In the next blog posts, we will dive on all possible infield chemical weathering reactions at different pH conditions, look at conditions necessary for carbonic acid to win over other acids, discuss the agronomic paradox of liming, and how downstream losses fall harder on limestone than on silicates, and many other interesting subjects.

References

Zeng, S., Liu, Z., Groves, C., 2022. Large-scale CO₂ removal by enhanced carbonate weathering from changes in land-use practices. Earth-Sci. Rev. 225, 103915. https://doi.org/10.1016/j.earscirev.2021.103915

What limestone is, and how much carbon does it carry?

Limestone already contains carbon. Agricultural lime (aglime) is ground limestone, made mostly of calcite (CaCO₃), or ground dolostone, made mostly of dolomite (CaMg(CO₃)₂).

Calcite is 12% carbon by mass. Each tonne holds 0.44 t of CO₂, locked in the rock since it formed. Dolomite holds slightly more, 0.48 t CO₂ per tonne. When either mineral dissolves, this carbon is released into solution or to the air.

How much carbon can be captured?

Limestone has been claimed as a ERW feedstock because its dissolution can potentially draw down atmospheric CO₂. But, weathering of limestone also mobilizes carbon.

Per tonne, calcite’s best case (0.44 t CO₂) is higher than that of a typical basalt (mixture of many minerals), which maximum is usually 0.2 to 0.3 t CO₂ per tonne. But calcite’s worst case is an emission of the same size. A silicate’s worst case is zero. Table 1 illustrates the range of withdrawal and emission of two carbonates (calcite and dolomite), in comparison with two silicate minerals. As can be seen, carbonate minerals have an inherent higher risk because they can also be a source of atmospheric CO₂.

Table 1. Carbon locked in carbonate minerals, and the maximum CO₂ uptake by carbonic-acid dissolution, compared with two silicates, plus the net balance when reacted with non-carbonic acids.

The three roles of limestone in an ERW projects

Limestone enters an ERW project in one of three roles:

1. Counterfactual

2. Feedstock

3. Confounder

The three roles can overlap, and its role(s) are not set by the mineral.

Limestone as a counterfactual

Limestone is part of the counterfactual whenever the land would have been limed anyway. Liming is long-standing agronomic practice. Where farmers lime acid soils routinely, liming belongs to the business-as-usual scenario, whatever the ERW project does. Three cases follow:

1. Silicate replaces lime: the farmer spreads basalt instead of the usual lime. The counterfactual is the lime that was not applied, with its own CO₂ balance. The control fields must be limed as usual.

2. Silicate is added on top of lime: the farmer limes as usual and also receives another ERW feedstock e.g. basalt. Lime is part of both treatment and control.

3. Liming is the project activity, but the farmer would have limed the same amount anyway: the lime is not additional. Its CO₂ balance belongs to the baseline, and possibly no removal can be credited for it.

Limestone as a feedstock

For the purpose of this commentary, a feedstock is understood as a rock powder material that can enable CO₂ capture and storage.

Limestone could be a feedstock if the site favours carbonic-acid dissolution. At least, all of the following should hold:

• Chemistry: pore-water pH, CO₂ pressure and strong-acid load put the dissolution in the carbonic-acid window. Strongly acid, heavily fertilized soils usually fail this test (see Table 1 above).

• Downstream capacity: receiving streams and rivers are undersaturated with respect to calcite, so the exported bicarbonate does not reprecipitate, which could zero again the uptake. Proposals to use liming as a removal strategy are bounded by exactly this capacity (e.g. Zeng et al., 2022).

• Measurement: The strong-acid fraction, the soil inorganic carbon and the export are measured, not assumed.

• Accounting: Current registry rules need to be checked for this. Some ERW methodologies exclude or restrict carbonate feedstocks.

Limestone as a confounder

Limestone confounds the measurement when carbonate is present but is neither the baseline nor the feedstock. Three common cases:

• Accessory carbonate in a silicate feedstock: many basalts carry a few percent calcite or dolomite. It dissolves quickly and dominates the early alkalinity signal.

• Carbonate already in the soil. Calcareous soils contain native or paedogenic carbonate. Its dissolution or precipitation changes the soil inorganic carbon, and the cation pools independently of the feedstock.

• A liming legacy. Lime applied in previous years may still be dissolving. Unless treatment and control share the same liming history, the legacy is attributed to the project.

Table 2. Decision of the role limestone has on an ERW projects (not an exhaustive list).