GES 175, GES 175, Science of SoilsScience of Soils

Lecture 6, Soil Biology

The Little Guys

CO2

O2

Detritus (Plant Debris)

FungiEarthworms

Bacteria

Soil Humus

energy

Soil Bacteria

Ruehr’s Rule: “The bugs get it first”.

CH2O CO2

NH4+ NO3

-

Fe2+ Fe3+

H2S SO42-

NO3- NH4

+

Fe3+ Fe2+

SO42- H2S

O2 H2O

EnergyYield

Electron Donor(food)

Electron Acceptor

Edonor - Eacceptor

EnergyEnergy

CH2O

CO2

O2

H2O

NO3-CH2O

CO2 N2

Fe(OH)3CH2O

CO2 Fe2+

CH2O

CO2

SO42-

H2S

Energy YieldsDonor Acceptor

700

400

100

Eh (mV)* Condition

oxic

suboxic

anoxic

*pH 7

Aerobic Organisms

Food Source• Organic carbon*

• Ammonium Ion (NH4+)

• Ferrous Iron (Fe2+)

• Hydrogen Sulfide (H2S)

Electron Acceptor• Molecular Oxygen (O2)

CH2O CO2

NH4+ NO3

-

Fe2+ Fe3+

H2S SO42-

O2 H2O

EnergyYield

Electron Donor(food)

Electron Acceptor

Energy

-400

-36

KJ/mol

Anaerobic Organisms

Food Source• Organic carbon*

• Ammonium Ion (NH4+)

• Ferrous Iron (Fe2+)

• Hydrogen Sulfide (H2S)

Electron Acceptor• Nitrate (NO3

-)

• Manganese (Mn4+)

•Ferric Iron (Fe3+)

• Sulfate (SO42-)

CH2O CO2

NO3- NH4

+

Fe3+ Fe2+

SO42- H2S

O2 H2O

EnergyYield

Electron Donor(food)

Electron Acceptor

Energy

-400

-36

KJ/mol

Microorganism-Plant Microorganism-Plant RelationshipsRelationships

legumes

Bacterial-Plant Symbiosis

- bacteria require plant for growth; plant gains ‘free’ source of available N

Nitrogen FixationNitrogen Fixation* Symbiotic relation between * Symbiotic relation between

bacteria and plants:bacteria and plants:-e.g., legumes + -e.g., legumes + RhizobiumRhizobium

* Grasses-Azospirillum/Azobactor

N-Fixation: Frankia

Frankia Nodules on Alder

- actinomycetes

- associates with desert shrubs (e.g., Ceanothus) and various tree species (e.g., alder)

Fungi-Plant Interaction

Mycorrhizae(root fungus)

- extension of root system

- fungus enhances nutrient and water intake

- plants provide carbon source

Nearly 90% of native plants have mycorrhizae association

EctomycorrhyzaeEctomycorrhyzae

- association at exterior of root- develop on evergreen trees and shrubs

MycorrhizaeMycorrhizae

- Associations occur in root interior between cells- Develop on deciduous trees, annual and herbaceous plants

Endomycorrhizae

Organic Matter Degradation (Carbon Cycling)

CO2 Detritus (Plant Debris)

Fungi EarthwormsBacteria

Soil Humus

Organic Matter

Biomass

Humin(insoluble)

Humic Acid(insoluble in acid)

Fulvic Acid(soluble)

degr

adat

ion

(nonliving, nontissue decay products)

(identifiable dead tissue)

(living organisms)

Pools of Soil Organic Matter

Soil Humus

Organic Matter Degradation: Nutrient Cycling

Biomass

Detritus (Plant Debris)

Nutrient Release

NutrientIncorporation

-reactive functional groups: -carboxyl, hydroxyl, phenolic

Soil Organic Matter: Humus

- High cation (anion) exchange capacity

- High water holding capacity

- Promotes soil aggregation

To continue…

Fertilizer Effects on Soil Organic Matter

Effect of manure on organic matter