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UW scientist member of team that finds nitrogen pollution hinders forest decomposers

A researcher at the University of Wyoming is a member of a team of scientists that found atmospheric pollution may be altering forest ecosystems in ways that are difficult to reverse, according to their study published in January in the journal Ecology.

Linda van Diepen, assistant professor of soil microbial ecology in the Department of Ecosystem Science and Management, collaborated with scientists from the University of New Hampshire and University of Wisconsin-Madison.

Van Diepen is lead author of the article and a faculty member in the College of Agriculture and Natural Resources.

The study is the first to investigate a three-part domino effect: long-term exposure to pollution causes organisms to evolve, altering their growth habits and functions, and, in turn, altering the ecosystem processes those organisms control.

“Only a small subset of fungi can actually decompose wood. It’s troubling to see this group so affected.” Serita Frey, professor of natural resources at the University of New Hampshire

The team focused on a group of tiny organisms with a disproportionately large impact on the ecosystem: soil fungi.

"They are the recyclers of the ecosystem — the primary decomposers of wood, leaves and other plant material," said Serita Frey, professor of natural resources and the environment at the University of New Hampshire, and team member.

Without them, Frey noted, dead material would not decompose.

Soil fungi are sensitive to environmental stress, and they evolve quickly in new conditions. In the lab, the team analyzed soil fungi samples, half reared in petri dishes polluted with nitrogen as an environmental stressor.

There was a twist. Some of the fungi samples were polluted to begin with – collected from a long-term study plot at the Harvard Forest in Petersham, Mass., where, for the past 28 years, nitrogen had been added to simulate chronic industrial pollution.

After 14 weeks in the lab, the scientists found the polluted samples were stymied in their ability to perform decomposition, their primary function in the ecosystem.

"The fungi from polluted plots had evolved," Frey said. "They weren't decomposing as much plant material as the same species collected from a less polluted area. Something about the organism had changed in a fundamental way."

Some species of fungi were more sensitive than others — particularly a species called Irpex, which decomposed up to 44 percent less plant material after being polluted long-term. This fungus is from a subset of fungi that decompose wood.

"Only a small subset of fungi can actually decompose wood," Frey said. "It's troubling to see this group so affected."

Although 28 years of nitrogen pollution has altered fungal community composition and diversity of the study plots at Harvard Forest, other pollution studies have shown resilience of the fungal community diversity with no significant changes in species composition.

"However, given the results of our study, we now have to look beyond effects on biodiversity. Functional abilities of the fungal community may be affected even in the absence of changes in fungal diversity," van Diepen said.

Because decomposition is central to ecosystem function, the scientists said, a critical next question is determining whether, and how, fungal communities can recover after long-term nitrogen pollution.

Nitrogen pollution deposited on Earth's ecosystems from the atmosphere has increased 200 percent since the beginning of the Industrial Revolution, they noted. F

–UW