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