Nitrogen Cycle

Nitrogen Cycle: Processes, Uses
and Impacts



Nitrogen
• Critical for survival of living beings
• Often acts as limiting factor in the primary
production in many ecosystems
• 79% of atmospheric gaseous volume
• Inaccessible in this form to plants and animals
• Needs to be transformed in to different states
to be accessible



Nitrogen cycle
• Cyclic transformation of N
2
in to different
states and return to atmosphere
• Gaseous bio-geochemical cycle
• Main reservoir is atmosphere
• Maintains amount of fixed N
2
in to ecosystem
• Essential for the growth of plants and animals




Schematic representation of nitrogen cycle
Different Steps of Nitrogen Cycle
• Nitrogen fixation
• Nitrification
• Denitrification
• Ammonification

Nitrogen Fixation
• Reduction of N
2
into biologically available
form (NH
4
+
)
N
2
+ 8H
+
+ 8e
-
+ 16 ATP 2 NH
4
+
+ 16 ADP + 16 Pi
• High energy process
• Requires 8 electrons and 16 ATP molecules
• N
2
converted in to biologically active form
• Now can be accessed by plants and animals


Different ways of nitrogen fixation
• Abiotic Processes- lightening, Haber’s process
• Biotic processes
Free living bacteria-
– Aerobic (Acetobacter, Azatobacter)
– Anaerobic (Clostridium, Rhodospirrilium)
 Cyanobacteria (Anabaena, Nostoc)
Symbiosis (Rhizobium in legumes)

Nitrogenase
• Enzyme for nitrogen fixation
• Sensitive to oxygen
• Oxygen need to be removed
• Different strategies-
 Presence of heterocyst in cyanobateria
 Leghaemoglobin protein in Rhizobium

Nitrification
• Oxidation of NH
4
+
into further oxidation states
• Aerobic process
• Mostly carried out by prokaryotes
NH
4
+
NO
2
-
NO
2
-
NO
3
-
• Plants convert NO
3
-
into NH
4
+
for assimilation

• Accumulated nitrites are toxic
• Highly soluble, enter groundwater

Nitrosomas
Nitrobacter
Denitrification
• Reduction of NH
4
+
and NO
3
–
back into N
2
• Returns fixed N
2
back to atmosphere
• Occur under anaerobic conditions and mostly
in soil
• Pseudomonas and Clostridium
• Use NO
3
–
as an electron acceptor in place of
O
2
during respiration
Ammonification
• Conversion of organic nitrogen into NH
4
+
when
plant or animal die
• Done by saprotrophs
• Becomes available for uptake by palnts and
other organisms

Uses of Nitrogen Cycle

Human Impacts on Nitrogen Cycle

References
• Bernhard, A. (2010) the nitrogen cycle:
processes, players and human impact. Nature
education knowledge 3 (10).25
• E. P. Odum, G.W. Barrett; Fundamentals of
Ecology, 5e, 2005, p(143-149)
•