Movement of Soil Gases

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Movement of Soil Gases

• Marc Kruse
• 2 April 03

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Gas Movement

• Change in temperature
• Change in atmospheric pressure
• Rainwater infiltration
• Wind over the soil surface
• Root effects

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Gas Movement

• Occurs by 2 processes
• Diffusion
• Convection

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Gas Movement Diffusion

• Movement of a gas from an area of high
  concentration to lower concentration
• More important than convection in soil aeration

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Gas Movement Diffusion

• Transport of oxygen and carbon dioxide occurs in
  the gas phase and liquid phase
• In liquid films, the process maintains O2
  supply and means of CO2 removal from live tissue

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Gas Movement Diffusion
Can be described by Ficks Law Jg Diffusive
Flux of a Gas (mass diffusing across a unit area
per unit time) D Diffusion coefficient
(area/time) c concentration (mass of a
diffusing substance per unit volume)
concentration gradient
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Gas Movement Diffusion

• In porous bodies, diffusion coefficient depends
  on fractional volume of continuous gas phase
• Not affected by shape of solid surface or
  distribution of pore size

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Gas Movement Diffusion
In aggregated soils, gas diffusion takes place in
interaggregated macropores due to the fact
macropores are readily drained Diffusion into an
aggregate formula Concentration difference in
the clod is proportional directly to respiration
rate (S) and inversely to diffusion coefficient
(Ds)
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Gas Movement Diffusion
Cylindrical form of the diffusion equation looks
at the diffusion of soil gases and the water
coating on the root r radial distance from
the root axis
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Gas Movement Diffusion
Increasing soil water decreases the diffusion
coefficient D(q) of a gas 1) Diffusion in water
is less than diffusion in air 2) Concentration
of oxygen in water is lower than the
concentration of oxygen in air
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Gas Movement - Convection

• Similar to water flow
• Flow is proportional to the pressure gradient
• Different than water flow
• Air is compressible and dependent on density
  and viscosity
• Gravity does not affect gas flow
• Gas is not attracted to mineral surfaces

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Gas Movement - Convection
Described by a formula similar to Darcys
Law Flux mass per unit area and unit time qv
volume convective flux of air (volume flow
though a unit cross section area per unit time k
permeability of air filled pore space
viscosity of soil air 3d gradient of soil
air pressure
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Gas Movement - Convection
Density of a gas depends on pressure and
temperature Assume soil air is an ideal gas PV
nRT P Pressure V Volume n number of moles
of the gas
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Gas Movement - Tortuosity
Air tortuosity is dependent on the fractional
volume of air filled pores Air filled porosity
increases as tortuous path length decreases
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Measuring Soil Gases

• Page 300-302 of Hillel shows laboratory
  examples to measure soil gases
• Personal research uses a LiCor-6400 Infrared
  Gas Analyzer (IRGA)

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Measuring Soil Gases

• The chamber initially measures the ambient air
  CO2 concentration
• The chamber then draws down the CO2 within it
  and begins measuring the flux back to the target
  (ambient) CO2 concentration
• Flux is measured in µmol CO2 / m2 / s

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Measuring Soil Gases

• 2002 data indicates as N rate (applied in 2001)
  increases, CO2 flux increases (soybeans)
• Flux due to microbial mass and root respiration
• Tillage increases CO2 flux for the first couple
  hours and then stabilizes
• Increased breakdown of soil organic matter due
  to microbial activity

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Soil Gas Fun Facts
Agriculture supposedly contributes 20 of all
greenhouse gas emissions Agriculture supposedly
contributes 40 of all methane emission Estimated
the ocean holds 1.5 3x as much C as living
vegetation Total soil carbon is estimated at
1.4-1.6 x 1012 metric tons
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