JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology. Abstract.Topography controls snowpack accumulation and hence growing-season length, soil water availability, and the distribution of plant communities in the Colorado Front Range alpine. Nutrient cycles in such an environment are likely to be regulated by interactions between topographically determined climate and plant species composition. We investigated variation in plant and soil components of internal N cycling across topographic gradients of dry, moist, and wet alpine tundra meadows at Niwot Ridge, Colorado. We expected that plant production and N cycling would increase from. dry to wet alpine tundra meadows, but we hypothesized that variation in N turnover would span a proportionately greater range than productivity, because of feedbacks between plants and soil microbial processes that determine N availability. Plant production of foliage and roots increased over topographic sequences from 280 g.m-2.yr--l in dry meadows to 600 gm-2 yr-I in wet meadows and was significantly correlated to soil moisture. Contrary to our expectation, plant N uptake for production increased to a lesser degree, from 3.9 g N-m-2 yr-' in dry meadows to 6.8 g N.m-2 yr-' in wet meadows. In all communities, the belowground component accounted for the majority of biomass, among communities. These results indicate that the topographic soil moisture gradient is in fundamental control of the patterns of N turnover among communities and that differences in plant species do not appear to be as important.

The alpine tundra of the Colorado Front Range supports a variety of plant communities whose distribution corresponds to their topographic position. Our objectives were to: (i) determine patterns in net N mineralization and microbial N pools among three communities, Kobresia, Acomastylis, and Carex meadows, that span a topographic gradient, and (ii) relate any patterns to soil microclimate differences among these communities. Average yearly net N mineralization rates, measured with an in situ core incubation technique, were 1.2 g N m−2 in 1991 and 1.0 g N m−2 in 1992. No differences were detected in yearly N mineralization rates among the three communities; however, net nitrification and other soil properties were found to differ among communities. Net N mineralization rates and microbial N showed strong temporal variation, and this variation was related to different variables for each community. Seasonal variation in N mineralization was related to soil water and microbial N in Kobresia meadows, to soil temperature and microbial N in Acomastylis meadows, and to soil water and temperature in Carex meadows. Seasonal changes in microbial N were related to soil water in Kobresia and Acomastylis meadows. Large fluctuations in microbial N indicate that periodic losses from the microbial pool may be important to N availability in this alpine tundra site.

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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.

Pseudomonas acidovorans and Pseudomonas sp. strain ANL but not Salmonella typhimurium grew in an inorganic salts solution. The growth of P. acidovorans in this solution was not enhanced by the addition of 2.0 jig of phenol per liter, but the phenol was mineralized. Mineralization of 2.0 jig of phenol per liter by P. acidovorans was delayed 16 h by 70 ,ug of acetate per liter, and the delay was lengthened by increasing acetate concentrations, whereas phenol and acetate were utilized simultaneously at concentrations of 2.0 and 13 jig/liter, respectively. Growth of Pseudomonas sp. in the inorganic salts solution was not affected by the addition of 3.0 jig each of glucose and aniline per liter, nor was mineralization of the two compounds detected during the initial period of growth. However, mineralization of both substrates by this organism occurred simultaneously during the latter phases of growth and after growth had ended at the expense of the uncharacterized dissolved organic compounds in the salts solution. In contrast, when Pseudomonas sp. was grown in the salts solution supplemented with 300 jig each of glucose and aniline, the sugar was mineralized first, and aniline was mineralized only after much of the glucose carbon was converted to CO2. S. typhimurium failed to multiply in the salts solution with 1.0 jig of glucose per liter. It grew slightly but mineralized little of the sugar at 5.0 jig/liter, but its population density rose at 10 jig of glucose per liter or higher. The hexose could be mineralized at 0.5 jig/liter, however, if the solution contained 5.0 mg of arabinose per liter. In solutions with this arabinose concentration and glucose levels too low to support growth, the percentage of glucose carbon incorporated into S. typhimurium cells was the same as when the bacterium was grown in solutions with high concentrations of glucose alone. When glucose was the only carbon source for S. typhimurium, the percentage of the glucose carbon assimilated and mineralized progressively declined as the sugar concentration was reduced to levels approaching the threshold for growth. These results indicate that second substrates and uncharacterized dissolved organic carbon may play an important role in controlling the rate and extent of biodegradation of organic compounds at low concentrations. Microorganisms can degrade a variety of synthetic organic compounds present in samples from natural environments at concentrations below 10 puglliter (1, 16). However, it is not certain whether microorganisms can grow at such low concentrations. It has been postulated that substrate uptake will just meet maintenance energy requirements and growth is not possible at sufficiently low substrate concentrations (12). * Corresponding author. to use the uncharacterized DOC were used. In addition, to understand the effect of DOC on the metabolism of synthetic organic substrates, studies were also conducted on the effects of second substrates on the biodegradation of low concentrations of organic compounds. MATERIALS AND METHODS Rad...