The nitrogen cycle describes the transformation of nitrogen between inorganic and organic forms. Key processes include nitrogen fixation by bacteria and archaea, which converts atmospheric nitrogen to ammonium; ammonification and nitrification, which convert organic and ammonium nitrogen to nitrates; and denitrification, where nitrates are converted back to nitrogen gas and released to the atmosphere. Nitrogen is essential for amino acids, proteins and nucleic acids and cycles through different reservoirs in the biosphere, hydrosphere, lithosphere and atmosphere.
6. Roles of Nitrogen
• Plants and bacteria use nitrogen in the
form of NH4
+
or NO3
-
• It serves as an electron acceptor in
anaerobic environment
• Nitrogen is often the most limiting
nutrient in soil and water.
7. Nitrogen is a key element for
• amino acids
• nucleic acids (purine, pyrimidine)
• cell wall components of bacteria (NAM).
11. 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
12. • Plants die or bacterial cells lyse release of
organic nitrogen
• Organic nitrogen is converted to inorganic
nitrogen (NH3)
• When pH<7.5, converted rapidly to NH4
• Example:
Urea NH3 + 2 CO2
13. 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
15. Nitrogen Fixation
• Energy intensive process :
• N2
+ 8H+ + 8e-
+ 16 ATP= 2NH3
+ H2
+ 16ADP
+ 16 Pi
• Performed only by selected bacteria and
actinomycetes
• Performed in nitrogen fixing crops
(ex: soybeans)
16. Microorganisms fixing
• Az o bacte r
• Be ije rinckia
• Az o spirillum
• Clo stridium
• Cyano bacte ria
• Require the enzyme
nitrogenase
• Inhibited by oxygen
• Inhibited by
ammonia (end
product)
17. Rates of Nitrogen Fixation
N2 fixing system Nitrogen Fixation (kg
N/hect/year)
Rhizobium-legume 200-300
Cyanobacteria- moss 30-40
Rhizosphere
associations
2-25
Free- living 1-2
18.
19. Applications to wetlands
• Occur in overlying waters
• Aerobic soil
• Anaerobic soil
• Oxidized rhizosphere
• Leaf or stem surfaces of plants
20. Bacterial Fixation
• Occurs mostly in salt marshes
• Is absent from low pH peat of northern
bogs
• Cyanobacteria found in waterlogged
soils
22. Nitrification
Two step reactions that occur together :
• 1rst
step catalyzed by Nitro so m o nas
2 NH4
+
+ 3 O2 2 NO2
-
+2 H2O+ 4 H+
• 2nd
step catalyzed by Nitro bacte r
• 2 NO2
-
+ O2 2 NO3
-
23. • 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?
25. Denitrification
• Removes a limiting nutrient from the
environment
• 4NO3
-
+ C6H12O6 2N2 + 6 H20
• Inhibited by O2
• Not inhibited by ammonia
• Microbial reaction
• Nitrate is the terminal electron acceptor
