The study focused on nitrogen, a plant nutrient that plays a critical role in maintaining everything from the health of local waterways to the global climate.
Standard thinking among ecologists had been 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.
The study, published in the Jan. 24 issue of Nature, concludes that the high levels of inorganic nitrogen in the United States, long thought to be the natural mainstay of the ecosystem, are really the result of acid rain and agricultural fertilizers. The authors argue that the ecosystems of South America, with their preponderance of organic nitrogen, are a window into the past, showing that United States ecosystems were very different before the industrial revolution.
"We traveled in time by traveling to South America," said Lars Hedin, professor of ecology and evolutionary biology, who recently came to Princeton from Cornell University, where he conducted the research with former graduate student Steven Perakis.
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 run-off.
"Nitrogen is a sort of master variable," said Perakis, who is the lead author of the paper and is now 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."
"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."
"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. "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."
One of the chief questions the study raises is how its findings will affect predictions about the way forests and the world climate are responding to the rapidly increasing concentration of carbon dioxide in the atmosphere, which leads to global warming. When trees grow and mature they remove carbon dioxide from the air, lessening the impact of fossil fuel burning. The ability of trees to grow and absorb more carbon is intimately related to the availability of nitrogen.
The findings also suggest that the impact of nitrogen pollution from acid rain and agriculture may be more dramatic in years to come than previously thought.
The nation's forests, which are currently recovering from past logging and agriculture, serve as a buffer that lessens the impact of nitrogen pollution. For every ton of nitrogen deposited into soils, some is taken up by trees as they mature and increase in bulk. When trees reach maturity, however, they will hold all the nitrogen they can and won't absorb any more from the environment. At that point, more inorganic nitrogen than ever will flow into rivers and groundwater, which will be further from the natural conditions than previously thought, Hedin said.
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 South American areas they studied, there is no fertilizer use and almost no influx of fossil fuel emissions.
To reach their conclusions, the researchers 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 to over 20,000 years.
Another interesting finding, said Perakis, who conducted much of the fieldwork, 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. We live in a transient world, a world that's changing because of many human activities, so many systems are responding in unique ways."
Perakis' work was funded by an Earth System Science Fellowship from NASA. The overall project was funded by grants from the Andrew Mellon Foundation and the National Science Foundation.
Journal
Nature