Transcript Nitrogen Cycle - University of Arizona
Nitrogen Cycle
Sources
• Lightning • Inorganic fertilizers • Nitrogen Fixation • Animal Residues • Crop residues • Organic fertilizers
Forms of Nitrogen
• Urea CO(NH 2 ) 2 • Ammonia NH 3 (gaseous) • Ammonium NH 4 • Nitrate NO 3 • Nitrite NO 2 • Atmospheric Dinitrogen N 2 • Organic N
Global Nitrogen Reservoirs
Nitrogen Reservoir Atmosphere Ocean
soluble salts Biomass Metric tons nitrogen
3.9*10 15 6.9*10 11 5.2*10 8
Actively cycled
No 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
N 2 N 2 O NH 4 NO NO 2 R-NH 2 NO 2 NO 3
Ammonification or Mineralization
N 2 N 2 O NH 4 NO 2 R-NH 2 NO NO 2 NO 3
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 organic nitrogen • Organic nitrogen nitrogen (NH 3 ) is converted to release of inorganic • 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
N 2
Nitrogen Fixation
N 2 O NH 4 NO 2 R-NH 2 NO NO 2 NO 3
Nitrogen Fixation
• Energy intensive process : •
N 2 + 8H+ + 8e 16ADP + 16 Pi + 16 ATP = 2NH 3 + H 2 +
• 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
Rhizobium-legume Cyanobacteria- moss
Nitrogen Fixation (kg N/hect/year)
200-300 30-40 Rhizosphere associations Free- living 2-25 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
N 2 N 2 O NH 4
Nitrification
R-NH 2 NO NO 2 NO 2 NO 3
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?
N 2 N 2 O NH 4
Denitrification
NO 2 R-NH 2 NO NO 2 NO 3
Denitrification
• Removes a limiting nutrient from the environment • 4NO 3 + C 6 H 12 O 6 • Inhibited by O 2 2N 2 + 6 H • Not inhibited by ammonia 2 0 • 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 Low [NH 4 ] [NH 4 ] HIGH Slow Diffusion Biodegradati on C/N <20 C/N >20
Surfac e water Oxidized layer Reduce d soil layer Low [NH 4 ] nitrificatio n [NH 4 ] HIGH Slow Diffusion [NO 3 ] high
Surfac e water Oxidized layer Reduce d soil layer N 2 [NO 3 ] high Leaching [NO 3 ] Low Denitrificatio n