Calcite Supersaturation in Soils as a Result of Organic Matter Mineralization

Solutions equilibrated with soils under laboratory conditions often exhibit Ca and CO^~ 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 HCO, as an end product of aerobic decomposition. The rate of HCO, 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 HCO, production. It was determined that the decomposition of organic anions associated with inorganic cations always results in HCO, as an end product. The rates of calcite dissolution and precipitation were greatly inhibited in the presence of decomposing organic matter. Additional Index Words: carbon dioxide, alkalinity, organic matter decomposition, mineral weathering, ion activity products, kinetAmrhein, C., and D.L. Suarez. 1987. Calcite supersaturation in soils as a result of organic matter mineralization. Soil Sci. Soc. Am. J. 51:932-937. S (1977) found calcite supersaturation (ion activity products three times that of calcite) in waters sampled from below the root zone in both field and lysimeter studies. Inskeep and Bloom (1986b) found that Calciaquolls in western Minnesota were up to 40-fold oversaturated with respect to pure calcite. Both studies found that these high levels of Ca and carbonate activities in solution could not be explained by Mg substitution in the calcite, calcite particle size, or the presence of a more soluble soil CaCO3 phase. Suarez (1977) attributed these large supersaturation values to the concentrating effects of evapotranspiration and the kinetic inhibitions of calcite precipitation, and further hypothesized that organic excretions from microorganisms blocked the crystal growth sites on calcite surfaces. Reynolds (1978) and Inskeep and Bloom (1986a) have shown that some organic matter constituents may inhibit calcite precipitation. Apparent calcite supersaturation in soil/water laboratory equilibrations have been noted by several investigators. In contrast to the field studies, calcite supersaturation in the laboratory equilibrations cannot be attributed to the concentrating effects of evapdtranspiration. Cole (1957), Olsen and Watanabe (1959) and Levy (1981) observed that Ca concentrations and calcite supersaturation increased as the soil-to-water ratio increased. Suarez and Rhoades (1982) found that calcareous soils which were reacted with water under controlled CO2 conditions resulted in calcite supersaturation when equilibrated from initially undersaturated conditions. However, calcite nodules isolated from soil gave CaCO3 ion activity product (IAP) values of 10~ to 10~. Organic complexes with Ca 1 Contribution from the U.S. Salinity Laboratory, USDA, ARS, 4500 Glenwood Drive, Riverside, CA 92501. Received 20 October 1986. 2 Soil Scientist and Geochemist, respectively. AMRHEIN & SUAREZ: CALCITE SUPERSATURATION IN SOILS 933 Table 1. Composition of each reaction bottle. Exp. Solution Organic material Calcite, g pCOj, kPa