Nitrogen cycle

The nitrogen cycle is the biogeochemical process by which Nitrogen circulates between the atmosphere, living organisms, soil, and water. It's essential for life: nitrogen is a key ingredient in proteins and DNA, yet most organisms cannot use atmospheric nitrogen directly.

The cycle has several stages. Nitrogen fixation converts inert atmospheric N₂ into ammonia, performed by specialized bacteria (free-living and symbiotic) and some Archaea. Nitrification oxidizes ammonia to nitrite and nitrate, which plants can absorb through their Root systems. Organisms at each Trophic Level pass nitrogen upward as they consume lower levels. Denitrification completes the loop, converting nitrate back to atmospheric nitrogen through microbial respiration, primarily in oxygen-poor soil and water.

Human activity has dramatically disrupted this cycle. Industrial fertilizer production, large-scale agriculture, and fossil fuel combustion have doubled the amount of reactive nitrogen in the biosphere, causing Eutrophication in waterways and contributing to climate instability.

Understanding the nitrogen cycle reveals how interconnected ecosystems truly are—and how fragile. It demonstrates the principle of mass conservation operating across living and non-living realms.

Related

Phosphorus cycle, Carbon cycle, Soil, Eutrophication, Nitrogen fixation, Sustainable agriculture