The Nitrogen
Cycle
A2 OCR Biology
Asking questions is a sign of
INTELLIGENCE 
Unfortunately
all questions
must wait until
the end of the
lecture.
By the end of this session I will:
• be able to describe the importance of nitrogen
in living systems.
• outline the role of micro-organisms in the cycling
of nitrogen
• Describe the nitrogen cycle using the
appropriate AS/A2 scientific language.
• (k) describe the role of decomposers in the
decomposition of organic material;
• (l) describe how microorganisms recycle
nitrogen within ecosystems. (Only
Nitrosomonas, Nitrobacter and Rhizobium need
to be identified by name).
Nitrogen the basics – needed for
• Nitrogen (N) is an
element like carbon. All
creatures need nitrogen
to survive.
• Nitrogen is needed for the
synthesis of proteins e.g.
enzymes.
• Nitrogen is also an
important component of
nucleic acids
Nitrogen the basics – chemistry of
• There are huge amounts of nitrogen gas in
the atmosphere, but most animals and
plants have no way of using it as it is very
unreactive due to the triple covalent bond.
• It needs to be fixed (put into a biologically
useful compound). After it is fixed, it can
then start to move through the cycles and
organisms in an ecosystem.
Nitrogen the basics – sources of
• Let's start with the main sources of nitrogen.
Nitrogen gas is the most abundant element in
our atmosphere.
• The other main source of nitrogen is in the
nitrates of soil. The nitrogen in the atmosphere
cannot be used while the nitrates in the soil can
be used by plants. Nitrogen can be converted
into useful nitrate compounds by bacteria,
algae, and even lightning. Once in the soil, the
nitrogen becomes biologically accessible.
Forms of Nitrogen
•
•
•
•
•
•
•
•
Urea  CO(NH2)2
Ammonia  NH3 (gaseous)
Ammonium  NH4
Nitrate  NO3
Nitrite  NO2
Atmospheric Dinitrogen N2
Organic N
CHEMISTS WE NOW NEED TO
CORRECT SOME OF THESE IONS!!
Nitrates are recycled via microbes
Animal
protein
Soil organic nitrogen
Ammonification
Plant
protein
Root
uptake
Ammonium NH4+
Nitrification
Nitrite NO2Nitrification
Nitrate
NO3-
Importance of bacteria in the
nitrogen cycle
• Nitrogen fixing
bacteria
• Nitrifying bacteria
• Denitrifying bacteria
Nitrogen fixing bacteria (Mr M
discuss nitrogenase here!!)
Function…….
…… converts nitrogen into
nitrogen containing
compounds
E.g. ….
…… Rhizobium
Found in ….
….. Roots nodules of
leguminous plants and
soil
Mutualism
Nitrifying bacteria
Function….
….. Convert
ammonia in
nitrite ions and
then into nitrate
ions.
E.g. …..
…..Nitrosomonas
and
Nitrobacter.
Found in….
….well aerated
soils.
Denitrifying bacteria
Function….
…. Convert
nitrates into
nitrogen gas
Found in….
…. Sewage
works,
compost and
wet soils
Nitrogen in
the air
nitrogen fixing plant
eg pea, clover
animal protein
plant made
protein
root nodules
(containing nitrogen
fixing bacteria)
nitrates absorbed
dead plants & animals
urine & faeces
denitrifying
bacteria
decomposition by bacteria & fungi
ammonia
nitrates
bacteria
(nitrifying bacteria)
nitrites
bacteria
Other ways in which nitrogen can
be fixed
Lightning
Haber process
Lightning
• The electrical energy of lightning causes
nitrogen gas (N2) to react with oxygen
(O2) in the atmosphere to produce nitrate
ions (NO3-) which reach the soil dissolved
in precipitation.
• Recent research suggests the amount of
nitrates produced by lightning may be as
high as 50%.
Haber process
Basic summary of the nitrogen cycle
Eutrophication
Yet another example of pollution, eutrophication is when lakes become
stagnant due to careless use of fertiliser. There are six steps:
1) Inorganic fertilisers
used on fields are
washed into the lake
3) This growth causes overcrowding
and many plants die due to lack of
enough light or food
2) The fertiliser causes increased growth in water plants
13/04/2015
Eutrophication
4) Microorganisms and bacteria
increase in number due to the
extra dead material
6) The lack of oxygen
causes the death of fish
and other aquatic animals
Can’t…breathe…
5) These microorganisms
use up the oxygen in the
lake during respiration
13/04/2015
Eutrophication
4) Microorganisms and bacteria
increase in number due to the
extra dead material
6) The lack of oxygen
causes the death of fish
and other aquatic animals
5) These microorganisms
use up the oxygen in the
lake during respiration
13/04/2015