Lecture 8Lecture 8

NITROGENNITROGEN

GES175, GES175, Science of SoilsScience of Soils

Slide 8.2

Slide 8.3

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

N-cycleN-cycleplant & animal residues

organic-N

NH4+NO2

-

NO3-

N2

1122

33

33

4455

55

Slide 8.6

Mineralization vs. ImmobilizationMineralization vs. Immobilization

Fate of N if added to soil???

Alfalfa, peas, grass

Low C:N (high N content)Slide 8.8

straw, bark, sawdust

High C:N (low N) Slide 8.9

Ammonia Volatilization Ammonia Volatilization - gaseous loss of N- gaseous loss of N

Ammonia VolatilizationAmmonia VolatilizationUrea: 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

Nitrogen (nitrate?) Leaching

Eutrification

Denitrification

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

Process:Process:

Rhizobium

nodule

(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

Nonsymbiotic N-fixation:Nonsymbiotic N-fixation:Free-living OrganismsFree-living Organisms

* Bacteria and blue-green algae

aerobic and anaerobic

small amounts: 5 - 50 kg/ha/yr

inhibited by available soil N