Nitrogen cycle, Microbial Recycling of N2
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This presentation is all about: 1. Nitrogen Cycle 2. Microbial activities 3. Microbes involved 4. Restrictions 5. Mechanisms of N2 Cycle Fixation Ammonification Nitrification Dinitrification This is not originally my own content thus usage as well modification of the content is not a problem from my side. Regards, M. Waqar Akhtar
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Nitrogen cycle, Microbial Recycling of N2
- 1. Microbial Cycling of Nitrogen M. Waqar Akhtar
- 2. Sources • Lightning • Inorganic fertilizers • Nitrogen Fixation • Animal Residues • Crop residues • Organic fertilizers
- 4. Forms of Nitrogen • Urea CO(NH2)2 • Ammonia NH3 (gaseous) • Ammonium NH4 • Nitrate NO3 • Nitrite NO2 • Atmospheric Dinitrogen N2 • Organic N
- 5. Global Nitrogen Reservoirs Nitrogen Reservoir Metric tons nitrogen Actively cycled Atmosphere 3.9×1015 No Ocean: •Soluble salts •Biomass 6.9×1011 5.2×108 Yes Yes Land: •Organic matter •Biota 1.1×1011 2.5×1010 Slow Yes
- 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).
- 8. Nitrogen Cycles • Ammonification/mineralization • Immobilization • Nitrogen Fixation • Nitrification • Denitrification
- 10. Ammonification or Mineralization NH3 NH4 NO2 NO3 NO2 NO N2O N2
- 11. Mineralization orAmmonification Decomposers: •Earthworms, termites, slugs, snails, bacteria, and fungi etc. Extracellularenzymes Initiate degradation of plant polymers Microorganisms use: •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 Examples •Azo bacte r •Be ije rinckia •Azo spirillum •Clo stridium •Cyano bacte ria The process: •Require the enzyme nitrogenase •Inhibited by oxygen •Inhibited by ammonia
- 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
- 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 : • Step1 Catalyzed by Nitro so m o nas 2NH4 + + 3 O2 2NO2 - +2H2O+ 4H+ • Step2 Catalyzed by Nitro bacte r • 2NO2 - + O2 2NO3 -
- 23. • Optimal pH is between 6.6-8.0 • If pH < 6.0 rate is slowed • If pH < 4.5 reaction is inhibited
- 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