The Nitrogen Cycle
Introduced| English | Meaning |
|---|---|
| nitrogen fixation/ˈnaɪtrədʒn fɪkˈseɪʃn/ | The conversion of nitrogen gas into ammonia or ammonium that plants can use. |
| denitrification/diːˌnaɪtrɪfɪˈkeɪʃn/ | The microbial conversion of nitrate or nitrite into gaseous nitrogen compounds, returning nitrogen from soil or water to the atmosphere. |
| nitrifying bacteria/ˈnaɪtrɪfaɪɪŋ bækˈtɪərɪə/ | nitrifying bacteria |
The locked-up gas
- The air is mostly nitrogen — about 78% of every breath.
- Nitrogen is essential for proteins and DNA.
- Yet almost no living thing can use the nitrogen gas directly.
- Getting nitrogen into life takes a special journey: the nitrogen cycle.
Why nitrogen must be fixed
- Nitrogen gas (N2) has a very strong triple bond, hard to break.
- So plants and animals cannot use it straight from the air.
- Nitrogen fixation 固氮作用 turns N2 into ammonia and nitrates.
- This job is done mainly by special bacteria in soil and roots.
The nitrogen cycle: fixation makes N₂ usable; denitrification 反硝化作用 returns it to the air
Why can most living things not use the nitrogen gas in the air directly?
Difficulty 1/5 · 100 coins + a small bonus on first completion today
N2 has a very strong triple bond, so it must first be fixed into usable forms.
Nitrogen fixation is carried out mainly by…
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Nitrogen fixation by bacteria turns N2 into ammonia and nitrates plants can use.
Nitrogen enters the food chain
- Plants absorb the fixed nitrates through their roots.
- They use the nitrogen to build proteins and DNA.
- Animals get their nitrogen by eating plants.
- So fixed nitrogen passes up through the whole food chain.
The nitrogen cycle
Step around the cycle - nitrogen gas is fixed into usable forms, used by life, then returned to the air.
Plants use fixed nitrogen to build ____, and animals get it by eating plants.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Nitrogen is essential for proteins and DNA; animals get theirs by eating plants.
Back to the air
- When living things die, decomposers release their nitrogen into the soil.
- Nitrifying bacteria 硝化细菌 convert it into nitrates plants can reuse.
- Finally, denitrification returns nitrogen to the air as N2 gas.
- The loop is closed, ready to begin again.
Denitrifying bacteria return nitrogen to the air, completing the cycle.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Denitrification converts nitrates back to N2 gas, closing the loop.
Select all true statements about the nitrogen cycle.
Difficulty 2/5 · 140 coins + a small bonus on first completion today
Most life cannot use N2 gas directly — it must be fixed first. The other three are correct.
The whole cycle depends on bacteria, not big organisms. Without nitrogen-fixing and denitrifying bacteria, the nitrogen in the air could never reach living things or return. These tiny microbes are the hidden engine of the entire nitrogen cycle.
Farmers and the nitrogen cycle:
- Crops strip nitrogen from the soil as they grow.
- Farmers restore it — by planting legumes whose root bacteria fix nitrogen, or by adding fertiliser.
- Both put usable nitrogen back into the soil so the next crop can build its proteins.
The nitrogen cycle moves nitrogen from air to life and back. Because N2 gas is hard to use, nitrogen fixation by bacteria turns it into usable nitrates. Plants use these for proteins; animals get theirs by eating plants; and denitrification returns nitrogen to the air. Bacteria drive every key step.