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2001-08-14 14:25:21 (UTC)

You are still learning about cycles

The Nitrogen Cycle

All living things require nitrogen to make proteins. About
78 percent of the atmosphere is nitrogen (N2). In the form
of a gas, nitrogen is useless to plants and animals. To
become useful, the nitrogen must be first converted into a
nitrate (NO3) ­ a combination of oxygen and nitrogen. Only
in the nitrate form can plants absorb the nitrogen they

Lightning forms some nitrates. As well certain nitrogen-
fixing bacteria (found in soil) and algae in wet habitats
can convert inorganic nitrogen into nitrates. Many of the
nitrogen-fixing bacteria live in little nodes on the roots
of clover, alfalfa, and other members of the pea (legume)
family. The resulting nitrates are absorbed by the roots of
plants for use in the production of proteins. The tissues
of plants use the nitrogen atoms to produce amino acids,
which are then built into the proteins. Animals, in turn,
get the nitrogen that they need to make proteins by eating
plants or other animals.

When the plant or animal dies, decomposing bacteria and
fungi cause the body of the plant to decay. As plants or
animals decay, the amino acids that contain nitrogen are
broken down and ammonia gas (NH3) is released. Nitrite
bacteria convert this poisonous ammonia into nitrate
molecules, and other nitrate bacteria add a third atom to
nitrites to form nitrates. Plants now have a usable form of
nitrogen again. Many plants can use ammonia directly. These
plants do not have to wait until the ammonia has been
converted into a nitrate before they absorb it.

Nitrogen can be removed from the nitrate in the soil by
denitrifying bacteria. In this case, the released nitrogen
is returned to the atmosphere and the nitrogen cycle is

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