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The Nitrogen Cycle
Nitrogen is an element that is part of
proteins (amino acids) and nucleic acids
(DNA/RNA). Like some other substances,
nitrogen is recycled in the environment.
Plants, animals, and bacteria are all
important to the nitrogen cycle.
Aerial nitrogen (nitrogen gas) is not usable
by most organisms. The nitrogen that
plants use comes from nitrogen-containing
minerals in the soil called nitrates (NO3-).
Green plants take in nitrates from the soil
through their roots. Inside plant cells,
nitrates are used to make amino acids, the
building blocks of proteins. Consumers
depend directly or indirectly on the food
produced by the plants to supply them with
protein.
The conversion of nitrogen gas (N2) to nitrate (NO3-) is known as nitrogen fixation. Nitrogen fixation is carried
out mainly by bacteria found in the root nodules of legumes such as peas and beans. These bacteria take
atmospheric nitrogen (N2) and change it to nitrates, which the plants can then use.
When plants and animals excrete waste or die, the decomposing bacteria convert their nitrogen compounds to
ammonia- a process called ammonification. Then, because most plants cannot use ammonia (NH3), it is
converted into nitrate (NO3-) in a two-step process called nitrification. One group of soil bacteria
(chemosynthetic nitrite bacteria) change the ammonia into nitrites (NO2-), and another group of bacteria
(chemosynthetic nitrate bacteria) then converts the nitrites into nitrates.
Other bacteria convert ammonia, nitrite and nitrate back into nitrogen gas- a process known as denitrification.
Thus, nitrogen as (N2) is returned to the atmosphere.
Complete the paragraph below using some of the following terms:
herbivores
chemosynthetic
nitrogen gas
legumes
denitrifying
wastes
nitrogen fixation
ammonia
nitrate
ammonification
decomposing
nitrification
soil
nodules
nitrite
In the nitrogen cycle, atmospheric nitrogen is converted into usable nitrogen compounds.
Certain bacteria live in the ___________________ on the roots of ______________________.
These bacteria take in atmospheric nitrogen and convert it to _____________________. This
process is known as ________________ __________________. Animals such as omnivores
and ___________________ eat these plants, thus consuming nitrates that their bodies can use.
When the legumes, animals, or other plants die or excrete _________________, bacteria in the
soil convert these nitrogen compounds to ammonia in a process called ____________________.
Since plants cannot use ammonia, _______________________ bacteria in the soil convert the
ammonia to nitrate in a two-step process called __________________________. To complete
the cycle, _____________________ bacteria return the nitrogen found in ammonia, nitrates and
nitrites back to the atmosphere as nitrogen gas.