Reactions & Processes Influenced by Soil Wetness

Topics for Consideration A. Organic matter (OM) accumulation

B. Some reduction effects induced by soil saturation

C. Redoximorphic features

D. Calcium carbonate accumulation at surface (marl)

E. Near-surface stratification (with special morphology)

F. Root death

Reactions and Processes Influenced by Soil Wetness A. Organic Matter (OM) Accumulation OM accumulation favored by near-surface saturation. Why? - Greater biomass production - Lower T - Anaerobic pathways << aerobic pathways in energy yield - Anaerobic decomposition therefore much slower - Possible limited availability of electron acceptors - Fewer fragmenting fauna and bacterial grazers

Consequently, wetland soils tend to have - Higher OM content - Lower value & chroma in surface horizons - Thicker surface horizons than associated upland soils. Caveats - Climate & vegetation affect OM-wetness link - Hydric indicators are therefore region specific - Hydric indicators can require more evidence (e.g., 70% coatings on sand grains) - Muck more definitive than mucky peat or peat in FL.

tricky questions - Would "A" horizons of hydric soils commonly consist of organic soil material? - Could a hydric soil have a mineral surface horizon? - Could a mineral soil have organic soil materials?

•(This sounds straightforward, but actually it’s ambiguous. I’ll explain why) - Could an organic soil ever NOT be a hydric soil?

What is “saturation”?

• Water fills a recently-dug hole. • Soil water pressure ≥

atmospheric pressure.

B. Some Reduction Effects Induced by Saturation

What does “reduced” mean?

• Reduced is a vague term meaning that reduced forms of oxygen, nitrogen, manganese, iron, sulfur, &/or carbon are present in soil solution unless one element is specified.

• In the wet soil arena “reduction” has come to mean that reduced iron is in solution. This is because of the importance iron plays in soil morphology & the relative ease of measuring for reduced iron.

Points of departure - a generalized reduction equation, a (oxidants) + b H+ + n e- = c (reductants) + d H2O - and the Nernst equation (as stolen from Dr. Reddy) where Eh is electrode potential (calibrated to Pt/H2, mV), E0

is standard electrode potential (unit activity, STP), R is gas constant, T is temperature (K), F is Faraday constant, n = number of electrons transferred, and [ ] is activity of chemical species in mol/L.

- The Nernst equation shows an increase in pH requires a decrease in Eh, all else being equal. - Half-cell reduction reactions at Ph 7 for successive (top to bottom)

electron acceptors in soils include:

Eh (mv) • 24e- + 6O2 +24H+ = 12H20 +350 • 10e- + 12H+ + 2NO3

- = N2 + 6H2O +220 • 2e- + 4H+ + MnO2

= Mn 2+ + 2H2O + 200

• e- + 3H+ + Fe(OH)3 = Fe 2+

+ 3H2O +120 • 8e- +10H+ + SO4

2- = H2S + 4 H2O - 150

• 8e- +8 H+ + CO2- = CH4 + 2 H2O - 250

• The next slide shows the redox state for two of these elements (Oxygen and Iron).

0

200

400

600

Eh/pH Graph for Determing Redox State for Oxygen and Iron

pH4 pH5 pH6 pH7 pH8

Iron is reduced

Iron is not reduced

Oxygen is not reduced

Oxygen is reduced Eh

Redox measurements enable these reactions to be expressed in terms of the Nernst equation.

Two examples are: Eh = 1224 - 59 log (Mn2+) - 118 pH Eh = 724 - 59 log (Fe2+) - 177 pH (for FeOOH) We can infer that: - Eh and pH are inversely related - Slope is slightly more negative (steeper) for Fe - Intercept is indeterminate without knowing metal activities - For comparable reduced species activities, Mn more easily reduced.

Eh vs. pH plots would have the following trends:

(Not so) trick question: - What are the implications of the above Eh vs. pH plot for Fe-related morphological indicators of wetness in alkaline soils?

Morphological implications: - Reduced Fe & Mn are soluble & colorless - Mn oxides are black - Fe oxides range from red to yellowish brown - Fe & Mn distributions are influenced by spatial and temporal variability in redox conditions - Morphological consequence - Redoximorphic Features Olfactory implications - Hydrogen sulfide odor for very reduce conditions (e.g. coastal marshes, where seawater supplies SO4)

Measuring Reduction in Soils

• Methods – Analyzing soil solution for reduced form of nitrogen, manganese,

iron, sulfur, &/or carbon - too expensive except for research – Using dyes sometimes good for routine field work.

• Solutions change color when they encounter reduced iron. Most common is αα-dipyridyl.

• When solution is sprayed onto a fresh slice of soil it turns red almost immediately if reduced iron is present.

• Will not work where soils are low in iron such as much of Florida.

– Measuring redox potential is complex but can be done in field. • More on this method later

– Measuring dissolved oxygen - too expensive except for research.

Using αα-dipyridyl dye

• αα-dipyridyl applied to fresh slice of soil – (1, on photo ) where no

reduced iron is present and (2) where reduced iron is present and the soil turned red.

Measuring Redox Potential

• Redox reactions create an electrical potential • Difference in electrical potential between the soil as determined

through use of a platinum electrode and a reference electrode (both of which are in contact with soil solution) can be measured.

• On the next slide the basic components needed to measure redox potential in soil are shown (platinum electrode, reference electrode, & volt meter).

• Normally 5 replicate readings are required because Eh measurements are variable.

• Reduction reactions occur when soil solution is at various redox potentials. These reactions also vary with pH and mineral species present.

Measuring Redox Potential (cont.)

•

Platinum Electrode

Reference Electrode

Volt meter

Soil

Air

Factors Controlling Reduction in Soils

1. Organic matter must be present (source of electrons). – Dead roots – Plant debris such as leaves – Pieces of roots – Root exudates – Dissolved organic carbon

2. Air must not enter the soil (soil is saturated). 3. Organisms must be decomposing organic matter. This activity

is termed “microbial active.” For most of the US, wet soils are microbial active throughout the year.

4. Dissolved oxygen in soil water must be removed.

Water Movement and Reduction

• Reduction occurs where water moves through soil slowly and occurs best where water does not move through the soil and most all pores are filled with water.

• Reduction does not occur where water flows too quickly. Fast moving water (right photo) carries oxygen which must be removed before reduction of other elements can occur.

• Soil may be saturated but not reduced if: – 1 organic matter content is low (less

than about 2 percent), – 2. microbial activity is lacking, and/or – 3. water moves too quickly (oxygen is

not removed).

Summary of Features Formed by the Redox Reactions

• Reducing/oxidation reactions, collectively known as redox reactions, leave signs in the soil that they have occurred as follows: • Element Sign that Reduction Occurred

Oxygen Carbon Accumulation/Differential Removal* Nitrogen None Manganese Redoximorphic Features** Iron Redoximorphic Features** Sulfur Rotten egg odor – *Remember this is an aerobic reduction reaction. It is the first

reducing reaction. The soil is anaerobic after most all of the oxygen has been reduced and microbes start reducing nitrogen.

– **A generalized term which included all the features produced by manganese and iron reduction reactions.

C. Redoximorphic Features

• What are they?

• Why are they important?

• What do they look like?

• How do they form?

• What are redoximorphic features?

– Features associated with wetness which result from alternating periods of reduction and oxidation of Fe and Mn in soils.

– Qualification: Should be validated regionally.

• Why are they important?

– As indicators that soil has been saturated, even if not saturated at the time observed!

• What do they look like?

Types of redoximorphic features: a. Redox Concentrations (i)) Nodules and Concretions (ii) Masses (iii) Pore Linings b. Redox Depletions (i)) Iron Depletions (i) Clay Depletions c. Reduced Matrix

Nodules

Examples of redox concentrations and depletions

Fe & Mn can be redistributed in same soil horizon.

Colors are revealing about soil drainage!

• How do Redoximorphic Features Form?

• Involves 5 components (Review):

– Organic matter: electron source.

– Microbes: electron “brokers”.

– Oxygen: ultimate electron acceptor.

– Redox-sensitive metals: respond colorimetrically to electrons.

– Water: barrier to oxygen.

• Required Conditions for Soil Reduction: – Exclusion of oxygen by water

– Presence of organic matter – source of electrons

– Microbial respiration – transfer of electrons

– Electron acceptors

• How do Redoximorphic Features Form (cont.)?

• How do Redoximorphic Features Form (Cont.)?

• Unsaturated - Oxygen is plentiful - Microbial respiration unrestricted

• Saturated

- Oxygen is depleted - Microbes need alternative electron acceptors - Fe and Mn reduced and mobilized

• Unsaturated (drained) again

- Oxygen re-enters - Fe and Mn oxidized and immobilized

• Chemical reduction of Fe and Mn results in:

- Loss of red, brown, yellow color (black for Mn). - Reduced soil zone becomes gray (unless blackened by OM). - Dissolution and potential mobilization.

• Re-oxidation results in:

- Return of higher chroma color. - Immobilization.

• Cycles or zonal variations in redox result in:

- Redistribution of Fe and Mn. - Patterns of color and consistence (morphological legacies).

• How do Redoximorphic Features Form (Cont.)? (another way to look at it …)

Broken soil ped (natural aggregate) Why is the ped interior red?

Review

Drainage poor <----------------------------------> good ---- ----

poor aeration, good aeration, reduced oxidized

Fe2+ + HCO3

-1 + ½ O2 + H2O <-----------> FeOOH + H2CO3

soluble, mobile insoluble, residual

gray orange, yellow, brown

D. Marl - secondary CaCO3 formation * CaCO3 indicates wet conditions in humid-region soils. - Carbonates would leach from well drained soils - Carbonates can form and remain stable in some wetlands - Near-sea-level limestone shelf of S. Florida is one example . Vertical movement of water is minimal . Lateral flow & evapotransporation prevalent . Underlying limestone provides Ca . Precipitation promoted by algal photosynthesis

Ca2+ + 2(HCO3)- = CaCO3 + H2CO3

H2O + CO2

E. Near-surface stratification

Caveat: This is a hydric soil indicator only when specific morphological criteria are met

F. Root Death

Stripped Matrix

From NRCS Field Indicators of HS in US