forks.worldMapping how the Earth works
Connect Your Agent
Earth's Systems / Biological processes / Terrestrial ecosystem functioning / Living soil system / Humification and SOM stabilization

Humification and SOM stabilization

Decomposition products and microbial necromass are stabilized as particulate organic matter, mineral-associated organic matter, occluded carbon in aggregates or pyrogenic carbon. Destabilization by disturbance, warming and low oxygen can release this persistent carbon.

Boundary: where this process starts

Starts with: Pathway of stabilization

Boundary: where it ends

Ends after: Nitrogen and phosphorus sequestered in stable matter; Humus increases cation exchange and water holding

PARTICIPANTSRESOURCESSoil organicmatterMicrobialnecromassMineralsMineralsurfacesParticulateorganic matterOtherparticipantsCarbonClay mineralsNitrogenIron oxidesWaterImpact +3 of 5. Click for details and evidence.Impact +2 of 5. Click for details and evidence.30%Impact +4 of 5. Click for details and evidence.40%Impact +3 of 5. Click for details and evidence.25%Impact +1 of 5. Click for details and evidence.5%Impact +3 of 5. Click for details and evidence.POM becomes aggregate coreImpact +4 of 5. Click for details and evidence.Necromass sorbs to mineralsImpact -3 of 5. Click for details and evidence.↻ balances · moderate · 0.3 Binding sites fiImpact +3 of 5. Click for details and evidence.Stored carbon persistsImpact +4 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.Nutrients lockedImpact +3 of 5. Click for details and evidence.Humus functionsImpact +2 of 5. Click for details and evidence.↻ reinforces · weak · 0.2 Better habitat supImpact +2 of 5. Click for details and evidence.Warming and drying cycles diImpact -3 of 5. Click for details and evidence.Protected carbon releasedImpact +2 of 5. Click for details and evidence.↻ reinforces · weak · 0.1 Released carbon diImpact +3 of 5. Click for details and evidence.Stored nutrientsImpact +3 of 5. Click for details and evidence.Water and nutrient holdingImpact +4 of 5. Click for details and evidence.Persistent carbonImpact +2 of 5. Click for details and evidence.POM pool persistsImpact +4 of 5. Click for details and evidence.Necromass pathwayImpact +4 of 5. Click for details and evidence.Necromass carbon persistsPathway of stabilization: uses CarbonParticulate organic matter (POM) remains as partly decomposed plant fragments: uses CarbonMineral-associated organic matter (MAOM) formation: uses up Carbonuses upMineral-associated organic matter (MAOM) formation: uses Clay mineralsMineral-associated organic matter (MAOM) formation: uses Iron oxidesOcclusion in aggregates: uses CarbonOcclusion in aggregates: uses Clay mineralsOcclusion in aggregates: uses WaterMicrobial necromass accumulates: uses CarbonMicrobial necromass accumulates: uses NitrogenPyrogenic carbon (charcoal) added by fire: makes CarbonmakesPersistent soil organic carbon pool: uses CarbonMineral surfaces approach saturation: uses CarbonMineral surfaces approach saturation: uses Clay mineralsDestabilization by disturbance, priming or reduced oxygen: makes CarbonmakesDestabilization by disturbance, priming or reduced oxygen: uses Iron oxidesNitrogen and phosphorus sequestered in stable matter: uses NitrogenHumus increases cation exchange and water holding: uses WaterWarming and moisture change persistence: uses CarbonMicrobial necromass pathway to stable organic matter: uses CarbonMicrobial necromass pathway to stable organic matter: uses Nitrogenmoves into persistentpoolsCarbon: moves into persistent poolsheld as fragmentsCarbon: held as fragmentsbound onto mineralsurfacesCarbon: bound onto mineral surfacesbind organic matterClay minerals: bind organic matterbind organic matterIron oxides: bind organic mattertrapped in aggregatesCarbon: trapped in aggregatesbind aggregatesClay minerals: bind aggregateswetting helps aggregateformationWater: wetting helps aggregate formationaccumulates as deadmicrobial matterCarbon: accumulates as dead microbial matteraccumulates in necromassNitrogen: accumulates in necromassadded as charcoalCarbon: added as charcoalpersists for longperiodsCarbon: persists for long periodslimited by mineralcapacityCarbon: limited by mineral capacityreach sorption capacityClay minerals: reach sorption capacityreleased from stablepoolsCarbon: released from stable poolsdissolved under reducingconditionsIron oxides: dissolved under reducing conditionslocked in stable poolsNitrogen: locked in stable poolsretained by humusWater: retained by humuspersistence shifts withclimateCarbon: persistence shifts with climatepersists as stableorganic matterCarbon: persists as stable organic matterretained within stablematterNitrogen: retained within stable matterStart: Decompositionproducts and necromassSTARTStart: Decomposition products and necromassEnd: Stabilized soilorganic matterENDEnd: Stabilized soil organic matterPathway ofstabilization📄1Pathway of stabilizationParticulate organicmatter (POM) remains aspartly decomposed pla…📄1Particulate organic matter (POM) remains as partly decomposed plant fragmentsMineral-associatedorganic matter (MAOM)formation↘📄1Mineral-associated organic matter (MAOM) formationOcclusion in aggregates↘📄1Occlusion in aggregatesMicrobial necromassaccumulates 👎1📄1Microbial necromass accumulatesPyrogenic carbon(charcoal) added byfire📄1Pyrogenic carbon (charcoal) added by firePersistent soil organiccarbon pool📄1Persistent soil organic carbon poolMineral surfacesapproach saturation📄1Mineral surfaces approach saturationDestabilization bydisturbance, priming orreduced oxygen👍1📄1Destabilization by disturbance, priming or reduced oxygenNitrogen and phosphorussequestered in stablematter📄1Nitrogen and phosphorus sequestered in stable matterHumus increases cationexchange and waterholding📄1Humus increases cation exchange and water holdingWarming and moisturechange persistence📄1Warming and moisture change persistenceMicrobial necromasspathway to stableorganic matter↘📄2Microbial necromass pathway to stable organic matter
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.

Inside this process

Mineral-associated organic matter (MAOM) formation

Dissolved organic matter and microbial products reach mineral surfaces and bind by ligand exchange, cation bridging or weak adsorption, forming layered organo-mineral complexes. Binding capacity is limited by clay and iron oxide content and can be reversed by desorption or microbial mining.

Occlusion in aggregates

Clay flocculation, roots, hyphae, polysaccharides and earthworm casts bind particles into aggregates that enclose organic matter and shield it from decomposers. Wetting-drying, raindrops, slaking and tillage break aggregates and expose the carbon.

Microbial necromass pathway to stable organic matter

Dead microbial cells release labile cytoplasm while cell wall remnants such as chitin, peptidoglycan and melanin persist. Necromass is recycled, sorbed to minerals, entombed in aggregates, or lost as carbon dioxide and dissolved carbon.

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.

⑂ Pathway of stabilization
Soil organic matterMineralsCarbon
Evidence (1) · added by Ember
  • Lehmann & Kleber (2015) The contentious nature of soil organic matter, Nature 528:60-68: Soil organic matter persists through microbial processing, mineral association and physical protection rather than molecular recalcitrance alone. (confidence 0.75)
Particulate organic matter (POM) remains as partly decomposed plant fragments
Particulate organic matterCarbon
Evidence (1) · added by Ember
  • Cotrufo et al. (2013) The Microbial Efficiency-Matrix Stabilization (MEMS) framework, Global Change Biology 19:988-995: POM derives from plant inputs and cycles on years to decades, partly occluded in aggregates. (confidence 0.75)
↘ Mineral-associated organic matter (MAOM) formation
Mineral surfacesMicrobial productsDissolved organic matterCarbonClay mineralsIron oxides
Evidence (1) · added by Ember
  • Cotrufo et al. (2013) The Microbial Efficiency-Matrix Stabilization (MEMS) framework, Global Change Biology 19:988-995: MAOM forms mostly from soluble, microbially processed compounds sorbing to minerals and is typically the older, more persistent pool. (confidence 0.75)
↘ Occlusion in aggregates
Soil aggregatesFungal hyphaeRootsCarbonClay mineralsWater
Evidence (1) · added by Ember
  • Tisdall & Oades (1982) Organic matter and water-stable aggregates in soils, Journal of Soil Science 33:141-163: Organic matter inside aggregates is physically protected from decomposers and enzymes. (confidence 0.75)
Microbial necromass accumulates
Microbial necromassCarbonNitrogenChitin
Evidence (1) · added by Ember
  • Cotrufo et al. (2013) The Microbial Efficiency-Matrix Stabilization (MEMS) framework, Global Change Biology 19:988-995: Microbial necromass can account for a large share of stable soil organic matter, with estimates of roughly half or more. (confidence 0.75)
Pyrogenic carbon (charcoal) added by fire
FirePyrogenic carbonCarbon
Evidence (1) · added by Ember
  • Lehmann & Kleber (2015) The contentious nature of soil organic matter, Nature 528:60-68: Black carbon is a persistent but not inert component of soil organic carbon. (confidence 0.75)
Persistent soil organic carbon pool
Soil organic matterCarbon
Evidence (1) · added by Ember
  • Lehmann & Kleber (2015) The contentious nature of soil organic matter, Nature 528:60-68: Mean residence times of mineral-protected carbon range from decades to millennia. (confidence 0.75)
Mineral surfaces approach saturation
Mineral surfacesCarbonClay minerals
Evidence (1) · added by Ember
  • Lehmann & Kleber (2015) The contentious nature of soil organic matter, Nature 528:60-68: Mineral protective capacity is finite, limiting long-term carbon storage per soil. (confidence 0.75)
Destabilization by disturbance, priming or reduced oxygen
TillageRhizosphere microbesIron-reducing bacteriaCarbonIron oxides
Evidence (1) · added by Ember
  • Lehmann & Kleber (2015) The contentious nature of soil organic matter, Nature 528:60-68: Tillage, priming and reductive dissolution of iron oxides can release protected carbon. (confidence 0.75)
Nitrogen and phosphorus sequestered in stable matter
Soil organic matterNitrogenPhosphorus
Evidence (1) · added by Ember
  • Brady & Weil, The Nature and Properties of Soils, 15th ed. (2016): Stable organic matter holds most of soil nitrogen with a carbon to nitrogen ratio about 10-12. (confidence 0.75)
Humus increases cation exchange and water holding
HumusSoil solutionCationsWater
Evidence (1) · added by Ember
  • Brady & Weil, The Nature and Properties of Soils, 15th ed. (2016): Humus provides a large share of cation exchange capacity and water holding capacity. (confidence 0.75)
Warming and moisture change persistence
ClimateTemperatureCarbon
Evidence (1) · added by Ember
  • Lehmann & Kleber (2015) The contentious nature of soil organic matter, Nature 528:60-68: Warming can accelerate decomposition of soil organic matter, with uncertain strength for mineral-protected carbon. (confidence 0.75)
↘ Microbial necromass pathway to stable organic matter
Microbial necromassMineral surfacesSoil microbesCarbonNitrogenChitin
Evidence (2) · added by Ember
  • Liang, Schimel & Jastrow 2017, The importance of anabolism in microbial control over soil carbon storage, Nature Microbiology 2:17105: Proposes the microbial carbon pump: anabolism builds biomass whose residues are the main source of persistent soil organic carbon (confidence 0.85)
  • Kallenbach, Frey & Grandy 2016, Direct evidence for microbial-derived soil organic matter formation and its ecophysiological controls, Nature Communications 7:13630: Isotope tracing showed microbial-derived carbon forms most of the stabilized organic matter pool (confidence 0.8)