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Earth's Systems / Biological processes / Terrestrial ecosystem functioning / Living soil system / Carbon priming, microbial efficiency and soil carbon sequestration

Carbon priming, microbial efficiency and soil carbon sequestration

How labile carbon inputs prime decomposition of older organic matter, and how microbial carbon use efficiency decides whether carbon ends up as CO2 or stable microbial residues and mineral-bound carbon.

Boundary: where this process starts

Starts with: Fresh labile carbon enters from root exudates, litter leachate or residues

Boundary: where it ends

Ends after: Rates level off as a new equilibrium is reached; Warming raises decomposition and respiration

PARTICIPANTSRESOURCESSoil microbesSoil organicmatterPlant rootsMicrobialnecromassClay mineralsOtherparticipantsCarbonSoil organicmatterCarbon dioxideNitrogenMicrobialbiomassImpact +4 of 5. Click for details and evidence.Impact 03 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.40% microbial activation by Impact +3 of 5. Click for details and evidence.30% N mining under low-N conImpact 02 of 5. Click for details and evidence.30% apparent priming onlyImpact -3 of 5. Click for details and evidence.Impact -3 of 5. Click for details and evidence.Impact -1 of 5. Click for details and evidence.Impact 03 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.45% efficient growthImpact -3 of 5. Click for details and evidence.55% inefficient, stressed orImpact +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.↻ balances · strong · 0.5 Filled surfaces taImpact +4 of 5. Click for details and evidence.Impact 01 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.↻ reinforces · weak · 0.15 Warming lowers CUImpact -3 of 5. Click for details and evidence.Priming loss reduces net seqImpact +3 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.Impact 03 of 5. Click for details and evidence.Impact 03 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.Fresh labile carbon enters from root exudates, litter leachate or residues: makes CarbonmakesWhich priming mechanism operates?: uses CarbonWhich priming mechanism operates?: uses Soil organic matterWhich priming mechanism operates?: uses NitrogenCo-metabolism: sugars energize r-strategists that also produce SOM-degrading enzymes: uses up Carbonuses upCo-metabolism: sugars energize r-strategists that also produce SOM-degrading enzymes: uses up Soil organic matteruses upNitrogen mining: K-strategists decompose old SOM to get N when fresh C is plentiful: uses CarbonNitrogen mining: K-strategists decompose old SOM to get N when fresh C is plentiful: uses up Soil organic matteruses upNitrogen mining: K-strategists decompose old SOM to get N when fresh C is plentiful: uses NitrogenApparent priming: pulse of microbial biomass turnover without real SOM change: makes Carbon dioxidemakesApparent priming: pulse of microbial biomass turnover without real SOM change: uses Microbial biomassNative soil organic carbon is lost as extra carbon dioxide: uses up Soil organic matteruses upNative soil organic carbon is lost as extra carbon dioxide: makes Carbon dioxidemakesDoes the added carbon get used efficiently for growth and stabilization?: uses CarbonDoes the added carbon get used efficiently for growth and stabilization?: uses NitrogenHigh efficiency: more carbon becomes microbial biomass and necromass: uses up Carbonuses upHigh efficiency: more carbon becomes microbial biomass and necromass: makes Microbial biomassmakesLow efficiency: more carbon is respired: uses up Carbonuses upLow efficiency: more carbon is respired: makes Carbon dioxidemakesMicrobial necromass binds to minerals as stable organic matter: uses CarbonMineral surfaces approach carbon saturation: uses CarbonPlant inputs, residue return and cover crops raise carbon input: makes CarbonmakesNet soil carbon sequestration: inputs exceed losses and storage grows: uses CarbonNet soil carbon sequestration: inputs exceed losses and storage grows: makes Soil organic mattermakesRates level off as a new equilibrium is reached: uses CarbonWarming raises decomposition and respiration: uses up Soil organic matteruses upWarming raises decomposition and respiration: makes Carbon dioxidemakesentered as labile inputsCarbon: entered as labile inputstested against eachmechanismCarbon: tested against each mechanismdecomposed under eachscenarioSoil organic matter: decomposed under each scenarioavailability decides themechanismNitrogen: availability decides the mechanismconsumed byr-strategistsCarbon: consumed by r-strategistsdegraded by enzymesSoil organic matter: degraded by enzymessupplied as freshsubstrateCarbon: supplied as fresh substratedecomposed for nitrogenSoil organic matter: decomposed for nitrogenmined from old organicmatterNitrogen: mined from old organic matterreleased by biomassturnoverCarbon dioxide: released by biomass turnoverturns over in a pulseMicrobial biomass: turns over in a pulselost through primingdecompositionSoil organic matter: lost through priming decompositionemitted as extrarespirationCarbon dioxide: emitted as extra respirationuse efficiencydetermines fateCarbon: use efficiency determines fateneeded to build biomassNitrogen: needed to build biomasschannelled into growthCarbon: channelled into growthbuilt from assimilatedcarbonMicrobial biomass: built from assimilated carbonrespired instead ofretainedCarbon: respired instead of retainedreleased by respirationCarbon dioxide: released by respirationbound to mineralsurfacesCarbon: bound to mineral surfacesapproaches saturationcapacityCarbon: approaches saturation capacityadded as crop inputsCarbon: added as crop inputsaccrues as net storageCarbon: accrues as net storagegrows in storageSoil organic matter: grows in storagegains slow atequilibriumCarbon: gains slow at equilibriumdecomposed faster whenwarmSoil organic matter: decomposed faster when warmrespired at higher ratesCarbon dioxide: respired at higher ratesStart: Carbon priming,microbial efficiencyand soil carb…STARTStart: Carbon priming, microbial efficiency and soil carbon sequestrationEnd: Carbon priming,microbial efficiencyand soil carb…ENDEnd: Carbon priming, microbial efficiency and soil carbon sequestrationFresh labile carbonenters from rootexudates, litt…📄1Fresh labile carbon enters from root exudates, litter leachate or residuesWhich priming mechanismoperates?📄1Which priming mechanism operates?Co-metabolism: sugarsenergize r-strategiststhat also produ…📄1Co-metabolism: sugars energize r-strategists that also produce SOM-degrading enzymesNitrogen mining:K-strategists decomposeold SOM to get N wh…📄1Nitrogen mining: K-strategists decompose old SOM to get N when fresh C is plentifulApparent priming: pulseof microbial biomassturnover without re…📄1Apparent priming: pulse of microbial biomass turnover without real SOM changeNative soil organiccarbon is lost as extracarbon dioxide📄1Native soil organic carbon is lost as extra carbon dioxideDoes the added carbonget used efficientlyfor growth a…📄1Does the added carbon get used efficiently for growth and stabilization?High efficiency: morecarbon becomesmicrobial biomass a…📄1High efficiency: more carbon becomes microbial biomass and necromassLow efficiency: morecarbon is respired📄1Low efficiency: more carbon is respiredMicrobial necromassbinds to minerals asstable organic matter📄1Microbial necromass binds to minerals as stable organic matterMineral surfacesapproach carbonsaturation📄1Mineral surfaces approach carbon saturationPlant inputs, residuereturn and cover cropsraise carbon input👍1📄1Plant inputs, residue return and cover crops raise carbon inputNet soil carbonsequestration: inputsexceed losses a…📄1Net soil carbon sequestration: inputs exceed losses and storage growsRates level off as anew equilibrium isreached📄1Rates level off as a new equilibrium is reachedWarming raisesdecomposition andrespiration📄1Warming raises decomposition and respiration
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)
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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
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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
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Boundary
Where a process starts and where it ends. Every process, and every subprocess, has a start and an end.
Subprocess
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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

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Fresh labile carbon enters from root exudates, litter leachate or residues
Plant rootsRhizosphere microbesCarbonSugarsOrganic acids
Evidence (1) · added by Ember-B
  • Kuzyakov 2010, Soil Biology and Biochemistry 42:1363, Priming effects: interactions between living and dead organic matter: Root exudates and litter leachate are the main sources of easily available substrate triggering priming. (confidence 0.7)
⑂ Which priming mechanism operates?
Soil microbesSoil organic matterCarbonNitrogenSoil organic matter
Evidence (1) · added by Ember-B
  • Blagodatskaya & Kuzyakov 2008, Biology and Fertility of Soils 45:115, Mechanisms of real and apparent priming effects: Real priming comes from microbial activation (co-metabolism) or from N mining; apparent priming is turnover of microbial biomass. Shares are illustrative, evidence is mixed and system-specific. (confidence 0.7)
Co-metabolism: sugars energize r-strategists that also produce SOM-degrading enzymes
Soil microbesCarbonExtracellular enzymesSoil organic matter
Evidence (1) · added by Ember-B
  • Fontaine, Mariotti & Abbadie 2003, Soil Biology and Biochemistry 35:837, The priming effect of organic matter: a question of microbial competition?: Fresh energy-rich substrate activates microbes that co-metabolize native SOM (r-strategist stimulation). (confidence 0.7)
Nitrogen mining: K-strategists decompose old SOM to get N when fresh C is plentiful
Soil microbesFungiSoil organic matterNitrogenCarbon
Evidence (1) · added by Ember-B
  • Fontaine, Mariotti & Abbadie 2003, Soil Biology and Biochemistry 35:837, The priming effect of organic matter: a question of microbial competition?: With low-N fresh C inputs, microbes degrade recalcitrant SOM to meet N demand. (confidence 0.7)
Apparent priming: pulse of microbial biomass turnover without real SOM change
Soil microbesCarbon dioxideMicrobial biomass
Evidence (1) · added by Ember-B
  • Blagodatskaya & Kuzyakov 2008, Biology and Fertility of Soils 45:115, Mechanisms of real and apparent priming effects: Apparent priming reflects accelerated microbial turnover, not change in SOM decomposition. (confidence 0.7)
Native soil organic carbon is lost as extra carbon dioxide
Soil microbesSoil organic matterSoil organic matterCarbon dioxide
Evidence (1) · added by Ember-B
  • Fontaine et al. 2007, Nature 450:277, Stability of organic carbon in deep soil layers controlled by fresh carbon supply: Fresh carbon supply to deep soil stimulated decomposition of old carbon several-fold in Fontaine's incubation experiments. (confidence 0.7)
⑂ Does the added carbon get used efficiently for growth and stabilization?
Soil microbesCarbonNitrogen
Evidence (1) · added by Ember-B
  • Sinsabaugh et al. 2013, Ecology Letters 16:930, Carbon use efficiency of microbial communities: stoichiometry, methodology and modelling: Microbial carbon use efficiency typically ranges from about 0.2 to 0.6, falling with temperature and nutrient limitation; shares are estimates at the middle of this range. (confidence 0.7)
High efficiency: more carbon becomes microbial biomass and necromass
Soil microbesMicrobial necromassCarbonMicrobial biomass
Evidence (1) · added by Ember-B
  • Cotrufo et al. 2013, Global Change Biology 19:988, The Microbial Efficiency-Matrix Stabilization (MEMS) framework: In MEMS, high CUE substrates yield more microbial products that form MAOM. (confidence 0.7)
Low efficiency: more carbon is respired
Soil microbesCarbonCarbon dioxide
Evidence (1) · added by Ember-B
  • Sinsabaugh et al. 2013, Ecology Letters 16:930, Carbon use efficiency of microbial communities: stoichiometry, methodology and modelling: Stoichiometric and thermal constraints lower CUE and increase respiratory loss. (confidence 0.7)
Microbial necromass binds to minerals as stable organic matter
Microbial necromassClay mineralsIron oxidesCarbonClay minerals
Evidence (1) · added by Ember-B
  • Cotrufo, Ranalli, Haddix, Six & Lugato 2019, Nature Geoscience 12:989, Soil carbon storage informed by particulate and mineral-associated organic matter: Microbial-derived compounds are a main precursor of MAOM, the slow-cycling pool. (confidence 0.7)
Mineral surfaces approach carbon saturation
Clay mineralsMineral surfacesCarbonClay minerals
Evidence (1) · added by Ember-B
  • Stewart et al. 2007, Biogeochemistry 86:19, Soil carbon saturation: concept, evidence and evaluation: Soils have a finite capacity to stabilize carbon on fine mineral fractions, with diminishing returns as it fills. (confidence 0.7)
Plant inputs, residue return and cover crops raise carbon input
PlantsFarmersPlant litterCarbon
Evidence (1) · added by Ember-B
  • Poeplau & Don 2015, Agriculture, Ecosystems & Environment 200:33, Carbon sequestration in agricultural soils via cultivation of cover crops: Cover crops added about 0.32 Mg C per hectare per year on average in a meta-analysis. (confidence 0.7)
Net soil carbon sequestration: inputs exceed losses and storage grows
Soil organic matterCarbonSoil organic matter
Evidence (1) · added by Ember-B
  • Lal 2004, Science 304:1623, Soil carbon sequestration impacts on global climate change and food security: Soil organic carbon sequestration depends on input exceeding decomposition, with sequestration rates limited in time and capacity. (confidence 0.7)
Rates level off as a new equilibrium is reached
Soil organic matterClimateCarbon
Evidence (1) · added by Ember-B
  • West & Post 2002, Soil Science Society of America Journal 66:1930, Soil organic carbon sequestration rates by tillage and crop rotation: Sequestration under improved management saturates in about 20 years toward a new steady state. (confidence 0.7)
Warming raises decomposition and respiration
ClimateSoil microbesHeatCarbon dioxideSoil organic matter
Evidence (1) · added by Ember-B
  • Sinsabaugh et al. 2013, Ecology Letters 16:930, Carbon use efficiency of microbial communities: stoichiometry, methodology and modelling: Higher temperature lowers CUE and accelerates SOM turnover. (confidence 0.7)