A young Knute Nadelhoffer as a PhD student at University of Wisconsin. Or, "Back when it was Crosby, Stills, Nash, and Nadelhoffer!"

Two men walk onto a bus. One says to the other: “Have you heard the one about soil organic matter?”

This may sound like the introduction to a uniquely esoteric joke, but, in fact, it is the origin story for an international research network. The two characters are the late Dr. Francis Hole, a beloved University of Wisconsin soil scientist, and would-be UMBS Director, Knute Nadelhoffer, then a PhD student. Their discussions on the bus and subsequent visits to Hole’s research site at the University of Wisconsin Arboretum led to the eventual establishment of the Detrital Input and Removal Treatment (DIRT) experimental network. The DIRT network now includes 13 sites in North America, Europe, and Asia, all focused on investigating the effects of plant litter and root inputs on the stability, accumulation, and chemical composition of soil organic matter in forests.

Dr. Francis Hole, "ambassador of soils."

To University of Wisconsin and beyond

Hole, a professor of soil science and geography and renowned “ambassador of soils”, had a particular interest in the important role of soil organic matter (SOM) in ecosystem productivity. SOM consists of decomposing plant and animal detritus, plus soil microbes and their various byproducts. It provides nutrients, habitat for diverse organisms from microbes to burrowing mammals, and the critical “stickiness” that resists erosion and increases water retention. In 1956, the young Professor Hole, with encouragement from renowned plant ecologist Dr. John Curtis, started a new project at the University of Wisconsin Arboretum in which experimental plots in established oak forests and restored prairie were subjected to differing amounts of leaf litter input, from no inputs at all (by raking and removal), to natural inputs, to experimentally doubled inputs after leaves die and accumulate on soil surfaces. Hole sought to determine how the amount of leaf litter entering soil might change organic matter contents over the long term. He diligently applied these experimental manipulations until his passing in 2002. A long list of volunteers have since carried the experiment into the present time.

The Soil Songs of Dr. Francis Hole, "undisputed poet-laureate-of-soils."

Enter Nadelhoffer, a graduate student interested in how soil influences ecosystem carbon storage and nutrient cycling. Nadelhoffer was aware of Hole’s pioneering work and enthusiastic promotion of soil. Aided by shared interests – and a shared bus route – they quickly struck up a friendship and regularly discussed ongoing research. Hole spoke often of his long-term project, piquing Nadelhoffer’s curiosity.

Hole’s intellectual earworm followed Nadelhoffer into his postdoc at The Ecosystems Center in Woods Hole, Massachusetts. Despite widespread discussion about the role of trees in sequestering carbon in biomass, Nadelhoffer’s interests – coupled with the influence of Hole’s experiment – led him to focus on the capacity of soils to accumulate and store, or “sequester”, carbon. Nadelhoffer knew that globally, soil stores more carbon than the atmosphere and all above-ground life combined. The mechanisms, however, were not well understood. He established a long-term study similar to Hole’s “Arboretum Experiment” in 1990 at the Harvard Forest in central Massachusetts, where Nadelhoffer had worked since the 1980s. Deciding that the experiment needed an acronym, he landed on “DIRT”: the Detrital Input and Removal Treatments.

Nadelhoffer adopted the DIRT name with a bit of trepidation. In their long-ago bus rides, Dr. Hole often grumbled that dirt is what you sweep out of the house, whereas soil is the “complex and beautiful, yet unseen, world beneath our feet.” However, on Hole's first and only visit to the second DIRT experiment at the Harvard Forest, Nadelhoffer mentioned that the Oxford English Dictionary traces the origin of “dirt” to drit, an Old Norse word for animal excrement. As a good deal of the organic material (or humus) in soil has passed through an animal’s gut at one time or another, Professor Hole deemed this a worthy acronym for the emerging network of experiments based on his design.

Nadelhoffer at the DIRT site in Síkfőkút, Hungary.

Why DIRT?

The rationale for DIRT is twofold. First, at the start of the DIRT project, investigations 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 process of asking basic scientific questions, designing experiments to answer them, and evaluating subsequent findings is inherently valuable. At the very least, it adds to the collective knowledge bank about the interconnected systems around us. At most, it leads 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. To indulge a pun – DIRT is climate change research from the ground up.

As it turns out, Nadelhoffer’s interest in addressing these questions was shared by other ecologists and biogeochemists studying Earth’s carbon and nutrient cycles. After establishing the Harvard Forest site, he continued to court collaborators - and come, they did. He won the buy-in of his peers, many of whom went on to initiate DIRT sites in China, Hungary, Germany, Quebec, and across the continental U.S. Particular DIRT methodologies and experimental designs were customized for specific environmental conditions, including dryland (D-DIRT) and grassland (Grass-DIRT) protocols. Soon, the curiosity-driven experiment dreamed up by Francis Hole had blossomed into a robust research network spanning international borders and a wide variety of landscapes.

DIRT at UMBS

When Nadelhoffer became the director of the University of Michigan Biological Station in 2003, DIRT came with him.

Map provided by Nadelhoffer Lab Manager John Den Uyl.

He established 27 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.

The DIRT site at UMBS - located in a northern temperate forest dominated by bigtooth aspen - features a coarse textured, sandy soil. In addition to climate, soil texture is a known factor in carbon and nitrogen dynamics, and its impact is evident across the multi-site framework in which DIRT operates.

Prior sampling of the UMBS plots has been completed in 2004, 2005, 2009, and 2014.

DIRT experimental design. Figure provided by Nadelhoffer Lab Manager John Den Uyl.
A LICOR Gas Analyzer used to measure how much carbon dioxide soil is releasing into the atmosphere over time.

The 15-Year DIRT Harvest

2019 marked the 15th consecutive year of litter manipulations at the UMBS DIRT site, and the fifth time a comprehensive soil sampling was completed.

In light of Nadelhoffer’s teaching and directorial responsibilities, much of the boots-on-the-ground DIRT sampling has fallen to his dedicated lab managers: Pat Micks in 2003-4, Jim LeMoine from 2004 to 2016, and John Den Uyl from 2017 to present.

After speaking with Den Uyl, it is apparent that the word “sampling” is a major distillation of the time and effort required to collect relevant DIRT parameters over a typical sampling year.

Nadelhoffer Lab Manager John Den Uyl (far right) with UROP students (left to right) Rhianna Lucas, Julien Malherbe, and Olivia Tu at the UMBS DIRT site.

“During the summer, I collect groundwater, conduct bi-weekly soil respiration measurements, and monitor soil moisture and temperature in each of our twenty-seven 5x5m plots,” he said. “Come October, we core and collect soil down to 30cm so we can see how the treatments have affected soil chemistry over time, particularly with respect to carbon and nitrogen.”

Under the guidance of Nadelhoffer, that is just what transpired last October. Den Uyl, LeMoine, and three Undergraduate Research Opportunity (UROP) students collected topsoil from each plot, as well as soil cores down to the aforementioned 30 centimeter mark. Samples were weighed and processed to remove roots, rocks, and woody debris. The remaining fine soil was then analyzed for percent carbon and nitrogen, and another important series of data points were effectively entered into the 15-year record book.

The future of DIRT

Although this anniversary is a landmark for DIRT at UMBS, it reflects a relatively short period in the lifespan of soil. Hence, much of the project data have yet to be fully analyzed and published.

“Now that we’ve hit the 15-year mark, we’ve started to combine and prepare data sets for analysis and archival,” says Den Uyl. “This will allow us to make cross-harvest comparisons and provide insight into how different inputs have affected soil chemistry over the life of the project.”

As for Nadelhoffer, the 15-year DIRT milestone mirrors another big transition. At the end of August 2020, he will retire as UMBS Director, followed by his retirement as Professor in the U-M Department of Ecology and Evolutionary Biology at the end of this calendar year.

“Retirement will free up time to work on projects I love,” he says. “DIRT and related questions about soils are high on the list, as are increases in my work to use science as an effective tool for informing public policy decisions.”

Among his emeritus projects, Nadelhoffer plans to prioritize DIRT data analysis and preparation for formal publication.

From humble beginnings some 64 years ago, DIRT has evolved from Francis Hole’s backyard curiosity, to a diverse, longitudinal, fruitful ecological study. But, this is still just the beginning.

Years ago, Nadelhoffer recalls asking Hole how long he thought the project should continue.

“I hope at least a couple thousand years,” he responded.

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Editorial note - A noncomprehensive list of dirt-related puns withheld in the interest of professionalism:

  • Researchers look forward to “digging into” DIRT data.
  • A “quick and dirty” summary of the project.
  • DIRT represents “grassroots” science at its finest.
  • Hole and Nadelhoffer laid the “groundwork” for an international research network.