Eos. “Microbial activity and decomposition rates are faster at higher temperatures.”
A Question of Texture
But an open question in soil science is whether soil texture—its fineness or coarseness—also affects the likelihood of carbon being transferred out of underground storage, said Hartley. In 2019, he and his colleagues set out to answer that question.
The researchers amassed data from more than 9,300 soil samples collected from every continent except Antarctica. The digital records, obtained from the World Soil Information database, included such measurements as the soil’s pH, its carbon content, its ability to hold on to certain ions, and its percentage of clay minerals. Hartley and his colleagues focused their analysis on so-called mineral soils, which consist of a combination of organic matter and minerals. (They specifically excluded peat soils, which tend to be extremely carbon rich but cover only a relatively small fraction of the planet.)
The mineral soils the team analyzed ranged from fine-textured, clay-rich soils to coarse soils poor in clay. Hartley and his collaborators found that carbon storage in fine-textured soils declined less with temperature than previously estimated. That was a surprise to the team. Fine-textured soils tend to consist of tiny agglomerations of organic matter and minerals in which carbon is essentially shielded from microbes, said Hartley, but such clusters ought to be vulnerable to disassociation at high temperatures. One would therefore expect that fine-textured soils would readily give up their carbon as temperatures rise, he said. “We saw the opposite to what we might have expected.”
The team also found that carbon storage in coarse-textured soils declined more rapidly with temperature than previously estimated. Together, these findings highlight significant differences between fine- and coarse-textured soils, Hartley said. “The effect of temperature on carbon storage in coarse-textured soils was more than 3 times greater than in fine-textured soils.”
Pinpointing Vulnerable Soils
These results, which were published in November in Nature Communications, are valuable because they highlight the types of soils most likely to relinquish their carbon to the atmosphere. “We’ve started to help identify which soil organic matter stocks may be the most vulnerable,” said Hartley. But the measurements don’t reveal a key parameter of interest, the researchers acknowledged. “We can’t take these data and work out how rapidly soils may lose carbon as the world warms,” said Hartley.
And that’s an important quantity to determine, said Keith Paustian, a soil scientist at Colorado State University in Fort Collins who was not involved in the research. “There’s a tremendous amount of carbon in soils. You could have really large emissions from ecosystem carbon.”
—Katherine Kornei (@KatherineKornei), Science Writer