Detrital Input and Removal Treatments (DIRT)

Soil stores more carbon than the atmosphere and all above-ground life combined. The mechanisms, however, are not entirely understood. The Detrital Input and Removal Treatments (DIRT), led by UMBS Director-emeritus Knute Nadelhoffer, investigates the effects of plant litter and root inputs on the stability, accumulation, and chemical composition of soil organic matter in forests. Since Nadelhoffer established the first site at Harvard Forest in 1990, DIRT has grown into a 13-site network spanning North America, Europe, and Asia -- including an anchor site at UMBS. At the start of the DIRT project, investigation of how soils respond to differing rates and types of plant inputs had been done mainly in agricultural systems, not in forests or natural grasslands, which together store large amounts of Earth’s organic carbon. The basic scientific questions behind DIRT seek to add to the collective knowledge bank about the interconnected systems around us, leading to discoveries that help solve ecological problems.

One such problem - and a key motivator for DIRT - is the increasing concentration of atmospheric carbon dioxide, an inorganic form of carbon produced by fossil fuel burning that is overheating our planet. While plants remove inorganic carbon dioxide from the atmosphere through photosynthesis and biomass production, long-term storage of plant-derived organic carbon in soils reduces the rate of atmospheric carbon dioxide increase, thereby lessening the harmful impacts of this major heat-trapping greenhouse gas. Terrestrial ecosystems -- mainly forests -- remove the equivalent of about one-fourth of human-derived carbon dioxide emissions from the atmosphere, thereby slowing what would otherwise be an even faster increase. One question that DIRT addresses is how changes in forest above- and below-ground inputs (i.e. leaves and roots) might change amounts of organic carbon storage in soil. After all, more organic carbon storage in soils can function to slow the rate of atmospheric carbon dioxide increase. The network of DIRT experiments helps establish a baseline of carbon storage activity under background (control) and manipulated conditions (experimental increases or decreases in plant organic matter inputs to soil), allowing for better understanding the mechanisms responsible for storing carbon in soils. This basic scientific work informs management strategies aimed at maximizing ecosystem carbon storage potentials.

DIRT at UMBS includes twenty-seven experimental plots. The 5x5m plots span nine treatments with three replicates per treatment, including double leaf litter, double litter plus fertilizer, no litter, no inputs (roots nor litter), no roots, a wood addition plot, and a fertilizer plot. Control plots, against which all other treatments are measured, are kept free of ground-layer vegetation to minimize the effects of ground-layer vegetation disturbances imposed by treatments. The reference plots remain totally untouched, with undisturbed ground-layer vegetation.

Sampling of the UMBS plots has been completed in 2004, 2005, 2009, 2014, and 2019. During the harvest, Nadelhoffer and collaborators collect topsoil from each plot, as well as soil cores down to 30cm. Samples are weighed and processed to remove roots, rocks, and woody debris. The remaining fine soil is then analyzed for percent carbon and nitrogen. Now that DIRT has reached its 15th birthday, the team is preparing data sets for analysis and archival. This will allow them to make cross-harvest comparisons and provide insight into how different inputs have affected soil chemistry over the life of the project.