Nitrogen Cycle

Understand Nitrogen's Cycle Through Nature

The nitrogen cycle describes the path of the element nitrogen through nature. Nitrogen is essential for life—it is found in amino acids, proteins, and genetic material. Nitrogen is also the most abundant element in the atmosphere (~78%). However, gaseous nitrogen must be "fixed" into another form so that it can be used by living organisms.

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Nitrogen Fixation

Lightning in the night sky during a thunderstorm

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There are two main ways nitrogen can become "fixed:"

  • Fixation by lightning: The energy from lightning causes nitrogen (N2) and water (H2O) to combine to form ammonia (NH3) and nitrates (NO3). Precipitation carries the ammonia and nitrates to the ground, where they can be assimilated by plants.
  • Biological fixation: About 90% of nitrogen fixation is done by bacteria. Cyanobacteria convert nitrogen into ammonia and ammonium: N2 + 3 H2 → 2 NH3. Ammonia can then be used by plants directly. Ammonia and ammonium may be further reacted in the nitrification process.
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Nitrification

A dairy cow chewing grass

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Nitrification occurs by the following reactions:

2 NH3 + 3 O2 → 2 NO2 + 2 H+ + 2 H2O
2 NO2- + O2 → 2 NO3-

Aerobic bacteria use oxygen to convert ammonia and ammonium. Nitrosomonas bacteria convert nitrogen into nitrite (NO2-), and then Nitrobacter converts nitrite to nitrate (NO3-). Some bacteria exist in a symbiotic relationship with plants (legumes and some root-nodule species), and plants utilize the nitrate as a nutrient. Meanwhile, animals obtain nitrogen by eating plants or plant-eating animals.

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Ammonification

Wilted and sun dried flowers on a bush

Simon McGill / Getty Images

When plants and animals die, bacteria convert nitrogen nutrients back into ammonium salts and ammonia. This conversion process is called ammonification. Anaerobic bacteria can convert ammonia into nitrogen gas through the process of denitrification:

NO3- + CH2O + H+ → ½ N2O + CO2 + 1½ H2O

Denitrification returns nitrogen to the atmosphere, completing the cycle.