
A rare, three-decade long study of forest soil, held at the Bousson Environmental Research Reserve was recently highlighted by Soil Society of America, a leading publication in the field.
The investigation, better known as Detritus Input and Removal Treatments (DIRT), was led by Professor of Environmental Science and Sustainability (ESS), Rich Bowden, to analyze the lasting function of tree leaves and roots within soil’s organic matter.
Soil organic matter is critical to soil health, providing a source of nutrients, water storage, and energy for soil organisms, and helps to prevent soil erosion. Forest soil organic matter also contains enormous stores of climate-polluting carbon, making forest soils critically important in keeping carbon out of the atmosphere where it would otherwise contribute to climate change.
Initial findings twenty years ago in this study suggested that leaves were the most important organic matter sources, and that roots were only small contributors. However, after thirty years, it is evident that roots are also very important sources in the long term and those results came from an unexpected glitch in the experiment.
According to Bowden, “We had plots where we severed the tree roots and put in barriers to exclude them, enabling us to see what would happen if there were no roots. Initially, we saw little response in soil organic matter, making us think that tree roots were of limited importance in these forest soils.”
After thirty years, roots had unexpectedly infiltrated the supposedly root-less plots, and organic matter also increased dramatically. “We were surprised,” said Bowden.
Clearly we needed to wait longer to know the big picture of how Mother Nature operates in these soils,” added Bowden. “Now we know that roots also have an important role in contributing to this important resource in soils.
The study also found that soil organic matter is very sensitive to change in inputs, suggesting that human impacts such as unsustainable logging techniques, fire, or climate change that alter forest productivity will influence the amount of organic matter in forest soils.
According to Bowden, “in plots where we excluded leaves, soil organic matter declined precipitously, but in plots where leaf additions were doubled experimentally, soil organic matter showed only small gains.”
According to Dr. Kevin Geyer, a soil scientist in the Allegheny ESS Department who has conducted a great deal of research on long-term soil experiments, “these results tell us that how we manage forests and what we do in them really matters to soil organic matter, which is very slow to accumulate, but easily lost if mismanaged.
Another important outcome of the work is that scores of Allegheny students have gained invaluable research experience as they helped to maintain the experiment and collect samples over the years.
Current student Jackie DiGiacomo ’26, a coauthor on the recent publication, said that it “has been so rewarding to be able to contribute to long-term research that has been touched by so many professionals and students. The lifelong skills that DIRT has given me and seeing our work incorporated into meaningful research is not an opportunity that all students have and something for which I am very grateful.”
Similarly, alumni Alexandrea Rice ’17, currently a Ph.D. candidate at the University of Virginia, where she is studying forest soils, said that her years-long involvement as an undergraduate research assistant on the project “provided my first taste of field research as a freshman, where I vividly remember the excitement I felt collecting samples and the intrigue of observing the experimental design. Through hands-on field and lab work, I became very interested in soil dynamics and forest ecosystems, which has continued to guide my research throughout my M.S. and soon to be Ph.D. The DIRT experiment not only provided me with valuable research skills but also ignited a lifelong passion for soil science and provided a foundation for the research I pursue today.”
The Allegheny College DIRT study is among the first sites in a network of similar experiments established throughout the country and internationally.
The study was further showcased in the publication Crop Science, Soil Science, and Agronomy News.