How forests respond to elevated nitrogen levels from atmospheric pollution is not always the same. While a forest is filtering nitrogen as expected, a higher percentage than previously seen is leaving the system again as the potent greenhouse gas nitrous oxide, say researchers.
Forests serve as important filters of nitrogen from atmospheric pollution like the burning of fossil fuels, buffering the nutrient from running off into streams and causing environmental damage.
"In the past, scientists have defined a limited number of ways forests could respond as they accumulate new nitrogen inputs from atmospheric pollution," Kaye said. "We see a lot of nitrogen coming in, but it seems to be leaving as a gas, and that's a new twist on our understanding of how forests are going to respond to atmospheric pollution."
Previous studies found forests absorb as much as 90 percent of nitrogen that falls from the atmosphere with rain. When forests become saturated, additional nitrogen is not absorbed as effectively and is transported downhill to streams, where it can leave the system and contribute to water pollution.
Some of the nitrogen is also consumed by microbes found in wet, low-oxygen soils near streams and wetlands, and is turned into the greenhouse gas nitrous oxide, which eventually returns to the atmosphere.
In the new study, recently published in the Journal of Geophysical Research: Biogeosciences, researchers found the forest was absorbing more than half of the nitrogen from atmospheric pollution, but that nitrous oxide production occurs at a higher rate than expected, and in upland portions of the watershed not previously studied. Upland nitrous oxide production was one of the highest outputs of nitrogen in the system.
"Moving to upland areas further away from the stream, you don't expect it to be as wet," said Julie Weitzman, a postdoctoral researcher at the CUNY Advanced Science Research Center and the Cary Institute of Ecosystem Studies. "But knowing the hydrology of the area, especially the swale and midslope with deep soil, it can definitely become saturated from runoff from the ridgetops and convergent flow moving to concave areas."
Researchers said the nitrogen budget they developed could lead to a better understanding of how large landscapes, like the forests of the Northeast U.S., will respond to rising nitrogen levels in the atmosphere.
"We want to be able to predict how forest ecosystems are going to buffer streams from atmospheric pollution," Kaye said. "We want to be able to project that into the future under lots of different scenarios. I think this work suggests that we really need to think about controls on nitrogen gas losses from uplands as we build that understanding."
The study is also one of the first to create a nitrogen budget that includes inputs from rock weathering. Researchers said about 10 percent of annual nitrogen inputs come from rock weathering. Because shale bedrock is widespread on Earth's surface, rock weathering represents an important part of the nitrogen budget that has been missing from previous studies, the researchers said.
Weitzman, who is lead author on the paper, conducted the research while a graduate student at Penn State. She collected field samples for two years from the nearby Susquehanna Shale Hills Critical Zone Observatory, part of a National Science Foundation-funded network promoting cross-disciplinary research on Earth surface science.
Susan Brantley, distinguished professor of geosciences in the College of Earth and Mineral Sciences and director of the Earth and Environmental Systems Institute at Penn State, is principal investigator on the Shale Hills CZO project.
"The CZO is unique in that there's so many different collaborators out there looking at different aspects of the critical zone," Weitzman said. "There was a lot of data and a lot of people looking at different aspects. You don't have to go out and try to tackle every tiny aspect of the nitrogen budget alone. We put this budget together with a little information from everyone who has worked out there."
The CZO features two forested sites near the Shaver's Creek Environmental Center in Huntingdon County. The project recently expanded to include an agricultural site owned by Herbert Cole, professor emeritus in the College of Agricultural Sciences.
"We are accumulating these headwater catchments that help us predict what's going on in Shaver's Creek," Kaye said. "We've got an understanding of the forested catchment. The water is moving down past agricultural landscapes, and we need to take the same nitrogen budget approach and apply it to Cole Farm and see what we learn there."
The National Science Foundation and the U. S. Department of Agriculture National Institute of Food and Agriculture fellowship program supported this work.