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An Improved Method for the Enumeration of Denitrifying Bacteria¹

ABSTRACT

An improved method for enumerating denitrifying bacteria from soil by use of the most-probable-number method is outlined. Difco nitrate broth was used, and culture tubes near the dilutions to extinction were analyzed for the disappearance of nitrate-nitrite. The distinction between denitrifying and nondenitrifying bacteria was clearly defined and thus minimized visual subjective errors. Earlier methods which assumed that a pH increase was due solely to denitrification were shown to be ambiguous by comparison between culture tubes for nitratenitrite disappearance; 1 out of every 4 tubes compared gave an erroneous reading for dentirification. The distribution of denitrifying bacteria within two different soil profiles showed a definite logarithmic decrease in numbers with depth.

¹ Contribution from the Dep. of Soil Sci. and Agr. Engineering, University of California, Riverside. 92502 This research was supported in part by RANN division of the National Science Foundation, Project GI-34733.

² Assistant Professor of Soil Microbiology and Staff Research Associate, respectively.

Received for publication February 2, 1973. Accepted for publication April 4, 1973.