The nitrogen cycle is a biogeochemical process that describes how nitrogen moves through the environment. It is essential for life because nitrogen is a key component of amino acids, proteins, and nucleic acids. The cycle involves several key steps:

• Nitrogen Fixation: Atmospheric nitrogen (N₂), which is unusable by most organisms, is converted into ammonia (NH₃) or ammonium (NH₄⁺). This process is primarily carried out by nitrogen-fixing bacteria in the soil and aquatic environments. Some nitrogen fixation also occurs through lightning.
• Nitrification: Ammonia or ammonium is converted into nitrite (NO₂^-) and then into nitrate (NO₃^-) by nitrifying bacteria. Nitrate is a form of nitrogen that plants can readily absorb.
• Assimilation: Plants absorb nitrate (NO₃^-) or ammonium (NH₄⁺) from the soil through their roots. This nitrogen is then incorporated into plant tissues, such as amino acids and nucleic acids. Animals obtain nitrogen by consuming plants or other animals.
• Ammonification: When plants and animals die, or when animals excrete waste, the organic nitrogen in their tissues is converted back into ammonia (NH₃) or ammonium (NH₄⁺) by decomposers (bacteria and fungi). This process returns nitrogen to the soil.
• Denitrification: Nitrate (NO₃^-) is converted back into atmospheric nitrogen (N₂) by denitrifying bacteria. This process occurs in anaerobic conditions (oxygen-poor environments) and completes the cycle by returning nitrogen to the atmosphere.

Human activities, such as the use of fertilizers and the burning of fossil fuels, can significantly alter the nitrogen cycle, leading to environmental problems like eutrophication and air pollution.

Sources:

• The Nitrogen Cycle: Processes, Players, and Human Impact - Nature
• The nitrogen cycle - Science Learning Hub