Reevaluation of Calcite Supersaturation in Soils

The calcite saturation status of soil-water systems in semiarid regions was reexantined taking into consideration organic ligand alkalinity and Ca-organic complexation. These factors have been forwarded as the reason for earlier reports of calcite Supersaturation in soilwater systems. We examined groundwaters beneath irrigated lands, soil-water suspensions in the laboratory using four soils and two levels of CO2, and soil water extracted from the root zone of an irrigated, cropped field. Total alkalinity and net inorganic C alkalinity were determined by titration. Calcium activity values calculated from total concentrations using the speciation program WATEQ4F were compared with Ca activity values measured with an ion-specific electrode. In all our systems, total alkalinity and inorganic C alkalinity were almost equivalent and thus organic C alkalinity was negligible. Calculated Ca activity was generally 5 to 25% greater than Ca activity determined with an ion-specific electrode. Although these data suggest the presence of additional Ca complexes not included in the speciation routines, the determined dissolved organic C values are too low for Ca-organic complexes to be the cause of these differences in Ca activity. Ion activity products based on either measured Ca* activity values or calculated Ca activities indicated that all systems were supersaturated, with saturation ratios of two to three. T SATURATION STATUS of calcite in soil-water systems has been the subject of numerous studies, often with apparently conflicting results and different conclusions. Past studies may not be as conflicting as a first examination might indicate because experimental conditions often differed. Also, differences in past studies are at least in part due to an examination of different problems. For example, early reports of calcite Supersaturation in soil—water reactions (Cole, 1957; Olsen and Watanabe, 1959) can be questioned because these studies did not correct for ion activity coefficient and complexation effects. Many studies have also been conducted that assumed soil-water reactions could be represented by solution studies without a solid phase or only by solution clay reactions. We do not examine these studies here because they do not directly relate to the questions formulated below. In order to properly interpret the published research on soil CaCO3 and its effect on soil solution composition, we considered two distinct questions. First, what is the thermodynamic stability of the CaCO3 phase present in soils? Secondly, what is the CaCO3 ion activity product, (Ca)(CO3-), in soil water? The former question is best answered by isolating the CaCO3 phase, if possible, and reacting it under controlled laboratory conditions of fixed temperature and CO2 partial pressure. The latter question is best resolved by direct examination of the natural system. Laboratory soil-water equilibrations have been by far the most common experiments reported, but provide only a limited evaluation of the natural system. Within this D.L. Suarez and J.D. Wood, USDA-ARS, U.S. Salinity Lab., 4500 Glenwood Dr., Riverside, CA 92501; and I. Ibrahim, Soil Salinity and Alkalinity Lab., Bacos, Alexandria, Egypt. Contribution from the U.S. Salinity Lab., Riverside, CA. Received 18 July 1991. *Corresponding author. Published in Soil Sci. Soc. Am. J. 56:1776-1784 (1992). framework, a distinction should be made among systems in which relevant ions may be released or removed from sources other than the carbonate phase and those in which this does not occur. Since we are dealing with a system also involving kinetic considerations, a distinction needs to be made between conditions where saturation is approached from undersaturated or supersaturated conditions. Calcite Supersaturation under field conditions was reported by Suarez (1977) for groundwaters below irrigated lands in two different hydrologic systems. All of the 62 well waters sampled were supersaturated with respect to calcite. This Supersaturation could be the result of either deviations of the AG°f of soil CaCO3 from that of pure well-crystallized calcite, or from kinetic factors that prevent solution equilibrium with the existing solid phase. Determination of the cause of apparent Supersaturation requires examination of the solid phase. Suarez and Rhoades (1982) extracted pedogenic CaCO3 from soils and identified it by XRD as lowMg calcite. This material, when reacted with deionized water, gave the same ion activity product as pure calcite. This appears to be the only study in which the stability of the CaCO3 solid phase present in a soil was determined directly, in the absence of other mineral phases and organic matter. Suarez and Rhoades (1982) also reacted soils containing calcite (including the soil from which the pedogenic calcite was isolated and reacted) in the laboratory with various solutions including deionized water and soil water extracts. All reactions resulted in calcite Supersaturation including those under controlled, high CO2 partial pressures. The degree of supersaturation depended on the extent of calcite dissolution, suggesting nonequilibrium conditions. Increases in Supersaturation with time implied that Ca or alkalinity were released from other sources, and that there was also inhibition of calcite precipitation. These results suggest that well-crystallized calcite, with a solubility comparable to specimen samples, is formed in these soils, and that the Supersaturation observed for soil water reactions may be due to other sources of Ca and alkalinity and slow precipitation kinetics. Additional experiments, conducted under ambient CO2 conditions, have also demonstrated Supersaturation. Marion and Babcock (1977) observed calcite supersaturation for two out of three soils reacted with distilled water in the laboratory. Their data show increasing alkalinity with time through 40 d of reaction. Inskeep and Bloom (1986a) found high degrees of Supersaturation in soil-water suspensions measured in the laboratory. Their calculations indicate that their samples were also supersaturated under field conditions. They related the persistence of Supersaturation to the inhibition of precipitation due to the presence of soluble organic C. Inhibition of calciteprecipitation by soluble organAbbreviations: XRD, x-ray diffraction; IAP, ion activity product; ICP, inductively coupled plasma emission photometry; DOC, dissolved organic carbon.