1. Lecture 8 NITROGEN GES175, Science of Soils

2. Slide 8.2

3. Slide 8.3

4. Oxidation States of Soil N N Form NameOxidation state organic-N-3 NH 4 + ammonium-3  N 2 dinitrogen gas 0 (oxidation) (reduction) NO 2 - nitrite+3   NO 3 - nitrate+5   Slide 8.4

5. Nitrogen Redox Processes Oxidation: loss of e - Reduction: gain of e - -3 +5 NH 4 +  NO 3 - 8 e - transfer

6. N-cycle plant & animal residues organic-N NH 4 + NO 2 - NO 3 - N2N2 1 2 3 3 4 5 5 Slide 8.6

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

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

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

10. Ammonia Volatilization - gaseous loss of N

11. Ammonia Volatilization Urea: CO(NH 2 ) 2  NH 3 +CO 2 + H 2 O urea soil enzymes & H 2 O - Most volatilization when:   coarse or sandy-textured soils   low clay and low organic matter (which adsorb NH 4 + )   dry alkaline surface

12. Nitrification NH 4 +  NO 2 -  NO 3 - ammonium nitrite nitrate - oxidation of N * Autotrophic bacteria obtain energy from N oxidation Nitrosomonas NH 4 +  NO 2 - + energy Nitrobacter NO 2 -  NO 3 - + energy

13. Nitrification (cont’d) * Rapid in well-aerated, warm, moist soils aerobic organisms (O 2 is required) little NO 2 - accumulation * Acid-forming process NH 4 + +3/2O 2  NO 2 - + 2H + + H 2 O

14. Nitrogen (nitrate?) Leaching  Eutrification

15. Denitrification

16. Denitrification Gaseous loss of N upon N reduction + e - + e - + e - + e - NO 3 -  NO 2 -  NO  N 2 O  N 2 nitric nitrous oxide oxide

17. Denitrification (cont’d) * Microorganisms responsible: facultative anaerobes - prefer O 2 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 O 2 (flooding) high O.M. (energy source) high NO 3 -

19. Denitrification (cont’d) * Metabolic reduction is not denitrification (no N gas formation) NO 3 - organisms NO 3 -  NH 4 +  organic-N - N is reduced for use in protein formation

20. Nitrogen Fixation N 2  (organisms)  NH 4 + * Symbiotic relation between bacteria and plants: - legumes + - rhizobium

21. 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) Nitrogen Fixation

22. Process: Process: Rhizobium nodule

24. (b) Process: C from plant photosynthesis  N from fixation of N 2   symbiosis   symbiosis Rhizobium organic-C N2N2N2N2 organic-N

25. Quantity of N Fixed 4 Alfalfa and clover provide  100 - 250 kg N/ha/yr ( mature stand, good fertility & pH) 4 Beans and peas  less fixation but high protein food with minimum N input 4 added N fertilizer  lowered N fixation

26. Symbiotic Nodules - Nonlegumes * Organisms  actinomycetes - Frankia * Plants  Alders and other trees 

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

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