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Annual Public Lectures Series > William Schlesinger

Scientists and Journalists:
Getting the Point Across
June 8-12, 2009

Monday, June 8, 3:30 p.m.
Chasing Nitrogen Atoms: The Global Nitrogen Cycle
William Schlesinger, Cary Institute of Ecosystem Studies


William H. Schlesinger will not find answers to his questions in a laboratory. "It's ecosystem science at its largest scale," he said, "The planetary system."

Schlesinger, the president of the Cary Institute of Ecosystem Studies, spoke Monday at the Metcalf Institute's Annual Public Lecture Series. His talk, Chasing Nitrogen Atoms: The Global Nitrogen Cycle, focused on the biotic and human contributions to the nitrogen cycle.

Because of the scale of his work - the entire planet - modeling is an essential tool. There are no replicate planets to study, no smaller-scale systems to observe. Instead, Schlesinger said, he uses all of the available data and looks backwards to see if he can accurately describe past conditions.

Using simplified, one-panel cartoons, Schlesinger explained to the audience of scientists, journalists and members of the general public, the natural components of the nitrogen cycle - plants, bacteria, and lightning. The picture became more complicated as he added human contributions - such as coal-fired power plants, cars, and fixed nitrogen compounds, used for weapons and fertilizer.

Schlesinger emphasized that the cartoons were good for more than a chuckle; they were a useful tool for communicating complex scientific issues - to journalists and the public. Schlesinger, who has testified before U.S. House and Senate Committees and worked with policy makers, said that the first thing a policy analyst will ask is "what difference do humans make?"

"These diagrams can show that clearly," he said. "And when simplified enough, they're kind of useful for the journalism world as well."

But the crux of Schlesinger's talk was one question: Where does the nitrogen go? Human activities have been increasing levels of nitrogen, so where is this excess being stored?

Schlesinger proposed five possible pathways to explain the fate of the extra 150 teragrams (or 10^12 grams) of nitrogen that humans apply to the earth's surface each year, beyond the pre-industrial levels of the element.

  • Some nitrogen (about 9 teragrams, Tg) is stimulating the growth of plants, and not just in agriculture. Schlesinger said nitrogen used in farms might also escape, ultimately ending up in forests.
  • Some nitrogen (between 30-40 Tg of the human input) is going into rivers - Schlesinger estimates that the amount of nitrogen being added to rivers and oceans has roughly doubled with the addition of human activity.
  • Although there is very little scientific literature to confirm the amount of nitrogen going into groundwater, Schlesinger assumes that all nitrogen found in groundwater (estimated at 15 Tg/year) is the product of human activity.
  • Denitrification (by which inorganic nitrate is converted to gaseous nitrogen), such as happens in wetlands, is another important pathway for nitrogen. Although it is difficult to accurately measure denitrification rates because of the large background of naturally occurring gaseous nitrogen in the atmosphere, Schlesinger estimates that this process accounts for 17 teragrams of nitrogen.
  • Referring to recent estimates, Schlesinger also cited atmospheric transport to the ocean as a pathway for 48 Tg of nitrogen per year.

After all is said and done, these five sinks for nitrogen do not quite match the total human-mediated nitrogen inputs, Schlesinger said. He went on to note that 249 Tg of nitrogen/year can be accounted for out of the total of 295 Tg entering the land-based nitrogen cycle each year. Still, he said, accounting for 84 percent of the total terrestrial nitrogen budget is "pretty good," considering the uncertainties inherent in this kind of global scale estimate.

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June 8, 2009