Soil Aeration

Ventilation of soil allowing gases to be exchanged with atmosphere

Gas is exchanged by: Mass flow: air forced in by wind or pressure Diffusion: gas moves back and forth from soil to

atmosphere acc. to pressure

Redox potential

Tendency of a substance to accept or donate electrons

Oxidation-reduction potential a way to characterize aeration Eh

Oxidation Loss of electrons Fe+2 Fe+3

+28

-25

Fe+3

+28

-26

Fe+2

e-

Reduction Gain of electrons Fe+3 Fe+2

+28

-25

Fe+3

+28

-26

Fe+2

e-

Oxidized/Reduced forms of…

Iron Fe+2 (ferrous)

Fe+3 (ferric)

Nitrogen N+3 in NH+4 (ammonium)

N+5 in NO3- (nitrate)

Manganese Mn+2 (manganous)

Mn+4 (manganic)

Sulfur S-2 (sulfide)

SO4-2 (sulfate)

Carbon CH4 (methane)

CO2O

R

O

R

Oxidation reaction

electrons that could

potentially be transferred to others

2FeO + 2H2O 2FeOOH + 2H+ + 2 e-

Fe+2 Fe+3

H+ ions formed

Redox potential Tendency of a substance to accept or

donate electrons Measured in volts or millivolts Depends on pH and presence of electron

acceptors (oxidizing agents) Used to quantify the degree of reduction in

a wetland soil

Oxidizing agent

Substance accepts electrons easily

Oxygen is very strong electron acceptor

Reducing agent

Substance donates electrons easily

Aerobic Respiration Oxygen is electron acceptor for organic

carbon, to release energy. As oxygen oxidizes carbon, oxygen in turn

is reduced (H2O)

O2 + C6H12O6 CO2 + H2O

Electron donorElectronacceptor

To determine Eh (See graph)

Insert electrode in soil solution: free dissolved oxygen present : Eh stays same oxygen disappears, reduction (electron gain)

takes place and probe measures degree of reduction ( mv)

As organic substances are oxidized (in respiration) Eh drops as sequence of reductions (electron gains) takes place:

Oxidized form Reduced form Eh (v)

O2 H2O .38 - .32

NO3-1 N2 .28 - .22

Mn+4 Mn+2 .22 - .18

Fe+3 Fe+2 .11 - .08

SO4-2 S-2 -.14 - -.17

CO2 CH4 -.2 - -.28

Graph (handout) shows: sequence of reductions that take place when well

aerated soil becomes saturated with water Once oxygen is gone, the only active

microorganisms are those that can use substances other than oxygen as electron acceptors (anaerobic)

Eh drops Shows Eh levels at which these reactions take place

Poorly aerated soil contain partially oxidized products:

Ethylene gas, methane, alcohols, organic acids

organic substrate oxidized (decomposed) by various electron acceptors:

O2

NO3-

Mn+4

Fe+3

SO4-2

rates of decomposition are most rapid in presence of oxygen

Aeration affects microbial breakdown:

Poor aeration slows decay Anaerobic organisms

Poorly aerated soils may contain toxic, not oxidized products of decomposition: alcohols, organic acids

Organic matter accumulates Allows Histosol development

Some conclusions about aeration:

1. Forms/mobility

2. Roots

3. Decomposition

Some conclusions about aeration:

1. Forms and Mobility

Soil aeration determines which forms of chemicals are present and how mobile they are

Redox colors in Poorly and Well-Aerated Soil Nutrient elements

1. Forms and Mobility: A) Poorly aerated soils

reduced forms of iron and manganese

Fe+2, Mn+2

Reduced iron is soluble; moves through soil, removing red, leaving gray, low chroma colors (redox depletions)

Reduced manganese : hard black concretions

1. Forms and MobilityB) Well-aerated soils:

Oxidized forms of iron and manganese

Fe+3 Mn+4

Fe precipitates as Fe+3 in aerobic zones or during dry periods

Reddish brown to orange (redox concentrations)

Plate 26  Redox concentrations (red) and depletions (gray) in a Btg horizon from an Aquic Paleudalf.

Plate 16  A soil catena or toposequence in central Zimbabwe. Redder colors indicate better internal drainage. Inset: B-horizon clods from each soil in the catena.

Plate 21  Effect of poor drainage on soil color. Gray colors and red redox concentrations in the B horizons of a Plinthaquic Paleudalf.

Manganese concretions

1. Forms and MobilityC. Nutrient Elements

Plants can use oxidized forms of nitrogen and sulfur

Reduced iron, manganese Soluble/”good” in alkaline soils More soluble in acid soils; can reach toxic levels

Some conclusions about aeration:

2. Root respiration

Good aeration promotes root respiration

Poor aeration: water-filled pores block oxygen diffusion into soil to replace what is used up in respiration

Some conclusions about aeration:

3. Decomposition

In aerated soils, aerobic organisms rapidly oxidize organic material and decomposition is rapid

In poor aeration, anaerobic decomposers take over and decomposition is slower

Hydric Soils

Wetland criteria

Hydrology Hydric soils Hydrophytic plants

Hydric soil

soil that is saturated, flooded, or ponded long enough during the growing season to develop anaerobic conditions in the upper part. Oxygen is removed from groundwater by

respiration of microbes, roots, soil fauna Biological zero = 5°C

Why is “during growing season” important part of definition?

If wet period is during COLD time of year (too cold for microbial growth and plant root respiration), might not have anaerobic conditions.

It is anaerobic conditions that cause a soil to be hydric, not just saturation!!!

Hydric soils support growth and regeneration of hydrophytic plants.

Hydric Soils and Taxonomy Histosols

(all Histosols except Folists) (all Histels except Folistels)

Aquic suborders and subgroups Definition of aquic soil moisture regime:

“reducing regime in soil virtually free of dissolved oxygen because it is saturated. Some soils are saturated at times while dissolved oxygen is present, either because the water is moving or the environment is unfavorable for microorganisms; such a regime is NOT considered aquic”.

Organic soils made up mostly of forest litter’ not saturated

Aquic Conditions:

Periodic or continuous saturation Redoximorphic features Verify by measuring saturation or reduction

Exception to Aquic conditions:

Artificial drainage Removal of free water from soils with aquic

conditions Artificially drained soils are included with aquic

soils Because soil Taxonomy is based on soil

GENESIS and minimizes human disturbance Pertains to Hydric soils also

Artificially wet soils are considered hydric

Artificially “dry” (drained) soils are considered hydric

Types of saturation endosaturation: all soil layers sat’d to 2 m

depth Episaturation: sat’d layers in upper 2 m

(perched) Anthric saturation: controlled flooding (rice,

cranberries)

Hydric soil indicators: Color

Chroma 1or 2 or gley (Fe++2 grey or green)

May have redox concentrations or concretions

Sulfidic materials (odor of rotten eggs) Sulfate reduction

Plate 30  Dark (black) humic accumulation and gray humus depletion spots in the A horizon are indicators of a hydric soil. Water table is 30 cm below the soil surface.

Figure 7.11  The relationship between the occurrence of some soil features and the annual duration of water-saturated conditions. The absence of iron concentrations (mottles) with colors of chroma >4, and the presence of strong expressions of the other features are indications that a soil may be hydric. [Adapted from Veneman et al. (1999)]

List of hydric soils

http://soils.usda.gov

Click on hydric soils

Oxidized rhizosphere In some poorly aerated soils:

Red, oxidized iron in root channels Oxygen diffused out of plant roots Some plants transport oxygen through

aerenchyma tissue in stems and leaves to roots (hydrophytic plants)

Plate 29  Oxidized (red) root zones in the A and E horizons indicate a hydric soil. They result from oxygen diffusion out from roots of wetland plants having aerenchyma tissues (air passages).

Black spruce

Pitcher plant