Unit 10 - Redox Reactions and Hydric Soils

Redox Reactions - Oxidation / Reduction reactions (redox) are widely considered the ____

most important chemical reactions on Earth

Redox Reactions - The most common reaction is CO2 + H2O + energy = ____

CH2O (carbohydrate) + O2 (when left to right, it’s photosynthesis)

Redox Reactions - ___ requires energy in the form of sunlight

photosynthesis

Photosynthesis is a ____ because elements are gaining and losing electrons

redox reaction

The oxidation state of carbon in CO2 is ___

+4

The oxidation state of O in H2O and CO2 is ___

-2

The redox reaction is in the transfer of ____ to e^4+ in CO2

4e’s from O2-

The oxidation state of O in O2 is ___

0

The oxidation state of C in organic matter is ____

0

When the photosynthesis reaction goes right to left, it’s called 

respiration

Respiration releases ___ in organic matter

energy stored

The oxidation of organic matter is also called ___

mineralization, decomposition, or decay

Organic matter is ____

electron rich

Microbial decay of soil organic matter releases ___

4e- per C atom

CH2O (om) + H2O = ____

CO2 (g) + 4e- + 4H+

The most common electron acceptor in nature is

elemental oxygen (O2)

Redox Ladder - Many microorganisms can take advantage of ____ in the absence of O2

alternative electron acceptors

The relative order in which these alternative electron acceptors are used is called the ____

“redox ladder”

O2 (ag) + 4e- + 4H+ > 2H2O

Anoxia equation:

NO3- (ag) + 5e- + 6H+ > ½N2 (g) + 3H2O

Denitrification equation:

Mn O2 (g) + 2e- + 4H+ > Mn2+ (ag) + 2H2O

Reductive dissolution of manganese dioxide:

Fe OOH (s) + e- + 3H+ > Fe2+ (ag) + 2H2O

Reductive dissolution of goethite:

SO4^2- (ag) + 8e- + 10H+ > H2S (g) + 4H2O

Sulfate reaction:

2CH2O (om) > CH4 (g) + CO2 (g)

Methane Production

Electron Activity - The electron activity of a solution can be measured using a ____ coupled with a reference electrode

Pt electrode

The Pt electrode measurement is called the ____ of the solution

“redox potential” (Eh)

Large positive Eh values indicate ___ and low e- activity

oxidizing conditions

Eh ranges for redox reactions in soils: Fully aerated water, saturated with dissolved oxygen

(+600 to +450 mV)

Eh ranges for redox reactions in soils: Denitrification 

(+200 to -100 mV)

Eh ranges for redox reactions in soils: Reductive Dissolution of FeOOH

(+170 to -100 mV)

Eh ranges for redox reactions in soils: Sulfate reduction

(-100 to -200 mV)

Eh ranges for redox reactions in soils: Methane Production

(-150 to -3,000 mV)

Eh ranges for redox reactions in soils:

______ form under conditions of saturation, flooding, or ponding

Hydric soils

Hydric Soils - The saturated conditions need to be long enough during the growing season to ____, especially in the upper part of the soil

develop anaerobic conditions

Hydric Soils - These soils support the growth of ____ under natural conditions

hydrophytic vegetation

Hydric soils exhibit properties that can be easily observed in the field known as

redoximorphic features

Accumulation of iron (Fe) and manganese (Mn) nodules

Zones of iron accumulation on pore surfaces and root channels

Zones of iron depletion > gray mottles or gley colors

Indicator tests positive for Fe2+ in soil solution

Examples of redoximorphic features

Eh measurements with Pt electrodes and pH are

Time consuming and difficult

Use of alpha - alpha - dipyridyl will

React with ferrous iron (Fe2+) and forms a pink color

Difficult to obtain (hazardous)

IRIS tubes

Indicator of Reduction In Soils

PVC tubes are coated with ferrihydrate paint

Fe3+ > Fe2+

Organic matter accumulation is favored in soils that are ____  (hydric soil conditions)

perennially wet

Organic Soils - _____, very common in peatlands

Histosols

Organic matter decomposition rates are low because of the lack of e- acceptors

(no O2, NO3, MnO2, Fe2 oxides, sulfates)

High organic matter content

Dark brown or black in color

Low bulk density

High water holding capacity

High CEC (lots of humus)

Corpse preservation

Peat can be harvested for fuel

properties of organic soils

Organic soil subside when drained due to

Shrinkage as the material dries

Consolidation because of a loss of groundwater buoyancy

Compact from tillage or traffic

Wind erosion, burning, oxidation

Microbial decomposers take advantage of the energy stored in organic C bonds, thereby altering the ____

original organic matter

____ results from decomposing organic matter

Humus

Anaerobic, decompositions processes are found in

Flooded soils

Anaerobic processes are found in

Well-drained upland soils

75% water

25% dry matter

• 45% cellulose

• 20% lignin

• 18% hemicellulose

• 8% protein

• 5% sugars and starches

• 2% fats and waxes

• 2% polyphenols

Typical composition of organic matter

Sugars, starches, simple proteins (easy to decompose)

Crude proteins

Hemicellulose

Cellulose

Fats and waxes

Lignins and phenolic compounds (hard to decompose)

Rate of decomposition

____ decomposition is complicated and slow

The reactions are carried out by fungi

Lignin

Residue particle size

• The smaller the particles, the more rapid the decomposition

Temperature

• The colder the temperature, the slower the decomposition

Soil water content

• Too much water, too little water

Carbon : Nitrogen ratio

Factors controlling rates of decomposition and mineralization