U.S. Ecology Dramatically Altered By Fertilizers And Acid Rain

The study, published in the Jan. 24 issue of 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 study finds 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 U.S. ecosystems were very different before the industrial revolution.

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.

"We traveled in time by traveling to South America," said Lars Hedin, a co-author of the study and a professor of ecology and evolutionary biology at Princeton University.

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 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 findings raise questions about our 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.

The remote areas of Chile and Argentina provided prime areas for conducting the research. In North America the ground is tainted with large amounts of inorganic nitrogen from widespread use of nitrogen-heavy fertilizers, as well as acid rain brought on by fossil fuel burning. In the South American areas the researchers studied, there is no fertilizer use and almost no influx of fossil fuel emissions.

To reach their conclusions, 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.

The results also 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. That's because North American forests are still young, after recovering from past logging and agricultural clear-cutting. Young trees use nitrogen from the soil for growth, but as they mature, they sequester less nitrogen from the environment. When that happens, more inorganic nitrogen will be available to run off into rivers and groundwater.

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. 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 a NASA Earth System Science Fellowship. The overall project was funded by grants from the Andrew Mellon Foundation and the National Science Foundation.