Mountain soils may hold more carbon than models suggest

Mountain terrain may be a larger soil carbon store than scientists have accounted for, according to a University of Oregon-led study published in Science Advances. The finding could affect how researchers identify places where natural climate strategies might protect or increase carbon held in the ground.

The study examined old landslide deposits in the Oregon Coast Range and found that deep, weathered soils in those areas can contain large amounts of organic carbon. The University of Oregon said the work challenges the assumption that steep, eroding landscapes hold relatively little carbon because their soils are thin or short-lived.

Soils store carbon that otherwise could remain in the atmosphere as carbon dioxide, but storage varies with depth, texture and mineral makeup, the university said. Brooke Hunter, who led the research while a doctoral student in University of Oregon earth scientist Josh Roering’s lab and is now an assistant professor at Appalachian State University, said soil carbon must be mapped more accurately to understand carbon budgets.

“When we think about terrestrial carbon, soil contains more carbon than vegetation and the atmosphere combined,” Hunter said, according to the University of Oregon. “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.”

Old landslides offered a test case

The researchers studied nearly 10,000 landslide remnants in the Oregon Coast Range, with ages ranging from 4 years to 480,000 years, according to the university. Those landslides had become stable deposits where soil and organic matter accumulated over time.

The team drilled into a representative group of six landslides to measure carbon density, then used those measurements to build an age-based model for the broader set of landslides. From there, the researchers estimated carbon storage across the study area, the university said.

Earlier soil-carbon models often used a 30-centimeter soil depth assumption, according to the University of Oregon. The new work found landslide deposits frequently held soil more than 5 meters deep, or about 16 feet.

Those thicker deposits also held more carbon than thinner soils, the researchers found. The university said fine-grained soil created through long weathering provides more surface area that can bind carbon.

Roering said the findings reverse a common view of steep terrain. “There was a misconception that mountainous areas would not hold much carbon because they're so rapidly eroding and there's not much soil,” he said, according to the university. “What we're saying is, it's actually the opposite.”

Models may need better terrain data

The researchers found that soil carbon stocks in deep-seated landslides were about twice as high as estimates from a prior global model, according to the University of Oregon. Roering said the result points to the value of better geomorphic maps, which describe landforms and how they change over time.

Mountainous areas have received less study partly because measuring soil depth and composition on steep ground is difficult, the university said. Much previous work focused on flatter agricultural regions, where erosion and deposition patterns are easier to predict.

The study could inform natural climate approaches, according to the University of Oregon. Roering cited efforts such as adding minerals to landscapes to speed rock weathering or adding nutrients to soils to improve carbon uptake.

Hunter said site-specific models can help determine which soil management approaches may work in particular places. Roering said land managers seeking to protect carbon stores would need to know where high-carbon soils are located before choosing practices, such as preserving tree cover to limit erosion.

This story draws on original reporting from Phys.org.