By Ethan Shaw | Updated Aug 30, 2022
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Nitrogen is the most abundant gas in Earth's atmosphere, yet it is a crucial building block for all living organisms. The nitrogen cycle—its journey through the atmosphere, geology, and biology—is one of ecology's most elegant processes.
Every cell relies on nitrogen to build proteins and amino acids. In plants, nitrogen is a core component of chlorophyll, the pigment that captures sunlight during photosynthesis. In animals, it is essential for DNA, enzymes, and cellular repair.
Chlorophyll contains nitrogen atoms that help transfer energy from light to the plant's metabolic pathways. Without sufficient nitrogen, plants struggle to form chlorophyll, leading to stunted growth and yellowing leaves.
Although atmospheric nitrogen makes up 78 % of the air we breathe, most organisms cannot use it directly. Through nitrogen fixation, microbes convert atmospheric N₂ into ammonia (NH₃) or nitrates (NO₃⁻), which plants and animals can absorb.
Root‑associated bacteria, often in symbiosis with legumes, and free‑living soil microbes drive this conversion. Their enzymatic machinery—nitrogenase—catalyzes the reduction of N₂, supplying the bulk of bioavailable nitrogen to ecosystems.
After plants assimilate nitrogen, it flows through the food chain. When organisms die, decomposers release nitrogen back into the soil or atmosphere as gases like ammonia, nitrous oxide, or nitrogen dioxide, completing the cycle.
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