.Nitrogen Cycle.ppt how the nitrogen are fixed in soil

Nitrogen cycle

1. Nitrogen Cycle

2. The Nitrogen Cycle
1)Mineralization
a)Aminization
b)Ammonification
c)Nitrification
2) Immobilization
3) Gains of nitrogen by soil
a)Biological nitrogen fixation
b)Addition through precipitation
c)Addition through manures and fertilizers
4) Fixation of nitrogen in soils
5) Losses of nitrogen from soils
a)Leaching
b)Volatilization
c)Denitrification
d)Runoff
e)Plant assimilation

4. R-NH
2
NH
4 NO
2
NO
3
NO
2
NO
N
2O
N
2

5. 1. Nitrogen mineralization
Theprocessofbreakdownoforganic-Ncompounds bythe
microorganismstothemineralNformsistermedas
mineralization.
I.Aminization:astheprocessbywhichthehydrolytic
decompositionofproteinsfromcombinednitrogenous
compoundsaswellasreleaseofaminesandaminoacids
takesplacebyheterotrophicsoilmicroorganisms.
Proteins R-NH
2 + CO
2+ energy + others
This process is also known as proteolysis.
End product under aerobic condition-CO
2, (NH
4)
2, SO
4, H
2O
Anaerobic condition-NH
3, NH
2, CO
2, organic acids, H
2S etc.

6. Ammonification or Mineralization
R-NH
2
NH
4 NO
2
NO
3
NO
2
NO
N
2O
N
2

7. II. Ammonification:
R-NH
2 + HOH NH
3+ NR-OH + energy
+H
2O
NH
4
+
+ OH
-
Ammonification increases pH of soil because
release of OH
-
and CO
2
-
.
This conversion process believed to slow.
Optimum temp.-46-60
0
C
Moisture-50-75 % of water holding capacity

8. Nitrification
R-NH
2
NH
4 NO
2
NO
3NO
2
NO
N
2O
N
2

9. III. Nitrification: is a two step process
A.Conversion of ammonium to nitrite (NO
2
-
)
2NH
4+ 3O
2 2NO
2
-
+ 2H
2O + 4H
+
Various genera like Nitrosomonas, Nitrosolubusand
Nitrosospira
B.Conversion of nitrite (NO
2
-
) to nitrate (NO
3
-
)
2NO
2
-
+ O
2 NO
3
-
Nitrosomonasand Nitrobacterare collectively k/a
Nitrobacteria.
BothammoniumandNO
2
-
oxidisersareobligatory
aerobic.Therefore,inwaterloggedsoilsthe
oxidationofNH
4
+
isthusrestricted.

10. 2. Immobilization
Nitrogen immobilization means the change of mineral
nitrogen to organic forms by soil microorganisms.
C:N ratio Process operates
Narrow (<20:1) Mineralization (release of inorganic
nitrogen)
Critical (20 to 30:1) Neither net mineralization or nor
netimmobilization
Wider (>30:1) Immobilization (production of
organic nitrogen)

11. Nitrogen Fixation
R-NH
2
NH
4 NO
2
NO
3
NO
2
NO
N
2O
N
2

12. 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)

13. Microorganisms fixing
•Azobacter
•Beijerinckia
•Azospirillum
•Clostridium
•Cyanobacteria
•Require the enzyme
nitrogenase
•Inhibited by oxygen
•Inhibited by
ammonia(end
product)

14. Rates of Nitrogen Fixation
N
2fixing systemNitrogen Fixation (kg
N/hect/year)
Rhizobium-legume 200-300
Cyanobacteria-moss 30-40
Rhizosphere
associations
2-25
Free-living 1-2

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

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

18. Nitrification
Two step reactions that occur together :
•1
rst
step catalyzed by Nitrosomonas
2 NH
4
+
+ 3 O
22 NO
2
-
+2 H
2O+ 4 H
+
•2
nd
step catalyzed by Nitrobacter
•2 NO
2
-
+ O
22 NO
3
-

19. •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?

20. Denitrification
R-NH
2
NH
4 NO
2
NO
3
NO
2
NO
N
2O
N
2

21. Denitrification
•Removesa limiting nutrient from the
environment
•4NO
3
-
+ C
6H
12O
62N
2+ 6 H
20
•Inhibited by O
2
•Not inhibited by ammonia
•Microbial reaction
•Nitrate is the terminal electron acceptor

22. Looking at the Nitrogen
cycle through the eye of
NH
4

24. 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

25. Surfac
e
water
Oxidized
layer
Reduce
d soil
layer
[NH
4]
HIGH
Low
[NH
4]
Slow Diffusion
nitrificatio
n
[NO
3]
high

26. Surfac
e
water
Oxidized
layer
Reduce
d soil
layer
[NO
3]
high
Leaching
[NO
3] Low
N
2
Denitrificatio
n