EFFECTS OF SOIL OSMOTIC POTENTIAL ON NITRIFICATION, AMMONIFICATION, N-ASSIMILATION, AND NITROUS-OXIDE PRODUCTION

Previous studies have examined the effects of soil osmotic potential ( Psi(s)) on net rates of mineralization and nitrification. Because net rates represent the difference between gross production and consumptio n processes, it is unclear which process is being affected. We used an N-15 isotopic dilution method to evaluate the effects of Psi(s) on gr oss rates of nitrification, ammonification, NH4+ assimilation, and NO3 - assimilation, and net rates of nitrous oxide production in a Penoyer sandy loam at field capacity. To avoid creating specific ion toxiciti es that normally do not occur in this soil, we used a chemical equilib rium model to predict how solute concentrations in the soil solution c hange during evapo-concentration; then we used solutions containing th ese mixtures of solutes to create individual Psi(s) treatments. A nitr ification potential assay was also performed to determine the effect o f Psi(s) on nitrification rates at high substrate concentrations. In s oil slurries with elevated NH4+ concentration (1110 mu M), nitrificati on rates declined exponentially with reduced Psi(s) (increased salt co ncentration); however, in soil samples incubated at field capacity wit hout added NH4+ (9.7 mu M, or 2 mg N kg(-1)), the gross nitrification rate was independent of Psi(s). The differential response between slur ries and soil at field capacity was attributed to differences in NH4concentrations, and indicated that the effects of Psi(s) were secondar y to NH4+ concentrations in controlling nitrification rates. Nitrifica tion rates in slurries declined more when a single salt (K2SO4) was us ed than when the mixture of salts that more closely approximated the s olute composition predicted to occur in the field was used to lower Ps i(s). This suggests that nitrifying bacteria are capable of adapting t o specific ion toxicities. Gross rates of ammonification declined expo nentially with decreased Psi(s) between 0 and -500 kPa but were indepe ndent of Psi(s) at potentials of -500 to -1750 kPa. Rates of microbial assimilation of NO3- exceeded NH4+ assimilation by a factor of 4, ind icating that under NH4+ limited conditions substantial NO3- assimilati on can occur. Microbial assimilation of both NH4+ and NO3- declined ex ponentially with decreased Psi(s), and were insignificant at <-1500 kP a Psi(s). Because NO3- assimilation declined more rapidly than gross n itrification, net nitrification rates actually increased with declinin g Psi(s). Rates of nitrous oxide (N2O) production were also inversely correlated with Psi(s). Our results indicate that in previous studies, measurement of net rates, use of inappropriate salts, and addition of substrate may have resulted in overestimation of the adverse effects of low Psi(s) on rates of N-transformations.