Is Soil Inorganic Carbon Critical to Include in Your Soil Carbon Accounting?

For two decades the soil carbon conversation has been a soil organic carbon conversation. The vocabulary, the protocols, the meta-analyses, and the credit methodologies all assume the relevant pool is the dark, microbially-cycled fraction in the topsoil. That assumption is convenient, defensible in many systems, and structurally wrong in many of the systems on which corporate Scope 3 commitments are increasingly being made: irrigated croplands in Mediterranean climates, calcareous vineyards, intensively N-fertilized cereal systems, dryland sourcing programmes, and any agroforestry on carbonate parent material.

Soil inorganic carbon is not a niche issue. The most recent global synthesis estimates 2,305 ± 636 Pg C in SIC to 2 m, comparable in magnitude to the SOC pool, with at least 1.13 ± 0.33 Pg C lost to inland waters annually and a forecast 23 Pg C decline over the next 30 years from N-deposition-driven acidification alone1. A bibliometric audit of 47,000 soil-carbon publications since 1905 found that less than 4% address SIC; the visibility gap is now larger than the empirical gap2. For a corporate accountant or project developer, the stakes are direct: in calcareous and carbonate-bearing systems, a soil carbon trend that ignores SIC is a number that does not survive contact with a careful auditor.

The pool not yet in your accounting framework

The GHG Protocol Land Sector and Removals (LSR) Standard, effective 1 January 2027, defines the soil carbon pool as comprising both soil organic carbon and soil inorganic carbon, where SIC includes elemental carbon and carbonate minerals (calcite, dolomite, aragonite, siderite, gypsum-associated phases)3. The accompanying LSR Guidance is being published in Q2 2026 and will set the operational rules for how Scope 3 reporters should quantify, partition, and disclose the two pools. The direction is no longer ambiguous: if your soil holds carbonate carbon and your management is changing it, that change belongs in your inventory.

Voluntary market methodologies are converging on the same point from a different angle. Verra's VM0042 v2.2, ICVCM-endorsed under the Core Carbon Principles in October 2025, focuses crediting on SOC, but the v3.0 Soil Sampling and Analysis Handbook in public consultation through 31 March 2026 is sharpening the distinction between organic and inorganic carbon measurement and the conditions under which each is admissible4. Enhanced rock weathering (ERW) protocols sit on the other side of the line entirely: they credit inorganic carbon formation, and their integrity now depends on cleanly separating the SIC accrual that came from atmospheric drawdown from the SIC accrual that came from accessory carbonates already in the feedstock56. In all three regimes, the analytical question is the same: do you actually know which carbon you measured?

When SIC is and is not material

The answer depends on four characteristics of the system. Treat them as filters, in order.

1. Does the soil contain inorganic carbon at all?

In acidic, non-calcareous soils (most humid temperate forest soils, weathered tropical Oxisols and Acrisols below pH 6, and the great majority of cocoa systems on West African shield substrates), SIC is negligible at the project scale and can be excluded after a confirming subsample analysis. In calcareous soils, SIC concentrations of 1–6% by mass are routine, and stocks of 100–250 Mg C/ha to 1 m are common in dryland and semi-arid contexts17.

2. Is the SIC pool stable, or is it actively cycling under your management?

Lithogenic carbonate inherited from parent material can be effectively inert over centuries. Pedogenic carbonate, formed in situ through dissolution and reprecipitation, is dynamic and responds to changes in pH, soil moisture, irrigation chemistry, and rooting depth on decadal timescales8. Drylands hold ~80% of the global SIC pool, but pedogenic carbonate dominates in grasslands (≈60% of SIC at 0–100 cm) while lithogenic carbonate dominates in deserts (≈55%); the management implication is opposite7.

3. Is the management plausibly perturbing the SIC pool?

Five interventions are known accelerators: nitrogen fertilization (acidification dissolves carbonates), liming (releases CO₂ during neutralization), irrigation with bicarbonate-bearing water in drylands (precipitates pedogenic carbonate but releases abiotic CO₂ in the process), enhanced rock weathering (deposits accessory carbonates and may form new ones), and any land-use change that shifts soil pH or rooting depth in carbonate-bearing soils. China lost ~9% of its 0–30 cm SIC stock from the 1980s to the 2010s, a ~1.37 Pg C decrease equivalent to 17.6–24% of China's terrestrial carbon sink over the same period and to 57% of the SOC accrual credited to its cropland9. Chinese cropland-specific SIC stocks fell 7% from 1980 to 2020, with 7 million hectares now carbonate-free, and a further 37% loss is forecast by 2100 under business-as-usual N application10.

4. Does your sampling depth see it?

In temperate calcareous croplands SIC is sometimes concentrated below the topsoil; in cropland in northeast China, SIC stocks at 100–150 cm contributed 23% of the total carbon stock to that depth, while SOC contributed only 6%11. A 0–30 cm protocol, still the default in many MRV frameworks, is structurally blind to this layer.

The specific risk: SIC can mask or inflate SOC trends

The most expensive mistake in soil carbon reporting is not under-measuring SIC. It is letting an unmeasured SIC trend ride silently inside a total-carbon number that gets reported as if it were SOC. The mechanism is mass-balance arithmetic. If a soil carbon analysis returns total carbon (TC) and the project assumes TC equals SOC (because no separate inorganic measurement was made), then any change in SIC is silently attributed to SOC. In a soil where SIC is comparable to or larger than SOC, this assumption produces directional, systematic, and sometimes order-of-magnitude error.

Three documented results make the mechanism vivid.

A 62-year recultivation chronosequence on carbonate-bearing agricultural soil in Germany showed SOC accruing at +0.30 Mg C/ha/yr while SIC fell at −0.61 Mg C/ha/yr; the SOC-to-TC ratio rose from 17% to 93% over the period12. A topsoil carbon report that did not separate the two pools would have shown a roughly stable total carbon trajectory, a slight gain, or a small loss depending on the exact decade. It would also have completely missed two facts: that SIC is being lost twice as fast as SOC is accruing, and that the soil's total carbon stock is decreasing on net by ~19.5 Mg C/ha. A RothC projection on the same site predicted SOC equilibrium at 38.6 Mg C/ha after 197 years; the SIC stock lost in the first 70 years was larger than that.

The Loess Plateau revegetation synthesis found that for every 1 kg of SOC accrual following cropland abandonment, 0.73 kg of SIC was lost in the 0–40 cm layer and 1.26 kg of SIC was gained in the 40–300 cm layer per square metre, depending on precipitation13. The aggregate signal across depths and rainfall gradients was small; the within-profile signals were large and pointed in opposite directions. A topsoil-only TC measurement would have understated the carbon co-benefit of revegetation in the deeper profile and overstated it in the topsoil.

Chinese cropland under business-as-usual N fertilization is losing SIC fast enough that the carbonate-dissolution flux now offsets a meaningful share of the SOC sink credited to its agricultural land use changes1014. A Scope 3 inventory built on those croplands that reports only SOC change is reporting a fraction of the soil carbon dynamics that the LSR Standard now requires to be quantified.

A subtler version of the same risk hits projects already using equivalent soil mass (ESM) accounting. ESM corrects stock-change estimates for changes in bulk density, but it operates on mass, not on the carbon's chemical form. If SIC is dissolving (mass leaving the soil profile as bicarbonate), ESM will register the mass change correctly, but if the project's analytical workflow then attributes the corresponding C reduction to SOC, the rigour added by ESM is undone by the failure to partition the pools. Fowler and colleagues (2023) flagged this in their original ESM framing, and Raffeld and colleagues (2024) showed that fixed-depth and ESM stock-change estimates can differ by more than 100% even within SOC alone. Without partitioning, a project that has correctly applied ESM can still be reporting a fundamentally wrong pool1516.

The analytical trap, before you sample

Most of the SIC error in commercial soil carbon programmes is fixed before a single core leaves the field. It is fixed in the analytical workflow specified in the protocol.

Total carbon by dry combustion is fast, cheap, and ubiquitous, but it does not distinguish SOC from SIC. Five common workarounds are in routine use, each with a directional bias:

• Assume TC = SOC. Defensible only after a confirming subsample analysis showing SIC is below detection. Not defensible by default in calcareous, dryland, irrigated, or ERW systems.
• Loss on ignition (LOI). Heat-decomposition of organic matter at 360–550 °C, with mass loss attributed to SOC. Inter-laboratory comparison shows LOI has the lowest agreement with reference methods (R² ≈ 0.83) and is sensitive to clay content, hydroxide minerals, and structural water17. In carbonate-bearing soils, decomposition of carbonate above 600 °C means residual heating envelopes overlap, biasing the result.
• Manocalcimetry / pressure-transducer methods for CaCO₃ equivalent. Industry-standard for inorganic carbon; SIC is then estimated as 12% of CaCO₃ equivalent and SOC by difference (TC − SIC). This is the USDA-NRCS Kellogg Soil Survey Laboratory's legacy approach and produces good results when carbonate is dominantly calcite, but introduces error in soils with siderite, dolomite, or mixed phases18.
• Dry combustion with pre-acidification. Acid wash removes carbonates before combustion, leaving SOC for measurement. Reliable in skilled hands, but acid-soluble organic matter is a documented loss pathway, biasing SOC low.
• Temperature-ramp dry combustion (TRDC). Direct, simultaneous quantification of SOC and SIC using staged combustion under controlled atmospheres. Most accurate option among combustion methods; slated to replace the legacy KSSL workflow18.
• Mid-infrared spectroscopy (MIR / FTIR). Predicts TC, SIC, and SOC directly from spectra with R² of 0.97, 0.99, and 0.90 respectively against dry-combustion references in a recent multi-laboratory study17. Cheaper per sample than wet chemistry once a regional spectral library is built; under-utilized in commercial MRV.

Practical recommendation by scenario

The matrix below is the operational version of the four-question test. It is intended for a Scope 3 reporter or project developer scoping a new soil carbon programme, working back from the GHG Protocol LSR Standard's requirements.

Scenario 1, humid temperate or tropical, acidic, non-calcareous topsoil (e.g. cocoa on Acrisol, coffee on Andosol, oak/spruce forest on Cambisol). SIC is below detection in the dominant horizons. Action: confirm with a single round of subsample carbonate analysis (manocalcimetry on 5–10% of cores), document the absence in the project plan, and proceed with an SOC-only programme. Risk: low, provided the confirming analysis is run.

Scenario 2, calcareous cropland under intensive N fertilization (Mediterranean cereal, North China Plain, German wheat-rapeseed). SIC is present in topsoil and subsoil; pH trajectory under N input is downward; pedogenic carbonate is actively dissolving. Action: measure SOC and SIC separately at every monitoring round, sample to at least 0–60 cm in three increments, and report both stock-change trajectories. Apply ESM to both pools using consistent depth references. Disclose carbonate dissolution as a CO₂ source line item. Risk: high if SIC is unmeasured; the SOC trend reported can be the wrong sign.

Scenario 3, calcareous Mediterranean perennial systems (vineyards, olives, cocoa-like agroforestry on calcareous parent material). SIC stocks are large, often dominate the subsoil, and respond to organic amendments and irrigation19. Action: measure SOC and SIC separately, sample to 0–60 cm minimum and 0–100 cm where the subsoil contains pedogenic carbonate, partition lithogenic from pedogenic where decadal trends are claimed, and use δ¹³C signatures to corroborate the partition where the project is making strong claims about pedogenic accrual.

Scenario 4, dryland and semi-arid pastoral / cropland systems (Sahel, Australian rangeland, Central Asian grassland). SIC dominates the soil carbon pool, often by a factor of 2 to 57. Action: dual SOC + SIC measurement is mandatory; SIC may be the larger sink or source depending on management. Irrigation with bicarbonate-bearing water can drive abiotic CO₂ release at the same time as it builds pedogenic SIC, and the net flux is the ledger entry, not the SIC accrual alone20. Subsoil sampling to 0–100 cm is the floor. Climate normalisation against a counterfactual is not optional.

Scenario 5, enhanced rock weathering deployment (basalt or wollastonite on cropland, regardless of climate). The credit is for inorganic carbon formation; the SOC pool is a co-pool that may rise or fall under the same intervention. Action: separate SOC and SIC measurement is mandatory. Account explicitly for accessory carbonates in the feedstock; basalt routinely contains 0.1–1% trace calcite and dolomite that dominate early DIC fluxes and can be misattributed to silicate weathering56. Track SOC dynamics independently; a recent meta-analysis finds ERW typically increases SOC by ~3.8% in low-latitude warm-humid systems and decreases it in cold-dry systems21. The CDR claim is the silicate-derived SIC accrual or DIC export net of the accessory-carbonate dissolution and any SOC loss.

Scenario 6, liming or pH-management programmes. Lime-induced CO₂ release is the second-largest pathway by which agricultural management transfers SIC to the atmosphere globally (~273 Tg C/yr)14. Action: report lime-derived CO₂ as a Scope 3 emissions line item; if subsequent in-situ pedogenic carbonate formation is claimed, document it with isotopic or mineralogical evidence and net the two flows in disclosure.

What the 2026/2027 frameworks actually require

The GHG Protocol Land Sector and Removals Standard is the foundational rule for corporate Scope 3 reporting from 1 January 20273. Its definition of the soil carbon pool includes SIC explicitly. Its reporting model requires gross emissions and gross removals to be separated. Its quantification model requires uncertainty to be propagated and disclosed across all relevant carbon pools, with measurement-based data preferred over modelled trajectories for productive agricultural removals. The LSR Guidance, publishing Q2 2026, will set the operational procedures. Companies relying on supplier-level soil carbon attestations on calcareous land have until then to verify that their MRV providers actually partition SOC and SIC, or to specify a remediation pathway.

Verra VM0042 v2.2 (ICVCM-endorsed under the Core Carbon Principles in October 2025) credits SOC change4. Its v3.0 Soil Sampling and Analysis Handbook, in public consultation through 31 March 2026, formalises ESM accounting and is sharpening the analytical-method requirements for SOC measurement on carbonate-bearing soils. Projects on calcareous land that ignore SIC are not eligible for the VM0042 credit pathway today. The auditor's question is unchanged: did you measure SOC, or did you measure SOC plus an unattributed SIC trend? The LSR auditor will ask the same.

The EU Carbon Removals and Carbon Farming Regulation (2024/3012), in force since December 2024, is preparing methodologies for afforestation, agroforestry, and peatland rewetting under its delegated act expected in summer 202622. Its QU.A.L.ITY criteria (quantification, additionality, long-term storage, sustainability) will require defensible separation of SOC and SIC trajectories where both are present; the long-term storage criterion in particular favours SIC, because pedogenic carbonate is a more durable sink than SOC, but only when its formation is genuinely additional and the gross flux from carbonate dissolution under acidification is netted against it.

ICVCM's Core Carbon Principles, with eight programmes and 38 methodologies CCP-approved as of late 2025 (and 22 rejected), have raised the floor on what defensible looks like for any voluntary credit4. Single-pool, single-depth, single-point soil carbon claims will not survive that floor.

The punchline

To the question itself, is soil inorganic carbon critical to include?, the practical answer is the same wherever the soil contains carbonate: if your supply chain or project sits on calcareous, dryland, irrigated, N-fertilized, limed, or ERW-amended soil, ignoring SIC misstates the soil carbon trend you are reporting. The risk is not symmetric. A project that overstates SOC accrual because SIC was lost silently fails verification. A project that partitions and reports both pools holds up under audit, and earns a price premium for the rigour.

The economics now favour rigour. The marginal cost of SIC partitioning (a manocalcimetry or temperature-ramp dry combustion run on the same cores already sent to the lab) is a small share of the total MRV bill, and a vanishing share if FTIR/MIR is in the workflow. The cost of not partitioning, when the LSR Guidance lands and Scope 3 buyers start asking the right question, is materially larger.

Key takeaways

• Global SIC stock to 2 m is comparable to SOC, but receives less than 4% of soil-carbon research effort. Less than 4% of attention is not less than 4% of the accounting risk.
• The GHG Protocol LSR Standard, effective 1 January 2027, defines the soil carbon pool as SOC plus SIC. The LSR Guidance publishes Q2 2026.
• SIC is material when carbonate is present, the pool is actively cycling, the management plausibly disturbs it, and the sampling depth captures it. Three of four conditions met means SIC is in scope.
• The masking risk is the dominant operational risk: in a soil where TC ≈ SOC + SIC and SIC is changing, a project that measures only TC will misattribute the SIC trend to SOC and report a number that can be the wrong sign.
• The analytical workflow is where most SIC error is created. LOI is unreliable on carbonate-bearing soils. Manocalcimetry and temperature-ramp dry combustion are the defensible options; FTIR/MIR is the most cost-efficient at scale once a regional library exists.
• ESM accounting alone does not protect against SIC misattribution. ESM corrects for bulk density; it does not partition the chemistry. Both are required.
• The scenario-by-scenario recommendation is consistent: confirm SIC absence on acidic non-calcareous soils with a subsample, then proceed with SOC only; everywhere else, partition and report SOC and SIC separately at the same monitoring depth and frequency.
• For ERW projects, accessory carbonate dissolution in feedstock dominates early DIC and cation fluxes and must be netted from any silicate-derived CDR claim.