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Oxidation States of Soil NOxidation States of Soil N
N Form Name Oxidation state
organic-N -3
NH4+ ammonium -3
N2 dinitrogen gas 0 (oxidation)
(reduction)
NO2- nitrite +3
NO3- nitrate +5
Slide 8.4
Nitrogen Redox ProcessesNitrogen Redox Processes
Oxidation: loss of e-
Reduction: gain of e-
-3 +5NH4
+ NO3-
8 e- transfer
Ammonia VolatilizationAmmonia 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
NitrificationNitrificationNH4
+ NO2- NO3
- ammonium nitrite nitrate
- oxidation of N
* Autotrophic bacteria • obtain energy from N oxidation
• Nitrosomonas NH4
+ NO2- + energy
• Nitrobacter NO2
- NO3- + energy
NitrificationNitrification (cont’d) (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
DenitrificationDenitrification
Gaseous loss of N upon N reduction
+ e- + e- + e- + e-
NO3- NO2
- NO N2O N2
nitric nitrous oxide oxide
DenitrificationDenitrification (cont’d) (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)
DenitrificationDenitrification (cont’d) (cont’d)
* Denitrification enhanced by:
• low O2 (flooding)
• high O.M. (energy source)
• high NO3-
DenitrificationDenitrification (cont’d) (cont’d)
* Metabolic reduction is not denitrification(no N gas formation)
NO3-
organisms
NO3- NH4
+ organic-N
- N is reduced for use in protein formation
Nitrogen FixationNitrogen FixationN2 (organisms) NH4
+
* Symbiotic relation between * Symbiotic relation between bacteria and plants:bacteria and plants:
- legumes- legumes + +
- - rhizobiumrhizobium
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 FixationNitrogen Fixation
(b) Process:(b) Process:
C from plant photosynthesis
N from fixation of N2 symbiosissymbiosis
RhizobiumRhizobium
organic-Corganic-C
NN22organic-Norganic-N
Quantity of N FixedQuantity 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 - NonlegumesSymbiotic Nodules - Nonlegumes
* Organismsactinomycetes - Frankia
* PlantsAlders and other trees
Symbiotic - without nodulesSymbiotic - 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