1.5 The Nitrogen Cycle

Nitrogen's giant reservoir

N2 gas makes up ~78% of the atmosphere but is unusable by most organisms because of its strong triple bond. Nitrogen must be converted into usable forms like ammonia (NH3), nitrites (NO2-), and nitrates (NO3-) through nitrogen fixation, done by lightning and nitrogen-fixing bacteria (some free-living, some symbiotic in legume root nodules).

Nitrification, assimilation, and ammonification

Nitrification is the conversion of ammonia into nitrites then nitrates by nitrifying bacteria, making nitrogen usable by plants. Plants take up nitrates through assimilation to build proteins and nucleic acids; this nitrogen moves through the food web. When organisms die or excrete waste, decomposers perform ammonification, returning nitrogen to soil as ammonia.

Denitrification and human impacts

Denitrifying bacteria, usually in anaerobic soils/water, convert nitrates back into N2 gas, completing the cycle. Humans disrupt this cycle by producing synthetic fertilizers (Haber-Bosch process) and burning fossil fuels, adding excess nitrogen. This causes eutrophication, algal blooms, hypoxia/dead zones, and acid rain.

Key terms

Nitrogen fixation

Conversion of atmospheric N2 into ammonia (NH3), done by lightning or nitrogen-fixing bacteria.

Nitrification

Conversion of ammonia into nitrites (NO2-) then nitrates (NO3-) by nitrifying bacteria.

Denitrification

Conversion of nitrates back into N2 gas by denitrifying bacteria, usually in anaerobic conditions, completing the cycle.

Ammonification

Decomposers break down dead organisms/waste, releasing nitrogen as ammonia into the soil.

Human impact on nitrogen cycle

Synthetic fertilizers and fossil fuel combustion add excess nitrogen, causing eutrophication, algal blooms, and dead zones.

Try an AP-style question

What is nitrogen fixation?

1. Plants absorbing nitrate from soil
2. Converting nitrate into N₂ gas
3. Converting ammonia into nitrate
4. Converting N₂ gas into ammonia