Limestone in Enhanced Weathering: Part II

This post dives into detailed chemical reactions. The intent is not to prove that things are overly complex, but to showcase how important it is to know the mechanisms of reactions for understanding our projects, interpret measurement results, and make informed decisions and accurate assumptions.

Carbonic acid at near-neutral pH (pH ~ 7)

The only route that takes up carbon dioxide (CO2). Each mole of calcite, i.e., calcium carbonate (CaCO3), consumes one mole of CO2 and produces two moles of bicarbonate (HCO₃⁻). One carbon comes from the feedstock calcium carbonate (CaCO3), and one from the soil “air”:

CaCO₃ (s)+ CO₂ (g) + H₂O (l) → Ca²⁺ (aq) + HCO₃⁻ (aq) (from feedstock) + HCO₃⁻ (aq) (from air↓) (1)

Carbonic acid at acidic pH (pH < 6.35)

CaCO₃ (s) + 2H⁺ (aq) → Ca²⁺ (aq) + H₂O (l) + CO₂ (g) (air↑) (2)

A strong acid at near-neutral pH (pH ~ 7)

Typically, results in no uptake, no release. One proton per mole of calcite converts the carbonate ion to bicarbonate. The bicarbonate stays in solution, but all its carbon comes from the feedstock. The most probable types of reactions in agricultural soils are with nitric acid, which could originate from fertilizers and manure:

CaCO₃ (s) + HNO₃ (aq) → Ca²⁺ (aq) + NO₃⁻ (aq) + HCO₃⁻ (aq) (feedstock) (3)

In more polluted areas, sulphuric acid could also be involved:

2CaCO₃ (s)+ H₂SO₄ (aq) → 2Ca²⁺ (aq) + SO₄²⁻ (aq) + 2HCO₃⁻ (aq) (feedstock) (4)

A strong acid in acid soil (pH< 6.35)

The feedstock carbon is emitted. Two protons per mole of calcite take the reaction to completion, and the rock carbon leaves as CO₂:

CaCO₃ (s) + 2HNO₃ (aq) → Ca²⁺ (aq) + 2NO₃⁻ (aq) + H₂O (l) + CO₂ (g) (air ↑) (5)

CaCO₃ (s) + H₂SO₄ (aq) → Ca²⁺ (aq) + SO₄²⁻ (aq) + H₂O (l) + CO₂ (g) (air ↑) (6)

Where sulfate and calcium are both high, as in acid sulfate soils, gypsum (CaSO₄.2H₂O) can precipitate. Gypsum can serve as a mineralogical marker of sulfuric-acid attack.

Soil exchangeable acidity acts like a strong acid (pH < 5.5)

In an acid soil, the Ca²⁺ released by CaCO3 displaces H⁺ and Al³⁺ from exchange sites, and the released acidity reacts with bicarbonate:

Exchange of H⁺

2X–H (s) + Ca²⁺ (aq) → X₂–Ca (s) + 2H⁺ (aq) (7a)

Exchange of Al³⁺

2X₃–Al (s) + 3Ca²⁺ (aq) → 3X₂–Ca (s) + 2Al³⁺ (aq) (7b)

Aluminium releases its acidity by hydrolysis

Al³⁺ (aq) + 3H₂O (l) → Al(OH)₃ (s) + 3H⁺ (aq) (7c)

Neutralisation of bicarbonate

3H⁺ (aq) + 3HCO₃⁻ (aq) → 3H₂O (l) + 3CO₂ (g) (air↑) (7d)

Net equations:

2X–H + CaCO₃ → X₂–Ca + H₂O + CO₂ (g) (air↑) (7e)

2X₃–Al + 3CaCO₃ + 3H₂O → 3X₂–Ca + 2Al(OH)₃ + 3CO₂ (g) (air↑) (7f)

This is the purpose of agronomic liming. To neutralize natural exchangeable acidity and aluminium in the soils. It is also why liming an acid soil first releases CO2, the “initial emission pulse” mentioned in Suhrhoff et al. (2026).

How strong-acid protons form

Nitrogen

In farmed soils, the main strong-acid source is nitrogen. Nitrification of ammonium produces two protons per mole of nitrogen:

NH₄⁺ (aq) + 2O₂ (g) → NO₃⁻ (aq) + 2H⁺ (aq) + H₂O (l) (8)

If the nitrate is taken up by the crop, uptake consumes a proton again. If the nitrate is leached, the acidity stays. Urea and ammonia first consume one proton when they form ammonium, so the net acidity is about one proton per mole of nitrogen leached as nitrate.

Sulfur

Sulfides in soils, mine wastes or the feedstock itself oxidize to sulfuric acid, and so does elemental sulfur applied as fertilizer:

FeS₂ (s) + 15/4 O₂ (g) + 7/2 H₂O (l) → Fe(OH)₃ (s) + 2SO₄²⁻ (aq) + 4H⁺ (aq) (9)

S (s) + 3/2 O₂ (g) + H₂O (l) → SO₄²⁻(aq) + 2H⁺ (aq) (10)

Other sources are atmospheric nitric and sulfuric acid deposition, crops taking up more base cations than anions (the cations leave with the harvest), and organic acids from roots, fungi and decomposing residues.

After CaCO3 dissolution/weathering

Dissolution is not the end of the carbon pathway. When calcium carbonate reprecipitates, in the soil or downstream, one CO₂ is released:

Ca²⁺ (aq) + HCO₃⁻ (aq) + HCO₃⁻ (aq) → CaCO₃ (s) + H₂O (l) + CO₂ (g) (air ↑) (11)

For a silicate, equation 11 is a partial transfer: half the captured carbon stays in the solid. For limestone, equation 11 is the exact reverse of equation 1. Reprecipitation returns the full CO₂ taken up, and the net removal falls to zero. Degassing of bicarbonate in transit, and cation retention on proton-occupied exchange sites, are further routes back to the atmosphere.

Acids in soil: strong, moderate and weak

For carbonate, any acid much stronger than carbonic acid behaves as a “strong” acid. Carbonic acid has a first dissociation constant of pK₁ ≈ 6.35. An acid with a lower pKₐ gives up its protons more readily, protonates carbonate and bicarbonate, and converts carbonate carbon to CO₂. Mineral acids are fully dissociated at any soil pH. Many organic acids have pKₐ values of 3 to 5, so they also act on carbonate as strong acids while they persist.

Organic acids are a special case

Organic acids dissolve carbonate like strong acids, but microbes later oxidize many organic anions, and that oxidation produces bicarbonate. Part of the effect is therefore reversed over time. The net outcome is hard to measure, and organic anions also corrupt alkalinity titrations.

Summary table of acids in agricultural soils and their effect on carbonate

The chemical reactions of calcite in the field

Carbonate reactions in soil are plenty and knowing exactly what is continuously happening in the field is not an easy task. Measurements are typically just a snapshot, which could catch the soil in the midst of any of these reactions. Different reactions may dominate at different times.

In Part III of these series we will explore why liming for CDR becomes an agronomical paradox.

Contact us for any questions or comments at contact@cdrexperts.org