What it is: The continuous movement of nitrogen atoms between the Earth's atmosphere, soil, and living organisms.

Importance: Nitrogen is an essential component of proteins, nucleic acids (DNA and RNA), and other vital organic molecules necessary for life.

Key Processes & Reservoirs:

Atmosphere: The largest reservoir of nitrogen, present as nitrogen gas (N₂), which is unusable by most organisms in this form. (Fun fact: the atmosphere is made up of approximately 78% Nitrogen gas.)

Nitrogen Fixation: The conversion of atmospheric nitrogen (N₂) into usable forms of nitrogen compounds (like ammonia, NH₃, or ammonium ions, NH₄⁺). This can occur through:

• Biological Fixation: Carried out by certain types of bacteria (Rhizobium, Azotobacter etc.) that live freely in the soil or in symbiotic relationships with the roots of leguminous plants (e.g., beans, peas). Symbiotic bacteria in root nodules fix nitrogen in exchange for food from plants. These bacteria use enzymes to convert N₂ into NH₃.
• Atmospheric Fixation: During lightning strikes, the high energy can cause N₂ and oxygen to react, forming nitrogen oxides that eventually dissolve in rainwater and reach the soil as nitrates (NO₃⁻).
• Industrial Fixation: Humans produce nitrogen fertilizers (e.g., ammonia, urea) from atmospheric nitrogen under high pressure and temperature.

Nitrification: The conversion of ammonia (NH₃ or NH₄⁺) into nitrites (NO₂⁻) and then into nitrates (NO₃⁻) by specific types of soil bacteria (nitrifying bacteria). Nitrates are the primary form of nitrogen absorbed by plants.

• NH₃ → NO₂⁻ (by Nitrosomonas bacteria)
• NO₂⁻ → NO₃⁻ (by Nitrobacter bacteria)

Assimilation: Plants absorb nitrates (NO₃⁻) from the soil through their roots and use them to synthesize proteins and nucleic acids. Animals obtain nitrogen by eating plants or other animals.

Ammonification (Mineralization): When plants and animals die, or when animals excrete waste products, the organic nitrogen compounds are broken down by decomposers (bacteria and fungi) into ammonia (NH₃) or ammonium ions (NH₄⁺), returning nitrogen to the soil.

Denitrification: The conversion of nitrates (NO₃⁻) back into nitrogen gas (N₂) by denitrifying bacteria, which occurs under anaerobic (oxygen-deficient) conditions, often in waterlogged soils. This process returns nitrogen to the atmosphere.

Diagram:

Human Impact:

• Excessive use of nitrogen fertilizers: Which can lead to water pollution (eutrophication) and the release of nitrous oxide (N₂O), a potent greenhouse gas.
• Deforestation: Can disrupt the nitrogen cycle in soil.

Key Terms to Remember:

• Nitrogen fixation (biological, atmospheric, industrial)
• Nitrification
• Assimilation
• Ammonification
• Denitrification
• Nitrogen-fixing bacteria
• Nitrifying bacteria (Nitrosomonas, Nitrobacter)
• Eutrophication
• Denitrifying bacteria
• Nitrates (NO₃⁻)
• Ammonia (NH₃)
• Ammonium ions (NH₄⁺)

Written by Kasiban Parthipan