Content and Composition of Soil Air

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Content and Composition of Soil Air

• Jamie Boehm
• February 5, 2003

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Objectives

• Define and discuss
• Soil air content and composition
• Soil aeration
• Soil respiration
• Methods for measurement of soil air content and
  composition and soil respiration

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Soil Aeration

• Gaseous exchange between the atmosphere and soil
  air
• Determinant of soil productivity
• Gasses move through air-filled pores and in the
  dissolve form in water allowing for root and
  microorganism respiration

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Poor aeration can be caused by

• Compaction
• Water-logged soils
• Poor drainage

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Poorly Aerated Soils

• Anaerobic conditions can cause a chemical
  reduction of minerals
• Mottling and gleyed zones occur
• Manganese reduction causes black nodules to form
  
• Poorly aerated zones can occur in well-aerated
  soils

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Soil Aeration Index

• fa f-?
• fa volume fraction of air
• f total porosity
• ? volume fraction of water

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Wetness Factors

• Field Capacity- a state of free drainage after
  the period of rapid drainage has ceased
• Field Air Capacity- Volume of air when a soil is
  at field capacity water content

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Air Content is Texture and Structure Dependent

• Sand 25
• Loam 15-20
• Clay Below 10
• Strongly Aggregated (gt5mm) 20-30
• Smaller Aggregates and dispersed clay 5

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Limiting plant growth

• Limiting value for for aeration has been
  estimated at 5-20
• Difficult to determine
• Field capacity and total pore space measurements
  vary
• Rate of exchange also important
• Composition of air is more important than volume

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Composition of Soil Air

• Influenced by
• Time of year
• Root growth
• Microbial activity
• pH
• Rate of exchange through soil surface
• Temperature
• Soil moisture
• Depth below surface

• Well aerated soils have air composition similar
  to the atmosphere
• CO2 levels are often 10 times greater than the
  atmospheric concentration

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Measurement of Soil Air Content

• Field measurements
• An undisturbed sample is taken 2 days after heavy
  rain, then pore space is measured
• Lab
• Soil is placed on a porous plate, water is added
  and tension is applied to obtain field capacity
  water content, then pore space is measured

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Measurement of Soil Air Composition

• Syringe extraction of .5mL of soil air and
  analyze using GC
• Membrane covered electrodes
• CO2 and O2 can be measured
• Problems can occur with equilibration and soil
  moisture

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Soil Respiration and Aeration Requirements

• Aeration Requirement- amount of O2 needed and CO2
  produced
• Varies spatially and differs among soils due to
  changes in temperature, pH, OM content
• Insufficient amounts of O2 causes oxygen stress,
  or anaerobiosis

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Seasonal Respiration

• Respiration rates vary greatly due to temperature
  changes
• Summer respiration rates can be 10 times greater
  than winter rates
• Other factors such as microbial activity increase
  spring respiration

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Seasonal Respiration
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Measurement of Soil Respiration

• Field respirometers used in field studies provide
  a controlled environment to measure soil
  respiration
• Using this method, the Respiratory quotient (RQ),
  the ratio of CO2 to O2, is obtained

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Oxidation-Reduction Processes in Soil

• Anaerobic conditions cause reduction reactions
• Denitrificatoin- NO3- NO2- N2O N2
• Manganese reduction
• Iron Reduction
• Sulfate Reduction

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Oxidation-Reduction Processes in Soil

• Organic compounds are also produced in the
  anaerobic decomposition of OM
• Compounds that are toxic to plants
• Hydrogen sulfide
• Methane
• Ethylene
• Acetic, butyric, and phenolic acids

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Oxidation-Reduction Processes in Soil

• Electron transfer occurs as soil organisms obtain
  energy
• Example glucose is an electron source when
  oxidized to pyruvic acid
• C6H12O6 2CH3COCOOH 4H 4e-

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Oxidation-Reduction Processes in Soil

• Oxygen is the final electron sink in aerobic
  respiration and forms water
• O2 4H 4e 2H2O
• Nitrate and sulfate ions accept electrons and
  lose oxygen when free oxygen is low

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Oxidation-Reduction Processes in Soil

• The tendency of a solution to donate electrons
  to a reducible substance or to accept electrons
  from an oxidizable substance is measured in terms
  of its redox potential
• Potential in volts required in an electric cell
  to produce oxidation at the anode and reduction
  at the cathode
• The more strongly reducing a substance, the lower
  its potential
• Depends on soil pH

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Questions?