Ch. 7 Soil Aeration

Air Composition with Depth

-02 decreases with depth -CO2 increases

Wetland chemistry (depending on the redox state of the system)

-Availability of nutrients and trace elements can increase/ decrease -Can aid in neutralization of acid drainage -Cain aid in removal of toxic forms of trace elements

Soil Heterogeneity

-Depth in profile -Tillage (short term drying at the surface -Macropores vs. micropores

O2 Decreases in soil:

-Displaced by H2O -Consumed by biological activity -Has an inherent low solubility and diffusion rates in H2O

Organic Matter Degradation

-Faster & more complete under oxidized conditions -Toxic by-products may accumulate (when OM reduces) -Ethylene gas, alcohols & organic acids

Soil Structure and Cultivation

-Presence of macropores is good for aeration -Maintenance of organic matter -Limit cultivation to maintain macroporosity & carbon -This is true for almost all plant types

Container-Grown Plants

-Roots are concentrated -Nutrients not as available, -Soil is easily waterlogged

Growing Season

-Soils must be warm enough for microbial respiration -Reduction rate increases with soil temperatures

Importance of Wetlands

-Species habitat -Flooding reduction -Water filtration -Shoreline protection -Commercial/rec. activities`

Surface Compation

-Tillage, timber harvesting, wheel traffic, etc. compact a A horizon and deaceas aeration -Puddling (compacting paddy soils decreases aeration

Redox Potential (Eh)

-Volts 1. Measures the tendency of soil to donate or accept electrons 2. More + the Eh the easier/more readily the substance can be used as a TEA 3. Measured using a platinum electrode 4. Related to reference state (half reaction)

Redox Principles

1. Directly related to aeration 2. Majority of reactions are microbially mediated during OM decomposition 3. OM is the major electron source 4. In aerobic soils, O2 is used as the primary TEA or oxidizing agent 5. As soils become anaerobic microbes must find other TEAs

Redox Sequence in Waterlogged Soils

1. Eh decreases as soil pores fill & stay full of water 2. O2 decreases in soil 3. Soil becomes anaerobic & microbes then use other substances as TEA 4. pH rises because most redox reactions consume protons

Redox of Elements Affects...

1. Element toxicity, availability & solubility/mobility 2. CH4 production 3. Upland plant growth (reduced in poorly aerated soils

Redoximorphic features

1. In O2 depleted soils microbes use Mn, Fe, & S as TEAs that leave color patterns when reduced/oxidized 2. Low chroma colors become dominantly grey as Mn & Fe reduce 3. Mn & Fe nodules sometimes form during oxidation if they precipitate in high concentration 4. Mottling related to redox processes w/ notable exceptions 5. Must occur near surface to help ID hydric soils & wetlands

Gaseous Exchange

1. Mass Flow 2. Diffusion

Soil Color

1. Redox state or aeration state of those minerals -Reduced Fe is grey -Oxidized Fe is yellow to red 2. Time for development of reducing condition longer a soil is wet the greyer it becomes.

Factors Affecting Soil Aeration

1. Soil drainage class 2. Microbial respiration rate increase as OM & temp. rise. 3. Soil Heterogeneity 4. Growing Season 5. Oxidized Rhizospheres 6. Surface Compation

Oxygen availability is a function of...

1. Soil macroporosity 2. Soil H2O content 3. O2 Depletion

Hydrophytic Vegetation

1. Vegetation that has adapted to low O2 conditions 2. Grasses w/ aerenchyma tissues (rice) 3. Trees w/ adventitious roots and knees (bald cypress

Wetland Hydrology

1. Water balance 2. Hydroperiod 3. Residence time 4. Indicators (hydric soils & hydrophytic vegetation)

Wetland Delineation

1. Wetland hydrology or water regime 2. Hydric soils 3. Hydrophytic plants

Oxidizing Agent

A substance that easily accepts e-

Reducing Agent

A substance that easily supplies e-

Aerenchyma tissues

Air channels that allow air exchange between shoots and roots

Anaerobic

All O2 is depleted

Redox

Electropotential soils -Influences solubility and availability of nutrients and the toxicity and mobility of contaminants

Half Reaction

H+ + e- = 1/2 H2 (Arbitrarily taken as zero)

Diffusion

Major Process -differences in partial pressure -gradients between different partial pressures -diffusion rate is 10,000x faster in air than in water

Oxidized Rhizosphere

Many plants are capable of oxidizing Fe in their rhizosphere

Mass Flow

Minor Process -Changes in soil moisture -barometric pressure changes -wind

Residence time

Need several weeks of saturation to get reduction

loses electrons

Oxidized

Oxidation/Reduction

Redox is short for...

Gains electrons

Reduced

O2 Depletion

Reduction and consumption during microbial OM decomposition

Hydric Soils

Soils that are wet within or immediately below the A horizon to such an extend that they become reduced and exhibit significant redoximorphic features

Terminal Electron Acceptor

TEA

Hydroperiod

Temporal pattern of water table changes in height

Aeration

Ventilation (gas exchange) -Most plants and microbes require oxygen from the soil

Wetlands

Wet, reduced soils with adapted vegetation

Constitute 14% of the world's largest land area

Wetlands

pH rises because...

most redox reactions consume protons