Carbonate Precipitation in Artificial Soils Produced from Basaltic Quarry Fines and Composts: An Opportunity for Passive Carbon Sequestration

The proportions of different carbon pools within artificial soils prepared by blending composts with dolerite and basalt quarry fines has changed over a period of 7 years, accumulating inorganic carbon as carbonate minerals newly formed within the soils. With no artificial energy inputs following construction, this is regarded as a passive mineral carbonation process. Carbon isotope data show that up to 40% of the carbon within the soil carbonate is derived from photosynthesis, mixed with carbon from geological sources (limestone present in the quarry fines). Organic matter within the soils shows very variable composition, with an apparent increase with time in the relative proportion of labile carbon relative to more stable forms, reflecting a change in the soil organic matter composition associated with the establishment of new plant communities. The rate of accumulation of inorganic carbon as carbonate minerals is estimated to be equivalent to 4.8 t C ha−1 annually to a depth of 0.3 m, consistent with rates observed for accumulations of carbonate carbon in urban soils containing demolition wastes (annually 3.0 t C ha−1 to 0.3 m). There appears to be substantial potential for artificial soils to be designed expressly for the purpose of carbon capture. The process is analogous to the use of reed beds for the removal of pollutants from contaminated waters.