Use of nitrifier activity measurements to estimate the efficiency of viable nitrifier counts in soils and sediments.

A procedure for estimating the efficiency of the most-probable number (MPN) technique for counting ammonium-oxidizing bacteria was tested on sediments and soils collected from Delaware Inlet, Nelson, New Zealand. The procedure involved estimating the nitrifier populations required to produce observed activities and comparing these estimates with the MPN-countable populations. MPN counts ranged between 0.15 x 10 to 3.0 x 10 cells g in sediments and between 4.4 x 10 to 19 x 10 cells g in soils. These counts were only 0.1 to 5.0% of the estimated populations that would be required to produce the observed activity. Similar efficiency calculations were made for data already in the literature, and these calculations gave much higher percentages. Thus, we concluded that for the soils and sediments we studied, the MPN counting technique greatly underestimated the populations present and that the efficiency calculation could be used as a counting efficiency index.