Soil holds the largest terrestrial pool of carbon on the planet, with tropical soils containing the most carbon of any type. Activities that cause soil to release its hold on this carbon can thus increase levels of carbon dioxide in the atmosphere and augment the effects of climate change.

Elizabeth Coward, who received her Ph.D. from the University of Pennsylvania on May 15, has spent her tenure at Penn investigating this pool of carbon and considering its vulnerabilities. She’s also turned her focus outward as an educator, engaging in innovative “active learning” courses at Penn and serving Philadelphia with her knowledge of soil health.

Coward grew up with an interest in environmental science and entered Haverford College as an undergraduate just as that institution launched an interdisciplinary program in the subject. Starting in her sophomore year, Coward, a biology major, began working with Haverford chemistry professor Helen White to study oil in the Gulf of Mexico that stemmed from the Deepwater Horizon spill in 2010.

“I was interested in applying fundamental science to environmental questions,” says Coward. White “got me interested in thinking about the intersection of biology, which is what my academic training was in, and the chemical processes that occur in the natural world. That’s what biogeochemistry is all about.”

Coward continued working in White’s lab for three years, investigating the “why” of the hydrocarbon’s persistence.

“Oil is essentially hydrocarbons,” says Coward, “and oil-derived carbon shouldn’t stick around in the ocean for long, as there are so many microbes and other primary producers that consume carbon as an energy source.”

After graduating from Haverford, Coward spent a year in the lab of Penn’s Frederic Bushman, the William Maul Measey Professor in Microbiology in the Perelman School of Medicine, studying microbiome communities.

Starting her doctoral degree in Penn’s Department of Earth and Environmental Science in the School of Arts & Sciences, working under advisor and associate professor Alain Plante as well as co-advisor Aaron Thompson of the University of Georgia, Coward returned to a question similar to the one she had pursued as an undergraduate: Why does carbon persist in the environment? This time, instead of a marine ecosystem, she looked to the tropical soils of northeastern Puerto Rico’s Luquillo Mountains.

“The transition was pretty easy; it was just a different system,” Coward says. “I think interdisciplinary science lets you do that; it lets you move from working in the ocean to working in soil because it’s really thinking about processes versus thinking about sites.”

During the past four years, Coward has examined soil samples from the National Science Foundation-supported Luquillo Critical Zone Observatory. In this hot, humid environment, with annual temperatures averaging 73 degrees Fahrenheit and 60 inches of rain per year, one would expect a lot of microbial activity to quickly degrade carbon. Yet some of the soil extracted from beneath the forest floor contains carbon that is hundreds or even thousands of years old.

Coward’s work hypothesized that persistent soil carbon may be stabilized by bonding to the surfaces of minerals, particularly those containing iron. Investigating that hypothesis, however, was not simple. Many methods of assessing iron in soil rely on carbon-containing reactions, which could contaminate the samples, so Coward and collaborators developed a method to identify and determine the stability of sometimes-complex carbon structures without introducing background carbon in the laboratory. In addition, many of the associations between carbon and iron in soils are taking place on the nanoscale.

“I was able to see that a substantial portion of the carbon that was in these soils was associated with minerals, so it can’t be reincorporated into microbial systems,” she says. “That’s probably what is keeping it from being bioavailable.”

The work not only helps assuage the researchers’ curiosity about why carbon can linger in the tropical soils but also has implications for what might occur as Puerto Rico and other tropical areas are subject to a changing climate and changing land-use patterns.

“One big question in soil-carbon research is what is going to happen with increasing human use of previously untouched land and whether mineral-stabilized soil carbon can be released as carbon dioxide,” Coward says.

While her research examined the carbon-stabilization mechanisms in soils, she found that, chemically, much of that carbon could be easily released into the atmosphere.

“Without the protection offered by mineral associations, much of that carbon is very accessible to microbes,” says Coward. “If it was exposed to a different setting, such as being brought up to the surface and exposed to greater oxygen levels or a warmer temperature, that could very easily affect what amount of carbon is being moved out of that soil into the atmosphere.”

While the CZO is in a protected area, many other unprotected forested lands on the island are undergoing shifts to more agricultural and urban uses, potentially freeing carbon in the process. Climate change, with increasingly frequent strong hurricanes and landslides, could also release some carbon currently stored in the island’s soils.

“What happens in Puerto Rico is important because tropical soils are very fragile ecosystems and are especially vulnerable to land-use changes or changes in temperature” she says.

Alongside her scientific pursuits in the lab, Coward has, since her undergrad days, devoted time to helping others gain a deeper understanding of and appreciation for science. At Haverford, she worked as a teaching assistant in chemistry classes and lab sections for three years. And because Haverford’s biology program emphasized cellular and molecular studies, she led workshops in anatomy and physiology to supplement her fellow student’s understanding of systems-level biology, enlisting graduate students at Penn’s School of Veterinary Medicine to supply their expertise in those subjects.

At Penn, she’s worked closely with Jane Dmochowski, a senior lecturer in the Earth and Environmental Science Department, assisting with a course that used an active learning format, which engages students in collaborative activities.

“Often they’re taking real data from the U.S. Geological Survey website or the Environmental Protection Agency website and answering questions after you do a series of manipulations on the data,” Coward says.

“It’s very hands on,” she adds. “People are working in groups at small tables, so you make your way through them and help them start thinking analytically. I had never been part of an active-learning classroom as a student so it was interesting to learn how it works from the other side. I was hooked on teaching after that.”

Coward began working with Penn’s Center for Teaching and Learning, helping train teaching assistants, and she’s also assisted Plante, her advisor, with a large introductory environmental science course, holding office hours and leading recitation groups.

Outside the classroom, she’s helped impart the importance of identifying and quantifying heavy metals in soil to the public through her participation on the Philadelphia City Soil Safety Working Group, which assayed soils in the community. In addition, she has participated for several years in the Soil Kitchen, an outreach event that invites members of the public to bring soils in for testing to ensure they can garden safely.

“The most interesting part was talking to people about what’s happening in their soil,” Coward says. “A lot of people have a rough idea that there are metals in their soil, but they don’t know what that means or where they are coming from, so it was a nice opportunity to talk with people in the neighborhood about what is going on in their own back yard.”

Having just successfully defended her dissertation, Coward will continue her work at Penn as a postdoctoral researcher during the summer and hopes to continue in a career path that incorporates both scientific research and teaching.

“I think if I did solely teaching I would miss my research, and if I did solely research I would miss teaching,” she says. “I need both of them in my life.”