Sequestration of inorganic carbon in soil and groundwater

Movement of CO2 from the atmosphere into land via photosynthesis and root respiration, the subsequent formation of bicarbonate in soil, and its storage in groundwater or precipitation as CaCO3 in dryland soils are major processes in the global carbon cycle. Together, inorganic carbon as soil carbonate (similar to 940 PgC) and as bicarbonate in groundwater (similar to 1404 PgC) surpass soil organic carbon (similar to 1530 PgC) as the largest terrestrial pool of carbon. Yet, despite general agreement about its huge size as a carbon pool, controversy about the potential of inorganic carbon to sequester atmospheric CO2 remains unresolved. We suggest that the controversy stems from the absence of a lexicon and propose a classification scheme that uses (1) calcium source illustrated by two widely recognized chemical reactions, and (2) the concept of carbonate generations. When calcium is derived from preexisting carbonate, an equilibrium reaction occurs that does not sequester carbon in soil carbonate but does sequester carbon in groundwater until bicarbonate precipitates as CaCO3. When calcium is derived from silicate minerals, a unidirectional reaction occurs that sequesters carbon in both soil carbonate and groundwater. The generations concept shows that carbon sequestration occurs only in the first generation when calcium is released directly from silicates. This classification not only enhances communication and provides a framework for quantifying amounts of fossil fuel carbon that can be sequestered within a geoengineering context, it provides more precise language for discussing the terrestrial carbon cycle through geologic time.