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Lecture 8 NITROGEN

GES175, Science of Soils. Lecture 8 NITROGEN. Slide 8.2. Slide 8.3. Slide 8.4. Oxidation States of Soil N. N Form Name Oxidation state organic-N -3 NH 4 + ammonium -3 â á N 2 dinitrogen gas 0 (oxidation) (reduction) NO 2 - nitrite +3 â á

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Lecture 8 NITROGEN

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  1. GES175, Science of Soils Lecture 8NITROGEN

  2. Slide 8.2

  3. Slide 8.3

  4. Slide 8.4 Oxidation States of Soil N N Form NameOxidation state organic-N -3 NH4+ ammonium -3 âá N2 dinitrogen gas 0 (oxidation)(reduction) NO2- nitrite +3 âá NO3- nitrate +5 âá

  5. Nitrogen Redox Processes Oxidation: loss of e- Reduction: gain of e- • -3 +5 • NH4+¾¾¾¾¾® NO3- • 8 e- transfer

  6. 5 Slide 8.6 N-cycle plant & animal residues 5 N2 4 organic-N NO3- 2 3 1 3 NO2- NH4+

  7. Mineralization vs. Immobilization Fate of N if added to soil???

  8. Slide 8.8 Low C:N (high N content) Alfalfa, peas, grass

  9. High C:N (low N) Slide 8.9 straw, bark, sawdust

  10. Ammonia Volatilization - gaseous loss of N

  11. Ammonia Volatilization Urea: CO(NH2)2¾¾¾¾¾¾® NH3 +CO2 + H2O urea soilenzymes & H2O - Most volatilization when: ücoarse or sandy-textured soils ülow clay and low organic matter (which adsorb NH4+) üdry alkaline surface

  12. Nitrification NH4+¾¾¾® NO2-¾¾¾® NO3- ammonium nitrite nitrate - oxidation of N * Autotrophic bacteria • obtain energy from N oxidation • Nitrosomonas NH4+¾® NO2- + energy • Nitrobacter NO2-¾® NO3- + energy

  13. Nitrification (cont’d) * Rapid in well-aerated, warm, moist soils • aerobic organisms (O2 is required) • little NO2- accumulation * Acid-forming process NH4+ +3/2O2¾® NO2- + 2H+ + H2O

  14. Nitrogen (nitrate?) Leaching  Eutrification

  15. Denitrification

  16. Denitrification Gaseous loss of N upon N reduction + e- + e- + e- + e- NO3-¾¾® NO2-¾¾® NO ¾¾® N2O ¾¾® N2 nitric nitrous oxide oxide

  17. Denitrification (cont’d) * Microorganisms responsible: • facultative anaerobes - prefer O2 but will use N for a terminal e- acceptor • mostly heterotrophic - use organic-C for energy source (reductions require energy)

  18. Denitrification (cont’d) * Denitrification enhanced by: • low O2 (flooding) • high O.M. (energy source) • high NO3-

  19. organisms NO3- NO3- Denitrification (cont’d) * Metabolic reduction is not denitrification (no N gas formation) ® NH4+® organic-N - N is reduced for use in protein formation

  20. Nitrogen Fixation N2¾(organisms)® NH4+ * Symbiotic relation between bacteria and plants: - legumes + - rhizobium

  21. Nitrogen Fixation Bacteria: Rhizobium genus (species specific) R. meliloti - alfalfa R. trifolii - clover R. phaseoli - beans - bacteria require plant to function - inoculation of seed (coat seed with proper bacteria)

  22. nodule Process: Rhizobium

  23. organic-C (b) Process: organic-N N2 C from plant photosynthesis î N from fixation of N2ì Rhizobium • Þsymbiosis

  24. Quantity of N Fixed • Alfalfa and clover provide » 100 - 250 kg N/ha/yr (mature stand, good fertility & pH) • Beans and peas » less fixation but high protein food with minimum N input • added N fertilizer  lowered N fixation

  25. Symbiotic Nodules - Nonlegumes * Organisms lactinomycetes - Frankia * Plants lAlders and other trees

  26. Symbiotic - without nodules * Azolla/Anabaena complex éé é blue-green algae (N-fixer) éin leaves floating fern in rice paddies * Rhizosphere organisms l use root exudates (C) l large areas

  27. Nonsymbiotic N-fixation:Free-living Organisms * Bacteria and blue-green algae l aerobic and anaerobic l small amounts: 5 - 50 kg/ha/yr l inhibited by available soil N

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