Soil Air and Temperature Characterizing Soil Air Factors Affecting Soil Air Ecological Impacts of Soil Aeration Soil Air and Wetland Hydrology Soil Temperature Thermal Properties of Soils Management of Soil Temperature

1. Soil Air Soil air and water share the pore space of soils So texture, structure, porosity, etc affect aeration Air (O2) is needed in the soil for root respiration as well as the activity of microorganisms in soil. Therefore a major management objective is to maintain a high level of air in the soil through a ventilation process.

Soil Aeration Process Soil Aeration is a process that involves the ventilation of the soil with gases moving both into and out of the soil The two most important gases in soil aeration are oxygen and carbon dioxide

Soil aeration in the field Poor soil aeration –O2 in the root zone is insufficient to support optimum growth of most plants and aerobic microorganisms (>80% pores filled with water) Waterlogged -When all soil pores (~100%) are filled with water, soil is said to be water saturated Anaerobic soil environment is when the oxygen supply is virtually exhausted Plants that are adapted to life in water saturated soils are called hydrophytes.

Gaseous Exchange Soil aeration process is achieved by gaseous exchange The more rapidly roots and microbes use up oxygen and release carbon dioxide, the greater is the need for exchange of gases between the soil and the atmosphere. Exchange of gases between soil and atmosphere is achieved through two mechanisms: Mass flow – gas exchange is due to fluctuations in water content of soil that force air in and out. Diffusion – gas exchange is by partial pressure.

Characterizing Soil Air Soil air can be characterized by Content of oxygen and other gases in soil Air-filled soil porosity Oxidation-reduction potential

a). Content of oxygen and other gases in soil The main differences between soil air and atmospheric air are the concentrations of oxygen and carbon dioxide. Soil air always has lower oxygen levels. But has greater Carbon dioxide levels (8 to 300 times greater because CO2 is given off as a result of microbial and root respiration in the soil). Gas % in atmosphere % in soil air O2 20.95 20.42 N2 78.08 Ar 0.934 CO2 0.035 0.56 Other 0.001 0.001 

b). Air-filled soil porosity Recall, ideal soil composition for plant growth = 50:50, air and water. Air filled porosity of less than 20% has been found to severely limit plant growth. High soil moisture cause oxygen deficiency by blocking diffusion of O2 to replace that used by respiration in the soil.

c). Oxidation-reduction potential The oxidation states of chemical elements depend on the soil aeration. If soil is well aerated, oxides of elements dominate- FeOOH, NO3- In poorly aerated soils, reduced forms of the elements are common. Oxygen plays very important role in oxidation-reduction of the other elements since it accepts electrons from many other elements. Other electron acceptors are N, Fe, Mn, and S

Factors Affecting Soil Air Drainage of water - Amount of Macropores Texture, Bulk density, Aggregate structure and stability, and Soil organic matter content. Rates of respiration in the soil - O2 and CO2 depend on microbial activity Manure, crop residue, sewage sludge. Soil depth (subsoil vs. topsoil) Subsoil usually wetter, higher bulk density, often less total and macropore space, lower OM and slower respiration.

Changes in a Soil Condition - Caused by: Tillage (short term introduces air; long term destroys macropores). Seasonal Differences (Moisture and Temperature) humid temperate regions have wet and cold soils More favorable tempts. Stimulate aeration. Vegetation Remove water by transpiration (can affect depth to water table) Affects soil temperature and therefore respiration rates (i.e shading)

Ecological Effects of Soil Aeration Effects on organic residue Poor aeration slows down the rate of decay. Oxidation reduction of elements O2 status through oxidation-reduction determines the forms of nutrients in soil (e.g., oxidized states of N, S, Fe, Mn), toxic elements (eg., Cr, As, Se), greenhouse gases, and soil colors. Effects on higher plants Poor aeration adversely affects plant growth, nutrient absorption, and formation of toxic compounds in soil.

Soil Air, Soil Water, Plants, and Wetlands Poorly aerated areas are called wetlands. The interrelationships of air, water, and plants are most critical in water-saturated soils (wetlands) They are water-saturated near the surface for prolonged periods that soil organisms experience insufficient O2 creating anaerobic conditions. Since uses and management of wetlands are regulated by most governments, they are protected by federal dollars. Wildlife habitat Water storage Water filtration etc Politics and money are involved. –wetland delineation/Banks.

What are wetlands? Wetlands are ecosystems that are transitional between land and water. They have anaerobic conditions in general. Three characteristics of Wetlands: Wetland hydrology or water regime Hydric soils Hydrophytic plants

a. Wetland Hydrology Water balance (inputs and outputs of water) Hydroperiod (temporal pattern in changes of the water table) Some wetlands may be flooded daily and some may never be flooded. Residence time (flow rate)

b. Hydric soils -mainly Histosols (OM accumulation due to slower decomposition) The properties that define hydric soils: Hydric soils are commonly saturated. Hydric soils experience reducing conditions Hydric soils have some indicators: Redoximorphic features (mottles) Redox depletion (primarily in Fe) Gray, low-chroma

c. Hydrophytic vegetation This distinguishes wetlands from other systems. Adaptative features include: Aerenchyma tissue Allows plants to transport O2 down to roots. Buttress roots Shallow roots

2. Soil Temperature Generally speaking, soil temperature affects many physical, chemical, and biological soil properties, and the plants growing in it. It affects plant and microorganism growth It affects many soil environmental processes such as soil drying by evaporation (therefore, water content) It affects water movement and retention in soils It affects soil formation

Processes Affected by Soil Temperature Specifically, processes affected by soil temperature include the following: Plant Processes Seed germination Root functions (i.e. winter burn from inhibited water uptake by roots in cold soils) Microbial processes Decomposition of OM (35-40 °C) Nitrification (NH4+ to NO3-) ~ 30 °C Frost heaving (soil formation)

Processes Affected by Soil Temperature Soil----------------Plants Microorganisms Soil physical and chemical properties

Source of Soil Heat Soil heat comes primarily from absorption of solar radiation. Incoming solar radiation is influenced by: Clouds and atmospheric particles Color--dark absorbs more radiation Aspect & slope--perpendicular to incoming rays absorbs more Rain Soil cover

How is soil heat utilized? Little of the solar energy reaching the earth actually results in soil warming. The energy is used primarily to evaporate water from the soil or leaf surfaces. Some is re-radiated or reflected upward--that's why temperatures near the ground are highest. The rest (about 10%) is used to warm the soil.

Thermal Properties of Soils Specific heat (heat capacity) The amount of energy required to raise the temperature of a substance by 1deg. C (cal/g). Wet soil change temperature more slowly than dry soils because the specific heat of water is higher than that of soil solid. Heat of vaporization The amount of energy required to vaporize water. Energy of vaporization of soil water comes from solar radiation or soil itself and result from soil cooling. Thermal Conductivity -the ease with which heat moves through a substance. Heat will pass from soil to water about 150 times easier than from soil to air because air is a good insulator, water is not. .

Management of Soil Temperature Two approaches: Practices that cover or mulch soil Mulches buffer extremes in soil temperature Mulches are important for temperate and tropical environments. Practices that reduce excess soil moisture. Poorly drained soils in temperate regions have been found to be about 5 deg. C cooler than comparable well drained soils. Removing water alleviates temperature depression. Use drainage systems Use ridge tillage systems.