Calcite Supersaturation in Soils as a Result of Organic Matter Mineralization

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Abstract
Solutions equilibrated with soils under laboratory conditions often exhibit Ca2+ and CO2‐3 activities that exceed calcite saturation. This supersaturation has been attributed to various possibilities, none of which have been fully able to explain the phenomenon. In this study we report on the attainment of calcite supersaturation in calcite + water + organic matter systems under controlled CO2 conditions. The decomposition of alfalfa shoots (Medicago sativa L.) and soybean roots (Glycine max L. merr.) in batch reactors under CO2 pressures of 1.0, 10.0, and 99.5 kPa was found to result in HCO3 as an end product of aerobic decomposition. The rate of HCO3 production from mineralizing organic matter exceeded calcite precipitation rates and, thus, calcite supersaturation resulted. The decomposition of pure organic compounds (dextrose, inositol, glucosamine, glucuronic acid, citric acid, and sodium citrate) was studied to elucidate the mechanism of HCO3 production. It was determined that the decomposition of organic anions associated with inorganic cations always results in HCO3 as an end product. The rates of calcite dissolution and precipitation were greatly inhibited in the presence of decomposing organic matter.