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Nitrogen Cycle

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.

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Nitrogen Cycle

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  1. Nitrogen Cycle

  2. Sources • Lightning • Inorganic fertilizers • Nitrogen Fixation • Animal Residues • Crop residues • Organic fertilizers

  3. Forms of Nitrogen • Urea  CO(NH2)2 • Ammonia  NH3 (gaseous) • Ammonium  NH4 • Nitrate  NO3 • Nitrite  NO2 • Atmospheric Dinitrogen N2 • Organic N

  4. Global Nitrogen Reservoirs

  5. 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.

  6. Nitrogen is a key element for • amino acids • nucleic acids (purine, pyrimidine) • cell wall components of bacteria (NAM).

  7. Nitrogen Cycles • Ammonification/mineralization • Immobilization • Nitrogen Fixation • Nitrification • Denitrification

  8. N2 N2O NH4 NO2 R-NH2 NO NO2 NO3

  9. Ammonification or Mineralization N2 N2O NH4 NO2 R-NH2 NO NO2 NO3

  10. 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

  11. 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

  12. 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

  13. Nitrogen Fixation N2 N2O NH4 NO2 R-NH2 NO NO2 NO3

  14. 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)

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

  16. Rates of Nitrogen Fixation

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

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

  19. Nitrification N2 N2O NH4 NO2 R-NH2 NO NO2 NO3

  20. Nitrification Two step reactions that occur together : • 1rst step catalyzed by Nitrosomonas 2 NH4+ + 3 O2 2 NO2- +2 H2O+ 4 H+ • 2nd step catalyzed by Nitrobacter • 2 NO2- + O2  2 NO3-

  21. 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?

  22. Denitrification N2 N2O NH4 NO2 R-NH2 NO NO2 NO3

  23. 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

  24. Looking at the Nitrogen cycle through the eye of NH4

  25. Surface water Low [NH4] Oxidized layer Biodegradation Reduced soil layer Slow Diffusion C/N <20 C/N >20 [NH4] HIGH

  26. Surface water nitrification Low [NH4] Oxidized layer [NO3] high Reduced soil layer Slow Diffusion [NH4] HIGH

  27. N2 Surface water Oxidized layer [NO3] high Leaching Reduced soil layer [NO3] Low Denitrification

  28. QUESTIONS ?

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