Nitrogen cycle Knowledge-Based Study

Study notes

1. The Importance of Nitrogen

   Nitrogen is an essential element required by all living organisms to synthesize vital macromolecules, including amino acids, proteins, and nucleic acids (DNA and RNA). Although molecular nitrogen gas (N2) constitutes approximately 78% of Earth's atmosphere, it is chemically inert due to a strong triple covalent bond between the two nitrogen atoms. Because eukaryotic organisms and most prokaryotes cannot directly utilize atmospheric N2, it must undergo chemical transformation into biologically accessible forms through the nitrogen cycle.

   Nitrogen Fixation

   The conversion of molecular nitrogen (N2) into ammonia (NH3) or ammonium ions (NH4+) is termed nitrogen fixation. This process occurs via biological, atmospheric, or industrial mechanisms:

   • Biological Fixation: Specialized prokaryotes containing the enzyme nitrogenase catalyze this reduction. These include free-living soil bacteria (such as Azotobacter) and symbiotic bacteria (such as Rhizobium) that reside inside root nodules of leguminous plants.
   • Atmospheric Fixation: High-energy physical events, such as lightning, break atmospheric N2 bonds, allowing nitrogen to combine with oxygen to form nitrates, which reach the soil through precipitation.
   • Industrial Fixation: Chemical processes, primarily the Haber-Bosch process, combine atmospheric nitrogen and hydrogen under high heat and pressure to manufacture synthetic agricultural fertilizers.

   Converting and Incorporating Nitrogen

   Once nitrogen is fixed into ammonia, it undergoes further transformations in the soil that determine how autotrophs absorb and use it.

   Nitrification

   Nitrification is a two-step aerobic process performed exclusively by chemoautotrophic soil bacteria:

   1. Ammonia Oxidation: Ammonia (NH3) or ammonium (NH4+) is oxidized into nitrite (NO2-) by ammonia-oxidizing bacteria, such as Nitrosomonas.
   2. Nitrite Oxidation: The resulting nitrite is subsequently oxidized into nitrate (NO3-) by nitrite-oxidizing bacteria, such as Nitrobacter.

   Nitrate is highly soluble and represents the primary, most readily available chemical form of inorganic nitrogen absorbed by terrestrial plants.

   Assimilation

   Assimilation is the biological uptake of inorganic nitrogen compounds by primary producers and their conversion into organic molecules:

   • Plant roots absorb nitrate (NO3-) and ammonium (NH4+) from the soil solution via active transport.
   • Inside plant cells, these inorganic ions are reduced and incorporated into organic compounds, including amino acids, proteins, and nucleotides.
   • Consumers then acquire these organic nitrogen complexes by feeding on plants or other animals along food webs.

   Recycling and Returning Nitrogen

   To maintain equilibrium within terrestrial and aquatic ecosystems, nitrogen must continuously cycle between organic biomass and inorganic reservoirs.

   Ammonification (Decomposition)

   When living organisms excrete nitrogenous wastes (such as urea or uric acid) or die, their organic tissue contains fixed nitrogen. Decomposers—predominantly heterotrophic bacteria and fungi—break down this organic matter, releasing excess nitrogen back into the soil matrix as ammonia (NH3) or ammonium (NH4+). This ammonification process ensures that essential nitrogen is continually recycled rather than locked away in dead biomass.

   Denitrification

   Denitrification is the concluding process that completes the nitrogen cycle by returning nitrogen to the atmosphere:

   • Under anaerobic conditions—such as waterlogged soils, wetlands, or deep aquatic sediments—specialized facultative anaerobic bacteria (such as Pseudomonas species) use nitrate (NO3-) as an alternative terminal electron acceptor in cellular respiration.
   • Nitrate is sequentially reduced to nitrite, nitric oxide, nitrous oxide, and ultimately to inert dinitrogen gas (N2).
   • This gaseous N2 diffuses back into the atmosphere, preventing excessive accumulation of nitrate salts in aquatic systems and sustaining the global atmospheric nitrogen reservoir.