Soil Aeration And Hydric Soils Why is soil

Soil Aeration And Hydric Soils

Why is soil aeration important? Five reasons 1. Allows gases to be exchanged between soil and atmosphere (like O 2 and CO 2)

2. Aeration allows water to move through soil. (Nutrients are carried to plants in water).

3. Allows roots to penetrate soil.

4. State of aeration determines types of decomposition (aerobic or anaerobic).

5. Determines form of nutrient elements (reduced or oxidized).

Compacted and Saturated Soils

Compacted soil is the opposite of wellaerated soil. Compacted soils are poorly-aerated and have high bulk density.

Can be corrected by a soil aerator.

Aerator sandals!

Saturated soils are also not well -aerated. How can we express / measure how aerated or saturated a soil is?

REDOX POTENTIAL (Eh) n Tells state of aeration of a soil. n Tendency of a substance to accept or donate electrons. n Used to quantify the degree of reduction in a wetland soil.

n Remember oxidation and reduction? ? ?

Oxidation of iron Loss of electrons n Fe+2 Fe+3 n ferrous ferric -26 +28 Fe+2 e- -25 +28 Fe+3

Oxidizing agent n Substance accepts electrons easily n Oxygen is very strong electron acceptor, but in the absence of oxygen, other substances act as electron acceptors

Reduction of iron Gain of electrons n Fe+3 Fe+2 n -26 +28 Fe+2 e- -25 +28 Fe+3

Reducing agent n Substance donates electrons easily

In aerobic respiration / decomposition Oxygen is electron acceptor for organic carbon, to release energy. n As oxygen oxidizes carbon, oxygen in turn is reduced (H 2 O) n O 2 + C 6 H 12 O 6 Electron acceptor Electron donor CO 2 + H 2 O

In anaerobic decomposition n After all free oxygen has been used, other electron acceptors are used. n They become reduced in the process.

Oxidized/Reduced forms of… Iron Fe+2 (ferrous) R Fe+3 (ferric) O Nitrogen N+3 in NH+4 (ammonium) R N+5 in NO 3 - (nitrate) O Manganese Mn+2 (manganous) R Mn+4 (manganic) O

Sulfur S-2 (sulfide) SO 4 -2 (sulfate) R O Carbon CH 4 (methane) CO 2 R O

To determine Eh n Insert electrode in soil solution: n When all free dissolved oxygen disappears, reduction takes place and probe measures degree of reduction ( mv) n Eh drops as sequence of reductions takes place: Oxidized form Reduced form Eh (v) O 2 H 2 O . 38 -. 32 NO 3 -1 N 2 . 28 -. 22 Mn+4 Mn+2 . 22 -. 18 Fe+3 Fe+2 . 11 -. 08 SO 4 -2 S-2 -. 14 - -. 17 CO 2 CH 4 -. 2 - -. 28

n n 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 n Poorly aerated soil contain partially oxidized products: n Ethylene gas, methane, alcohols, organic acids Sulfate standard and wild rice

Additional significance of aeration: 1. 2. 3. Soil aeration determines which forms of chemicals are present and how mobile they are. Root respiration Decomposition rates

1 A) Poorly aerated soils n reduced forms of iron and manganese Fe+2, Mn+2 n Reduced iron is soluble; moves through soil, removing red, leaving gray, low chroma colors (redox depletions)

Manganese concretions

B) Well-aerated soils: n 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.

2. Root respiration Ø Good aeration promotes root respiration Ø n Poor aeration: water-filled pores block oxygen diffusion into soil to replace what is used up in respiration.

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 n Hydric soils n Hydrophytic plants n

Hydric soil n soil that is saturated, flooded, or ponded long enough during the growing season to develop anaerobic conditions in the upper part. n n 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? n If wet period is during COLD time of year (too cold for microbial growth and plant root respiration), might not have anaerobic conditions. n It is anaerobic conditions that cause a soil to be hydric, not just saturation!!!

How can a saturated soil be aerobic? n If water is flowing n If microbes and plant roots are not active

n Hydric soils support growth and regeneration of hydrophytic plants.

Hydric soil indicators: n Color n Chroma 1 or 2 or gley (Fe++2 grey or green) n May have redox concentrations or concretions n Sulfidic materials (odor of rotten eggs) n 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.

Hydric Soils and Taxonomy n Histosols n n n (all Histosols except Folists) (all Histels except Folistels) Aquic suborders and subgroups n 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 n Redoximorphic features n Verify by measuring saturation or reduction n

Exception to Aquic conditions: n Artificial drainage n n Removal of free water from soils with aquic conditions Artificially drained soils are included with aquic soils n n Because soil Taxonomy is based on soil GENESIS and minimizes human disturbance Pertains to Hydric soils also

n Artificially wet soils are considered hydric n Artificially “dry” (drained) soils are considered hydric

Types of saturation endosaturation: all soil layers sat’d to 2 m depth n Episaturation: sat’d layers in upper 2 m (perched) n Anthric saturation: controlled flooding (rice, cranberries) n

Oxidized rhizosphere n In some poorly aerated soils: n Red, oxidized iron in root channels n n 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).

n Crops requiring saturated conditions: Rice: wild rice, paddy rice n Cranberries n

n "In waters with high sulfate, we've struggled to find any wild rice, “ n John Pastor damage to wild rice probably occurs when sulfate is converted to hydrogen sulfide. In an oxygen-starved environment such as the sediment under wild-rice beds, bacteria "breathe in" sulfate and "exhale" hydrogen sulfide, which can be toxic to plants. n Pastor knows from previous research that the availability of adequate nitrogen is the biggest limiting factor for the growth of wild rice. n Now he is seeing wild-rice plants exposed to high sulfate that "didn't look poisoned. They looked starved. " Pastor's hypothesis is that sulfate transformed to sulfides is affecting root growth and blocking nutrients from getting into plants. Now he will see if the research supports his hypothesis.

Paddy rice n Important food staple for over half of world’s population n Important for food security, produced overwhelmingly in developing nations Asia: 90 % of world production n China 30% n India 21 % n

Process

Black spruce

Pitcher plant