Environmental Engineering Reference
In-Depth Information
during which bacteria help convert the nitrogen into useable forms. Nitrogen is essential for life because it
helps to develop proteins, DNA, and RNA, and provide for plant growth. It can also be a limiting factor in
plant growth.
The nitrogen cycle begins in the atmosphere as a gas (N
2
). It then goes through many steps throughout its
cycle.
• For it to be made useable by organisms, N
2
needs to be "fixed." This can occur by lightning or with the
help of nitrogen-fixing bacteria. Nitrogen-fixing bacteria live in the soil and in nodules on the roots of
legumes.
• When N
2
is fixed, it's combined with hydrogen to form ammonia (NH
3
). This fixing process is called
ammonification.
• The water-soluble ion of NH
3
is ammonium (NH
4
+
), which can be taken in by plants through their roots.
• NH
4
+
then goes through a process known as nitrification, in which it is converted into nitrite ions (NO
2
-
)
and then nitrate ions (NO
3
-
). This process is conducted by specialized bacteria called nitrifying bacteria.
These ions can also be taken in by plants since they, too, are water soluble.
• The ammonium, nitrite, and nitrate ions can be assimilated and taken in by plants. Animals then receive
nitrogen through consuming plants. Decomposers receive nitrogen through the decomposition of waste
and decaying plants and animals.
• Decomposers process the nitrogen substances they take in and return the nitrogen to the soil as ammoni-
um ions. Decomposition makes the nitrogen available to go through nitrification again.
• For nitrogen to return to the atmosphere in its gaseous N
2
form, it must be denitrified. Denitrifying bac-
teria convert nitrates into N
2
.
Humans have intervened in the nitrogen cycle by developing a way to fix nitrogen artificially, creating fertil-
izer. This fixation process, called the
Haber-Bosch process,
conducted on an enormous scale, has negatively
altered the nitrogen cycle by nearly doubling the amount of nitrogen fixation occurring on the Earth. Excess ni-
trogen in an aquatic environment can lead to the eutrophication of the ecosystem.
