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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 NH 4 + ammonium -3 â á N 2 dinitrogen gas 0 (oxidation) (reduction) NO 2 - nitrite +3 â á

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oxidation states of soil n

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

nitrogen redox processes
Nitrogen Redox Processes

Oxidation: loss of e-

Reduction: gain of e-

  • -3 +5
      • NH4+¾¾¾¾¾® NO3-
  • 8 e- transfer
n cycle

5

Slide 8.6

N-cycle

plant & animal residues

5

N2

4

organic-N

NO3-

2

3

1

3

NO2-

NH4+

mineralization vs immobilization
Mineralization vs. Immobilization

Fate of N if added to soil???

slide8

Slide 8.8

Low C:N (high N content)

Alfalfa, peas, grass

slide9

High C:N (low N)

Slide 8.9

straw, bark, sawdust

ammonia volatilization
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

nitrification
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

nitrification cont d
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

denitrification
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

denitrification cont d
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 d1
Denitrification (cont’d)

* Denitrification enhanced by:

• low O2 (flooding)

• high O.M. (energy source)

• high NO3-

denitrification cont d2

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

nitrogen fixation
Nitrogen Fixation

N2¾(organisms)® NH4+

* Symbiotic relation between bacteria and plants: - legumes + - rhizobium

nitrogen fixation1
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)

b process

organic-C

(b) Process:

organic-N

N2

C from plant photosynthesis î

N from fixation of N2ì

Rhizobium

  • Þsymbiosis
quantity of n fixed
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
Symbiotic Nodules - Nonlegumes

* Organisms

lactinomycetes - Frankia

* Plants

lAlders and other trees

symbiotic without nodules
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

nonsymbiotic n fixation free living organisms
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