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A solution containing 100 µg ml⁻¹ of NO₃⁻-N, 200 µg ml⁻¹ of glucose-C, and 0.01N CaSO₄ was supplied continuously to soil in acrylic plastic columns (6-cm inside diam × 10-cm length) for up to 96 hours while N₂ gas was passed through the system. Concentrations of NO₃⁻-N in the effluent decreased to 20 µg ml⁻¹after 60 hours. Simultaneously, NO₂⁻-N concentrations increased from traces to 65 µg ml⁻¹ and more than 70% of applied C had been utilized after 60 hours. These results indicate that most C oxidation was coupled with NO₃⁻ reduction although nearly all applied NO₃⁻ was ultimately denitrified after 96 hours. Denitrifiers estimated by a most probable number (MPN) technique increased from 1.1 × 10³g⁻¹ oven-dry soil to maximum populations of 1.9 × 10⁷ g⁻¹ during 96 hours of infiltration. From measured biomass increases and soluble N concentrations as functions of soil depth at “quasi” steady states during the course of the experiment, specific denitrification rates per microbe, rd, were determined. These values ranged from 1.3 × 10⁻⁵ to 6.8 × 10⁻⁷ (µg N ml⁻¹) hour⁻¹ per denitrifier for 18 to 96 hours respectively; the decrease may have been due to the activities of different denitrifier species. A heat shock treatment of soil which had been sampled before, during, and after the experiment and was then suspended in sterile distilled water indicated that most denitrifiers were present as vegetative cells, especially after 18 hours. In addition, after 96 hours a gram negative rod unclassified CDC VA-1, very closely related to Pseudomonas cepacia, capable of growth on yeast extract agar and in vitro denitrification was the sole denitrifier species isolated at dilutions near extinction using a MPN technique. These data indicate that, irrespective of the physiological diversity of denitrifier species present prior to anaerobiosis, a single bacterial type may become dominant as a result of a given set of environmental conditions. |
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