Question

You are a molecule of nitrogen. Choose a starting point in the nitrogen cycle and describe the process you would go through to move through the entire cycle.

Answer 1

I am a nitrogen molecule. I’ll start my journey as dinitrogen gas (N≡N) in the atmosphere and narrate, step by step, how I can move through the whole nitrogen cycle and eventually return to the air.

1. Atmospheric N2 — biological nitrogen fixation
- I drift down and land near the root of a legume. Inside root nodules, symbiotic Rhizobium bacteria use the enzyme nitrogenase to break my triple bond and reduce me to ammonia (NH3). (Lightning and industrial Haber–Bosch fixation are alternative ways to convert atmospheric N2 into reactive nitrogen.)

2. Ammonia → ammonium and assimilation into plant tissue
- In the slightly acidic soil solution, NH3 picks up a proton and becomes ammonium (NH4+). The plant’s roots take up NH4+ (or sometimes nitrate) and assimilate it into amino acids (e.g., glutamine, glutamate) and other organic nitrogen compounds — now I’m part of plant protein.

3. Move through the food web
- An herbivore eats the plant. I’m incorporated into the animal’s proteins and nucleic acids. If the animal is a carnivore later on, I can move up the food chain into other organisms’ bodies.

4. Excretion and decomposition (ammonification)
- The animal excretes wastes and eventually dies. Decomposer microbes (fungi, bacteria) break down the organic nitrogen in tissues and waste, returning it to inorganic form as ammonia — the process is called ammonification. So I become NH3/NH4+ again in the soil.

5. Nitrification (oxidation to nitrite and nitrate)
- In oxic soil, chemoautotrophic bacteria perform nitrification in two steps:
- Ammonia-oxidizing bacteria (e.g., Nitrosomonas) oxidize NH3/NH4+ to nitrite (NO2-).
- Nitrite-oxidizing bacteria (e.g., Nitrobacter) oxidize NO2- to nitrate (NO3-).
- I am now present as NO3-, a highly mobile form of nitrogen in soil water.

6. Three possible fates for nitrate
- A: Plant uptake: roots take up NO3- and reduce it back to NH4+ inside plant cells, then assimilate it into organic molecules — I re-enter biomass.
- B: Leaching/runoff: NO3- is soluble and can be leached into groundwater or run off into rivers and lakes, where phytoplankton assimilate me or I contribute to eutrophication.
- C: Denitrification or anammox (return to atmosphere): under anaerobic conditions (waterlogged soil, sediments), microbes use NO3- (or NO2-) as an electron acceptor.

7. Denitrification — returning to atmospheric N2 (and N2O)
- Facultative anaerobes (e.g., Pseudomonas, Paracoccus) enzymatically reduce nitrate stepwise: NO3- → NO2- → NO → N2O → N2. If reduction goes to completion, I become N2 gas and escape back to the atmosphere. Some incomplete denitrification releases nitrous oxide (N2O), a potent greenhouse gas.

8. Anammox (alternative atmospheric return)
- In oxygen-poor sediments, anammox bacteria (Planctomycetes) combine NH4+ and NO2- directly to produce N2 (NH4+ + NO2- → N2 + 2H2O). This is another pathway by which I can return to the air without going through classical denitrification.

9. Back to atmospheric N2
- Having been reduced to N2 by denitrification or anammox, I rejoin the atmosphere, completing the cycle.

Notes and human impacts
- Humans alter the cycle: industrial fixation (Haber–Bosch) creates lots of NH3 fertilizer that increases nitrate leaching, eutrophication, and N2O emissions. Burning fossil fuels adds reactive nitrogen oxides (NOx) to the atmosphere. These changes increase the rates and alter the balances of the steps above.

Summary (my path)
- As a quick recap of the specific route I took: atmospheric N2 → fixed to NH3/NH4+ by Rhizobium → assimilated into plant protein → eaten by an animal → returned to soil by decomposition (NH4+) → nitrified to NO2- and NO3- → denitrified to N2 → back to atmosphere.

That’s the nitrogen cycle — a loop of fixation, assimilation, transformation, and return, mediated largely by microbes and living organisms.