Kinetic Characteristics of Ammonium-Oxidizer Communities in California Oak Woodland-Annual Grassland

We must have information on the kinetic characteristics of soil biological processes if we are accurately to predict how environmental change 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 occurs in NH4+-oxidizer communities within a single ecosystem. Therefore, we evaluated how NH4+-oxidizer communities from an oak woodland-annual grass ecosystem in north-eastern California respond to changes in temperature, osmotic potential and substrate concentrations. We used nitrification potential assays to determine the effects of temperature and osmotic potential on rates of NH4+ oxidation in two soil depths, under two types of vegetative cover and during two seasons. We determined the kinetics of substrate utilization using soil slurries and enrichment cultures. In slurries, NH4+ oxidation rates were measured using 15N-isotope dilution to avoid confounding the effects of NO3− consumption with NO3− production. Ammonium-oxidizer communities beneath the canopies of oaks had lower temperature optima, greater activities and greater seasonal fluctuations in activity than communities in open grassy interspaces. Temperature optima of communities beneath oak canopies (31.8°C) and in open grassy interspaces (35.9°C) were as different as those reported for communities from tropical and temperate climatic zones. Ammonium-oxidizer communities from beneath oak canopies and open interspaces showed no difference in tolerance of low osmotic potential. The effect of substrate concentration on NH4+ oxidation rates in slurries were best described by the Michaelis-Menten equation. Rates in liquid cultures were best described by the Haldane equation because of substrate inhibition. The half-saturation constant (Km) for NH4+ oxidation in these soils averaged 15 μm NH4+ (=0.012 μm NH3), which is substantially lower than values reported in the literature for agricultural soils, sediments and sewage sludge. Enrichment cultures were inhibited by lower substrate concentrations (1600 μm NH4+ or 1.3 μm NH3) than reported for isolates from sewage systems. These results suggest that NH4+-oxidizer communities in uncultivated soils are more oligotrophic in nature, and thus kinetic parameters reported in the literature for agricultural soils, sediments and sewage sludge are not appropriate for describing NH4+ oxidation rates in these soils