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Earth's Systems / Biological processes / Terrestrial ecosystem functioning / Living soil system / Soil gas exchange and redox / Gas diffusion and air-filled porosity control of soil aeration

Gas diffusion and air-filled porosity control of soil aeration

Gas moves through soil mainly by diffusion in connected air-filled pores, so air-filled porosity and pore connectivity set how fast oxygen is resupplied and how fast carbon dioxide and trace gases escape.

Boundary: where this process starts

Starts with: Fick's law: gas flux = -Ds dC/dz, where Ds is the soil gas diffusion coefficient

Boundary: where it ends

Ends after: Aggregates wider than roughly 5-10 mm can hold anoxic cores in wet soil because O2 diffusing through water cannot meet microbial demand

PARTICIPANTSRESOURCESAir-filledporesAerobicmicrobesSoil poresAtmospherePlant rootsOtherparticipantsOxygenCarbon dioxidePore spaceWaterNitrous oxideImpact +3 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.Impact -3 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.Impact -3 of 5. Click for details and evidence.Impact -3 of 5. Click for details and evidence.Impact -3 of 5. Click for details and evidence.Impact -3 of 5. Click for details and evidence.↻ balances · moderate · 0.3 High CO2 and lowImpact -2 of 5. Click for details and evidence.↻ balances · weak · 0.2 Anoxic cores consumeFick's law: gas flux = -Ds dC/dz, where Ds is the soil gas diffusion coefficient: uses OxygenFick's law: gas flux = -Ds dC/dz, where Ds is the soil gas diffusion coefficient: uses Carbon dioxideGases diffuse about 10,000 times faster in air than in water (O2 about 2e-5 versus 2e-9 m2/s), so water-filled pores block aeration: uses OxygenGases diffuse about 10,000 times faster in air than in water (O2 about 2e-5 versus 2e-9 m2/s), so water-filled pores block aeration: uses WaterMillington-Quirk model Ds/D0 = e^(10/3)/phi^2: at porosity 0.5, cutting air-filled porosity from 0.30 to 0.10 reduces Ds/D0 from about 0.07 to 0.002: uses OxygenMillington-Quirk model Ds/D0 = e^(10/3)/phi^2: at porosity 0.5, cutting air-filled porosity from 0.30 to 0.10 reduces Ds/D0 from about 0.07 to 0.002: uses Pore spaceAeration is adequate above about 10% air-filled porosity, equivalent to an oxygen diffusion rate above about 0.2 microgram per cm2 per minute: uses OxygenAeration is adequate above about 10% air-filled porosity, equivalent to an oxygen diffusion rate above about 0.2 microgram per cm2 per minute: uses Pore spaceRespiration consumes O2 and produces CO2, so the steady-state CO2 excess scales with respiration divided by Ds: halving Ds doubles the excess: uses up Oxygenuses upRespiration consumes O2 and produces CO2, so the steady-state CO2 excess scales with respiration divided by Ds: halving Ds doubles the excess: makes Carbon dioxidemakesSoil air at depth commonly holds 0.3-3% CO2, about 10 to 100 times atmospheric (0.04%): uses Carbon dioxideWetting cuts Ds: aerobic activity peaks near 60% water-filled pore space and N2O production rises above it as denitrification takes over: uses OxygenWetting cuts Ds: aerobic activity peaks near 60% water-filled pore space and N2O production rises above it as denitrification takes over: uses WaterWetting cuts Ds: aerobic activity peaks near 60% water-filled pore space and N2O production rises above it as denitrification takes over: makes Nitrous oxidemakesAggregates wider than roughly 5-10 mm can hold anoxic cores in wet soil because O2 diffusing through water cannot meet microbial demand: uses OxygenCompaction and surface seals cut air-filled porosity and connectivity, lowering Ds and raising N2O emissions and lowering methane uptake: uses Pore spaceCompaction and surface seals cut air-filled porosity and connectivity, lowering Ds and raising N2O emissions and lowering methane uptake: makes Nitrous oxidemakesmoves down a gradientOxygen: moves down a gradientmoves up a gradientCarbon dioxide: moves up a gradientdiffuses far slower inwaterOxygen: diffuses far slower in waterblocks gas diffusionWater: blocks gas diffusiondiffusion falls as airdecreasesOxygen: diffusion falls as air decreasesair-filled fractionlimits diffusionPore space: air-filled fraction limits diffusionsupplied above thresholddiffusionOxygen: supplied above threshold diffusionair-filled fraction setsadequacyPore space: air-filled fraction sets adequacyconsumed by respirationOxygen: consumed by respirationexcess rises as Ds fallsCarbon dioxide: excess rises as Ds fallsaccumulates in deep soilairCarbon dioxide: accumulates in deep soil airconsumed by aerobicactivityOxygen: consumed by aerobic activitywetting reduces gasdiffusionWater: wetting reduces gas diffusionrises as denitrificationdominatesNitrous oxide: rises as denitrification dominatescannot reach aggregatecoresOxygen: cannot reach aggregate corescut by compaction andsealingPore space: cut by compaction and sealingemissions rise withcompactionNitrous oxide: emissions rise with compactionStart: Gas diffusionand air-filled porositycontrol of so…STARTStart: Gas diffusion and air-filled porosity control of soil aerationEnd: Gas diffusion andair-filled porositycontrol of so…ENDEnd: Gas diffusion and air-filled porosity control of soil aerationFick's law: gas flux =-Ds dC/dz, where Ds isthe soil gas diffusi…📄1Fick's law: gas flux = -Ds dC/dz, where Ds is the soil gas diffusion coefficientGases diffuse about10,000 times faster inair than in water (…📄1Gases diffuse about 10,000 times faster in air than in water (O2 about 2e-5 versus 2e-9 m2/s), so water-filled pores block aerationMillington-Quirk modelDs/D0 = e^(10/3)/phi^2:at porosity 0.…📄1Millington-Quirk model Ds/D0 = e^(10/3)/phi^2: at porosity 0.5, cutting air-filled porosity from 0.30 to 0.10 reduces Ds/D0 from about 0.07 to 0.002Aeration is adequateabove about 10%air-filled porosit…📄1Aeration is adequate above about 10% air-filled porosity, equivalent to an oxygen diffusion rate above about 0.2 microgram per cm2 per minuteRespiration consumes O2and produces CO2, sothe steady-state C…📄1Respiration consumes O2 and produces CO2, so the steady-state CO2 excess scales with respiration divided by Ds: halving Ds doubles the excessSoil air at depthcommonly holds 0.3-3%CO2, about 10 to 1…📄1Soil air at depth commonly holds 0.3-3% CO2, about 10 to 100 times atmospheric (0.04%)Wetting cuts Ds:aerobic activity peaksnear 60% water-fill…📄1Wetting cuts Ds: aerobic activity peaks near 60% water-filled pore space and N2O production rises above it as denitrification takes overAggregates wider thanroughly 5-10 mm canhold anoxic cores …📄1Aggregates wider than roughly 5-10 mm can hold anoxic cores in wet soil because O2 diffusing through water cannot meet microbial demandCompaction and surfaceseals cut air-filledporosity a…📄1Compaction and surface seals cut air-filled porosity and connectivity, lowering Ds and raising N2O emissions and lowering methane uptake
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Fick's law: gas flux = -Ds dC/dz, where Ds is the soil gas diffusion coefficient
Air-filled poresAtmosphereOxygenCarbon dioxide
Evidence (1) · added by Fern-C
  • Hillel 1998, Environmental Soil Physics: Diffusive flux is proportional to the concentration gradient with a soil-specific coefficient Ds below the free-air value D0. (confidence 0.85)
Gases diffuse about 10,000 times faster in air than in water (O2 about 2e-5 versus 2e-9 m2/s), so water-filled pores block aeration
Soil waterAir-filled poresOxygenWater
Evidence (1) · added by Fern-C
  • Glinski & Stepniewski 1985, Soil Aeration and Its Role for Plants, CRC Press: Diffusion coefficients of O2 are about four orders of magnitude lower in water than in air. (confidence 0.8)
Millington-Quirk model Ds/D0 = e^(10/3)/phi^2: at porosity 0.5, cutting air-filled porosity from 0.30 to 0.10 reduces Ds/D0 from about 0.07 to 0.002
Air-filled poresSoil poresPore spaceOxygen
Evidence (1) · added by Fern-C
  • Millington & Quirk 1961, Transactions of the Faraday Society 57:1200-1207: Relative diffusivity Ds/D0 = epsilon^(10/3) / phi^2 (epsilon air-filled and phi total porosity); the quoted values follow by substitution. (confidence 0.75)
Aeration is adequate above about 10% air-filled porosity, equivalent to an oxygen diffusion rate above about 0.2 microgram per cm2 per minute
Plant rootsAir-filled poresOxygenPore space
Evidence (1) · added by Fern-C
  • Grable & Siemer 1968, Soil Science Society of America Journal 32:180-186: Root elongation of corn declined when oxygen diffusion rate fell below about 0.2 microgram cm-2 min-1, which occurs near 10% air-filled porosity. (confidence 0.65)
Respiration consumes O2 and produces CO2, so the steady-state CO2 excess scales with respiration divided by Ds: halving Ds doubles the excess
Plant rootsAerobic microbesOxygenCarbon dioxide
Evidence (1) · added by Fern-C
  • De Jong & Schappert 1972, Soil Science 113:328-333, Calculation of soil respiration and activity from CO2 profiles in the soil: A diffusion-production model gives CO2 concentration at depth proportional to production rate divided by Ds. (confidence 0.7)
Soil air at depth commonly holds 0.3-3% CO2, about 10 to 100 times atmospheric (0.04%)
Soil airPlant rootsCarbon dioxide
Evidence (1) · added by Fern-C
  • Brady & Weil, The Nature and Properties of Soils, 15th ed., soil aeration section: Soil air CO2 is often several tenths to a few percent, much higher than the atmosphere, with O2 correspondingly lower. (confidence 0.65)
Wetting cuts Ds: aerobic activity peaks near 60% water-filled pore space and N2O production rises above it as denitrification takes over
Soil microbesSoil waterWaterOxygenNitrous oxide
Evidence (1) · added by Fern-C
  • Linn & Doran 1984, Soil Science Society of America Journal 48:1267-1272: CO2 production was greatest near 60% water-filled pore space, whereas N2O production increased at higher values. (confidence 0.75)
Aggregates wider than roughly 5-10 mm can hold anoxic cores in wet soil because O2 diffusing through water cannot meet microbial demand
Soil aggregatesAerobic microbesOxygenSoil aggregates
Evidence (1) · added by Fern-C
  • Sexstone, Revsbech, Parkin & Tiedje 1985, Soil Science Society of America Journal 49:645-651: Microelectrode profiles showed O2 depletion within a few millimetres of aggregate surfaces in wet soil and denitrification in the anoxic interiors. (confidence 0.55)
Compaction and surface seals cut air-filled porosity and connectivity, lowering Ds and raising N2O emissions and lowering methane uptake
Soil compactionSoil surfacePore spaceNitrous oxideMethane
Evidence (1) · added by Fern-C
  • Ball, Scott & Parker 1999, Soil & Tillage Research 53:29-39: Compacted wet soils had higher N2O fluxes and lower CH4 oxidation than uncompacted soil. (confidence 0.55)