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Components of Rates of Ammonium Oxidation in Soil¹

ABSTRACT

Diversity in the growth and the activity of ammonia (NH₃) oxidizers in a population of 132 soil microaggregates was documented. Each microaggregate was inoculated in a poorly buffered liquid medium containing bromthymol blue and selective for NH₃ oxidizers. The kinetics of ammonium oxidation was measured by monitoring the changes in percent transmittance of the medium concomitant to its acidification. Mean specific rates of NO^–₂ production were low: 0.321 d^–1, 0.403 d^–1, 0.246 d^–1, and 0.069 d^–1 at pH 7.50, 7.00, 6.00, and 5.50, respectively. At pH 7.5 only a few microaggregates harbored NH₃ oxidizers with generation times similar to those reported for pure cultures in liquid media. Most microaggregates were still active at pH 5.9 but became inactive at pH 5.1. The lag phase before detection of activity correlated to the specific rate of ammonium (NH⁺₄) oxidation. A statistical theory of nitrification in soil was formulated and tested. Nitrification in soil was assumed to proceed from pulses of ammonium oxidation generated by microbial clusters. The kinetics of nitrate accumulation in soil coincided with the cumulative distribution of the microaggregates' lag phase. On the average, each active microaggregate (0.036 mg) contained 2.01 clusters of NH₃ oxidizers, and had generated 0.0088 g NH⁺-N when their activity was stopped by self-induced acid toxicity.

¹ Contribution from the Dept. of Soil Science, Univ. of Minnesota, St. Paul, MN 55108. Minnesota Agric. Exp. Stn. Paper no. 14,001.

² Professor of Soil Microbiology, Univ. of Minnesota, St. Paul.

Received for publication June 14, 1984. Accepted for publication October 20, 1984.

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