Nitrogen Cycle

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Nitrogen Cycle
Sources
• Lightning
• Inorganic fertilizers
• Nitrogen Fixation
• Animal Residues
• Crop residues
• Organic fertilizers





Forms of Nitrogen
• Urea  CO(NH2)2
• Ammonia  NH3 (gaseous)
• Ammonium  NH4
• Nitrate  NO3
• Nitrite  NO2
• Atmospheric Dinitrogen N2
• Organic N
Global Nitrogen Reservoirs
Nitrogen
Reservoir
Metric tons
nitrogen
Actively cycled
Atmosphere 3.9*10
15
No
Ocean 
soluble salts
Biomass

6.9*10
11
5.2*10
8


Yes
Yes
Land  organic
matter
 Biota


1.1*10
11
2.5*10
10


Slow
Yes
Roles of Nitrogen
• Plants and bacteria use nitrogen in the
form of NH
4
+
or NO
3
-
• It serves as an electron acceptor in
anaerobic environment
• Nitrogen is often the most limiting
nutrient in soil and water.
Nitrogen is a key element for

• amino acids
• nucleic acids (purine, pyrimidine)
• cell wall components of bacteria (NAM).
Nitrogen Cycles
• Ammonification/mineralization
• Immobilization
• Nitrogen Fixation
• Nitrification
• Denitrification

R-NH
2
NH
4
NO
2
NO
3
NO
2
NO
N
2
O
N
2
Ammonification or Mineralization
R-NH
2
NH
4
NO
2
NO
3
NO
2
NO
N
2
O
N
2
Mineralization or Ammonification
• Decomposers: earthworms, termites, slugs,
snails, bacteria, and fungi
• Uses extracellular enzymes  initiate
degradation of plant polymers
• Microorganisms uses:
• Proteases, lysozymes, nucleases to degrade
nitrogen containing molecules

• Plants die or bacterial cells lyse  release of
organic nitrogen
• Organic nitrogen is converted to inorganic
nitrogen (NH
3
)

• When pH<7.5, converted rapidly to NH
4

• Example:

Urea NH
3
+ 2 CO
2

Immobilization
• The opposite of mineralization
• Happens when nitrogen is limiting in the
environment
• Nitrogen limitation is governed by C/N ratio
• C/N typical for soil microbial biomass is 20
• C/N < 20 Mineralization
• C/N > 20 Immobilization
Nitrogen Fixation
R-NH
2
NH
4
NO
2
NO
3
NO
2
NO
N
2
O
N
2
Nitrogen Fixation
• Energy intensive process :

• N
2
+ 8H+ + 8e
-
+ 16 ATP = 2NH
3
+ H
2
+
16ADP + 16 Pi

• Performed only by selected bacteria and
actinomycetes
• Performed in nitrogen fixing crops
(ex: soybeans)
Microorganisms fixing
• Azobacter
• Beijerinckia
• Azospirillum
• Clostridium
• Cyanobacteria
• Require the enzyme
nitrogenase
• Inhibited by oxygen
• Inhibited by
ammonia (end
product)
Rates of Nitrogen Fixation
N
2
fixing system Nitrogen Fixation (kg
N/hect/year)
Rhizobium-legume 200-300
Cyanobacteria- moss 30-40
Rhizosphere
associations
2-25
Free- living 1-2

Applications to wetlands
• Occur in overlying waters
• Aerobic soil
• Anaerobic soil
• Oxidized rhizosphere
• Leaf or stem surfaces of plants


Bacterial Fixation
• Occurs mostly in salt marshes
• Is absent from low pH peat of northern
bogs
• Cyanobacteria found in waterlogged
soils

Nitrification
R-NH
2
NH
4
NO
2
NO
3
NO
2
NO
N
2
O
N
2
Nitrification
Two step reactions that occur together :

• 1
rst
step catalyzed by Nitrosomonas
2 NH
4
+
+ 3 O
2
 2 NO
2
-
+2 H
2
O+ 4 H
+

• 2
nd
step catalyzed by Nitrobacter
• 2 NO
2
-
+ O
2
 2 NO
3
-


• Optimal pH is between 6.6-8.0
• If pH < 6.0  rate is slowed
• If pH < 4.5  reaction is inhibited
In which type of wetlands do
you thing Nitrification occurs?
Denitrification
R-NH
2
NH
4
NO
2
NO
3
NO
2
NO
N
2
O
N
2
Denitrification
• Removes a limiting nutrient from the
environment
• 4NO
3
-
+ C
6
H
12
O
6
 2N
2
+ 6 H
2
0
• Inhibited by O
2
• Not inhibited by ammonia
• Microbial reaction
• Nitrate is the terminal electron acceptor
Looking at the Nitrogen
cycle through the eye of
NH
4
Surfac
e
water
Oxidized
layer
Reduce
d soil
layer
[NH
4
]
HIGH
Low
[NH
4
]
Slow Diffusion
Biodegradati
on
C/N <20
C/N >20
Surfac
e
water
Oxidized
layer
Reduce
d soil
layer
[NH
4
]
HIGH
Low
[NH
4
]
Slow Diffusion
nitrificatio
n
[NO
3
]
high
Surfac
e
water
Oxidized
layer
Reduce
d soil
layer
[NO
3
]
high
Leaching
[NO
3
] Low
N
2
Denitrificatio
n

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