Science 20F
The Nitrogen Cycle
Life depends on the cycling of nitrogen.
Nitrogen assists in making proteins (building blocks of cells) and DNA.
Nitrogen gas (N2) makes up 79% of the Earth’s atmosphere. Unfortunately it is a
very stable gas and as a result quite difficult to introduce into the cycle.
To be useful to organisms, nitrogen must be supplied in another form, the nitrate
ion (NO3-) or sometimes called nitrates.
Atmospheric Nitrogen is converted into nitrates by two methods:
1) Nitrogen Fixation by Lightning
2) Nitrogen Fixation by Bacteria
Nitrogen Fixation Cycle by Lightning
Step 1: Energy from lightning causes nitrogen gas to react with oxygen in the air
producing nitrates.
Step 2: The nitrates dissolve in rain or surface water.
Step 3: Water, rich with nitrates, enters the soil and then move into plants
through their roots.
Step 4: Plants convert nitrates to DNA and proteins.
Step 5: Plants are consumed by animals passing on the much needed proteins.
Step 6: Decomposers break down the nitrogen containing chemicals into ammonia.
Step 7: Bacteria convert ammonia into nitrates using oxygen.
Note: At this point the cycle repeats itself beginning at step 3.
Step 8: Certain bacteria convert the nitrates into nitrogen gas (without using
oxygen). This process is called Denitrification. Denitrification ensures
the balance among soil nitrates, nitrates, and atmospheric nitrogen, and
completes the nitrogen cycle.
Nitrogen Fixation Cycle by Bacteria
Step 1: Certain bacteria found in soil converts nitrogen into nitrates. The nitrogen
fixing bacteria is also found in small lumps called nodules on the roots of
various plants such as soybeans, peas, or alfalfa.
The cycle continues similar to previous cycle beginning at step 5.