Week 2: Soil Aeration and Temperature - Textbook Information

As aeration is reduced in a soil, what is the redox potential range?

0.3 to 0.35 volts.

In a well-aerated soil, what is the redox potential range?

0.4 - 0.7 volts.

How much more concentrated is CO2 in soils compared to the atmosphere?

10 times as concentrated.

How much faster does oxygen diffuse in a pore filled with air compared to that of a similar pore filled with water?

10,000 times faster.

What is the "partial pressure gradient?"

A concentration gradient for one individual gas.

What does the term "poor soil aeration" mean?

A condition in which the availability of oxygen in the root zone is insufficient to support optimal plant growth and aerobic microorganisms.

What is a hydrophyte? Could you provide one example, while at the same time defining aerenchyma?

A plant adapted to living in waterlogged soils. Examples: sedges, or certain grass species, including rice, that transport oxygen for respiration down to their roots via hollow structures in their stems known as aerenchyma tissues.

What types of roots do mangrove and other hydrophytic trees produce, and why?

Aerial roots and other structures that allow their roots to obtain oxygen while growing in waterlogged soils.

When no O2 is available in a soil, what type of microorganism can survive? What must they do?

Anaerobic organisms. They must use substances other than O2 as the terminal electron acceptors for their metabolism, like iron in soil minerals.

What could we incorporate into a soil that would alter the soil air composition appreciably?

Animal manure, crop residues, compost, or sewage sludge.

Once a soil becomes essentially devoid of O2, the soil redox potential falls to what levels?

Below levels of about 0.38 to 0.32 volts.

How does compaction of soils cut off atmospheric gas exchanges?

By decreasing pore size and total pore space, even if the soil is not very wet and has a large percentage of air-filled pores.

How does the majority of gases interchange in soils?

Diffusion.

If organic matter-rich soils are flooded under warm conditions, how low can the redox potential go? Why?

Down to -0.3 volts, because oxygen is completely depleted.

In what direction does each gas move in a soil when influenced by diffusion, and how?

Each gas moves in a direction determined by its own partial pressure. The partial pressure of a gas in a mixture is simply the pressure this gas would exert if it alone were present in the volume occupied by the mixture.

Which gas helps plants sense the depletion of O2 in their roots?

Ethylene, which is produced by microorganisms within the soil.

True/False (explain): the O2 content of soil is NOT continually replenished by flowing water.

False, it is replenished by flowing water. Roots and microbes would completely deplete the O2 supply under stagnant water, water that doesn't move.

True/False: the concentrations of gases such as methane (CH4) and hydrogen sulfide (H2S) are NOT formed as organic matter decomposes in a soil.

False, these gases ARE formed.

True/False (explain): all aerobic respiration requires N2 to accept the electrons released as living organisms oxidize organic carbon to provide energy for life.

False. O2, not N2, does this.

True/False (explain): ethylene, in high concentrations, is very good for plant growth.

False. When gas exchange rates between the atmosphere and the soil are slow that ethylene begins to accumulate, even at 0.0001% concentration the number of plants has been shown to be inhibited.

How does soil moisture content enhance the mass flow of air? What is another factor that enhances mass flow?

Fluctuations in soil moisture content force air in or out of the soil. Another factor is pressure.

When anaerobic organisms are present, what color soil will you find? Why?

Gray, because of reduced iron and minerals exposed by the removal of iron oxide coatings. Reduction reactions caused by anaerobic organisms used iron for their electron acceptors.

In a reduction reaction, which ion is consumed that rises soil pH?

H+.

In an oxidation reaction, what ion is formed that lowers soil pH?

H+.

How could there be a reduction in oxygen content in a well-drained soil?

If heavy rain were to occur.

If there is a higher concentration of oxygen in the atmosphere, what will the net movement be: in, or out of a soil? Why?

In a soil. Oxygen is following its partial pressure gradient.

Respiration by the roots of upland plants usually depletes the O2 in the soil outside of the root. When is the opposite true?

In wet soils growing hydrophytic plants, where aerenchyma tissues transport surplus O2 into the roots.

Briefly describe how soil ventilation connects soil processes to the atmosphere.

It allows for the exchange of gases between the soil and the atmosphere, supplying enough oxygen for respiration, and preventing accumulation of toxic gases such as carbon dioxide, methane, and ethylene.

List, but do not explain, the two mechanisms that facilitate soil ventilation.

Mass flow and diffusion.

What are the concentrations of both O2 and CO2 largely dependent on in soils?

Microbial activity, which in turn depends on the availability of organic carbon compounds as food.

When air-filled porosity falls below 20% of the pore space, what is primarily inhibited in most soils?

Microbiological activity and plant growth become severely inhibited.

What does the term "anaerobic" mean?

No oxygen left in the soil environment.

How could a recently flooded soil become anaerobic in the upper 50 to 100 cm horizon, but remain aerobic deeper in the soil profile?

O2 can diffuse fast enough to replace that used by respiration if organic substrates are in low supply.

If the pH is 6.5, which gas will be reduced first? What will it be followed by? Use this graph: https://prnt.sc/uy8jou

O2, NO3-, MnO2, Fe(OH)3, iron in FeOOH, and finally sulfur in (SO4)2- and carbon in CO2.

If there are higher concentrations of carbon dioxide and water vapor in the soil air, what will the net movement be: in, or out of a soil? Why?

Out of a soil. These gases are following their partial pressure gradients.

List, and briefly explain in a few words, the three principal factors that regulate oxygen availability in field soils.

Soil macro-porosity (as affected by texture and structure), soil water content (as it affects how much of the pores are filled with air), and oxygen consumption (by plant roots and microorganisms).

What are the soil properties that help determine macropore content, and in turn, soil aeration?

Soil texture, bulk density, aggregate stability, organic matter content, and biopore formation.

The fact that the lines on the graph of redox potential vs. pH are sloping downwards, suggests what? Use this graph: https://prnt.sc/uy8jou

That the Eh (redox potential) for a reaction is lower when the pH of the soil solution is higher.

List, but do not explain, the three ways the aeration status of a soil can be characterized.

The content of oxygen and other gases in the soil atmosphere, the air-filled soil porosity, and the chemical oxidation-reduction potential.

What is the "oxidizing agent?"

The electron acceptor.

What is the "reducing agent?"

The electron donor.

What is ventilation? When is there a great need for soil ventilation?

The exchange of gases between the soil and the atmosphere. When more roots and microbes use up oxygen and use carbon dioxide.

When 80 to 90% of the soil pore space is filled with water, what does this mean for air storage and atmospheric gas exchanges?

The high soil water content leaves less than 10 to 20% of the pore space for air storage, but water also blocks the pathways by which gases could exchange with the atmosphere.

What can soil aeration help determine?

The specific chemical species present in soils and, in turn, the availability, mobility, and possibly toxicity of many chemical elements.

What is "redox potential?"

The tendency for electrons to be transferred from one substance to another.

If an upland plant succumbs to oxygen deficiency, what are the two ways they can adapt to these anoxic conditions?

They can sense low oxygen levels and adapt their physiology by developing aerenchyma or slowing their oxygen use, within a few days or weeks.

True/False: oxygen rapidly accepts electrons from many other elements.

True.

True/False: soil air usually is much higher in water vapor than atmospheric air.

True.

True/False: very small pores in a clayey or compacted soil may slow the diffusion of O2 and thereby lower redox potentials within that layer, whether or not the soil is very wet.

True.

True/False (explain): anaerobic conditions may occur in the center of an aggregate, only a few millimeters from well-aerated conditions near the surface of the same aggregate.

True. O2 will diffuse much more slowly through the small, largely water-filled pores within a soil aggregate than through the largely air-filled pores between aggregates.

In what unit is redox potential usually expressed?

Volts or millivolts.

How do high water contents cause oxygen deficiencies for roots?

Water-filled pores block the diffusion of oxygen into the soil to replace that used by respiration.

In what type of environment are waterlogged conditions typical?

Wetlands.

What does it mean for a soil to be "water saturated" or "waterlogged?"

When all of the soil pores are filled with water.

Quantitatively, when does poor aeration become a serious impediment to plant growth?

When oxygen concentration drops below 0.1 L/L. This often occurs when more than 80 to 90% of the soil pore space is filled with water.

How can redox potential be measured?

With a platinum electrode attached to a millivolt meter.