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🌿 How does the nitrogen cycle work?

Locked N₂, microbial keys, food-web handoffs, and fertilizer spills that rewrite a planetary loop.

5
lessons
~20 min
to learn
🔬 Science
subject
Adults
level
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What you’ll learn

  1. Nitrogen everywhere — and mostly unusableExplain why abundant N₂ does not equal usable nitrogen for most life.Air is ~78% N₂, but the triple bond locks it; the cycle moves atoms into reactive, bioavailable forms.
  2. Fixation: opening the atmospheric vaultDescribe biological, lightning, and industrial fixation pathways.Nitrogenase, lightning, and Haber-Bosch open N₂; legumes host microbial partners in nodules.
  3. Assimilation, food webs, and return to soilTrace nitrogen through plants, animals, and decomposers.Plants assimilate mineral N; animals remodel organic N; decomposers ammonify dead matter back to ammonium.
  4. Microbial redox: nitrification and denitrificationLink nitrification, denitrification, and related pathways to oxygen and oxidation state.Microbes oxidize ammonium to nitrate and can reduce nitrate back toward N₂ when oxygen is low.
  5. Humans rewired the global nitrogen cycleConnect fertilizer, pollution, and field budgets to amplified reactive nitrogen.Industrial fixation raised yields and leaks; eutrophication, air chemistry, and budgets are the spill story.

Questions this course answers

Why can ecosystems be nitrogen-limited even though air is mostly N₂?

N₂ is abundant but chemically inert for most life; only specialized pathways make bioavailable forms.

Roughly what percent of dry air is nitrogen gas by volume?

Dry air is about 78% N₂ by volume — the famous four-fifths figure taught in basic Earth science.

Match each fixation path to its agent

Microbes, lightning, and industry open N₂ by different means; nodules are a symbiotic workplace for microbial fixation.

Order these ideas from locked vault to plant-usable story

Fixation unlocks N₂; assimilation builds biomolecules; consumption redistributes organic N.

What do decomposers primarily contribute in the nitrogen cycle?

Decomposers break organic nitrogen compounds and return ammonium to soils and waters.

Why must animals obtain nitrogen from food rather than from breathing air?

Breathing supplies N₂ that animals cannot fix; dietary protein and related compounds supply usable nitrogen.

Grounded in trusted sources

  • USGS and EPA public education on the nitrogen cycle, nutrients, and water quality
  • NOAA resources on coastal hypoxia and nutrient pollution
  • Open educational chemistry/biology texts on nitrogen fixation, nitrification, and denitrification
  • IPCC and related assessment summaries on nitrous oxide as a greenhouse gas (overview level)

Every Wunder lesson is built from real, reputable sources — never invented.

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