A new study by researchers from the Institute of Applied Ecology of the Chinese Academy of Sciences has found that soil acidity plays a critical role in determining the effectiveness of enhanced rock weathering (ERW) and the accuracy of carbon removal estimates.

The study was published in Environmental Science & Technology.

Enhanced rock weathering is an emerging carbon dioxide removal (CDR) technology that aims to remove atmospheric carbon dioxide by applying silicate minerals to soils. During weathering, these minerals release alkaline cations, which can react with carbon dioxide to form dissolved inorganic carbon and contribute to long-term carbon storage.

However, current ERW carbon removal assessments mainly estimate carbon sequestration based on the release of calcium (Ca²⁺) and magnesium (Mg²⁺) ions, assuming their release results primarily from carbonic acid-mediated weathering. These assessments also assume that each mole of divalent cation released corresponds to the fixation of two moles of carbon dioxide. This approach does not fully consider the role of soil acidity and non-carbonic sources of protons (H⁺) in mineral weathering. This can lead to an overestimation of ERW's carbon removal potential.

To address this issue, the researchers conducted a long-term soil column experiment using wollastonite in agricultural soils with different acidity levels. By measuring dissolved ions, carbon formation, and carbon dioxide emissions, they were able to quantify how soil conditions influence ERW efficiency.

They found that lower soil pH accelerated wollastonite dissolution but did not proportionally increase carbon removal efficiency. Specifically, cation-based estimates of carbon dioxide removal were 1.5 to 3.1 times higher than the actual inorganic carbon formed.

Further analysis revealed that the source of protons affects the effectiveness of coupling mineral weathering with carbon removal. In acidic soils, protons from non-carbonic sources promoted the rapid dissolution of wollastonite and increased calcium release. However, this process did not consume atmospheric carbon dioxide. In contrast, weathering driven by carbonic acid contributed to the formation of inorganic carbon and the removal of carbon.

The study showed that for every mole of divalent cations released during wollastonite weathering, only 0.63 to 1.30 moles of carbon dioxide were fixed, which is substantially lower than the theoretical value of two moles. 

When the experimentally measured range of carbon fixation efficiency was applied to global agricultural ERW carbon sink estimates, the researchers found that conventional cation-based approaches could overestimate carbon removal potential by approximately 34 percent to 67 percent.

They also found that applying wollastonite to acidic soils promoted the dissolution of introduced carbonate minerals and accelerated soil organic carbon mineralization. This resulted in additional carbon dioxide emissions. After accounting for these emissions, they found that net carbon removal ranged from 0.07 to 0.46 grams of carbon per kilogram of soil, depending on whether the soil was acidic or alkaline.

The researchers suggest that soil acidity should be incorporated into future ERW carbon accounting frameworks to improve carbon removal assessments.