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Asynchronous Activity of Ammonium Oxidizer Clusters in Soil¹

ABSTRACT

Ammonium oxidation by 468 large (0.49 mg) and 416 small (0.12 mg) soil aggregates from an Ochric Epipedon was determined by the Winogradsky silica gel-CaCO₃ plate technique. Activity was spread over 8 weeks. The histogram for the percentage of active soil aggregates presented a maximum after 3 to 4 weeks of incubation. Only 48 and 22% of the large and small aggregates oxidized ammonium, which corresponded to an average of 0.65 and 0.34 clusters of activity per large and small aggregates, respectively. On the average, each cluster in the large and small aggregates harbored 31 and 14 ammonium oxidizer cells, as computed from the MPN values for the ammonium oxidizer cells per g of soil. Enrichment of the soil by preincubation for 8 weeks after the addition of 100 ppm NH₄⁺ induced for the large and small aggregates a 1.5- and 1.4-fold increase in the number of clusters but a 4.6- and 4.8-fold increase in the number of ammonium oxidizer cells per cluster.

The results indicate that the kinetics of ammonium oxidation in soil, for the nonsteady state situation, represents the resultant average of pulses of activity from small isolated and asynchronous ammonium oxidizer clusters, in contrast with present models which assume one homogenous, synchronous population of ammonium oxidizer cells in soil.

¹ Contribution from the Department of Soil Science, University of Minnesota, St. Paul, MN 55108; Paper no. 10,238 of the U of M Agric. Exp. Stn.

² Associate Professor, Dept. of Soil Science, Univ. of Minnesota, St. Paul.

³ Professor, Laboratoire de Biometrie, Place Croix du Sud, 2, B-1348, Louvain-La-Neuve, Belgium.

⁴ Adjunct Professor, Laboratorie d'Ecologie Generale, Louvain-La-Neuve. Present address: Corn Product Co., Havenstraat; 81, Vilvoorde, Belgium.

Received for publication October 16, 1978. Accepted for publication April 27, 1979.