Earth as a System: Energy, Matter, and Life
Nitrogen and oxygen cycles
Follow nitrogen and oxygen as they cycle through the Earth.
Living things need nitrogen to make proteins and nucleic acids (DNA). The air is 78% nitrogen, but nitrogen gas (N₂) hardly reacts: its two atoms are held by a strong triple bond (Chapter 9). Plants and animals can't use it directly, just as you can't breathe the oxygen dissolved in water the way a fish can. It must first be changed into compounds that dissolve in water.
The nitrogen cycle, step by step (one helper for each arrow):
| Step | What happens | Who does it |
|---|---|---|
| Nitrogen fixation | N₂ from the air → ammonia (NH₃) | Rhizobium (in the root nodules of pea and bean plants), Azotobacter (in soil); a little by lightning |
| Nitrification | Ammonia → nitrite (NO₂⁻) → nitrate (NO₃⁻) | Nitrosomonas (first step), Nitrobacter (second step) |
| Assimilation | Plants take in nitrates; animals get nitrogen by eating plants or other animals | Plants and animals |
| Ammonification | Dead bodies and wastes → ammonia again | Decomposers (bacteria and fungi) |
| Denitrification | Some nitrate → back to N₂ gas | Pseudomonas |
"Bread from air." In the early 1900s, the Haber-Bosch process began making ammonia from the nitrogen in the air, for fertilisers. It made India's Green Revolution possible and feeds billions. More than half the nitrogen atoms in your body came from it. But it uses about 1 to 2% of all the world's energy, and too much fertiliser has damaged soil and water.
Try this: take your last meal, say dal and roti. The carbon in it came from carbon dioxide in the air, through photosynthesis. The nitrogen came from the air too, fixed by bacteria or by a fertiliser factory, then taken up by the plant.
The oxygen cycle. About 21% of the air is oxygen. It is used up by:
- respiration (plants and animals breathing);
- combustion (burning fuel);
both of which give out carbon dioxide. It is put back by photosynthesis. Oxygen is also locked up in metal oxides and minerals in rocks, and in carbon dioxide.
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