By Claire Gillespie • Updated Aug 30, 2022

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Nitrogen constitutes about 78% of the Earth’s atmosphere, yet organisms cannot absorb it directly. Through the nitrogen cycle, specialized microbes transform atmospheric N₂ into biologically usable forms that support life.

TL;DR

Nitrogen is a core nutrient for all living beings. It forms proteins, DNA, chlorophyll, and essential enzymes. The nitrogen cycle converts atmospheric nitrogen into compounds that plants and animals can assimilate.

Why Humans and Other Animals Need Nitrogen

Every gram of human tissue—muscle, skin, hair, nails, and blood—is rich in protein. Protein synthesis, enzyme function, and cellular repair depend on nitrogen-containing amino acids. Because mammals cannot uptake atmospheric nitrogen, they rely on dietary sources. Key protein foods include meat, fish, legumes, eggs, dairy, and nuts.

The body continuously recycles nitrogen from amino acids. Excess nitrogen is converted into urea and excreted, while nitrogen also forms non‑protein molecules such as the heme group in hemoglobin, which transports oxygen throughout the body.

How Plants Acquire Nitrogen

Plants synthesize proteins, chlorophyll, and nucleic acids— all nitrogen‑rich molecules—essential for growth and reproduction. Roots absorb nitrogen from the soil as nitrates (NO₃⁻) and ammonium (NH₄⁺). A deficiency manifests as chlorosis (yellowing) and stunted growth, reducing fruit and flower yield.

The Nitrogen Cycle Explained

• Fixation: Diazotrophic bacteria, such as Rhizobium, use the enzyme dinitrogenase to reduce atmospheric N₂ to ammonia (NH₃).
• Nitrification: Ammonia is oxidized by nitrifying bacteria (Nitrosomonas and Nitrobacter) to nitrite (NO₂⁻) and then nitrate (NO₃⁻), the forms plants absorb.
• Assimilation: Plants incorporate nitrate into organic compounds. Animals obtain nitrogen by consuming plants or other animals.
• Decomposition: Microbial decomposition releases ammonia back into the soil.
• Denitrification: Anoxic bacteria convert nitrate back to N₂ gas, completing the cycle.

Understanding this cycle is critical for sustainable agriculture and ecosystem management.