The Dirt on Allegheny College’s DIRT Experiment

Walking through Bousson Experimental Forest reveals towering trees, small streams, occasional glimpses of white-tail deer, and frequent cries of songbirds. Yet we usually give little thought to what lies underfoot.

To most, it’s just dirt. But forest soils are critically important in climate change investigations. Forests absorb enormous amounts of the greenhouse gas carbon dioxide, storing much as carbon in trees, but most in soil, which contains 80 percent of the total carbon in a forest.

Allegheny College has been involved in a three-decade study at Bousson to understand controls on soil carbon. Understanding those controls is needed to wisely manage forests to help reduce atmospheric carbon dioxide.

student working with soil samples in a labThe Detrital Input and Removal Treatment (DIRT) Project assesses the role of plant detritus (dead leaves and roots) on carbon accumulation in forest soils by annually adding or excluding leaves, and excluding tree roots, to long-term plots. The experiment, begun in 1991, was among the first sites established in the U.S.; additional sites have since been established across the U.S., Europe, and Asia.

Students and faculty in the Department of Environmental Science and Sustainability (ESS) have been sampling these soils, with a major sampling effort conducted recently at the 30-year anniversary of the Bousson experiment. This effort also involves forest scientists at Oregon State University and the University of Toronto. Katie Brozell ’23, an ESS major and biology minor, has spent many hours at Bousson and in the lab collecting and processing these recently collected soils. “This work has exposed me to the real-world applications of research while providing me with valuable experiences both in the field and the lab,” she said.

The project has shown that soil carbon is controlled by tree species and soil characteristics. In some forests, like Bousson, leaves are more important than roots; in others, it is the reverse. This demonstrates that management efforts need to be tailored to the local ecosystem. A surprising finding was that increased organic matter inputs to soil did not increase soil carbon because soil bacteria and fungi increased, rapidly decomposing these increased inputs. According to Eric Pallant, Christine Scott Nelson ’73 Endowed Professor of ESS and teacher of the popular Soil-to-Plate course, “soil organic matter from leaves and roots are critically important in controlling the productivity of forest ecosystems, and soil bacteria and fungi are crucial in controlling carbon in that organic matter.”

The study also found that soils differ in their ability to retain carbon. Excluding leaves and roots dramatically reduced soil carbon at Bousson, but not at a DIRT site in an old-growth forest in Oregon, where soil minerals helped to retain carbon. Thus, management activities or environmental changes that mistreat soil will threaten carbon storage.

This project underscores the importance of collaboration at multiple sites, especially for processes that require long-term study and different kinds of expertise. According to Beth Choate, ESS associate professor and chair and frequent co-teacher of the Soil-to-Plate course, “Long-term collaborative efforts such as these show our students the value of cooperation among scientists, and gives them important research experience that enhances classroom learning.”

For additional information, contact