Soil Aeration and Temperature (8)

Permafrost

25% global land areas underlain by it; rising soil temperatures can cause upper layers to thaw

Soil temperature is

a measure of the thermal state of the soil with respect to its ability to transport heat.

The more water you have in soil pores, the less

air you have.

Factors affecting absorption and losses of solar energy include

albedo, aspect, rain, soil cover

Aspect

angle at which sun's rays strike soil; heat energy absorption greatest when sun is directly overhead

Respiration rates in the soil

availability of organic carbon; microbial activity; concentration of oxygen and carbon dioxide

Soil cover

bare soils warm up more quickly and cools off more rapidly than soils covered with vegetation

Soil heating by fire

breakdown and movement of organic compounds; may increase hydrophobicity in soils

Organic mulches and plant residue management

buffers extremes in soil temperatures, conservation tillage, cover crop, cool climates

Plastic mulches

can extend growing season in temperate regions

There is an inverse relationship between

carbon dioxide and oxygen in the soil; as high as 10% can be toxic

Rain

cools soils in summer

Thermal conductivity of soils

dependent on the various components of the soil (moisture, compaction)

Drainage of excess water

determined by macro porosity; water drains out under gravity and is replaced by air through diffusion; influenced by factors affecting pore size distribution

Factors affecting soil aeration and redox potential (Eh)

drainage of excess water; respiration rates in soil; soil profile; soil heterogeneity, seasonal differences, effects of vegetation

Moisture control

drainage systems, tillage

High albedo

dry, less OM, light in color

Diffusion

ease of movement of a specific gas into or out of the soil; most gaseous interchange; each gas moves in direction determined by partial pressure

Mass flow is where

ease of movement of all gases into or out of soil; less important than diffusion.

Albedo

fraction of incident radiation reflected by the land surface

Aerial roots

grab oxygen from atmosphere; roots below water table

Specific heat of soils

higher in wet soils than in dry soils

Effects of vegetation

increased transpiration (low water table)

Poor soil aeration is characterized by

insufficient oxygen, which is caused by high water content, which causes compaction (less pore spaces).

Oxygen

is a strong oxidizing agent (electron acceptor); an electron acceptor in aerobic respiration

Heat of vaporization

large amount of energy needed to evaporate water from soil surface; evaporation cools soils

The higher the pore tortuosity is, the

lower the mass flow.

Gaseous interchange is facilitated by

mass flow and diffusion.

Oxygen

may be below twenty percent in upper layers, so more macropores.

Other gases in soil include

methane, hydrogen sulfide; ethylene under anaerobic conditions (toxic)

Air filled porosity severely inhibits

microbial activity and plant growth when below 20% of pore spaces or 10% of total soil volume.

Organic residue degradation

microbial activity.

Low albedo

moist, more OM , dark in color

Excess moisture

nearly all pores filled with water such as in wetlands and short periods of time on upland sites; hydrophytes (rice; love water)

Soil temperature is dependent on

net amount of heat energy soil absorbs; heat capacity of the soil, energy required for processes such as evaporation

Oxidation-reduction of elements

nutrients, toxic elements, soil colors, greenhouse gases

Ways to control soil temperature include

organic mulches and plant residue management; plastic mulches; moisture control

Ecological effects of soil aeration

organic residue degradation, oxidation-reduction of elements, activities of higher plants

Gaseous composition of soil air

oxygen, carbon dioxide, other gases

Hydric soils (aquic suborder)

periods of saturation that inhibit the diffusion of oxygen into the soil; undergo reduced conditions for substantial periods of time

Soil temperature affects

physical, biological, and chemical processes in soil; may affect processes in plants growing on soil; related to soil water and aeration

Activities of higher plants include

plant growth (reduced root and shoot growth), nutrient and water uptake (reduced root growth, toxic substances produced by anaerobic microorganisms), soil compaction

Soil aeration is an important factor for

plant growth and microbial activity.

Processes affected by soil temperature include

plant processes, seed germination, microbial processes, permafrost, soil heating by fire, contaminant removal

Partial pressure is the

pressure a gas in a mixture would exert if it alone were present in the volume occupied by the mixture

Diffusion is influenced by

same factors as mass flow; concentration gradient; partial pressure of gas; water saturation

Mass flow is influenced by

soil bulk density, percent pore space, soil structure, soil texture, soil water content, temperature, soil cover, soil pore tortuosity

Soil aeriation is regulated by

soil macro porosity, soil water content, and oxygen consumption by respiring organisms

Plant processes

soil temperature for optimal growth differs for many plants

Soil aeration is affected by

soil texture, structure, and porosity; also can be affected by chemical and biological factors

The primary source of energy is

solar radiation.

Thermal properties of soils

specific heat, heat of vaporization, thermal conductivity

Seed germination

specific soil temperature needed to trigger it.

Soil profile

subsoils: lower pore spaces (macropores); higher water content, tortuosity

Seasonal differences

temperate humid regions; early spring = wet soil, low use of oxygen and release of carbon dioxide; summer = lower moisture content, quick depletion of oxygen and increased carbon dioxide

Redox potential

tendency/potential for electrons to be transferred from one substance to another

Soil pore tortuosity is

the crooked-ness of the pore; the higher it is, the lower the mass flow

Redox reactions occurs when

the reduced state of an element is changed to the oxidized state.

Soil heterogeneity is affected by

tillage (right after tillage, after some time), pore sizes (water saturation), and plant roots (oxygen depletion)

Wetland

transitional ecosystems between land and water;

Anaerobic conditions occur when the supply of oxygen is

virtually exhausted.

Microbial processes are greater at

warm temperatures; doubles for every 10 degrees C