Download The Nitrogen Cycle

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Principles of Ecology
The Nitrogen Cycle—Note Guide, KEY
Nitrogen is the most abundant component of earth’s atmosphere. Plants use nitrogen to make protein, and animals eat
protein to make flesh. Nitrogen is converted between various forms by the nitrogen cycle. Nitrogen is not usable in
atmospheric form (N2) so it must be fixated into one of its biological forms—nitrite (NO2-), nitrate (NO3-), or ammonium
(NH4+). Nitrogen-fixing bacteria convert nitrites into nitrates, a critical step of fixation because nitrites can be toxic to
plants.
1. Denitrification converts biological nitrogen (proteins—NO3- & NH4+) into atmospheric nitrogen.
Nitrification by bacteria convert NH4+ into NO2- and then into NO3-.
Fixation converts N2 to biologically active forms of nitrogen (NO3- & NH4+).
2. Biological fixation: bacteria in the roots of legumes fix nitrogen.
3. Atmospheric fixation: lightning converts N2 to NO3- & NH4+.
4. Industrial fixation: factories make fertilizer from atmospheric nitrogen.
Why is nitrogen important?
Nitrogen is necessary for all life on earth as it is a key element that forms nucleic acids that are a part of DNA,
RNA, and amino acids (protein).
How can humans alter the nitrogen cycle?
Overuse or misuse of nitrate fertilizer on crops can result in runoff to bodies of water. This buildup of nitrate can
then lead to eutrophication. Human use of nitrogen fertilizers has greatly increased the amount of biological nitrogen in
the biosphere. This results in production of nitrous oxide, a greenhouse gas, and acid rain. Nitrous oxide contributes to
the destruction of the ozone layer.
What organisms fix atmospheric nitrogen?
Bacteria are the only organisms that can biologically fix nitrogen. Some of these bacteria live symbiotically in the
root nodules of legumes (peas, lentils, beans, clover, alfalfa, peanut). The most common bacterium found in the root
nodules of leguminous plants is Rhizobium.
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