What Is Fixed Nitrogen or Nitrogen Fixation?

Living organisms need nitrogen to form nucleic acids, proteins, and other molecules. However, the nitrogen gas, N₂, in the atmosphere is unavailable for use by most organisms because of the difficulty breaking the triple bond between nitrogen atoms. Nitrogen has to be 'fixed' or bound into another form for animals and plants to use it. Here is a look at what fixed nitrogen is and an explanation of different fixation processes.

Fixed nitrogen is nitrogen gas, N₂, that has been converted to ammonia (NH₃, an ammonium ion (NH₄, nitrate (NO₃, or another nitrogen oxide so that it can be used as a nutrient by living organisms. Nitrogen fixation is a key component of the nitrogen cycle.

Read More

Nitrogen Cycle

By Anne Marie Helmenstine, Ph.D.

How Is Nitrogen Fixed?

Nitrogen may be fixed via natural or synthetic processes. There are two key methods of natural nitrogen fixation:

• Lightning
  Lightning provides energy to react water (H₂O) and nitrogen gas (N₂) to form nitrates (NO₃) and ammonia (NH₃). Rain and snow carry these compounds to the surface, where plants use them.
• Bacteria
  Microorganisms that fix nitrogen are known collectively as diazotrophs. Diazotrophs account for about 90% of natural nitrogen fixation. Some diazotrophs are free-living bacteria or blue-green algae, while other diazotrophs exist in symbiosis with protozoa, termites, or plants. Diazotrophs convert nitrogen from the atmosphere into ammonia, which can be converted into nitrates or ammonium compounds. Plants and fungi use the compounds as nutrients. Animals obtain nitrogen by eating plants or animals that eat plants.

There are multiple synthetic methods for fixing nitrogen:

• Haber or Haber-Bosch process
  The Haber process or Haber-Bosch process is the most common commercial method of nitrogen fixation and ammonia production. The reaction was described by Fritz Haber, earning him the 1918 Nobel Prize in Chemistry, and adapted for industrial use early in the 20th century by Karl Bosch. In the process, nitrogen and hydrogen are heated and pressurized in a vessel containing an iron catalyst to produce ammonia.
• Cyanamide process
  The cyanamide process forms calcium cyanamide (CaCN₂, also known as Nitrolime) from calcium carbide that is heated in a pure nitrogen atmosphere. Calcium cyanamide then is used as a plant fertilizer.
• Electric arc process
  Lord Rayleigh devised the electric arc process in 1895, making it the first synthetic method of fixing nitrogen. The electric arc process fixes nitrogen in a lab in much the same way lightning fixes nitrogen in nature. An electric arc reacts oxygen and nitrogen in air to form nitrogen oxides. The oxide-laden air is bubbled through water to form nitric acid.