ECOLOGISTS SEE GLOBAL THREAT IN
'FIXED' NITROGEN EMISSIONS FROM HUMAN ACTIVITIES

DURHAM, N.C. -- Many ecologists like Duke University's William Schlesinger are alarmed by the growing amounts of chemically active nitrogen in Earth's soils, water and atmosphere -- a new environmental threat that they blame on human activities like fertilizer production and automotive emissions.

"This perturbation of the nitrogen cycle, which has pretty much been overlooked by a lot of people, is significant," said Schlesinger, a James B. Duke professor at the department of botany and the Nicholas School of the Environment. "It's global, and it's going to have a lot of impacts."

A new Ecological Society of America report authored by eight scientists, including Schlesinger, said this "fixed" or "mobilized" kind of nitrogen has doubled in the postwar years and is continuing to climb worldwide.

The huge surge, the report warned, "is having serious impacts on ecosystems around the world because nitrogen is essential to living organisms and its availability plays a crucial role in the organization and functioning of the world's ecosystems."

Impacts that the authors said they've "identified with certainty" include growing concentrations of nitrous oxide gas that could contribute to global warming; loss of nutrients essential to long-term soil fertility; acidification of soils, streams and lakes in several regions; and "greatly increased" nitrogen pollution of estuaries and coastal waters.

And the authors said they're also "confident" that the disruptions have increased losses of biological diversity in plants, animals and microorganisms; altered ecological processes in coastal waters; and contributed to "long-term declines in coastal marine fisheries."
The panel of authors was headed by Peter Vitousek of Stanford University. Other contributors, in addition to Schlesinger, included John Aber of the University of New Hampshire; Robert Howarth of Cornell University; Gene Likens of the Cary Arboretum in Millbrook, N.Y.; Pamela Matson of the University of California, Berkeley; David Schindler of the University of Alberta in Canada; and David Tilman of the University of Minnesota.

Our planet has always been awash in nitrogen, since it naturally makes up about 78 percent of the air that we breathe.

But that kind of atmospheric nitrogen -- two-atom molecules chemically known as N₂ -- is "essentially inert," Schlesinger said in an interview.

"Plants can't use it directly. Essentially, it is biologically unavailable."

However, nitrogen becomes chemically active when it is "fixed," bonding with oxygen or hydrogen to form compounds like nitric oxide (NO) or ammonia (NH₃).

"Fertilizer production and fixation of nitrogen in fossil fuel combustion are two ways we take nitrogen out of the atmosphere and make it available for living things," the Duke professor said.

Nature itself has always fixed a limited amount of nitrogen. Atmospheric N₂ can be converted to nitric oxide by the white-hot temperatures of lightning bolts, or converted to ammonia by the quiet actions of bacteria in the root systems of plants like soybeans.

Fixed nitrogen is necessary for plants to make vital proteins, Schlesinger said, and animals also ingest it when they eat plants. After passing through plants and animals, naturally fixed nitrogen then gets reused by other organisms or is otherwise recycled within the environment. A portion is even converted back to N₂.

Scientists call nature's way the "nitrogen cycle."

But huge increases in fertilizer production since World War II have added massive amounts of additional fixed nitrogen to the world pool in support of the chemically intensive farming practices that feed the burgeoning global population, the new report said.

Atmospheric nitrogen is also fixed in a way that mimics the effects of lightning within the combustion chambers of automobiles. In addition, the burning of fossil fuels liberates fixed nitrogen which has been stored underground for millions of years.

Humans introduce yet more fixed nitrogen into the air and water when they burn vegetation, clear land, drain wetlands, increase livestock herds, and plant more nitrogen-fixing crops, the ecological society report added.
Using a figure suggested in Schlesinger's 1991 textbook, Biogeochemistry, the report estimated that about 154 million tons of nitrogen were fixed by plants each year worldwide before humans began significantly disrupting the natural cycle.

Human activities have since about doubled the output of fixed nitrogen on land, and while some areas have been impacted more than others, "no region remains unaffected," the report added. The added nitrogen load "is readily detectable, even in cores drilled from the glacial ice of Greenland," it said.

Another source the report's authors used -- a study by Schlesinger and his former graduate student, Anne Hartley -- estimated that worldwide annual emissions of ammonia alone had risen by 1992 to about 82 million tons.
One major reason for all that extra ammonia is farmer tendencies to apply much more nitrogen fertilizer than crops can use. In fact, said Schlesinger, the 1992 study showed that as much as 10 percent of applied nitrogen fertilizer gets released as ammonia before a plant can even take it up.
Nitrogen in commercial ammonia fertilizer is usually fixed using energy from fossil fuels, which when burned emit carbon dioxide gas. And scientists think carbon dioxide released through human activities will cause the Earth's climate to warm in the next century by trapping extra solar heat in a "greenhouse effect."

"So the problem begins right away with the production and use of fertilizer," said Schlesinger, who also studies the effects of carbon dioxide on forests.
After fertilizers are over-applied, excess fixed nitrogen not taken up by plants can dissolve in water and be carried away to rivers and estuaries, where it can nurture the growth of microscopic plants called algae, he added. Decomposing algae can then consume all the dissolved oxygen, leading to fish kills.

Meanwhile, even more fixed nitrogen can reach the water in runoff carrying manure from livestock, which pass along most of the nitrogen they consume in fertilizer-grown feed grains.

The ecological society study also warned that fertilizer overuse can stimulate some plant species into runaway growth that crowds out competing species, thus reducing biodiversity and altering the environment.

And the study said excess fixed nitrogen may reach the environment in the form of damaging levels of nitric acid. Or it may retard plant growth by causing vital calcium and magnesium to leach from plant tissues.

The report said another possible concern is the increasing concentrations of nitrous oxide being recorded in the atmosphere. While the sources of nitrous oxide still are under investigation, that gas has the potential to both enhance the greenhouse effect and damage the stratospheric ozone layer that shields our planet from harmful solar ultraviolet radiation, the report added.

"Nitrous oxide has an average residence time in the atmosphere of about 150 years," said Schlesinger. "So if we end up finding that it really is bad as a greenhouse gas and a destroyer of ozone, and we haven't made a policy decision to slow it down, we're going to have to live with the consequences for a long time."

For additional information, contact Tim Lucas at the Nicholas School’s Office of Communications, at (919) 613-8084 or tdlucas@duke.edu.