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, with at least 1.13 ± 0.33 Pg C lost to inland waters annually and up to 23 Pg C of topsoil SIC (0–30 cm) forecast to be lost over the next 30 years to acidification from nitrogen deposition and fertilization1. To 2 m the global SIC and SOC stocks are close to equal, and yet 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, the latter defined as carbon in soil carbonates and other mineral carbon forms in soil3. The accompanying LSR Guidance was published on 30 June 2026. It sets no procedure for partitioning the two pools, and it takes the position that soil inorganic carbon cannot be built up over decadal timescales but can be lost through management and natural factors, principally through acidification. 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, issued 21 October 2025, focuses crediting on soil organic carbon storage and already requires SOC stock changes from direct measurement to be reported on an equivalent soil mass basis; a v3.0 revision went to public consultation from 11 February to 31 March 2026, moving the sampling and analysis requirements into a separate handbook4. 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. Where carbonate is present the pool is large enough to change the answer. Across soils at pH 7 to 9, SIC content has a median near 6 g C/kg and an interquartile range of roughly 1 to 16 g C/kg, with values above about 40 g C/kg uncommon1, and SIC density averages 100 Mg C/ha to 1 m across drylands, about 130 Mg C/ha in desert soils and about 56 Mg C/ha in semi-arid steppe7.

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 running at a rate equal to 17.6–24% of China's terrestrial carbon sink; within cropland alone, the SIC loss of 0.016 Pg C/yr offsets 57% of the SOC sink accruing there9. In the upper 40 cm, Chinese cropland SIC stocks fell 7% from 1980 to 2020, 7 million hectares lost their carbonate to that depth, 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 transect found that, on average, 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 metre13. 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 offsets a meaningful share of the SOC sink accruing in that cropland91014. 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) showed how far bulk-density change alone distorts fixed-depth stock change, and Raffeld and colleagues (2024) showed that fixed-depth and ESM stock-change estimates can differ by more than 100% even within SOC alone. Neither analysis separates the organic and inorganic pools, so 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 °C or higher, with mass loss attributed to SOC. In a method comparison, LOI showed the lowest agreement with the reference protocol (R² ≈ 0.83) and systematically underestimated SOC; it stays precise but drifts with exposure time and final furnace temperature, neither of which is standardised between laboratories17. 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 because the method is calibrated against CaCO₃ the output is an estimate wherever the carbonate includes magnesite, 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. Two variants split the work, gas-switching for calcareous samples and non-gas-switching for the rest, selected by a routine effervescence pre-test. More precise than the legacy KSSL workflow and slightly more accurate on calcareous samples, and still under evaluation against it at KSSL18.
• 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 an elemental-analyser and pressure-transducer reference chain, with SOC taken by difference17. 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 and dominate the profile, running eight to ten times SOC concentrations on calcareous parent material, and SIC-derived CO₂ can reach 60% of subsoil respiration19. 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). Whether SIC dominates depends on where along the dryland gradient the site sits: across northwestern China's drylands the SIC stock runs 2.5 times the SOC stock, rising to 5.5 times in desert soils but falling to 0.76 times in grassland7. 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; the basalts used in large-scale ERW trials carry 0.17–1.25% calcite, enough to dominate early cation and DIC fluxes and to be misconstrued as 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 largest pathway by which agricultural management transfers SIC to the atmosphere globally, at ~273 Tg C/yr against ~7.5 Tg C/yr for the acidification-driven dissolution of indigenous carbonates14. 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 supplements the Corporate and Scope 3 Standards for agriculture and CO₂ removal technologies from 1 January 20273. Its glossary definition of the soil carbon pool includes SIC explicitly, though its required pool list names only soil organic carbon. It requires removals to be accounted for and reported separately from emissions, with gross fluxes available as a further, optional disaggregation. It requires quantitative uncertainty estimates for reported removals, propagated from the input data and carrying a confidence interval and a justification that the value does not overestimate the removal. It accepts regular measurement, calibrated modelling, or hybrid approaches, with no preference between them, provided sampling is repeated at least every five years. The LSR Guidance, published 30 June 2026, sets the operational procedures. Companies relying on supplier-level soil carbon attestations on calcareous land have until the effective date to verify that their MRV providers actually partition SOC and SIC, or to specify a remediation pathway.

Verra VM0042 v2.2 credits SOC change and already requires SOC stock changes from direct measurement to be reported on an equivalent soil mass basis4. Its carbonate-specific rules are narrow: inorganic C must be separately accounted for under inelastic neutron scattering, and samples must be acid-washed under laser-induced breakdown spectroscopy. Nothing in its applicability conditions excludes calcareous land, and no carbonate removal is required of the recommended dry-combustion route, which is the exposure a project on calcareous land carries 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 26 December 2024, names afforestation, agroforestry, and peatland restoration among the practices its certification methodologies are to cover, and leaves those methodologies to delegated acts the Commission has still to adopt for carbon farming22. Its quality criteria (Articles 4 to 7: quantification, additionality, storage, sustainability) will require defensible separation of SOC and SIC trajectories where both are present; the 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 have raised the floor on what defensible looks like for any voluntary credit. 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, now that the LSR Guidance has landed and Scope 3 buyers are starting to ask the right question, is materially larger.

Key takeaways