DETERMINATION OF KINETIC PARAMETERS FOR SULFUR PROCESSING POTENTIALS: VERIFICATION OF THE CONSTANT SPECIFIC ACTIVITY APPROACH A. R. AUTRY and J. W. FITZGERALD

A modification of the "heterotrophic activity method" of Wright and Hobbie (1966) was used to determine kinetic parameters for organic S formation and sulfate adsorption by forest soil. In this modification, the parameters V^,. , (Kt + SJ and T,, represent the maximum rate of sulfate incorporation into organic matter, the sum of the halfsaturation constant (Kt) and the endogenous sulfate pool (S,,) and the time required for complete mineralization of the organic S, respectively. As noted by Azam and Hodson (1981), if S, is not known, then the sum (Kt + SJ provides an upper limit on Kt. Doubling this sum provides an upper limit on the concentration of added sulfate required for saturation. Owing to the linearity of organic S formation with time, Kmax can be multiplied by 48 to yield saturation potentials for organic S formation over a 48 h period (Autry and Fitzgerald, 1990). Kinetic parameters for sulfate adsorption, including saturation potentials and the concentration of added sulfate yielding saturation, can be determined by visual inspection of the saturation curves. This approach is possible because of the close similarity between Langmuir adsorption isotherms and saturation curves (Wright and Hobbie, 1966). The major modification of the heterotrophic activity method was that the specific activity of the S-label was held constant at all concentrations of added sulfate, while simultaneously increasing the amount of S-labeled SOJ" across the concentration range examined (Autry and Fitzgerald, 1990). In the original procedure (Wright and Hobbie, 1966), the specific activity of the labeled compound of interest was reduced as the total concentration of the added compound was increased, thus maintaining a constant amount of added label across the concentration range examined. Because the label is used as a marker, maintenance of a constant specific activity while increasing the amount of S better reflects conditions prevalent in forest soils which receive gradually increasing amounts of sulfate in acidic precipitation. Previous applications of the heterotrophic activity method utilized isotope dilution when measuring glucose mineralization in aquatic and soil ecosystems (Azam and Hodson, 1981; Ferroni et al., 1985; Coody et al., 1986). Apart from the work of Autry and Fitzgerald (1990), no published reports involving the heterotrophic activity method have appeared in which specific activity was held constant. The uncertainty involved in this modification must be determined before the validity and interpretation of kinetic data derived from this approach can be fully ascertained. Kinetic parameters for organic S formation and sulfate adsorption were therefore determined for the same soil samples by both isotope dilution and constant specific activity methods. Samples were collected from the A (0-1 Ocm), B (10-25 cm), BC (25-40 cm) and C (40-55 cm) horizons of an eastern white pine forest (Watershed 1) located at the Coweeta Hydrologic Laboratory near Otto, N.C. Soils are in the fine loamy Fannin series of Typic Hapludults. Kinetic parameters for organic S formation and sulfate adsorption were determined as follows. Briefly, two sets of triplicate field-moist samples from each horizon were exposed to S-sulfate solutions (ICN Pharmaceuticals, 4 x 10'° Bq mmol") having amounts of sulfate ranging from 7.5nmol to 400 ̂ mol g"' wet wt. For one set of samples, the specific activity was held constant at 8.33 x 10Bqmmor' (constant specific activity method). For the other set, the specific activity was lowered over the concentration range from 8.33 x 10 to 15.6Bqmmol-' (isotope dilution method). Samples were extracted to yield water, salt, acid and base extracts. The amount of sulfate adsorbed was determined as S present in the salt extract, whereas the amount of organic S formed was determined as the S present in the acid and base extracts (Fitzgerald el al., 1988). Organic S formation data for both sets of samples were linearized according to the technique of Wright and Hobbie (1966). The linear portion of the resulting Wright-Hobbie plots and the sulfate adsorption saturation curves for both sets of samples were compared separately based on the fitting of full and reduced linear regression models to determine whether the slopes and ^-intercepts were significantly different. If two points of two lines are not significantly different, it can be inferred that both the lines and all parameters derived from them are not significantly different (Neter et al., 1989). For organic S formation, saturation with respect to increasing sulfate was obtained with all horizons irrespective of the technique (Table 1). As stated previously (Autry and Fitzgerald, 1990), an horizon is saturated if the mean amounts of organic S formed are not significantly different over 25% of the concentration range examined. Further, multiphasic uptake kinetics for sulfate, indicated by curvilinear relationships in the Wright-Hobbie plots, were observed for all horizons irregardless of method (data not shown). These results indicate that more than one microbial population was responsible for incorporation of sulfate into organic matter (Azam and Hodson, 1981). The fact that these kinetics were found further supports the functional similarity between the constant specific activity method and the traditional isotopic dilution approach. Because of multiphasic uptake kinetics, calculations were based on the initial linear portion of each curve. Thus, the results reported represent the lower limits for this process. When the slope and ^-intercepts of the regression lines derived from Wright-Hobbie plots were compared, no significant difference in either regression parameter was detected for any horizon analyzed, irrespective of technique (P > 0.26; Table 1). The constant specific activity modification generated regression lines and subsequent