Nitrogen Fixation: Nitrogen fixation is
one process by which molecular nitrogen is
reduced to form ammonia.
Ammonification: The chemical
transformation of organic nitrogen into
ammonia by the metabolic breakdown of
proteins and amino acids.
Nitrification: The oxidation (loss of
electrons) of ammonia by specialized
bacteria
Draw and label a graphic showing your
understanding of the carbon cycle.
Nitrogen Cycle
Why is nitrogen important to
organisms?
 Organisms use nitrogen to carry out life functions
 Nitrogen is an important component of DNA, RNA, proteins, and
chlorophyll
Nitrogen Cycle
 The earth’s atmosphere is composed of almost 80% nitrogen, but
it is mostly in the form of nitrogen gas (N2)
 N2 is unavailable to plants and hence to consumers of plants
Nitrogen Cycle
Nitrogen Cycle
 Lightning discharges convert some of the molecular nitrogen
into forms that plants can use
 Most nitrogen in the atmosphere enters the biological
pathways of the nitrogen cycle through nitrogen fixation
Nitrogen Fixation
 Biological absorption of atmospheric nitrogen to form
organic nitrogen containing compounds
 Nitrogen fixation is one process by which molecular
nitrogen is reduced to form ammonia
Ammonification
 Plants obtain nitrogen from the soil either as ammonium or
as nitrate
 With an input of energy, plants reduce nitrogen to an organic
form such as ammonium
 Organic nitrogen is the most reduced form of the nitrogen
atom, with the highest potential chemical energy
 Organic nitrogen is used to construct proteins
Ammonification
 Proteins are eventually metabolized (broken down to yield
energy)
 Excess nitrogen is excreted into the environment as waste
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Hydrolysis: A chemical process that splits molecules by the addition of
water
Oxidation: The loss of elections from a substance involved in a redox
reaction
Redox Reaction: A chemical reaction involving the transfer of one or more
elections from one reactant to another
Ammonification
 Ammonification occurs through the breakdown of proteins
into amino acids by hydrolysis and oxidation of the carbon in
those amino acids
 This process results in the production of ammonia
 Ammonia is usually transformed to ammonium
Nitrification
 Nitrification: The oxidation (loss of electrons) of ammonia by
specialized bacteria
 Nitrification yields nitrite and nitrate
 Oxidation steps release much of the potential chemical
energy of organic nitrogen
 Each step is carried out by specialized bacteria
Denitrification
 In water-logged anaerobic soils and in oxygen depleted
bottom waters, organic matter is broken down through
denitrification

Nitrate is converted back to atmospheric nitrogen
Nitrogen Fixation
 The loss of readily available nitrogen
through denitrification is offset by
nitrogen fixation
 Nitrogen fixation is accomplished by
specialized bacteria (prokaryotes)
 Azobacter: a free living species
 Rhizobium: occurs in a symbiotic relationship
with the roots of some legumes
Nitrogen Fixation
 Convert atmospheric nitrogen to minerals that can be used to
synthesize nitrogenous organic compounds
 Some cyanobacteria fix nitrogen in aquatic ecosystems
 Organisms that fix nitrogen are fulfilling their own metabolic
requirements, but the excess ammonia they release becomes
available to other organisms
Nitrogen Fixation
 The direct product of nitrogen fixation is ammonia
 Most soils are at least slightly acidic, and ammonia released
into the soil picks up a hydrogen ion to form ammonium
 Ammonium can be used directly by plants
Nitrogen Fixation
 Ammonia is a gas and can evaporate back into the
atmosphere
 Ammonia lost from the soil may then form ammonium in the
atmosphere
 As a result, ammonium concentrations in rainfall are
correlated with soil pH over large regions
 Most of the ammonium in the soil is used by certain aerobic
bacteria as an energy source (nitrification)
What must happen to nitrogen before
plants and animals can use it?
 Nitrogen must be converted, or “fixed”, into forms
that are useable by plants (ammonium or nitrate
ions). Then, animals eat the plants and assimilate the
nitrogen from the plants.
What organisms are able to “fix”, or
convert, nitrogen to be usable by
plants? How else might nitrogen be
“fixed” or converted?
 Symbiotic and free-living bacteria are able to fix nitrogen to
be usable by plants. Nitrogen can also be fixed by lightning
strikes, fires, or through industrial methods. However, it is
mostly done by bacteria.
Important Concepts of the nitrogen
cycle
 nitrogen in the atmosphere
 nitrogen in decaying matter and waste
 nitrogen assimilating to animals
 nitrogen fixation by bacteria
 decomposition by bacteria and fungi
 denitrification and nitrification.