1. GES175, Science of Soils
Lecture 8 NITROGEN

2. Slide 8.2

3. Slide 8.3

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

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

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

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

8. Low C:N (high N content)
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)2 ¾¾¾¾¾¾® NH3 +CO2 + H2O
urea soil enzymes
& H2O
- Most volatilization when:
ü coarse or sandy-textured soils
ü low clay and low organic matter
(which adsorb NH4+)
ü dry alkaline surface

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

20. Nitrogen Fixation
N2 ¾(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. Process:
nodule
Rhizobium

24. (b) Process: C from plant photosynthesis î N from fixation of N2 ì
organic-C
organic-N N2
Rhizobium
C from plant photosynthesis î
N from fixation of N ì
Þ symbiosis

25. Quantity of N Fixed Alfalfa and clover provide Beans and peas
» 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

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

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

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