The nitrogen cycle
Introduced| English | Meaning |
|---|---|
| nitrogen cycle/ˈnaɪtrədʒn ˈsaɪkl/ | The movement and conversion of nitrogen between organisms, soil, water and the atmosphere. |
| nitrates/ˈnaɪtreɪts/ | A compound containing the nitrate ion, NO3 with a single negative charge. |
| nitrogen-fixing/ˈnaɪtrədʒn ˈfɪksɪŋ/ | nitrogen-fixing |
| nitrifying/ˈnaɪtrɪfaɪɪŋ/ | nitrifying |
| denitrifying/diːˈnaɪtrɪfaɪɪŋ/ | denitrifying |
Recycling nitrogen
- Living things need nitrogen to make proteins and DNA — but they can't use the nitrogen gas that makes up most of the air.
- The nitrogen cycle 氮循环 turns it into a usable form and back again.
Why can't plants use nitrogen gas from the air directly?
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N₂ is unreactive; it must be turned into nitrates.
The problem with nitrogen gas
- The air is about 78% nitrogen gas (N₂), but it is unreactive — plants and animals can't use it directly.
- It must be turned into nitrates 硝酸盐 first.
The nitrogen cycle: microorganisms move nitrogen between the air, soil and living things
The nitrogen cycle
Nitrogen is recycled between the air, the soil and living things by several types of bacteria.
Nitrogen-fixing bacteria:
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They fix nitrogen into compounds plants can use.
The key bacteria
- Nitrogen-fixing 固氮 bacteria turn nitrogen gas into compounds plants can use.
- Decomposers release nitrogen from dead matter; nitrifying 硝化 bacteria make nitrates.
Plants take up nitrogen from the soil mainly as:
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Plants absorb nitrates to make proteins.
Denitrifying bacteria return nitrogen gas to the air.
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They complete the cycle by releasing N₂.
Animals obtain their nitrogen by ______ plants or other animals.
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Nitrogen passes along the food chain.
Into and out of living things
- Plants absorb nitrates from the soil to make proteins; animals get nitrogen by eating plants.
- Denitrifying 反硝化 bacteria return nitrogen gas to the air, completing the cycle.
Worked example. A dead leaf is broken down by decomposers, releasing nitrogen compounds. Nitrifying bacteria turn these into nitrates, which a new plant absorbs to build its own proteins — nitrogen recycled.
You've got it
- nitrogen gas (N₂) is unreactive; it must become nitrates for plants to use
- nitrogen-fixing and nitrifying bacteria make usable nitrogen; decomposers release it from dead matter
- plants absorb nitrates; animals get nitrogen by eating; denitrifying bacteria return N₂ to the air