Figure 9.21: The carbon cycle.Nitrogen cycle DUMMY
Nitrogen (N 2 ) makes up most of the gas in the atmosphere (about 78%). Nitrogen is impor-
tant to living organisms and is used in the production of amino acids, proteins and nucleic
acids (DNA, RNA).- Nitrogen gas present in the air is NOT available to organisms and thus has to be made
available in a form absorbable by plants and animals. - Only a few single-cell organisms, like bacteria can use nitrogen from the atmosphere
directly. - For multicellular organisms, like plants and animals, nitrogen has to be changed into
other forms, eg. nitrates or ammonia. This process is known as nitrogen fixation.
The nitrogen cycle involves the following steps:- Lightning: nitrogen can be changed to nitrates directly by lightning. The rapid growth
of fungi and algae after thunderstorms is because of this process, which increases the
amount of nitrates that fall onto the earth in rain water, acting as fertilizer. - Decomposition: during decomposition, bacteria and fungi break down proteins and
amino acids from plants and animals. - Ammonification: the nitrogenous breakdown products of amino acids are converted
to into ammonia (NH 3 ) by these decomposing bacteria. - Nitrification: is the conversion of the ammonia to nitrate (NO 3 −) by nitrifying bacte-
ria. - Absorption: ammonia and nitrates are absorbed by plants through their roots.
- Ingestion: humans and animals get their nitrogen supplies by eating plants or plant-
eating animals. - Denitrification: in a process called denitrification, bacteria convert ammonia and ni-
trate into nitrogen and nitrous oxide (N 2 O). Nitrogen is returned to the atmosphere to
start the cycle over again.
284 9.7. Nutrient cycles