Hilly and mountainous landscapes have a much greater ability to store carbon in the soil than previously thought, according to a new study co-led by scientists at the University of Oregon.
The finding, published June 12 in Science Advances, could help scientists better target natural approaches to addressing atmospheric carbon dioxide and other greenhouse gases that contribute to climate change.
Soil can hold onto carbon that might otherwise exist as atmospheric carbon dioxide, but the amount varies based on the soil's thickness, texture and mineral composition.
"There was a misconception that mountainous areas would not hold much carbon because they're so rapidly eroding and there's not much soil," said Josh Roering , an earth scientist at the UO. "What we're saying is, it's actually the opposite. These areas can be impressive reservoirs of soil organic carbon."
The research was led by Brooke Hunter during her time as a doctoral student in Roering's lab. Hunter is now an assistant professor at Appalachian State University.
"When we think about terrestrial carbon, soil contains more carbon than vegetation and the atmosphere combined," Hunter said. "In order to have an accurate understanding of carbon budgets, we need to know how much carbon is in the soil and where it's most concentrated."
Geomorphology, Roering's specialty, is the study of landforms and the processes that shape them, including the weathering of rock into soil and the erosion of soil from the landscape.
"We have our own form of bookkeeping to determine the rate at which rock is converted into soil and how long the soil hangs out before it is transported into rivers and beyond," Roering said.
Mountainous areas have been understudied because it's difficult to traverse the landscape and accurately measure the depth and composition of the soil.
That's made it harder to protect areas that are good carbon reservoirs by, for example, maintaining tree cover to prevent erosion.
In addition, "there is a lot of movement these days on natural climate solutions for greenhouse gases," Roering said. Examples include sprinkling minerals on the landscape to enhance rock weathering or seeding soils with nutrients so they better sequester carbon dioxide from the atmosphere.
Better information about the amount of carbon already in the landscape can help scientists more accurately determine how much carbon might be stored through these efforts.
The researchers studied the remnants of nearly 10,000 landslides in the Oregon Coast Range, ranging in age from 4 to 480,000 years, that have become stable repositories of soil and organic material. They augured holes into a representative half-dozen landslides to measure the density of carbon. From that data they created a timeline for all of the landslides and extrapolated a model to estimate carbon stored in landslides across the entire study area.
Their results show that scientists have dramatically underestimated the amount of organic carbon in the soil of landslides. While past models for carbon storage in soil have typically assumed soil depths of 30 centimeters, the researchers found that landslides often contain soil deposits more than 5 meters, or 16 feet, deep.
Thicker soils, they found, also contain higher carbon stocks than thinner soils. This is due to large amounts of fine-grained soils that provide more surface area to fix carbon due to thousands of years of weathering.
"These deep weathering zones are really good at holding carbon," Roering said. "The older they are, the more weathered they are and the thicker they are, and the more carbon they can store."
The researchers determined that stocks of carbon in the soil in deep-seated landslides are about twice as large as predicted by a previous global model. "Our research really emphasizes the importance of making better geomorphic maps and integrating our field with models making those predictions," Roering said.
Much of the past work focused on flat agricultural regions where deposit and erosion of soil is more predictable. By using models that take into account the shape of the landscape and how it has changed over time, scientists can more accurately determine soil depth and carbon density in mountainous regions, potentially opening up new approaches to natural climate solutions.
"When it comes to soil management and natural climate solutions, there isn't one miracle fix," Hunter said. "Incorporating these models can help determine what specific methods might be effective at specific sites."
Added Roering: "If you are going to manage the landscape for carbon, you would want to know where the areas with high amounts of carbon are and prioritize management practices that preserve them."
