Nitrogen Cycle
The nitrogen cycle is the movement of nitrogen through
different environmental segments.
Nitrogen is essential for life. It is found in amino acids, proteins, and genetic
Nitrogen in our environment
Nitrogen is the most abundant element in the atmosphere (~78%). However, gaseous
nitrogen must be 'fixed' into another form so that it can be used by living organisms.
Forms of Nitrogen useable for plants
 Urea  CO(NH2)2
 Ammonia  NH3 (gaseous)
 Ammonium  NH4
 Nitrate  NO3
 Nitrite  NO2
Nitrogen Fixation
Nitrogen fixation means making atmospheric nitrogen (N2) available for plant consumption through
ammonia and nitrates. Nitrogen fixation occurs naturally in the following ways.
 Lightening
 Biological fixation
Fixation by Lightning
 The energy from lightning
causes nitrogen (N2) and
water (H2O) to combine to
form ammonia (NH3) and
nitrates (NO3). Precipitation
carries the ammonia and
nitrates to the ground, where
they can be assimilated by
Biological Fixation
 About 90% of nitrogen fixation is done by bacteria.
Cyanobacteria convert nitrogen into ammonia and
N2 + 3H2 → 2NH3
 Ammonia can be used by plants directly. Ammonia and
ammonium may be further reacted in the nitrification
 When plants and animals die, bacteria convert nitrogen
nutrients back into ammonium salts and ammonia. This
conversion process is called ammonification.
Amino acids + 11/2O2  CO2 + H2O + NH3
+ 736kJ
 This process liberates a lot of energy which can be used by
the Saprotrophic microbes
 Nitrification occurs by the following reactions:
2 NH3 + 3 O2 → 2 NO2 + 2 H+ + 2 H2O
2 NO2- + O2 → 2 NO3 Aerobic bacteria use oxygen to convert ammonia and
ammonium. Nitrosomonas bacteria convert nitrogen into
nitrite (NO2-) and then nitrobacter convert nitrite to nitrate
The nitrogen fixers
 Cyanobacteria are nitrogen fixers that also fix carbon
(these are photosynthetic)
 Rhizobium bacteria are mutualistic with certain plant
species e.g. Legumes
 They grow in root nodules
 Azotobacter are bacteria associated with the rooting zone
(the rhizosphere) of plants in grasslands
Nitrogen fixation by bacteria
 Atmospheric nitrogen is chemically
fixed by bacteria to ammonium
(NH4+1) , which is used by plants as a
 Some bacteria exist in a symbiotic
relationship with plants (legumes and
some root-nodule species). Plants
utilize the nitrate as a nutrient.
Animals obtain nitrogen by eating
plants or plant-eating animals.
Only prokaryotes show nitrogen fixation
 These organisms possess the nif gene complex which make the proteins, such as
nitrogenase enzyme, used in nitrogen fixation
 Nitrogenase is a metalloprotein, protein subunits being combined with an iron,
sulphur and molybdenum complex
 The reaction involves splitting nitrogen gas molecules and adding hydrogen to
make ammonia
N2  2N
- 669 kJ
2N + 8H+  NH3 + H2 + 54 kJ
 This is extremely energy expensive requiring 16 ATP molecules for each
nitrogen molecule fixed
 The microbes that can fix nitrogen need a good supply of energy
 This involves two oxidation processes
 The ammonia produced by ammonification is an energy rich
substrate for Nitrosomas bacteria
They oxidise it to nitrite:
NH3 + 11/2O2  NO2- + H2O
+ 276kJ
This in turn provides a substrate for Nitrobacter bacteria oxidise
the nitrite to nitrate:
NO3- + 1/2O2  NO3-
+ 73 kJ
 This energy is the only source of energy for these
prokaryotes so they are called chemoautotrophs.
Return to the atmosphere: Denitrification
 Nitrates and nitrites can be used a source of oxygen for
Pseudomonas bacteria
 Favourable conditions: Cold waterlogged (anaerobic) soils
2NO3-  3O2 + N2
2NO2-  2O2 + N2 
providing up to 2385kJ
 The liberated oxygen is used as an electron acceptor in the
processes that oxidise organic molecules, such as glucose
 These microbes are, therefore, heterotrophs
Industrial N-Fixation
 The Haber-Bosch Process
N2 + 3H2  2NH3 - 92kJ
The Haber process uses an iron catalyst
High temperatures (500°C)
High pressures (250 atmospheres)
The energy required comes from burning fossil fuels (coal, gas or
Hydrogen is produced from natural gas (methane) or other
Nitrogen Cycle
Atmospheric Pollution
 The exhaust emissions of cars contribute a lot to atmospheric
pollution in the form of NOx
These compounds form photochemical smogs
They are green house gases
They dissolve in rain to contribute to acid rain in the form of
nitric acid
The rain falling on soil and running into rivers
They contribute to the Eutrophication of water bodies
How to make things better?
 The need for synthetic fertilisers can be reduced by cultural
Avoiding the use of soluble fertilisers in sandy (free draining soil)
prevents leaching
Rotating crops permits the soil to recover from nitrogen hungry
crops (e.g. wheat)
Adding a nitrogen fixing crop into the rotation cycle
Ploughing aerates the soil and reduces denitrification
Draining water logged soil also helps reduce denitrification

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