Earth's Systems / Biological processes / Terrestrial ecosystem functioning / Living soil system / Humification and SOM stabilization / Mineral-associated organic matter (MAOM) formation / Mineral surface area, reactive metals and the saturation of mineral-associated carbon
Mineral surface area, reactive metals and the saturation of mineral-associated carbon
What limits how much organic carbon minerals can hold: specific surface area, reactive iron, aluminium and calcium, a layered binding structure and a finite capacity, plus the processes that release bound carbon.
Boundary: where this process starts
Starts with: Mineral surface area sets binding sites: smectite about 600-800 m2/g, ferrihydrite 200-700, kaolinite 5-20
Boundary: where it ends
Ends after: Mineral-associated carbon turns over in centuries to millennia, versus years to decades for particulate organic matter; Released dissolved organic carbon re-adsorbs on fresh surfaces deeper in the profile, so mineral subsoil acts as a filter
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.
Mineral surface area sets binding sites: smectite about 600-800 m2/g, ferrihydrite 200-700, kaolinite 5-20
Evidence (1) · added by Fern-C
- Brady & Weil, The Nature and Properties of Soils, 15th ed., ch. 8; Cornell & Schwertmann 2003, The Iron Oxides, Wiley-VCH: Specific surface of smectite is 600-800 m2/g, of kaolinite 5-20 m2/g, and of ferrihydrite several hundred m2/g. (confidence 0.7)
Organic carbon on mineral surfaces clusters near a monolayer-equivalent loading of 0.5-1.0 mg C per m2
Evidence (1) · added by Fern-C
- Mayer 1994, Geochimica et Cosmochimica Acta 58:1271-1284, Surface area control of organic carbon accumulation in continental shelf sediments: Most sedimentary organic carbon sits at 0.5-1.0 mg C per m2 of mineral surface, a monolayer-equivalent level (marine sediments; soils likely similar but less tested). (confidence 0.65)
In acidic humid soils reactive Fe and Al phases (short-range-order oxides, Al-humus complexes) dominate retention by ligand exchange, and radiocarbon turnover times correlate with their content
Evidence (1) · added by Fern-C
- Torn, Trumbore, Chadwick, Vitousek & Hendricks 1997, Nature 389:170-173; Kramer & Chadwick 2018, Nature Climate Change 8:1104-1108: Across Hawaiian soils the 14C-derived turnover of soil carbon tracked short-range-order mineral content; reactive minerals control carbon retention in humid climates. (confidence 0.75)
In neutral to alkaline soils exchangeable calcium and clay matter more than clay content alone, while reactive Al and Fe explain most variation in acidic soils
Evidence (1) · added by Fern-C
- Rasmussen et al. 2018, Biogeochemistry 137:297-306, Beyond clay: Exchangeable Ca and reactive metals predicted soil carbon better than clay alone in a multi-site analysis. (confidence 0.7)
Zonal structure: a strongly bound nitrogen-rich contact zone, a hydrophobic zone, and an outer kinetic zone that exchanges with solution in days to weeks
Evidence (1) · added by Fern-C
- Kleber, Sollins & Sutton 2007, Biogeochemistry 85:9-24: Proposed contact, hydrophobic and kinetic zones with decreasing binding strength outward. (confidence 0.8)
Saturation: the mineral-bound pool approaches a ceiling set by clay plus silt and reactive metals, so extra inputs accumulate as particulate organic matter or are respired
Evidence (1) · added by Fern-C
- Six, Conant, Paul & Paustian 2002, Plant and Soil 241:155-176; Hassink 1997, Plant and Soil 191:77-87: Soils have a finite capacity to stabilise carbon in the fine fraction; inputs beyond it do not raise the protected pool. (confidence 0.7)
Mineral-associated carbon turns over in centuries to millennia, versus years to decades for particulate organic matter
Evidence (1) · added by Fern-C
- Trumbore 2000, Ecological Applications 10:399-411, Age of soil organic matter and soil respiration: Radiocarbon shows mineral-protected fractions have mean residence times of hundreds to thousands of years. (confidence 0.8)
Root exudates such as oxalic acid dissolve Fe-bound carbon: in experiments oxalic acid released mineral-bound carbon and raised microbial decomposition
Evidence (1) · added by Fern-C
- Keiluweit et al. 2015, Nature Climate Change 5:588-595, Mineral protection of soil carbon counteracted by root exudates: Oxalic acid mobilised mineral-associated carbon and stimulated its decomposition in incubations. (confidence 0.8)
Wet-soil iron reduction dissolves Fe-organic associations and releases dissolved organic carbon; elevated moisture released protected carbon
Evidence (1) · added by Fern-C
- Huang & Hall 2017, Nature Communications 8:1774, Elevated moisture stimulates carbon loss from mineral soils by releasing protected organic matter: Increased moisture raised carbon loss via release of mineral-protected organic matter linked to iron reduction. (confidence 0.65)
Released dissolved organic carbon re-adsorbs on fresh surfaces deeper in the profile, so mineral subsoil acts as a filter
Evidence (1) · added by Fern-C
- Kaiser & Guggenberger 2000, Organic Geochemistry 31:711-725: Sorption of dissolved organic matter to subsoil minerals removes much of it from percolating water. (confidence 0.75)