Date of Award:

5-1996

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Plants, Soils, and Climate

Department name when degree awarded

Plants, Soils and Biometeorology

Committee Chair(s)

Lynn M. Dudley

Committee

Lynn M. Dudley

Committee

John Stark

Committee

Dani Or

Abstract

An isotopic dilution method was used to test the effects of osmotic potential, (𝛹s), 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 NH4+ concentrations of the soil. The differential response was attributed to the variable NH4+ substrate quantities. The effects of osmotic potential were secondary to NH4+ 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 -500 kPa. Ammonification rates were independent of osmotic potential at potentials between -500 and -1800 kPa. Immobilization rates of both NH4+ and NO3- declined exponentially as osmotic potential decreased. Absolute rates of NO3- immobilization exceeded those of NH4+ by a factor of 4, indicating that under NH4+ limited conditions, substantial NO3- assimilation occurred. However, the generalization that NH4+ is the preferred N source was in fact supported by the data, where immobilization rates relative to the respective pool sizes clearly favored NH4+ assimilation. Nitrous oxide production rates increased linearly as osmotic potential decreased. An NH4+ dependence indicated the evolved N2O was derived from nitrification rather than denitrification.

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