Very few organisms can use nitrogen found in the atmosphere
directly. Most plants absorb nitrogen only in the form of nitrates NO2.
Atmospheric nitrogen is fixed (converted) by various bacteria. Plants absorb
nitrogen compounds either directly from nitrogen fixing bacterial or indirectly
from inorganic compounds that the bacteria excrete into the soil or water.
When an organism dies, the nitrogen that it contains in
organic molecules is converted back into inorganic forms by different decay
organisms.
Nitrogen is returned to the atmosphere as a gas by
denitrifying bacteria that break up organic molecules for their own food,
releasing any nitrogen they contain.
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