NRES 201 Lecture 21 Outline of Last Lecture I. Conservation tillageII. Significance of wind erosionIII. Mechanics of wind erosionIV. Factors affecting wind erosionV. Predicting wind erosionVI. Controlling wind erosionOutline of Current Lecture VII. Why is this topic important?VIII. Soil aeration: Basic ConceptsIX. Composition of soil airX. Oxidation - reduction (redox) potential XI. What happens when a soil is waterloggedXII. Factors affecting soil aeration and EbCurrent LectureWhy is this topic important?- Aeration ventilates the soilo Supplies O2 for respiration - by plant roots and by microorganisms o Removes CO2 and other toxic gases- Soil temperature affectso Plant and microbial growtho Soil drying by evaporationo Soil aerationSoil aeration: Basic conceptsGas exchange processes - - Mass flowo Movement with a mediumo Promoted by: soil moisture changes, wind, changes in barometric pressureo Of minor importance for soil aeration, mainly impacting: surface horizons, shallow soils, soils with extensive macro porosity - Diffusiono Movement of an individual gas in response to its own partial pressure gradiento Net effect for soil: gain of O2, loss of CO2o The major process by which soil aeration occursThese 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 supplyo Can limit subsoil rooting, especially in wet soilso High soil moisture reduces - air filled porosity, O2 diffusion Composition of Soil Air- Generalizationso Compared to ambient air, soil air has: about the same level of N2, Less O2, More CO2o O2 and CO2 changes are greatest for wet soilsOxidation - Reduction potential- Definitionso Original meaning for reactions involving O2o Oxidations: Addition of oxygen example: C + O2 = CO2o Reduction: removal of oxygen example: 2H2oo Current meaning Oxidation: loss of electrons Reduction: gain of electrons- Oxidizing and reducing propertieso Different substances vary widely in their affinity for electrons: Oxidizing agents take electrons Reducing agents give electronso The electron affinity of different substances can be compared in a relative sense -using an electrochemical cell- Importance to soilo Mineral solubilityo Microbial activitieso Composition of the soil atmosphere Aerobic conditions- Plenty of O2- High Eh- Organic matter decomposes Anaerobic conditions- Absence of O2- Low Eh- Organic matter accumulatesWhat happens when a soil is waterlogged? Initial changeso 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 watero Eh decreaseso Anaerobic microbes utilize terminal electron acceptors other than O2o pH increasesFactors affecting soil aeration and Eh Soil drainageo Mainly determined by macro pore volume which depends on: Texture, bulk density, aggregate stability, organic matter content, biopore formation Respiration rateso Microbial activities largely determine soil O2 and CO2 levels Roots and soil animals also have an effecto 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 deptho Compared to the surface soil, subsoils are usually lower in O2 and higher in CO2o These differences are increased by: Higher soil moisture Higher temperature Presence of organic
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