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
Write from nitrates and nitrogen gas
- Plants cannot use nitrogen gas directly.
- Another group of bacteria turns nitrates back into nitrogen gas.
What do plants absorb, and what do denitrifying bacteria do?
Bacteria convert nitrogen between the gas and the soil forms.
What do plant roots absorb to obtain nitrogen?
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Nitrates are the usable form named in the lesson.