The Natural Nitrogen Cycle

• The nitrogen cycle is extremely important for the health of every terrestrial ecosystem on the planet. The atmosphere is mostly nitrogen, but plants cannot absorb the diatomic nitrogen, N2, which is in the air. They must rely on certain nitrogen-fixing bacteria, many of which live in symbiosis with legumes -- such as peanuts, soy and alfalfa -- to transform atmospheric nitrogen into ammonia, which plants can absorb. Ammonia is also turned into nitrite, or NO2-, and then into nitrate or NO3-, by different bacteria called nitrifying bacteria. These nitrates are then either taken up by plants, dissolved in groundwater or rainwater runoff, or released back into the atmosphere as diatomic nitrogen after metabolism by denitrifying bacteria.
  
  

Human Sources of Nitrogen

• Humans disrupt the normal nitrogen cycle by adding large amounts of nitrogen in the form of fertilizers and animal wastes from livestock and poultry. Agriculture is responsible for much of this added nitrogen, but fertilizers on private lawns, landscaped areas and golf courses also contribute to the problem. Nitrate is highly soluble in water, so it can easily leach into groundwater or be carried into streams and rivers by rainwater runoff, especially when it is improperly applied as fertilizer.
  
  

Factors Influencing Leaching

• The harmful impact of nitrogen fertilizers can be minimized. Slow-release fertilizers allow plants to absorb more of the nitrogen before it is leached away. By applying only as much as the plants can use, groundwater pollution can be reduced. Leaching is also more of a problem in highly irrigated fields, in sandy soils, and in the fall when plants absorb less nitrogen. Ideally, artificial fertilizers can be foregone entirely and nitrogen needs can be met through crop rotation with legumes or by planting legumes alongside normal crops.
  
  

Effects in Groundwater

• High levels of nitrates in groundwater can impact human health. Nitrate exposure can cause a condition called methemoglobinemia, in which the nitrates cause the iron in red blood cells to be oxidized, destroying their ability to carry oxygen. Infants are most susceptible to nitrate poisoning, which can increase the risk of blue baby syndrome, in which infants die from a lack of oxygen.
  
  

Effects in Surface Waters

• High levels of nitrate in freshwater ecosystems or in marine ecosystems near the shore can be directly harmful to fish, and may cause growth deficiencies, immune impairment or death. Even more destructively, nitrates contribute to eutrophication, a term for nutrient overload in aquatic ecosystems. Just as nitrate fertilizers cause plants to grow on land, they also cause algae to grow in water. Nitrate pollution can lead to explosive population growth, or algal blooms. The excess of algae darkens the water and reduces sunlight penetration, which in turn reduces photosynthesis and reduces dissolved oxygen concentrations. As the algae die, their decomposition feeds bacteria, which further deplete the dissolved oxygen supply, sometimes to the point where fish can no longer survive.