U.S. Ecosystems Altered by Nitrogen Pollution

By Cat Lazaroff

WASHINGTON, DC, January 25, 2002 (ENS) - Much of the nitrogen in ecosystems throughout the United States comes not, as previously believed, from natural sources, but from acid rain and agricultural fertilizers. A study of ancient and unpolluted South American forests promises to upend longstanding beliefs about ecosystems and the effects of pollution in the Northern Hemisphere.


Burning fossil fuels like coal produces nitrogen pollution (Photo courtesy National Renewable Energy Lab)
Ecologists previously thought that nitrogen containing minerals, referred to collectively as inorganic nitrogen, have always been the dominant nutrient in forests worldwide. The study of South American forests, however, showed a sharply different picture: complex, organic compounds are the main form of nitrogen in unpolluted ecosystems.

"It's one of those things where everybody's intuition that they've gotten from studying the world is wrong," said Stephen Pacala, professor of ecology and evolutionary biology at Princeton University. "It's still not clear what the consequences are, but as a pattern it's completely different. You know there have to be lots of implications."

The study, published in the January 24 issue of the journal "Nature," focused on nitrogen, a plant nutrient that plays a critical role in maintaining everything from the health of local waterways to the global climate.

The authors argue that the ecosystems of South America, with their preponderance of organic nitrogen, are a window into the past, showing that U.S. ecosystems were very different before the industrial revolution.


Agricultural runoff contains large amounts of nitrogen laden fertilizer (Photo by Jack Dykinga, courtesy Agricultural Research Service)
"We traveled in time by traveling to South America," said Lars Hedin, a coauthor of the study and a professor of ecology and evolutionary biology at Princeton University.

Conditions in North America are different from those in South America for two reasons, the researchers said. First, burning fossil fuels produces great quantities of nitrogen and oxygen compounds, which wash out of the air as acid rain.

Second, during World War I the chemist Fritz Haber invented a process for extracting nitrogen from the atmosphere for use in making explosives. The Haber process allowed the mass production of fertilizers, which has fueled a boom in agricultural productivity.

In the remote areas of Chile and Argentina that the researchers studied, there is no fertilizer use and almost no influx of fossil fuel emissions.

The information they uncovered could have far reaching impacts in many areas of ecology, from predicting the pace of global climate change to understanding the consequences of acid rain and agricultural runoff.

"I think it is potentially very important research," said Nico van Breemen of the University of Wageningen in the Netherlands. "It raises points that are fundamental for our understanding of very big global environmental issues."


Many forested areas in the United States are populated with young trees, because the old growth forests have been logged (Photo courtesy American Lands)
The findings raise questions about science's understanding of global warming, which is partly caused by fossil fuel burning and increasing levels of heat trapping carbon dioxide in the atmosphere. When trees grow and mature they remove carbon dioxide from the air.

The ability of trees to grow and absorb more carbon is intimately related to the availability of nitrogen.

"Nitrogen is a sort of master variable," said Steve Perakis, the paper's lead author and a research scientist with the U.S. Geological Survey. "If we don't get the fundamental elements of the nitrogen cycle right, we can't answer many other ecological questions."

The scientists spent five years preparing experiments in remote Chilean temperate forests and another five years conducting detailed analyses of water in those forests. They also conducted one time tests in a dozen other remote areas in Chile and Argentina to prove that the preponderance of organic nitrogen they observed was not unique to the site they were studying.

At the same time, they repeated their measurements in three U.S. virgin forests, two in the Smokey Mountains and one in Pennsylvania. All of the areas studied contained unlogged primary forests, in ecosystems that have developed in place for 4,000 years to over 20,000 years.


By contrast, many Chilean forests, like this stand of araucaria trees, are far more ancient (Photo courtesy Ancient Forests International)
The results suggest that in North America the impact of nitrogen pollution from acid rain and agriculture may be more dramatic in years to come than previously thought.

North American forests are mostly young, recovering from past logging and agricultural clearcutting. Young trees use nitrogen from the soil for growth, serving as a buffer that lessens the impact of nitrogen pollution.

As trees mature, they sequester less nitrogen from the environment. When that happens, more inorganic nitrogen will be available to run off into rivers and groundwater, changing conditions even more from their natural state.

Another interesting finding, said Perakis, was that the nitrogen cycle - the way nitrogen compounds are exchanged between plants, soil, waterways and the atmosphere - in South America is more uniform than it is in the United States.

"We found that even though there were some noticeable variations in South America, they were pretty small compared to the variations caused by air pollution," Perakis explained. "We live in a transient world, a world that's changing because of many human activities, so many systems are responding in unique ways."