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Nitrogen cycle
- 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 R-NH2 NH4 NO2 NO3 NO2 NO N2O N2
- 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 • Azo bacte r • Be ije rinckia • Azo 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
- 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
- 26. Looking at the Nitrogen cycle through the eye of NH4
- 28. Surfac e water Oxidized layer Reduced soil layer [NH4] HIGH Low [NH4] Slow Diffusion Biodegradatio n C/N <20 C/N >20
- 30. Surfac e water Oxidized layer Reduced soil layer [NO3] high Leaching [NO3] Low N2 Denitrificatio n