Tallying Nitrogen's Increasing Impact

For most of agricultural history, nitrogen has been a precious commodity. Only specialized bacteria and lightning could convert atmospheric nitrogen into biologically usable forms. Today, however, fertilizers and fossil fuels have made nitrogen so freely available that it has become too much of a good thing.

 In a review of nitrogen's effects across the environmental spectrum, a team of ecologists headed by Peter M. Vitousek of Stanford University has concluded in no uncertain terms that human activities have dramatically increased the flow of nitrogen into the biological world -- doubling the natural rate at which it is made available on land -- with "serious and long-term" consequences.

"We are now the dominant force in the nitrogen cycle," says ecologist G. David Tilman of the University of Minnesota in St. Paul, one of the report's eight authors. "Humans are controlling more nitrogen than all natural processes."

 The Ecological Society of America is releasing a version of the report this week at the American Association for the Advancement of Science meeting in Seattle. The full report is slated to appear in the August Ecological Applications.

 Although none of the data or processes summarized in the report is new, a synthesis was needed, says John M. Blair. "When people think of global change, they usually think of climate change and increasing carbon dioxide," says Blair, a soil ecologist at Kansas State University in Manhattan. But the growth and reach of human population has a global impact in other ways. "The nitrogen cycle is a terrific example of that."

 The ecologists trace most of the new nitrogen in the system to three human activities. The use of commercial fertilizer is the biggest source, and it is increasing sharply, especially in developing countries. Of all the manufactured fertilizer used through 1990, half was applied to crops in the 1980s.

 Increased global cultivation of legumes and other crops that harbor nitrogen-fixing bacteria also adds to the influx. The burning of fossil fuels provides the third major source of newly available nitrogen compounds. These activities funnel about 140 million metric tons of nitrogen into the environment each year, the ecologists estimate -- an amount roughly equivalent to 10 million semi trucks of dry nitrogen fertilizer, says Tilman.

 The clearing of wildlands liberates perhaps another 70 million metric tons of nitrogen that had been stored in biomass.

 The nitrogen glut is evident throughout the biogeochemical cycle, according to the report. Nitrous oxide, a potent greenhouse gas, is accumulating in the atmosphere and can eat away at the stratospheric ozone layer. Other nitrogen compounds contribute to smog and acid deposition. They alter the pH and nutrient balance of soils and waters, triggering a cascade of effects.

 Researchers now think that the excess nitrogen is diminishing biological diversity in some areas. European heathlands, long adapted to nitrogen-poor conditions, are giving way to Eurasian grasses under the fertilizing effects of nitrogen. Such changes in species composition may be the newest and most surprising of nitrogen's consequences, says Vitousek.

 The trends are likely to continue, in step with the growing, urbanizing world population, the ecologists say. They see a need for more efficient fertilizer use and greater control of nitrogen emissions.