Nitrogen Fixation Definition and Processes

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Nitrogen Fixation Definition and Examples
Nitrogen fixation involves a set of natural and artificial processes that convert nitrogen into a form organisms can use.

Nitrogen is an essential component of amino acids, proteins, and DNA, making it fundamental for life. Yet, despite making up approximately 78% of Earth’s atmosphere, atmospheric nitrogen (N2) is not directly usable by most living organisms. This is where the crucial process of nitrogen fixation comes into play.

A Simple Definition of Nitrogen Fixation

Nitrogen fixation converts or ‘fixes’ nitrogen into a form organisms can use. It is the conversion of atmospheric nitrogen (N2) into ammonia (NH3) or related nitrogenous compounds, which are assimilated by plants and subsequently enter the food chain.

Why Is Nitrogen Fixation Important?

Even though nitrogen is abundant in the atmosphere, its triple bond makes it chemically stable and challenging for most organisms to utilize. Only specific bacteria and certain processes break this bond and ‘fix’ nitrogen into a biologically usable form.

So, the primary importance of nitrogen fixation is that it converts nitrogen into a form humans, other animals, plants, and other organisms use. Nitrogen is essential in proteins, nucleic acids, and other molecules.

  1. Bioavailability: Most organisms cannot directly use atmospheric nitrogen. Nitrogen fixation converts it into compounds like ammonia, which plants can absorb and utilize. Animals get their nitrogen by eating plants or by eating other animals.
  2. Fertility: In natural ecosystems, the availability of fixed nitrogen often limits plant growth. Nitrogen-fixing organisms thus play a key role in maintaining soil fertility.

History

Our understanding of nitrogen fixation goes back to the 18th century and continues today:

  • 1768: Carlos Linnaeus: This famous Swedish botanist was among the first to note that certain plants, which were later identified as legumes, grew well in soils that were considered too poor to support most other plants.
  • 1823: Jean-Baptiste Boussingault: A French agricultural chemist, Boussingault was the first to conclusively demonstrate that plants do not directly absorb free nitrogen from the air. His studies also hinted that legume plants had a unique method of nitrogen acquisition.
  • 1886: Hermann Hellriegel and Hermann Wilfarth: These German agronomists discovered the key to how legumes acquire nitrogen. They found that nodules on legume roots fix atmospheric nitrogen. When these nodules were absent or ineffective, the plants couldn’t thrive in nitrogen-deficient soils.
  • 1901: Martinus Beijerinck: This Dutch microbiologist identified that the bacteria living within these root nodules were the real heroes of the nitrogen fixation process. He showed that Rhizobium bacteria in the nodules converted atmospheric nitrogen into a form plants could use.
  • 1918: Fritz Haber: While his work wasn’t directly related to biological nitrogen fixation, this German chemist’s invention of the Haber Process revolutionized nitrogen fixation on an industrial scale, allowing for large-scale synthesis of ammonia. This had major implications for agriculture, providing an artificial source of fixed nitrogen for fertilizers.

The Role of Nitrogen Fixation in the Nitrogen Cycle

The nitrogen cycle is a series of processes that converts nitrogen in various forms through the environment. Nitrogen fixation plays a key role in the cycle:

  1. Nitrogen fixation turns nitrogen gas (N2) into ammonia (NH3).
  2. Nitrification converts ammonia to nitrites (NO2) and then nitrates (NO3).
  3. Assimilation sees plants absorbing these nitrates.
  4. Denitrification returns N2 to the atmosphere, completing the cycle.

Processes

Nitrogen fixation involves both biological and physical processes, plus there are commercial processes.

Nitrogen Fixing Bacteria

Bacteria (cyanobacteria or blue-green algae, green sulfur bacteria, purple sulfur bacteria, and anaerobic or methanogenic bacteria) and archaea achieve most of the biological nitrogen fixation. The bacteria are either free-living in soil or in a symbiotic relationships with plants or lichens.

  • Free-living Bacteria: Examples include Azotobacter and Clostridium. They fix nitrogen without forming symbiotic relationships.
  • Symbiotic Bacteria: Symbiotic bacteria form mutually beneficial associations with plants, ensuring a continuous supply of nutrients. The bacteria fix nitrogen, providing the plant with essential nutrients. In return, the plant supplies the bacteria with sugars and other organic compounds. Examples include:
    • Legume-root nodules: Rhizobium and Bradyrhizobium bacteria with legumes like beans and peas.
    • Non-leguminous plants: Frankia bacteria with alder trees and some other plants.

The overall chemical reaction for biological nitrogen fixation is:

N2 + 16 ATP + 16 H2O + 8 e– → 2NH3 + H2 + 16 ADP + 16 Pi

Most plants do not fix nitrogen. Of those that do, some use all of the nitrogen the bacteria produce. Others leak extra fixed nitrogen into the soil. Nitrogen also enters the soil when plants die. Animals get their nitrogen from plants (indirectly, if they eat other animals).

Nitrogen Fixation by Lightning and UV

During thunderstorms, the energy from lightning breaks nitrogen molecules apart, allowing the atoms to combine with oxygen and form nitrates. Ozone, also formed by lightning, facilitates these reactions. Rainfall carries the nitrates to the ground where it is absorbed by plants.

Ultraviolet light from the Sun also breaks some atmospheric nitrogen so that it forms new compounds.

Industrial Nitrogen Fixation

The Haber process is the prevalent industrial nitrogen fixation process, but there are other methods.

  • Frank-Caro Process: Frank and Caro developed a process in 1898 that fixes nitrogen in the form of calcium cyanamide.
  • Birkeland-Eyde Process: Invented in 1903 (but based on Henry Cavendish’s 1784 experiments), this process uses electrical arcs to oxidize nitrogen from the air, producing nitrogen oxides which are then converted to nitric acid.
  • Haber Process (Haber-Bosch Process): Developed by Fritz Haber in 1909, this process synthesizes ammonia from nitrogen and hydrogen under high temperatures and pressures, using iron as a catalyst.
  • Homogeneous Catalysis: A more modern method involves using soluble catalysts to produce ammonia under milder conditions than the Haber process.

References

  • Burris, R.H.; Wilson, P.W. (June 1945). “Biological Nitrogen Fixation”. Annual Review of Biochemistry. 14 (1): 685–708. doi:10.1146/annurev.bi.14.070145.003345
  • Erisman, J.W.; Sutton, M.A.; et al. (October 2008). “How a century of ammonia synthesis changed the world”. Nature Geoscience. 1 (10): 636–639. doi:10.1038/ngeo325
  • Hill, R.D.; Rinker, R.G.; Wilson, H.D. (1979). “Atmospheric Nitrogen Fixation by Lightning”. J. Atmos. Sci. 37 (1): 179–192. doi:10.1175/1520-0469(1980)037<0179:ANFBL>2.0.CO;2
  • Postgate, J. (1998). Nitrogen Fixation (3rd ed.). Cambridge: Cambridge University Press. ISBN 9780521648530.
  • Raymond, J.; Siefert, J.L.; Staples, C.R.; Blankenship, R.E. (March 2004). “The natural history of nitrogen fixation”. Molecular Biology and Evolution. 21 (3): 541–554. doi:10.1093/molbev/msh047