The effects of osmotic potential on ammonification, immobilization, nitrous oxide production, and nitrification rates in Penoyer soil

An isotopic dilution method was used to test the effects of osmotic potential, {dollar}rm(Psisb{lcub}s{rcub}),{dollar} upon nitrification, ammonification, N-immobilization, and nitrous oxide production rates in soil at solute concentrations encountered in Penoyer soil. A nitrification potential assay was also performed to approximate maximum nitrification rates.; Nitrification potential rates in soil slurries exponentially declined in response to decreased osmotic potential. However, nitrification was independent of salt concentration at the ambient NH{dollar}sb4sp+{dollar} concentrations of the soil. The differential response was attributed to the variable NH{dollar}sb4sp+{dollar} substrate quantities. The effects of osmotic potential were secondary to NH{dollar}sb4sp+{dollar} substrate levels in controlling nitrification rates. Ammonification rates declined exponentially as a function of decreased osmotic potential; however, the 33% reductions in ammonification rates were restricted to a range of osmotic potentials between 0 and {dollar}-{dollar}500 kPa. Ammonification rates were independent of osmotic potential at potentials between {dollar}-{dollar}500 and {dollar}-{dollar}1800 kPa. Immobilization rates of both NH{dollar}sb4sp+{dollar} and NO{dollar}sb3sp-{dollar} declined exponentially as osmotic potential decreased. Absolute rates of NO{dollar}sb3sp-{dollar} immobilization exceeded those of NH{dollar}sb4sp+{dollar} by a factor of 4, indicating that under N{dollar}sb4sp+{dollar} limited conditions, substantial NO{dollar}sb3sp-{dollar} assimilation occurred. However, the generalization that NH{dollar}sb4sp+{dollar} is the preferred N source was in fact supported by the data, where immobilization rates relative to the respective pool sizes clearly favored NH{dollar}sb4sp+{dollar} assimilation. Nitrous oxide production rates increased linearly as osmotic potential decreased. An NH{dollar}sb4sp+{dollar} dependence indicated the evolved N{dollar}sb2{dollar}O was derived from nitrification rather than denitrification.