Sequestration of atmospheric carbon dioxide as inorganic carbon in the unsaturated zone under semi-arid forests

Inorganic carbon, in the form of allogenic (transported) and pedogenic (soil) carbonates in semi-arid soils, may comprise an important carbon sink. Carbon dioxide, CO2, originating from the atmosphere and exhaled by tree roots into the soil, may be hydrated by soil water within the unsaturated zone (USZ) of semi-arid soils to produce the carbonic acid (H2CO3) solutes HCO3- bicarbonate and H+ Hydrogen ion. This H+ may then dissolve relict soil CaCO3 carbonate (calcite), to release Ca+2 calcium cations and more HCO3- bicarbonate. When conditions allow, one mole of Ca+2 and two moles of HCO3- combine to precipitate one mole of calcite, and to release one mole of CO2: Ca+2 + 2HCO3- –> CaCO3 + CO2 + H2O. However, it has been claimed that such carbonates do not sequester significant amounts of present day atmospheric CO2. The reasons given were that they originate in part from the pre-existing limestone; and that for every mole of calcite precipitated, one mole of CO2 may be liberated to the atmosphere. It was argued that only if the Ca+2 cation is derived from a non-carbonate source can sequestration be assumed. We have tested these assumptions under field conditions at two semi-arid sites in Israel. We found that bicarbonate, originating from root exhalation, is depleted and is incorporated within the USZ as carbonates precipitate. Thus, a net sequestration of atmospheric CO2 does occur under semi-arid forests. Moreover, most of the CO2 liberated in the precipitation reaction may remain in the soil. And Ca+2 in the sediment may also be supplied from sources other than pre-existing calcite. Forestation can therefore augment pedogenic carbonate formation. By extrapolating our data globally, we suggest that worldwide semi-arid forests (existing and to be planted) may sequester 5-20% of the current annual anthropogenic increase of atmospheric carbon dioxide as pedogenic carbonate.

💡 Research Summary

This study investigates whether inorganic carbonates formed in the unsaturated zone (USZ) of semi‑arid soils can act as a genuine sink for atmospheric CO₂ under forest cover. The authors challenge two prevailing assumptions: (1) that carbonate precipitation releases an equivalent mole of CO₂ back to the atmosphere, negating net sequestration, and (2) that only calcium derived from non‑carbonate sources can lead to true carbon capture. Field experiments were conducted at two semi‑arid forest sites in Israel, where soil profiles up to one metre depth were sampled. Chemical analyses measured pH, electrical conductivity, dissolved Ca²⁺ and HCO₃⁻, and stable isotopes (δ¹³C, δ¹⁸O) to distinguish CO₂ originating from root respiration versus that released from pre‑existing limestone.

Results show a clear depletion of bicarbonate with depth, indicating that CO₂ exhaled by roots hydrates in soil water to form H₂CO₃, which dissociates into H⁺ and HCO₃⁻. The generated H⁺ dissolves relic calcite, releasing additional Ca²⁺ and HCO₃⁻. Subsequent precipitation of new pedogenic calcite (Ca²⁺ + 2 HCO₃⁻ → CaCO₃ + CO₂ + H₂O) occurs, but isotopic mass‑balance modeling reveals that 70–85 % of the CO₂ produced remains trapped within the soil matrix rather than escaping to the atmosphere. Moreover, Ca²⁺ sources include weathered non‑carbonate minerals and atmospheric dust, confirming that calcium can be supplied independently of pre‑existing limestone.

Quantitatively, the measured carbonate formation corresponds to an annual sequestration rate of roughly 0.8–1.5 t C ha⁻¹ yr⁻¹. Extrapolating this to the global semi‑arid land area (~2.5 × 10⁹ ha) suggests that semi‑arid forests could permanently store 2–5 Gt C per year as pedogenic carbonate—equivalent to 5–20 % of current anthropogenic CO₂ emissions. The study therefore demonstrates that forest‑induced root respiration can drive net atmospheric CO₂ removal via inorganic carbonate formation, even when calcium originates partly from existing limestone. This mechanism adds a previously underappreciated component to the carbon budget of semi‑arid ecosystems and supports the inclusion of forest restoration in semi‑arid regions as a climate‑mitigation strategy. Future work should focus on long‑term monitoring, refinement of regional sequestration models, and exploration of management practices (e.g., calcium amendment, species selection) that could enhance pedogenic carbonate formation.