Measurement of calcite ion activity products in soils.

Soils often show supersaturation with respect to calcite. This study was undertaken to test alternative methods of measuring Ca2+ and COjactivities to assess their effects on measured ion-activity products (IAP) of calcite in soils. Soils were reacted with atmospheric CO,(g) and distilled H 2 0 for 1,3, and 7 d. The soil extracts were analyzed for pH, electrical conductivity (EC), cations, anions, and dissolved organic carbon (DOC). Carbonate activities in soil extracts were obtained by two methods. The first consisted of titrating the soil extracts with standard HCl to pH 4.0 (direct-titration method). The second method consisted of acidifying soil extracts to pH 4.0 and trapping the evolved CO, gas in a NaOH + BaCI, solution (C02 gas release method). The IAP values for calcite were calculated in two ways. First, Ca2+ activity was obtained by speciating total soluble Ca while Cogactivity was obtained from the direct titration of soil extracts. This method gave mean log IAP values of calcite ranging from -7.90 to -7.78 compared with a log KO (Ca2+)(C03-) (equilibrium constant) of 8.48 for calcite. These results suggest supersaturation with respect to calcite. Secondly, when Ca2+ activity was measured with a Ca2+-specific ion electrode and COjactivity was estimated by the C02 gas release method, the mean log IAP values of calcite ranged from 8.45 to 8.35, which is very near equilibrium with calcite. The apparent supersaturation in the first case was probably caused by DOC, which both a m plexed Ca2+ and contributed to titratable alkalinity over that of carbonate and bicarbonate alone. ALCITE (CaCO,) strongly buffers soils near pH 8 C and affects the equilibrium relationships of many other elements (Lindsay, 1979). Therefore, calcite solubility relationships in soils need to be clearly defined. Several researchers have examined calcite solubility in soils and have reported supersaturation (Cole, 1956; Olsen and Watanabe, 1959; and Levy, 1981). Suarez ( 1977) examined the relationships among pH, Ca2+, and HCOj concentrations in waters below the root zone and reported that attainment of calcite equilibrium in soil waters was very slow. Suarez and Rhoades ( 1982) attributed calcite supersaturation in soils to silicate minerals such as anorthite, which are more soluble than calcite. Inskeep and Bloom (1 986) suggested that supersaturation of calcite in soils occurs as a result of inhibition of calcite precipitation due to soluble organic components. Recently, Amrhein and Suarez (1 987) attributed calcite supersaturation to organic-matter mineralization. These authors use a double-titration method to determine alkalinity: once in the presence of C02 and once in its absence. They showed that calcite-organic systems showed supersaturation to calcite, but they did not extend their studies to include soils. In all of these studies except Amrhein and Suarez (1 987), Ca2+ K.J. Reddy, Wyoming Water Research Center, P.O. Box 3067, Univ. of Wyomin Laramie, WY 82071; W.L. Lindsay and S,M. Workman, Dep. opAgronomy, Colorado State Univ., Fort Collins, CO 80523; and J.I. Drever, Dep. of Geology and Geophysics, Univ. of Wyoming, Laramie, WY 8207 1. Received 10 Dec. 1988. *Corresponding author. Published in Soil Sci. SOC. Am. J. 54:67-71 (1990). activity was calculated from the Ca concentration measured by atomic absorption (AA) or ICP, and Cogactivity was calculated from total alkalinity. It is hypothesized that most methods overestimate Ca2+ and Cogactivities because dissolved organic species form complexes with Ca, and also contribute to total alkalinity in soil extracts. The objective of the present study was to test other methods of measuring Ca2+ and COP activities in soil and to determine their effect on the measured IAP of calcite in soils. The method makes use of C0,-gas release from soil extracts as a measure of total carbonate species in solution, which are then speciated to give Cogactivity. In addition, the specific Ca-ion electrode was used to measure Ca2+ activity in soil extracts. MATERIALS AND METHODS Three soils were used in this study. Soil 1 was collected at Fort Collins, CO; Soil 2 at Loveland, CO; and Soil 3 at Laramie Range, WY. Soil 1 is classified as Otero, a coarseloamy, mixed (calcareous), mesic Ustic Torriorthent. Soil 2 is classified as Nunn, a fine, montmorillonitic, mesic Aridic Argiustoll. Soil 3 is classified as a fine-loamy over sandy or sandy-skeletal Borollic Haplargid. These soils were analyzed for selected properties including texture (Soil Conservation Service, 1984, p. 15-17), CaC03 equivalent (Workman et al., 1988), and organic C (Snyder and Trofymow, 1984). Selected properties of these soils are presented in Table 1. Samples were air dried and ground to pass a 0.25-mm sieve. The sieved samples were used for x-ray diffraction analysis and for the equilibrium studies. For x-ray diffraction analysis, the soil samples were powdered finely by grinding under acetone and x-rayed using Cu h radiation at 2" 28 per minute. Calcite was identified by the presence of a sharp peak at 29.50" 28. Duplicate 25-g samples of soil were suspknded in 250-mL flasks with 100 mL of distilled deionized H20. Laboratory air was moisturized by passing it through distilled deionized H20, which was then bubbled through the soil suspensions. All samples were shaken at 25 "C on a mechanical shaker at 100 oscillations per minute. Three drops of toluene were added to the soil suspensions to suppress microbial activity. After reacting for 1, 3, and 7 d, the soil suspensions were centrifuged. The supernatant solutions were filtered using 0.45-pm Millipore filters. The clear filtrates were measured immediately for pH, EC and total carbonates, and later the same day for cations, anions, and DOC. The pH was measured with a combination glasscalomel electrode, and EC was measured with a conductivity meter. Measurements of Ca, Mg, Al, Sr, Ba, Na, K, Si, B, and Mo were made by inductively coupled plasma optical-emission spectrometry (ICP-OES). Specific-ion electrodes were used to measure C1and Factivities, while SO, was measured by the turbidometric BaC1,-precipitate method, NO, by ion Table 1. Selected properties of the soils used in the study.? Prowrtv Soil 1 Soil 2 Soil 3 Texture Sandy loam Clay Sandy clay loam CaC03 equivalent, Yo 4.80 1 1.60 6.5 Organic C, 46 0.92 1.47 0.42 Soil 1 is Otero series (a Ustic Torriorthent), Soil 2 is Nunn series (an Aridic Argiustoll), and Soil 3 is a Borollic Haplargid.