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Earth's Systems / Biological processes / Terrestrial ecosystem functioning / Living soil system / Soil gas exchange and redox / Redox ladder: sequential electron acceptors in waterlogged soil

Redox ladder: sequential electron acceptors in waterlogged soil

The order in which microbes use electron acceptors when oxygen is gone: nitrate, then manganese and iron, then sulfate, then carbon dioxide.

Boundary: where this process starts

Starts with: Respiration outpaces oxygen diffusion through water-filled pores; Oxygen disappears

Boundary: where it ends

Ends after: Drainage reoxidises Fe2+ and Mn2+: mottles, rust coats and nodules; CO2 is reduced to Methane below about -200 mV

PARTICIPANTSRESOURCESIron-reducingbacteriaSulfate-reducingbacteriaMethanogenicarchaeaDenitrifyingbacteriaAerobicmicrobesOtherparticipantsOxygenWaterManganeseoxidesIron oxidesIronImpact +3 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.Impact +2 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.Impact -2 of 5. Click for details and evidence.↻ balances · moderate · 0.35Impact +2 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.45% flood persistsImpact -3 of 5. Click for details and evidence.55% drained within weeksImpact +3 of 5. Click for details and evidence.Impact -2 of 5. Click for details and evidence.Impact +2 of 5. Click for details and evidence.↻ reinforces · weak · 0.3Impact 02 of 5. Click for details and evidence.Impact +2 of 5. Click for details and evidence.Respiration outpaces oxygen diffusion through water-filled pores; Oxygen disappears: uses up Oxygenuses upRespiration outpaces oxygen diffusion through water-filled pores; Oxygen disappears: uses WaterManganese(IV) oxides are reduced to soluble Mn2+: uses up Manganese oxidesuses upIron(III) oxides are reduced to Fe2+; pH converges toward 6.5-7.5: uses up Iron oxidesuses upIron(III) oxides are reduced to Fe2+; pH converges toward 6.5-7.5: makes IronmakesFlood persists beyond several weeks?: uses WaterSulfate is reduced to sulfide; FeS gives black colour: uses up Ironuses upDrainage reoxidises Fe2+ and Mn2+: mottles, rust coats and nodules: uses up Oxygenuses upDrainage reoxidises Fe2+ and Mn2+: mottles, rust coats and nodules: makes Manganese oxidesmakesDrainage reoxidises Fe2+ and Mn2+: mottles, rust coats and nodules: makes Iron oxidesmakesused faster thandiffusion suppliesOxygen: used faster than diffusion supplieslimits oxygen diffusionWater: limits oxygen diffusionreduced to soluble formManganese oxides: reduced to soluble formreduced to soluble ironIron oxides: reduced to soluble ironreleased as Fe2+Iron: released as Fe2+keeps soil saturatedWater: keeps soil saturatedprecipitated as sulfideIron: precipitated as sulfideoxidises ferrous andmanganous ionsOxygen: oxidises ferrous and manganous ionsprecipitate as blacknodulesManganese oxides: precipitate as black nodulesprecipitate as rustcoatingsIron oxides: precipitate as rust coatingsStart: Redox ladder:sequential electronacceptors …STARTStart: Redox ladder: sequential electron acceptors in waterlogged soilEnd: Redox ladder:sequential electronacceptors …ENDEnd: Redox ladder: sequential electron acceptors in waterlogged soilRespiration outpacesoxygen diffusionthrough water-fill…📄1Respiration outpaces oxygen diffusion through water-filled pores; Oxygen disappearsNitrate is reducedfirst (about +250 mV atpH 7)📄1Nitrate is reduced first (about +250 mV at pH 7)Manganese(IV) oxidesare reduced to solubleMn2+📄1Manganese(IV) oxides are reduced to soluble Mn2+Iron(III) oxides arereduced to Fe2+; pHconverges towa…👍1📄1Iron(III) oxides are reduced to Fe2+; pH converges toward 6.5-7.5Flood persists beyondseveral weeks?📄1Flood persists beyond several weeks?Sulfate is reduced tosulfide; FeS givesblack colour📄1Sulfate is reduced to sulfide; FeS gives black colourCO2 is reduced toMethane below about-200 mV📄1CO2 is reduced to Methane below about -200 mVDrainage reoxidisesFe2+ and Mn2+: mottles,rust coats and nodules📄1Drainage reoxidises Fe2+ and Mn2+: mottles, rust coats and nodules
Line thickness = size of impact (1 to 5)% on a line = chance a fork takes that branch📄 = pieces of evidence on a stepNegative effectLoops backHypothesisResource used up (arrow into the step)Resource used, not used upResource made or released (arrow out of the step)
What am I looking at? The concepts on every map
Participant
Someone or something that takes part in a process or influences it: a plant, a microbe, a river, a machine, a force. Participants are the rows of a map.
Resource
Something a step uses or uses up: water, oxygen, sunlight, a tool. Each step has an arrow down to the resources it uses.
Step
One thing that happens in a process. Every step has at least one participant and carries evidence.
Decision fork
A step where the process can go several ways. Every outcome is labeled with a percentage, and the percentages add up to 100%. It is not limited to yes or no.
Loop
A line that carries the process back to an earlier step. We record whether it reinforces or balances, how strong it is (weak, moderate, strong), and its gain (0 to 1): the share of the effect that returns on each pass.
Size of impact
Every line says how much the earlier step affects the next one (1 to 5), in which direction (+, -, 0), and why. Thicker lines mean bigger impact.
Evidence
The support for a claim: a source and what it shows. Every step, line, change and vote needs it. Click any step or line to read its evidence.
Boundary
Where a process starts and where it ends. Every process, and every subprocess, has a start and an end.
Subprocess
A step that is itself a whole process, with its own boundary, steps and lines. Click a step marked ↘ to open it. Subprocesses nest as deep as the evidence goes.
Description
A plain-language summary of what a process is and what it covers.
Hypothesis
A proposed new connection between two parts of the map that nobody has linked. It carries a prediction, a way to test it, and a count of papers on each topic and on both. It is an unverified idea until someone tests it.

Every step, with its evidence

The same information as the map, as a list you can search and share. Click a participant or resource to see everywhere it appears.

Respiration outpaces oxygen diffusion through water-filled pores; Oxygen disappears
Aerobic microbesSoil waterOxygenWater
Evidence (1) · added by Fern
  • Glinski & Stepniewski 1985, Soil Aeration and Its Role for Plants (CRC Press): Oxygen diffuses about 10,000 times more slowly in water than in air, so saturated soil runs out of oxygen within hours to days. (confidence 0.7)
Nitrate is reduced first (about +250 mV at pH 7)
Denitrifying bacteriaNitrateNitrous oxide
Evidence (1) · added by Fern
  • Patrick & Jugsujinda 1992, Soil Science Society of America Journal 56:1071: In flooded soil, nitrate disappears before Mn and Fe are reduced, at roughly +200 to +300 mV. (confidence 0.7)
Manganese(IV) oxides are reduced to soluble Mn2+
Manganese-reducing bacteriaManganese oxidesManganese
Evidence (1) · added by Fern
  • Patrick & Jugsujinda 1992, SSSAJ 56:1071: Mn2+ appears at about +200 mV after nitrate is exhausted. (confidence 0.65)
Iron(III) oxides are reduced to Fe2+; pH converges toward 6.5-7.5
Iron-reducing bacteriaIron oxidesIronHydrogen ions
Evidence (1) · added by Fern
  • Ponnamperuma 1972, Advances in Agronomy 24:29: Fe reduction consumes protons so acid soils rise and alkaline soils fall in pH toward near neutral within weeks of flooding. (confidence 0.75)
⑂ Flood persists beyond several weeks?
Water tableSoilWater
Evidence (1) · added by Fern
  • Reddy & DeLaune 2008, Biogeochemistry of Wetlands (CRC Press), ch. 4: Duration of saturation determines how far down the redox ladder soil goes. Shares are estimates (40-60%). (confidence 0.4)
Sulfate is reduced to sulfide; FeS gives black colour
Sulfate-reducing bacteriaSulfateSulfideIron
Evidence (1) · added by Fern
  • Reddy & DeLaune 2008, ch. 8: Sulfate reduction proceeds below about -100 to -150 mV and precipitates iron sulfides. (confidence 0.65)
CO2 is reduced to Methane below about -200 mV
Methanogenic archaeaCarbon dioxideMethaneHydrogen
Evidence (1) · added by Fern
  • Conrad 1996, Microbiological Reviews 60:609: Methanogenesis starts only after nitrate, Fe(III) and sulfate are depleted; methane production needs strongly reducing conditions. (confidence 0.7)
Drainage reoxidises Fe2+ and Mn2+: mottles, rust coats and nodules
Iron-oxidising bacteriaAtmosphereOxygenIron oxidesManganese oxides
Evidence (1) · added by Fern
  • Vepraskas 2004, Redoximorphic Features for Identifying Aquic Conditions, NC State Tech. Bulletin 301: Fe oxides precipitate along root channels and pore walls where oxygen re-enters, forming diagnostic redox features. (confidence 0.7)