Although terrestrial ecosystems are vulnerable to the global nitrogen glut, aquatic ecosystems in lakes, rivers, and coastal estuaries have probably suffered the most so far. They are the ultimate receptacle of much of the nutrient overload, which tends to accumulate in runoff or to be delivered directly in the form of raw or treated sewage. (Sewage is very high in nitrogen from protein in the human diet.) In these aquatic systems, excess nitrogen often greatly stimulates the growth of algae and other aquatic plants. When this extra plant matter dies and decays, it can rob the water of its dissolved oxygen, suffocating many aquatic organisms.
|Excess nutrients may spur algal blooms|
|Major or recurring harmful algal blooms, before and after 1972|
|Source: Donald Anderson, “Expansion of HAB Problems in the U.S.,” National Office for Marine Biotoxins and Harmful Algal Blooms, Woods Hole Oceanographic Institution. Available online at: http://habserv1.whoi.edu/hab/HABdistribution/habexpand.html (February 10, 1998).
Note: The increase in algal blooms may be attributable to a number of causes, including an increase in waterborne nutrients from human activities.
This overfertilization process, called eutrophication, is one of the most serious threats to aquatic environments today, particularly in coastal estuaries and inshore waters where most commercial fish and shellfish species breed  . Partially enclosed seas such as the Baltic Sea, the Black Sea, and even the Mediterranean have also been hard hit by nitrogen-caused eutrophication, and an extensive â€œdead zoneâ€ of diminished productivity has developed at the mouth of the Mississippi River in the Gulf of Mexico because of the large influx of nitrogen from agricultural runoff . One of the more troubling aspects of this nutrient assault on aquatic systems has been a steady rise in toxic algal blooms, which can take a heavy toll on fish, seabirds, and marine mammals . (See Excess Nutrients May Spur Algal Blooms.)
The nitrogen glut also impinges on the health of the atmosphere when the nitrogen-containing gases nitric oxide and nitrous oxide are released into the air, either from fossil fuel burning, land clearing, or agriculture-related activities. Nitric oxide, for example, is a potent precursor of smog and acid rain, and nitrous oxide is a long-lived greenhouse gas that traps some 200 times more heat than carbon dioxide. Nitrous oxide can also play a role in depleting the stratospheric ozone layer. Nitrous oxide concentrations in the atmosphere are rising rapidly – about 0.2 to 0.3 percent per year  .
Curbing the world’s nitrogen overload will mean acting on several fronts. Making fertilizer applications more efficient is one of the most promising options. Agriculture accounts for by far the largest amount of human-generated nitrogen – some 86 percent . Fertilizer use was scant until the 1950s but since then has increased exponentially. (See More Fertilizer: More Food, But More Pollution Too.)
In fact, one half of all the commercial fertilizer ever produced has been applied since 1984 . The problem is that about one half of every metric ton of fertilizer applied to fields never even makes it into plant tissue but ends up evaporating or being washed into local watercourses . A combination of better timing of fertilizer applications, more exact calculation of doses, and more accurate delivery could cut this waste substantially.
|More fertilizer: More food but more pollution too|
|Trends in fertilizer use, 1961-94
Source: Food and Agriculture Organization of the United Nations (FAO), FAOSTAT Statistical Database (FAO, Rome, 1997).
Note: Data for Asia and Europe do not include the countries of the former Soviet Union until 1991.
Cutting airborne nitrogen emissions from fossil fuels will also be important and will benefit from many of the same strategies used to reduce carbon dioxide emissions, including a greater emphasis on energy efficiency, a gradual shift toward alternative energy sources, and the use of low-nitrogen technology in power plants and cars. Other strategies make sense as well, such as restoration of wetlands, which are natural nutrient traps that sponge up excess nitrogen before it can damage aquatic systems.
But none of these steps is easy or obvious, and there seems little likelihood of concerted action until the nitrogen threat is elevated to a higher global profile. While the risks of global warming from a buildup of greenhouse gases in the atmosphere are fairly common knowledge today, the dangers of the world’s heavy nitrogen habit have gone largely unheralded so far, although this habit may be as pervasive and hard to address as cutting greenhouse gas emissions.
References and notes
13. Op. cit. 1, p. 11.
14. Robert J. Diaz and Rutger Rosenberg, “Marine Benthic Hypoxia: A Review of Its Ecological Effects and the Behavioral Responses of Benthic Macrofauna,” Oceanography and Marine Biology: An Annual Review, Vol. 33 (1995), p. 245.
15. “<@145>Dead Zone’ Plagues Gulf Fishermen,” The Washington Post (August 24, 1997), p. A-1.
16. Donald M. Anderson, “Red Tides,” Scientific American (August 1994), pp. 62-68.
17. Tony Socci, “Ecological Consequences of Human-Induced Changes in the Global Nitrogen Cycle,” briefing paper for lecture by William Schlesinger and David Tillman on global nitrogen cycle, U.S. Global Change Research Program (February 26, 1997), p. 2.
18. Op. cit. 1, pp. 6-7.
19. Op. cit. 2.
20. Op. cit. 17.
21. Op. cit. 1.