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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


NH4+
ammonium
N2
dinitrogen gas
NO2-
nitrite
+3


NO3-
nitrate
+5


-3
0 (oxidation) (reduction)
Nitrogen Redox Processes
Oxidation: loss of eReduction: gain of e-3
+5
NH4+  NO38 e- transfer
Slide 8.6
N-cycle
plant & animal residues
5
5
NO3-
4
N2
organic-N
2
3
1
NO2-
3
NH4+
Mineralization vs. Immobilization
Fate of N if added to soil???
Low C:N (high N content)
Alfalfa, peas, grass
Slide 8.8
High C:N (low N)
straw, bark, sawdust
Slide 8.9
Ammonia Volatilization
- gaseous loss of N
Ammonia Volatilization
Urea:
CO(NH2)2  NH3 +CO2 + H2O
urea
soil enzymes
& H2O
- Most volatilization when:
 coarse or sandy-textured soils
 low clay and low organic matter
(which adsorb NH4+)
 dry alkaline surface
Nitrification
NH4+  NO2-  NO3ammonium
nitrite
nitrate
- oxidation of N
* Autotrophic bacteria
• obtain energy from N oxidation
• Nitrosomonas
NH4+  NO2- + energy
• Nitrobacter
NO2-  NO3- + energy
Nitrification (cont’d)
* Rapid in well-aerated,
warm, moist soils
• aerobic organisms
(O2 is required)
• little NO2- accumulation
* Acid-forming process
NH4+ +3/2O2 NO2- + 2H+ + H2O
Nitrogen (nitrate?) Leaching
 Eutrification
Denitrification
Denitrification
Gaseous loss of N upon N reduction
NO3-
+ e+ e+ e+ e NO2-  NO  N2O  N2
nitric
oxide
nitrous
oxide
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)
Denitrification (cont’d)
* Denitrification enhanced by:
• low O2 (flooding)
• high O.M. (energy source)
• high NO3-
Denitrification (cont’d)
* Metabolic reduction is not denitrification
(no N gas formation)
organisms
NO3-
NO3- NH4+ organic-N
- N is reduced for use in protein formation
Nitrogen Fixation
N2 (organisms) NH4+
* Symbiotic relation between
bacteria and plants:
- legumes
+
- rhizobium
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)
Process:
nodule
Rhizobium
(b) Process:
organic-N
N2
Rhizobium
organic-C
C from plant photosynthesis 
N from fixation of N2

 symbiosis
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
Symbiotic Nodules - Nonlegumes
* Organisms
l
actinomycetes - Frankia
* Plants
l
Alders and other trees
Symbiotic - without nodules
* Azolla/Anabaena complex




blue-green algae (N-fixer)
in leaves
floating fern in rice paddies
* Rhizosphere organisms
use root exudates (C)
l large areas
l
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