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. 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. 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.
Curbing the world's nitrogen overload will mean acting on several fronts. Making fertilizer applications more efficient is one of the most promising options. Fertilizer use was scant until the 1950s but since then has increased exponentially. A combination of better timing of fertilizer applications, more exact calculation of doses, and more accurate delivery could cut this waste substantially. 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.