Carbonate Precipitation In Artificial Soils as a Plant-Driven Carbon Capture Process

Plant root exudation provides a source of unstable organic compounds to the soil solution. These decompose to give carbon dioxide, occurring in part in solution as bicarbonate. Plant root exudates enhance dissolution rates of calcium-bearing silicate minerals, especially feldspars, releasing calcium (and other ions) to solution.

In a coupled plant-soil system where calcium silicate minerals occur, calcite precipitates provided saturation is achieved. This process occurs in artificial soils of two types: blends of compost and dolerite quarry fines, and urban soils based on demolition wastes.

Soils prepared in 2002 from dolerite quarry fines (60%) and compost (40%) were initially carbonate-free. On sampling in 2007, they contained up to 7.5% calcite. Stable isotope data show that the calcite carbon in these soils is significantly more negative than in local geological sources of carbonate, consistent with an origin through precipitation of C derived from plants growing on the soils.

Sampling of soil from an urban site where buildings were demolished in 1998 yielded bricks with visible surface precipitates of calcite; the fine grained fraction of the soil contains up to 20% calcite. C and O isotope data again indicate that a proportion of the calcite carbon is derived from plant growth at the soil surface.

The occurrence of pedogenic carbonates in artificial environments offers the possibility of designing soils with a carbon capture function, analogous to the use of wetlands for passive remediation of contaminated waters. In an urban setting, it may be uneconomic to send to landfill fines resulting from recycling of demolition materials. In these circumstances, landscaping incorporating fines from concrete crushing within soils provides a carbon capture function that can reduce a development's carbon footprint. It is estimated that the carbon capture potential of such soils is of the order of 750 kg CO₂ per hectare annually.