Soil Carbon Is Supply-Limited

01The evidence

• The mineral-associated organic matter saturation concept has come under explicit pressure, with a 2024 review arguing that the framing needs updating because observations do not consistently show the asymptotic behaviour it predicts — and direct tests at continental scale have concluded that production, not surface area, sets the size of the pool.Soil Biology and Biochemistry, 2024

02The tension

Both camps have data. Saturation experiments show that adding carbon to a mineral-rich soil stops increasing the mineral-bound pool. Continental gradients show carbon tracking production. The disagreement is usually treated as a dispute about mechanism rather than about where on a curve the measurements were taken.

03The theory

The two findings describe different regimes of one function. At low production, mineral surfaces are far from filled, so the bound pool grows in proportion to carbon inputs. At high production, surfaces fill and additional carbon either leaches or remains particulate, so the bound pool flattens.

This predicts a breakpoint rather than a smooth asymptote. Below the breakpoint, the carbon-to-clay ratio is set by production; above it, by mineralogy. A dataset that samples mostly low-production soils will look supply-limited; one that samples mostly high-production soils will look saturation-limited.

The breakpoint should be predictable from clay type, because the density of sorption sites per gram differs by an order of magnitude between kaolinite and smectite. That gives a quantitative test: the same production should support different carbon stocks on different clays only above the breakpoint, and identical stocks below it.

The consequence for policy is direct. If most working land sits below the breakpoint, paying for carbon sequestration through mineral amendments is misdirected, and the lever is production and residue retention instead.

04What this is not

05Predictions

• P1Across a production gradient, the carbon-to-clay ratio should be linear at low production and flat above a breakpoint, with the breakpoint fitted rather than assumed.
• P2Adding labile carbon to low-production soils should raise the mineral-bound pool without approaching saturation.
• P3The breakpoint should shift with clay type in the direction predicted by site density.

06What would refute it

Saturation at low production, or a breakpoint that does not move with clay type, would collapse the piecewise account into one of the existing models.

07What it would change

It would settle a policy-relevant dispute, and redirect carbon farming subsidies from mineral amendments to production and residue management where the response is largest.