Division S-3-notes Hydrologic Tracer Effects on Soil Microbial Activities

While much research has shown that the commonly used hydrologic tracers Br ", Cl ~ , and pentafluorobenzoic acid (PFBA) are not affected by soil microbial processes, much less work has gone into analysis of the effects of these tracers on microbial activity. In this study, we analyzed the effects of Br ', Cl ~, and PFBA at 100 mg kg ~' concentration on soil respiration, denitrification, N mineralization, and nitrification in a forest soil under laboratory conditions. Chloride and Br ~ inhibited all activities other than denitrification and PFBA stimulated respiration and denitrification. Mechanisms for the observed effects may be related to the effects of these anion tracers on soil solution cation activities and microbial membrane processes. The results suggest that effects on microbial activities should be considered when tracers are used in studies of the fate and transport of pollutants that are affected by microbial processes. A VARIETY OF COMPOUNDS are used to trace the movement of water through environmental media. While a large body of work has gone into determining that these compounds are not degraded by microbial processes, much less work has gone into analysis of the effects of hydrologic tracers on microbial activity. These effects could be important when tracers are used in studies of the fate and transport of pollutants in the environment. In such studies, it is important to determine if tracer compounds affect microbial processes that influence pollutant dynamics. Tracer effects on soil microbial processes could be important in several contexts. Small anion tracers such as Cl" or Br~ are often added as Na, K, Ca, or Li salts. High salt concentrations have been found to inhibit general microbial activity and Cl~ has been found to be a potent inhibitor of nitrification, the oxidation of NrU to NO3~, (Hahn et al., 1942; Heilman, 1975; Johnson and Guenzi, 1963; Agarwal et al., 1971; Roseberg et al., 1986), and plant growth (Tisdale et al., 1985, p. 395). Organic tracers (e.g., fluorobenzoates) may be susceptible to microbial degradation under anaerobic conditions (Suflita et al., 1982; Bowman and Gibbens, 1992) and can therefore stimulate microbial activity in anaerobic media. Effects of tracers on overall microbial activity will affect general degradation and cometabolism reactions of pollutants. Effects on specific activities such P.M. Groffman, Inst. of Ecosystem Studies, Box AB, Millbrook, NY 12545; A.J. Gold and G. Howard, Dep. of Natural Resources Science, Univ. of Rhode Island, Kingston, RI 02881. Received 10 Dec. 1993. "Corresponding author ([email protected]) Published in Soil Sci. Soc. Am. J. 59:478-481 (1995). as nitrification and denitrification (the conversion of NOf to N gas) can be important to the fate of NOf and organic pollutants such as halogenated aliphatic compounds that are degraded by specific microbial groups (Vanelli et al., 1990; Rasche et al., 1991). In this study, we evaluated the effects of Cl~, Br", and PFBA on soil respiration, denitrification, nitrification, and N mineralization in a forest soil under laboratory conditions. Our objective was to determine if any of these tracers had significant effects on microbial activity that should be considered when choosing tracers for studies of the fate and transport of pollutants hi soils.