NRES 201 Lecture 21 Outline of Last Lecture I Conservation tillage II Significance of wind erosion III Mechanics of wind erosion IV Factors affecting wind erosion V Predicting wind erosion VI Controlling wind erosion Outline of Current Lecture VII Why is this topic important VIII Soil aeration Basic Concepts IX Composition of soil air X Oxidation reduction redox potential XI What happens when a soil is waterlogged XII Factors affecting soil aeration and Eb Current Lecture Why is this topic important Aeration ventilates the soil o Supplies O2 for respiration by plant roots and by microorganisms o Removes CO2 and other toxic gases Soil temperature affects o Plant and microbial growth o Soil drying by evaporation o Soil aeration Soil aeration Basic concepts Gas exchange processes Mass flow o Movement with a medium o Promoted by soil moisture changes wind changes in barometric pressure o Of minor importance for soil aeration mainly impacting surface horizons shallow soils soils with extensive macro porosity Diffusion o Movement of an individual gas in response to its own partial pressure gradient o Net effect for soil gain of O2 loss of CO2 o The major process by which soil aeration occurs These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Oxygen supply o Can limit subsoil rooting especially in wet soils o High soil moisture reduces air filled porosity O2 diffusion Composition of Soil Air Generalizations o Compared to ambient air soil air has about the same level of N2 Less O2 More CO2 o O2 and CO2 changes are greatest for wet soils Oxidation Reduction potential Definitions o Original meaning for reactions involving O2 o Oxidations Addition of oxygen example C O2 CO2 o Reduction removal of oxygen example 2H2o o Current meaning Oxidation loss of electrons Reduction gain of electrons Oxidizing and reducing properties o Different substances vary widely in their affinity for electrons Oxidizing agents take electrons Reducing agents give electrons o The electron affinity of different substances can be compared in a relative sense using an electrochemical cell Importance to soil o Mineral solubility o Microbial activities o Composition of the soil atmosphere Aerobic conditions Plenty of O2 High Eh Organic matter decomposes Anaerobic conditions Absence of O2 Low Eh Organic matter accumulates What happens when a soil is waterlogged Initial changes o Dissolved O2 decreases Due to respiration by aerobic microbes that use organic C as an energy source Organic C supply determines the rate of decrease Do not replace by slow O2 diffusion in water o Eh decreases o Anaerobic microbes utilize terminal electron acceptors other than O2 o pH increases Factors affecting soil aeration and Eh Soil drainage o Mainly determined by macro pore volume which depends on Texture bulk density aggregate stability organic matter content biopore formation Respiration rates o Microbial activities largely determine soil O2 and CO2 levels Roots and soil animals also have an effect o These activities Increase with soil temperature And require organic C as an energy source so there is a stimulating effect of organic C inputs such as Plant residues or manure Profile depth o Compared to the surface soil subsoils are usually lower in O2 and higher in CO2 o These differences are increased by Higher soil moisture Higher temperature Presence of organic substrates
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