Biogeochemical cycles - Nitrogen cycle

Nitrogen Cycle

Sources of Nitrogen

• 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 Nitrogen

Role/Importance 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.
• Nitrogen is a key element for:
• amino acids
•  nucleic acids (purine, pyrimidine) 
• cell wall components of bacteria

Nitrogen Cycles

• Ammonification/mineralization
• Immobilization
• Nitrogen Fixation 
• Nitrification
• Denitrification

Ammonification or Mineralization or Nitrogen fixation

• 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

When plants or bacterial cells die, they release of organic nitrogen 

Organic nitrogen is converted to inorganic nitrogen (NH3)

When pH is less than 7.5, NH3 is converted rapidly to NH4

Immobilization

• It is the opposite of mineralization
• Takes place 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 takes place
• C/N > 20 Immobilization takes place

Nitrogen Fixation is

• Energy intensive process
• Performed only by selected bacteria and actinomycetes
• Performed in nitrogen fixing crops (ex: soybeans)

Microorganisms responsible for fixing nitrogen are:

• Azobacter
• Azospirillum
• Clostridium
• Cyanobacteria

They

• Require the enzyme nitrogenase
• Inhibited by oxygen and ammonia (end product)

Nitrification

Two step reactions that occur together :

First step catalyzed by Nitrosomonas

Second step catalyzed by Nitrobacter

Optimal pH is between 6.6-8.0

Denitrification

Removes a limiting nutrient from the environment

Inhibited by O2

Not inhibited by ammonia

Microbial reaction

Nitrate is the terminal electron acceptor