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Sarah Hobbie

U biologist Sarah Hobbie wants to better understand the effects of decomposition and global change on each other.

The rotten truth

By Mary Hoff

Jan. 26, 2007

What happens to living things when they're no longer alive may seem a rather after-the-fact matter for a biologist. But Sarah Hobbie knows otherwise. Associate professor in the Department of Ecology, Evolution and Behavior (EEB), Hobbie studies the role that degradation of organic matter plays in the cycles of carbon and nutrients that ultimately sustain all of life.

"I joke with my classes, it's hard to go tell your grandparents you work on decomposition," she says. But once she explains how pertinent it is to everything from growing prized tomatoes to global warming, the laughs subside. Decomposition, she says, is critical to recycling plant nutrients. It's the main pathway by which carbon dioxide fixed by plants is returned to the atmosphere. And it's the source of organic matter needed for healthy soil.

Hobbie, who EEB head Claudia Neuhauser calls "one of the [department's] leading researchers" in the field of biogeochemistry, began studying decomposition as a graduate student looking at nutrient cycling on the Arctic tundra. She soon realized that it's a key--and often neglected--variable linked to human-caused global changes such as increases in atmospheric carbon dioxide and fixed nitrogen. Since then she has led a number of studies aimed at understanding the effects of decomposition and global change on each other.

In one project, Hobbie is looking at nutrient cycling in monoculture stands of 14 tree species that were planted in Poland three decades ago. When she started the study, she thought the amount of lignin and nitrogen in the leaf litter, which affects soil microorganisms' ability to break it down, would control the decomposition rate. "We found out we were completely wrong," she says. The wrench in the hypothesis turned out to be nightcrawlers, which prefer leaves with lots of calcium. This same nightcrawler species is now invading Minnesota forests, so the research "helps us understand what's currently going on in Minnesota," Hobbie says.

Hobbie is also studying decomposition at Cedar Creek Natural History Area. Past research has shown that the higher the nitrogen in plant material, the faster it rots. "That suggests that humans increasing the input of nitrogen should speed up the rate," she says. But when she added nitrogen fertilizer to her research plots, decomposition accelerated on only a few--and even slowed on some.

"I still don't understand those results," she says. Because of the prevalence of fertilizer use, she plans to conduct additional experiments until she does.

When all is said and done, Hobbie says, she hopes to have "contributed some understanding of the basic ecological response to human-caused global change." And, she adds, "It would be nice if that information actually related to societal decisions about global change."