Nutrient Dynamics

Essential Plant Nutrients

16 Elements are Essential for Plants

O, C, H,

N, K, P, Ca, Mg, S,

Cl, Fe, B, Mn, Zn, Cu, Mo,

Si, Na

Macronutrients

Micronutrients

Species dependent

Lecture 8Lecture 8

NITROGENNITROGEN

GES175, GES175, Science of SoilsScience of Soils

Oxidation States of Soil NOxidation States of Soil N

N Form Name Oxidation stateorganic-N -3

NH4+ ammonium -3

N2 dinitrogen gas 0

NO2- nitrite +3

NO3- nitrate +5

Fertilizer: N (first of three digit rating)

Nitrogen Nitrogen RedoxRedox ProcessesProcesses

Oxidation: loss of e-

Reduction: gain of e-

-3 +5NH4

+ NO3-

8 e- transfer

NN--cyclecycleplant & animal residues

organic-N

NH4+NO2

-

NO3-

N2

11

22

33

33

4455

(mineralization)

(nitrification)

(nitrification)

(denitrification)

(fixation)

(immobilization)

CO2

Soil

atmosphere

Vegetation

Lakes and Oceans

Carbon Pools

CO2

O2

Detritus (Plant Debris)

FungiEarthworms

Bacteria

Soil Humus

Polysaccharides (cellulose…)Organic Acids

Amino acids, Peptides, ProteinsLipids

NucleotidesPeptidesLignin

CelluloseProteinsLignin

Humus

CO2

NH3

NH3

C:N

> 25:1

C:N

10:1

Organic Matter Mineralization

4-12Bacteria

9-16Alfalfa

20-80Grass

130-400Wood

60-70Pine Needle

45-70Maple Leaf

C:NMaterial

Carbon to Nitrogen Ratios

MineralizationMineralization vs. Immobilizationvs. Immobilization

Fate of N if added to soil???

Alfalfa, peas, grass

Low C:N (high N content)

straw, bark, sawdust

High C:N (low N)

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

Ammonia VolatilizationAmmonia VolatilizationUrea:

CO(NH2)2 → NH3 +CO2 + H2Ourea 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

• NitrobacterNO2

- → NO3- + energy

NitrificationNitrification (cont’d)(cont’d)

* Rapid in well-aerated,warm, moist soils

• aerobic organisms(O2 is required)

• little NO2- accumulation

* Acid-forming processNH4

+ +3/2O2→ NO2- + 2H+ + H2O

Nitrogen (nitrate?) Leaching

Eutrification

Hypoxic Zone: Gulf of Mexico/ MS River Delta

oxygen depleted zone

N In

put (

met

ric to

ns x

106 )

N Input Sources to Mississippi River

Soil Mineralization

Fertilizer

Denitrification

NONO33--

NN22

DenitrificationDenitrification

Gaseous loss of N upon N reduction

+ e- + e- + e- + e-

NO3- → NO2

- → NO → N2O → N2

nitric nitrousoxide oxide

DenitrificationDenitrification (cont’d)(cont’d)* Microorganisms:

• facultative anaerobes- prefer O2 but will use NO3

-

for a terminal e- acceptor• heterotrophic

- use organic-C for energy source

* Denitrification enhanced by:• low O2 (flooding)• high O.M. (energy source)• high NO3

-

DenitrificationDenitrification (cont’d)(cont’d)* Metabolic reduction: Denitrification?

(no N gas formation)

NO3-

organisms

NO3- → NH4+ → organic-N

- N is reduced for use in protein formation

Biological Nitrogen FixationBiological Nitrogen Fixation

N2 (organisms)→ NH4+

* Symbiotic relation between * Symbiotic relation between bacteria and plants:bacteria and plants:

-- legumeslegumes++

-- rhizobiumrhizobium

Bacteria: Rhizobium genus (species specific)

R. meliloti - alfalfaR. trifolii - cloverR. phaseoli - beans

- bacteria require plant to function as N-fixers

Nitrogen FixationNitrogen Fixation

Process:Process:

Rhizobium

nodule

(b) Process:(b) Process:

C from plant photosynthesis

N from fixation of N2⇒⇒ symbiosissymbiosis

RhizobiumRhizobium

organicorganic--CC

NN22organicorganic--NN

Quantity of N FixedQuantity of N FixedAlfalfa and clover

≈ 100 - 250 kg N/ha/yr(mature stand, good fertility & pH)

Beans and peasless fixation but high protein food

with minimum N input

added N fertilizerlowered N fixation

Symbiotic Nodules Symbiotic Nodules -- NonlegumesNonlegumes* Actinomycetes - Frankia +

Alders and other trees

Symbiotic Symbiotic -- without noduleswithout nodules* Azolla/Anabaena complex

blue-green algae (N-fixer) in leaves offloating ferns

NonsymbioticNonsymbiotic NN--fixation:fixation:FreeFree--living Organismsliving Organisms

* Bacteria (Azotobacter, Azospirillum) and blue-green algae (Anabaena)

aerobic and anaerobic

small amounts: 5 - 50 kg/ha/yr

inhibited by available soil N

Microbial Immobilization

NH4+

NO3-

C-NH2

microbes