Quartz-Gobbling Worms Are Weathering Earth’s Soils – GWC Mag

Worms are in the business of breaking down dead things and freeing nutrients as they wiggle their way through the soil. As they chomp, they also weather minerals in a big way. New research has shown that worms may be contributing more to Earth’s weathering cycle than previously thought.

Earthworms’ short-term effects on plant growth and soil’s organic nutrient storage capability are well understood. Ask any gardener. But how worms affect mineral weathering and resulting nutrient availability over longer time periods hasn’t been studied as much.

“We’re trying to bring worms into the geology world, and taking a more long-term view,” said Adrian Wackett, the lead researcher on the project and a doctoral student in geological sciences at Stanford University. “We’re asking: How do worms break down the primary minerals that formed deep in Earth’s crust?” he said.

Mineral Munchers

Earth’s weathering cycle moves minerals formed in Earth’s crust up into soil, where animals and plants can use them. Weathering is driven mainly by abiotic processes such as the movement of water and, especially, changes in temperature. But plants and animals also contribute.

“Biology accelerates and plays a key role in Earth’s weathering cycle.”

Earthworms, like birds, have gizzards—organs in their digestive tract that grind up food with the aid of particles of rock or grit. Worms ingest mineral grains to keep in their gizzards, which helps them break down the rest of the soil to release nutrients and other minerals.

To understand how much earthworms contribute to mineral weathering, Wackett and his colleagues analyzed quartz-laden soils in Puerto Rico’s El Yunque National Forest, where some soils host hundreds of earthworms per square meter.

They used measurements of beryllium isotopes, which form when cosmic radiation bombards mineral grains in soil, to determine how long the quartz had spent at different soil depths. They then compared the differences in the sizes of quartz grains at those depths to determine the weathering rate in each soil layer.

In soil depths where earthworm burrowing occurred, the team found that quartz grain sizes had been reduced by nearly half over 10,000–20,000 years. Grain sizes in soil depths without earthworm activity had not been reduced.

“Once worms enter the picture, you see this rapid shrinking [of grains],” Wackett said. “Biology accelerates and plays a key role in Earth’s weathering cycle.” Quartz grains in worms’ gizzards grind other soil components, which could break down the quartz grains themselves. Microbes in the worms’ guts could also help chemically dissolve quartz grains, according to the researchers.

The team estimated that worms may cause about 2% of the silicate weathering in El Yunque soils. That’s a “very substantial” contribution, Wackett said. He and his colleagues will present these results today at AGU’s Annual Meeting 2023 in San Francisco.

Global Worming Underway

Worms can increase soil weathering rates wherever they go, Wackett said. That may soon include much more of Earth’s soils: As a result of warmer temperatures, earthworms are expanding their territories to higher altitudes and latitudes.

Human activities have quickened the spread of earthworms, too, as fishers and gardeners release earthworms into previously worm-free soils. But worm communities move only a few meters per year, meaning the worldwide worm takeover could take a while. 

To support their findings in El Yunque, Wackett and his research team also analyzed changes in mineral grain sizes across areas in Alaska, Minnesota, Finland, and Sweden. Earthworms have lived in El Yunque for millions of years, but were brought to the other sites by humans only about 100–150 years ago.

The team found that after earthworms arrived, there were significant shifts in median mineral grain sizes. 

Warmer temperatures aren’t just allowing worms to move around—they’re also spurring scientists to think about climate solutions, including those involving mineral weathering. Some weathering reactions consume carbon dioxide, helping to throttle global temperatures. “If earthworms are indeed true engines of weathering, we could potentially harness this effect” to boost carbon dioxide drawdown, Wackett said.

Earthworm Enigma

The new research suggested that worms are breaking down the quartz in El Yunque soils, but that may not necessarily be the case, said Mark Hodson, a mineralogist who studies earthworm ecology at the University of York and was not involved in the new research. It’s possible that the smaller grain sizes are due to bioturbation—worms preferentially ingesting smaller quartz grains at depth and bringing them to the surface, he said.

“Earthworms can be a small but significant part of the weathering engine that gives rise to soils.”

If the worms are instead breaking down the quartz, as the researchers suggested, the 2% figure “shows that earthworms can be a small but significant part of the weathering engine that gives rise to soils,” Hodson said.

The researchers plan to distinguish between true worm weathering and bioturbation by searching for unique markings on quartz grains in soils where earthworms live. The worms’ gizzards could be fracturing quartz in such a way that creates new forms of quartz grains that wouldn’t be created by bioturbation. 

The team also plans to use another form of beryllium isotope dating to gather similar data that would determine whether the surface of the quartz grains had been affected by worms. Finding those worm “fingerprints” would strengthen the evidence for worms’ weathering abilities, Wackett said. 

—Grace van Deelen (@GVD__), Staff Writer

Citation: Van Deelen, G. (2023), Quartz-gobbling worms are weathering Earth’s soils, Eos, 104, https://doi.org/10.1029/2023EO230426. Published on 14 December 2023.

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