Hydric Soil REDOX Flashcards
What are redoximorphic feature?
Features formed by the reduction, movement, and oxidation of Fe and Mn oxides
OIL - RIG
each complete redox reaction contains ________ an oxidation and reduction half-rxn
Oxidation is Loss
Reduction is Gain
BOTH
In ________ soils, organic compounds like glucose can be oxidized by CO2 as catalyzed by ________________ during ____________ respiration with organic tissues being used as the ___________
aerobic
heterotrophic microbes
aerobic respiration
carbon source
In ______ soils, substances are oxidized by other e- acceptors, these e- acceptors are themselves __________. In these soils, microbes will oxidize organic compounds in the following order . . .
anerobic
reduced
No → NO₃⁻ (Nitrate)
More → MnO₂ (Manganese Dioxide)
Fishy → Fe(OH)₃ (Ferric Hydroxide)
Smelling → SO₄²⁻ (Sulfate)
Carbon → CO₂ (Carbon Dioxide),
How → H2O (water)
Pungent → PO₄
What is the order in which soil OM is reduced to simpler monomers across aerobic and anerobic conditions? What are the associated mv?
Chemical analyses measure the concentration of __________ species in the soil solution such as __________ to determine if a soil is anaerobic
reduced
Fe 2+
_______________ measurements are a voltage that can be measured in the soil and used to predict the types of reduced species that would be expected
Redox Potential
How are redox potential measurements taken?
The device has a Pt-tipped electrode that is chemically inert and a reference electrode that creates a standard set of conditions. Reduced soils transfer e- to the Pt electrode while oxidized soils tend to take e- from the Pt electrode
Your redox potential measurement is -200 mV, how are the e- flowing from either electrode?
Your redox potential measurement is 600 mV, how are the e- flowing from either electrode?
Dyes such as ______ can measure reduced Fe 2+. What are this dye’s positive and negative results?
a,a’-dipyridyl
(+ result, RED!) soil is assumed to be reduced in terms of Fe and must be anaerobic
(- result) no reaction soil may be anaerobic but not reduced or may be aerobic
What are the 3 categories of redox features?
- Redox Concentrations
- Redox Depletions
- Reduced Matrix
Redox concentrations are . . . ?
What are the 3 kinds of redox concentrations?
Accumulations of Fe/Mn oxides
- Nodules and concretions (hard bodies)
- Masses (soft bodies)
- Pore linings (coatings along pores)
Boundaries of nodules and
concretions may be: _____ or ______.
Which of these would indicate that the nodules were moved, and may be relict?
- diffuse (fade out)
- sharp
SHARP boundaries = movement
What is an Oxidized rhizosphere?
Pore lining with live root
Redox depletions are . . . ?
What are the two kinds of depletions?
Zones of low chroma (< 2) where Fe/Mn oxides have been removed, or where both Fe/Mn oxides and clay have been removed.
- Fe depletions
- Clay depletions (rarer)
What are Fe depletions and where do they occur?
Low chroma bodies with a clay content similar to that of the matrix.
May occur in soil matrix or just along pores
What is a reduced matrix?
Soil matrices that have a low chroma color in situ, because Fe2+ is present, but change (redden) upon exposure to air
Why is it important to indicate where a redox feature occurred?
Redox depletions show areas that
were saturated and reduced.
Must describe which features lie along
channels and cracks, and which lie
in the matrix.
This can be in the cracks, channels,
matrix or all three.
How do Fe depletions form along a root channel?
1. Root Growth
2. Oxygen Release: Roots release oxygen into the surrounding soil, promoting Fe³⁺ (ferric) precipitation as iron oxides.
3. Root Death & Decay: When roots die, organic matter accumulates, increasing microbial respiration.
4. Anaerobic Conditions: Water saturation leads to oxygen depletion, causing microbial reduction of Fe³⁺ to Fe²⁺ (ferrous), which is soluble.
5.Fe Mobilization & Leaching: Soluble Fe²⁺ is transported away from the root zone, forming Fe-depleted zones.
6. Formation of Fe Depletions: Over time, the loss of Fe results in pale-colored Fe depletions along root channels.
How do Fe depletions form along unstable channels?
1. Initial Root Development: Similar to stable root channels, roots create pathways for oxygen diffusion and Fe oxidation.
2. Collapse of Root Channel: Soil compaction or decomposition of organic material leads to collapse, restricting oxygen diffusion.
3. Prolonged Water Saturation: The collapsed area remains saturated longer, promoting more extended reducing conditions.
4. Enhanced Fe Reduction: Increased anaerobic microbial activity reduces Fe³⁺ to Fe²⁺ over a larger area.
5. Greater Fe Loss: Because oxygen diffusion is lower in collapsed areas, Fe²⁺ is more extensively mobilized and leached, forming more pronounced Fe depletions.
How do Fe depletions form due to microsites?
Microsites are localized zones in the soil with distinct redox conditions, influencing Fe depletion formation.
1. Heterogeneous Saturation: Patches of soil may experience varying water saturation due to texture, organic matter, or microtopography.
2. Localized Anaerobic Conditions: In waterlogged microsites, microbial activity depletes oxygen, triggering Fe³⁺ reduction to Fe²⁺.
3. Solubilization of Fe²⁺: The reduced iron is dissolved and mobilized within the microsite.
4. Redistribution or Leaching: Water movement removes Fe²⁺, leading to Fe depletion in some areas while Fe may precipitate elsewhere.
5. Formation of Fe Depletions: The affected microsite develops a characteristic pale color due to Fe loss.
How do Fe depletions form in sands?
Sandy soils differ from other soil types due to their high permeability, rapid drainage, and lower organic matter content; Fe depletions form a stripped matrix
1. Rapid Drainage & Water Table Fluctuation: Sands drain quickly but can experience temporary waterlogging, creating alternating aerobic and anaerobic conditions.
2. Fe Reduction During Saturation: When water saturates the sand, oxygen depletion allows microbial reduction of Fe³⁺ to Fe²⁺ on coated sand grains.
3. Fe Mobilization: Soluble Fe²⁺ moves easily through the sandy matrix due to low retention capacity. Channels also collapse if roots were present.
4. Fe Loss & Depletion Formation: As Fe²⁺ is leached away, light-colored Fe-depleted zones develop in the sand.
5. Lack of Fe Re-Precipitation: Because sandy soils lack sufficient clay and organic matter, Fe does not readily re-precipitate, leading to persistent Fe depletion zones.
What are the four steps in formation of Mn and Fe concentrations?
1. Saturation & Oxygen Depletion
Microbes consume oxygen through respiration, further reducing oxygen availability.
2. Reduction of Fe and Mn
Fe³⁺ (ferric) and Mn⁴⁺ (manganese oxides) are reduced to Fe²⁺ (ferrous) and Mn²⁺, which are soluble.
This process is driven by microbial activity under anaerobic conditions.
3. Mobilization & Movement
The soluble Fe²⁺ and Mn²⁺ diffuse through the soil or move with water.
They accumulate in zones where conditions change, such as near roots, cracks, or aerated zones.
4. Reoxidation & Precipitation
When oxygen is reintroduced (due to drainage, root oxygen release, or air diffusion), Fe²⁺ oxidizes back to Fe³⁺, forming iron oxides.
Mn²⁺ oxidizes to Mn⁴⁺, forming manganese oxides.
These precipitates create reddish-brown Fe concentrations and black Mn concentrations.
