Download Aeration and Hydric Soils

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
-26
+28
Fe+2
e-
-25
+28
Fe+3
Reduction


Gain of electrons
Fe+3
Fe+2
-26
+28
Fe+2
e-
-25
+28
Fe+3
Oxidized/Reduced forms of…
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)
Iron
Sulfur
S-2
R
(sulfide)
SO4-2 (sulfate)
O
Carbon CH4 (methane)
CO2
R
O
Oxidation reaction
electrons that could
potentially be transferred to others
2FeO + 2H2O
Fe+2
2FeOOH + 2H+ + 2 eFe+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
Electron
acceptor
Electron donor
CO2 + H2O
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.
2.
3.
Forms/mobility
Roots
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 Mobility
B) 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 Mobility
C. 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 watersaturated 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