Permanence Factors (F_perm)
Chapter on inventory permanence factors: how estimates soil C stock change from biochar and Biochar Soil GHG Inventory Model turn production temperature (or H/C_org) into the fraction of biochar organic carbon remaining after a chosen time. Credit methodologies that cite these tables are mapped in Permanence Science for Credits—this note stays on the science/model. Report context: 2026-09-16.
Related notes: Permanence · H-Corg and O-Corg · Soil Temperature and Environment · CDR Potential and IPCC Accounting · Quantitative Benchmarks · Science Hub · Key Sources
1. What (F_\mathrm{perm}) is
[ F_\mathrm{perm}(t,T_\mathrm{soil}) = \text{fraction of biochar organic C still unmineralized after time } t \text{ at soil temperature } T_\mathrm{soil} ]
Both IPCC App. 4 and Woolf fit multi-pool exponential decay (minimum two-pool) to incubation and field datasets lasting ≥1 year, then evaluate remaining C at policy-relevant horizons (IPCC PDF; Woolf DOI).
Do not mix IPCC and Woolf tables in one calculation—Woolf re-derived factors with newer data and explicit soil-temperature adjustment; values differ (estimates soil C stock change from biochar “Use with care”).
2. Temperature classes (shared vocabulary)
| Class | Peak pyrolysis temperature | Notes |
|---|---|---|
| Low | 350–450 °C | IPCC / Woolf bin |
| Medium | 450–600 °C | |
| High | ≥600 °C (IPCC also flags high-T gasification) |
Materials below ~350 °C (torrefaction, typical HTC) are out of scope for these permanence tables (Woolf 2021 §2.1.1; IPCC definition).
Process engineering context: Pyrolysis Systems and Temperatures.
3. IPCC 2019 Appendix 4 (100-year factors)
Source: estimates soil C stock change from biochar · PDF (full text checked in vault).
| Production class | 100-yr (F_\mathrm{perm}) | Uncertainty |
|---|---|---|
| High (>600 °C) | 0.89 | ±13% |
| Medium (450–600 °C) | 0.80 | ±11% |
| Low (350–450 °C) | 0.65 | ±15% |
Temperature basis: factors adjusted to a conservative ~20 °C ambient (against ~10 °C mean land-surface temperature in the appendix background). Scope: mineral soils in cropland/grassland only—not organic soils, forests with organic horizons, wetlands, or wildfire chars (same PDF). Status: basis for future Tier 1 development, not complete good-practice inventory guidance yet.
Carbon content (F_{C_p}) is tabulated separately by feedstock/process (Neves et al. regressions + ash correction)—see Production, Woody vs Manure Ag Feedstocks.
4. Biochar Soil GHG Inventory Model (Woolf et al. 2021, temperature- and H/C-resolved)
Source: Biochar Soil GHG Inventory Model · DOI · open PDF (full text checked).
4.1 Headline 100-year values at global cropland mean (14.9 °C)
| Class | (F_\mathrm{perm}) (100 yr, 14.9 °C) |
|---|---|
| Low | 0.63 (SE 0.045) |
| Medium | 0.71 (SE 0.03) |
| High | 0.82 (SE 0.028) |
Abstract phrasing: 63–82% of initial biochar C remains after 100 years at that temperature (DOI).
4.2 Longer horizons at 14.9 °C (Table 3)
| Horizon | Low | Medium | High |
|---|---|---|---|
| 500 yr | 0.19 | 0.32 | 0.44 |
| 1000 yr | 0.084 | 0.16 | 0.25 |
4.3 Soil-temperature sensitivity (100 yr, high class examples)
| Soil T (°C) | High-class (F_\mathrm{perm}) (100 yr) |
|---|---|
| 5 | 0.94 |
| 10 | 0.88 |
| 14.9 (global cropland mean) | 0.82 |
| 20 | 0.79 |
| 25 | 0.76 |
US cropland mean 10.9 °C also tabulated (high 100-yr = 0.87). Full grid: Woolf Table 3 (open PDF). Deep dive: Soil Temperature and Environment.
4.4 H/C_org pathway
When H/C_org is measured, use Woolf eq. 5 with Table 3 regression coefficients rather than temperature bins (H-Corg and O-Corg). Preference order in Woolf §3.4: measured FC + H/C_org → else FC from Table 2 + T-class (F_\mathrm{perm}) → else conservative low-T (F_\mathrm{perm}).
4.5 Inventory equation (soil C term)
[ \mathrm{GHG_{bc}} \approx \frac{44}{12},M_\mathrm{bc},F_C,F_\mathrm{perm} ;+; \text{(optional year-1 N₂O term)} ]
(Woolf eq. 6). Optional N₂O: −23% first year only, only if application >10 Mg C ha⁻¹—usually small vs C sequestration; not the focus of this permanence chapter (GHG Fluxes).
5. Why Woolf numbers are lower than IPCC’s
| Factor | IPCC App. 4 | Woolf 2021 (this model) |
|---|---|---|
| Dataset vintage | Pre-2019 appendix synthesis | Updated decay compilation |
| Climate basis | ~20 °C conservative | Explicit T grid; headline at 14.9 °C |
| H/C_org option | Noted as future option | Fully parameterized |
| FC | Separate feedstock table | Table 2 by feedstock × T / gasification |
Using IPCC 0.89 with a Woolf-style cool-climate narrative (or vice versa) silently mis-states certainty. Pick one method stack and document it.
6. Worked intuition (from Woolf §3.4.1)
Maize stover pyrolysed at 500 °C (medium), FC = 0.68 (Table 2), sequestered at 10 °C soil → (F_\mathrm{perm}=0.79). With 15,000 Mg biochar + conditional N₂O term, Woolf’s example yields 29,710 Mg CO₂e (open PDF §3.4.1). Scale and boundaries matter for LCA vs inventory (CDR Potential and IPCC Accounting).
7. Link to carbon markets (pointer only)
Registries interpret these science tables differently (100y vs 200y vs 1000y; T-class vs H/C_org vs Ro). Do not paste methodology thresholds here—use Permanence Science for Credits · Verra VM0044 Permanence and Quantification · Isometric Permanence Pathways 200y and 1000y · Puro CORC200+ · CSI Global Biochar C-Sink.
8. Practice takeaway
- Quote horizon + soil T + method (IPCC vs Woolf) every time you cite (F_\mathrm{perm}).
- Prefer Woolf + H/C_org when lab data exist; IPCC T-class when only kiln temperature is known.
- Remember 1000-year remaining fractions are much lower than 100-year ones at the same T (Woolf Table 3)—central to Permanence Debates.
Primary links
- IPCC 2019 App. 4 PDF · estimates soil C stock change from biochar
- Woolf et al. 2021 · open PDF · Biochar Soil GHG Inventory Model
- Supporting spreadsheet (Woolf SI): linked from ACS page