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Lecture 8 NITROGEN

This lecture focuses on the nitrogen cycle, exploring various nitrogen compounds and their oxidation states in soil. Key topics include ammonium, nitrate, nitrite, and the processes of nitrification, denitrification, and nitrogen fixation. The lecture emphasizes the roles of microorganisms in these processes, highlighting the importance of conditions such as oxygen availability and organic matter content. Understanding these transformations is crucial for managing soil fertility and addressing environmental issues like eutrophication and nitrogen loss.

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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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