The rates of mineralization of ['4C]benzoate by an induced population of Pseudomonas sp. were measured at initial substrate concentrations ranging from 10 ng/ml to 100 jig/ml. Plots of the radioactivity remaining in the culture were fit by nonlinear regression to six kinetic models derived from the Monod equation. These models incorporate only the variables of substrate concentration and cell density. Plots of the mineralization kinetics in cultures containing low, intermediate, and high initial substrate concentrations were well fit by first-order, integrated Monod, and logarithmic kinetics, respectively. Parameters such as maximum specific growth rate, half-saturation constant, and initial population density divided by yield agreed between cultures to within a factor of 3.4. Benzoate mineralization by microorganisms in acclimated sewage was shown to fit logistic (sigmoidal), Monod, and logarithmic kinetics when the compound was added at initial concentrations of 0.1, 1.0, and 10 jig/ml, respectively. The mineralization of 10 ,ug of benzoate per ml in sewage also followed logarithmic kinetics in the absence of protozoa. It is concluded that much of the diversity in shapes of mineralization curves is a result of the interactions of substrate concentration and population density. Nonlinear regression with models incorporating these variables is a valuable means for analysis of microbial mineralization kinetics.

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We developed 12 models of kinetics to describe the metabolism of organic substrates that are not supporting bacterial growth. These models can be used to describe the biodegradation of organic compounds that are not supporting growth when the responsible populations are growing logistically, logarithmically, or linearly or are not increasing in numbers. Nonlinear regression analysis was used to fit patterns of mineralization by two bacteria to these kinetic models. Pseudomonas acidovorans mineralized 1 ng of phenol per ml while growing exponentially at the expense of uncharacterized organic carbon in a synthetic medium. Phenol at a concentration of 1 ng/ml did not affect the growth of P. acidovorans. These data were best fit by the model that incorporates the equation for logarithmic growth and assumes a concentration of test substrate well below its Km value. In the absence of a second substrate, glucose at concentrations below those supporting growth was mineralized by Salmonella typhimurium in a manner best described by pseudo first-order kinetics. In the presence of different concentrations of arabinose, however, the kinetics of glucose mineralization by S. typhimurium reflected linear, logistic, or logarithmic growth of the population on arabinose. We conclude that the kinetics of mineralization of organic compounds at concentrations too low to support growth are best described either by the first-order model or by models that incorporate expressions for the kinetics of growth of the metabolizing population on other substrates. When growth is at the expense of other substrates, the kinetics observed reflect such growth, as well as the concentration of the substrate of interest. The models also may be useful for analysis of the kinetics of cometabolism.

The spatial variation in field denitrification rates was studied in an acid soil where the sole denitrification end product was N2O. The rate of N2O accumulation was measured under a soil cover at 30 fixed sampling locations arranged in a grid. Water and NO3 were added prior to all measurements to provide soil water contents at or above field capacity and excess NO3 for denitrification. No significant differences in N2O production rates were observed in samples incubated with or without C2H2, indicting that nitrification is not a significant source of N2O in this soil. Denitrification was found to occur in “hot spots” in moist soil and throughout the soil volume if the soil was flooded. Denitrification rates followed a skewed frequency distribution when soil water content was at field capacity: most locations had low activity, and a few locations exhibited high activity. The frequency distribution of denitrification rates was less skewed in samples after flooding, following large additions of glucose, or when the vegetation underwent decay in the fall. The intensity of denitrification was much higher after flooding than following glucose addition. A few sample locations occasionally showed outstandingly high denitrification rates when at field capacity, as well as when flooded. The activity in these hot spots approximated the potential denitrification activity of that soil.