CASPER: A modelling framework to link mineral carbonation with the turnover of organic matter in soil

Rapid formation of stable soil carbonates offers a potential biologically-mediated strategy for removing atmospheric CO₂ and forms a part of the negative emissions debate in a bid to maintain global temperatures of 1.5 °C. Microbial respiration in soil and respiration by plant roots leads to high partial pressure of CO₂ below ground. Given adequate supply of calcium in soil solution the sequestration of C into the mineral calcite (CaCO₃) can occur at rapid rates. We have coupled an established soil C model RothC to a simplified geochemical model so that this strategy can be explored and assessed by simulation. The combined model CASPER partitions CO₂ respired belowground into soil solution as HCO₃− and simulates its reaction with Ca²⁺ based on a particular dissolution rate for Ca-bearing minerals, with precipitation of calcite into soil pores as a consequence. Typical model output matches observed field rates of calcite accumulation over 5 years, namely 81 t ha⁻¹, with 19 t CO₂ ha⁻¹ sequestered into the soil.