KINETIC CHARACTERISTICS OF AMMONIUM-OXIDIZER COMMUNITIES IN A CALIFORNIA OAK WOODLAND-ANNUAL GRASSLAND

We must have information on the kinetic characteristics of soil biolog ical processes if we are accurately to predict how environmental chang e will affect soil nutrient cycles. Little information is available on the characteristics of NH4+-oxidizing bacteria in uncultivated soils. In addition, little is known about how much physiological diversity o ccurs in NH4+-oxidizer communities within a single ecosystem. Therefor e, we evaluated how NH4+-oxidizer communities from an oak woodland-ann ual grass ecosystem in north-eastern California respond to changes in temperature, osmotic potential and substrate concentrations. We used n itrification potential assays to determine the effects of temperature and osmotic potential on rates of NH4+ oxidation in two soil depths, u nder two types of vegetative cover and during two seasons. We determin ed the kinetics of substrate utilization using soil slurries and enric hment cultures. In slurries, NH4+ oxidation rates were measured using N-15-isotope dilution to avoid confounding the effects of NO3- consump tion with NO3- production. Ammonium-oxidizer communities beneath the c anopies of oaks had lower temperature optima, greater activities and g reater seasonal fluctuations in activity than communities in open gras sy interspaces. Temperature optima of communities beneath oak canopies (31.8 degrees C) and in open grassy interspaces (35.9 degrees C) were as different as those reported for communities from tropical and temp erate climatic zones. Ammonium-oxidizer communities from beneath oak c anopies and open interspaces showed no difference in tolerance of low osmotic potential. The effect of substrate concentration on NH4+ oxida tion rates in slurries were best described by the Michaelis-Menten equ ation. Rates in liquid cultures were best described by the Haldane equ ation because of substrate inhibition. The half-saturation constant (K -m) for NH4+ oxidation in these soils averaged 15 mu M NH4+ (=0.012 mu M NH3), which is substantially lower than values reported in the lite rature for agricultural soils, sediments and sewage sludge. Enrichment cultures were inhibited by lower substrate concentrations (1600 mu M NH4+ or 1.3 mu M NH3) than reported for isolates from sewage systems. These results suggest that NH4+ oxidizer communities in uncultivated s oils are more oligptrophic in nature, and thus kinetic parameters repo rted in the literature for agricultural soils, sediments and sewage sl udge are not appropriate for describing NH4+ oxidation rates in these soils. Copyright (C) 1996 Elsevier Science Ltd