CUE × MAOC Accrual — Accrued Carbon

How fast does your soil carbon bucket fill?

Durable soil carbon sits in the MAOC fraction — century-scale turnover, low reversal risk. How much accumulates depends on two things: the empty space in the mineral soil (saturation deficit) and how efficiently microbial activity converts carbon inputs into necromass. Adjust the parameters to see how the numbers change.

Soil parameters

Starting MAOC stock 15 tC/ha

2 tC/ha40 tC/ha

MAOC maximum capacity 35 tC/ha

10 tC/ha70 tC/ha

Carbon use efficiency (CUE) 0.30

0.100.65 · natural ecosystems: 0.27–0.52

Annual carbon inputs 2.0 tC/ha/yr

0.5 tC/ha/yr6 tC/ha/yr

Crediting period 10 yr

5 yr30 yr

The bucket analogy

▭Bucket size — total MAOC capacity, set by clay and silt content

░Empty space — saturation deficit: tonnes still available to store durably

↓Flow rate — CUE × C inputs: necromass reaching mineral surfaces per year

~Leakiness — (1 − CUE): substrate respired as CO₂ before stabilisation

↗Overflow — gains beyond capacity enter POM: less durable, higher reversal risk

! Where the data gap sits +

In natural ecosystems, CUE can be estimated from climate variables (temperature, precipitation, pH) — it ranges from ~0.27 in high-productivity biomes to ~0.52 in arid systems (Cui et al. 2026).

In managed agricultural systems, that relationship breaks down. Only 11 studies globally have measured how tillage practice affects CUE (Holm et al. 2025), none confirming a link to SOC gains. In agricultural soils, MAOC formation is driven by mineral saturation deficit, not CUE alone (Sokol et al. 2025). The CUE slider reflects this uncertainty — use it to explore sensitivity, not to predict.

Model: annual MAOC increment = CUE × C inputs, capped at remaining capacity. Excess to POM. Does not model MAOC turnover or priming.

Durable MAOC gain

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tC/ha over period

Overflow to POM

—

tC/ha · higher reversal risk

Years to saturation

—

MAOC share

—

% of total gain that is durable

Bucket — MAOC at end of crediting period

Overflow → POM fraction (less durable)

Starting stock

Projected MAOC gain

Remaining capacity

Overflow → POM

Producer

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tCO₂/ha credible as permanent sequestration

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tCO₂/ha in POM — disclose separately or exclude

Base sustainability claims on MAOC gain only. POM is real sequestration but carries reversal risk that undermines a permanent removal story.

Project developer

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tCO₂/ha issuable as high-permanence credits

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credit stream status at saturation

POM tonnes may be issuable under some methodologies but attract buffer pool contributions and a permanence discount at market.

Ratings / investor

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% of total gain in durable MAOC fraction

—

permanence signal — MAOC turnover century-scale; POM decade-scale

Projects below 60% MAOC share warrant deeper scrutiny of permanence claims regardless of methodology used.

MAOC accrual trajectory

MAOC stock

Cumulative POM overflow

MAOC capacity ceiling

Simplified model: annual MAOC increment = CUE × C inputs, capped at remaining mineral capacity. Excess enters POM. Does not model MAOC turnover or priming effects. CUE for managed agricultural systems is largely unmeasured — use as sensitivity analysis, not prediction.