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Published byThaddeus Agers Modified over 10 years ago
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Nitrogen Cycle
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Sources Lightning Inorganic fertilizers Nitrogen Fixation Animal Residues Crop residues Organic fertilizers
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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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Global Nitrogen Reservoirs Nitrogen Reservoir Metric tons nitrogen Actively cycled Atmosphere3.9*10 15 No Ocean soluble salts Biomass 6.9*10 11 5.2*10 8 Yes Land organic matter Biota 1.1*10 11 2.5*10 10 Slow Yes
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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.
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Nitrogen is a key element for amino acids nucleic acids (purine, pyrimidine) cell wall components of bacteria (NAM).
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Nitrogen Cycles Ammonification/mineralization Immobilization Nitrogen Fixation Nitrification Denitrification
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R-NH 2 NH 4 NO 2 NO 3 NO 2 NO N2ON2O N2N2
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Ammonification or Mineralization R-NH 2 NH 4 NO 2 NO 3 NO 2 NO N2ON2O N2N2
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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
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Plants die or bacterial cells lyse release of organic nitrogen Organic nitrogen is converted to inorganic nitrogen (NH 3 ) When pH<7.5, converted rapidly to NH 4 Example: Urea NH 3 + 2 CO 2
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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
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Nitrogen Fixation R-NH 2 NH 4 NO 2 NO 3 NO 2 NO N2ON2O N2N2
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Nitrogen Fixation Energy intensive process : N 2 + 8H+ + 8e - + 16 ATP = 2NH 3 + H 2 + 16ADP + 16 Pi Performed only by selected bacteria and actinomycetes Performed in nitrogen fixing crops (ex: soybeans)
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Microorganisms fixing Azobacter Beijerinckia Azospirillum Clostridium Cyanobacteria Require the enzyme nitrogenase Inhibited by oxygen Inhibited by ammonia (end product)
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Rates of Nitrogen Fixation N 2 fixing systemNitrogen Fixation (kg N/hect/year) Rhizobium-legume200-300 Cyanobacteria- moss30-40 Rhizosphere associations 2-25 Free- living1-2
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Applications to wetlands Occur in overlying waters Aerobic soil Anaerobic soil Oxidized rhizosphere Leaf or stem surfaces of plants
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Bacterial Fixation Occurs mostly in salt marshes Is absent from low pH peat of northern bogs Cyanobacteria found in waterlogged soils
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Nitrification R-NH 2 NH 4 NO 2 NO 3 NO 2 NO N2ON2O N2N2
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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 -
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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?
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Denitrification R-NH 2 NH 4 NO 2 NO 3 NO 2 NO N2ON2O N2N2
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Denitrification Removes a limiting nutrient from the environment 4NO 3 - + C 6 H 12 O 6 2N 2 + 6 H 2 0 Inhibited by O 2 Not inhibited by ammonia Microbial reaction Nitrate is the terminal electron acceptor
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Looking at the Nitrogen cycle through the eye of NH 4
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Surfac e water Oxidized layer Reduce d soil layer [NH 4 ] HIGH Low [NH 4 ] Slow Diffusion Biodegradati on C/N <20 C/N >20
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Surfac e water Oxidized layer Reduce d soil layer [NH 4 ] HIGH Low [NH 4 ] Slow Diffusion nitrificatio n [NO 3 ] high
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Surfac e water Oxidized layer Reduce d soil layer [NO 3 ] high Leaching [NO 3 ] Low N2N2 Denitrificatio n
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