A method was developed to determine the ammonium oxidation rate (potential) of unenriched natural samples by measuring the nitrite produced in shaken slurries. Addition of chlorate to the samples prevented nitrite from being oxidized to nitrate. The effectiveness and specificity of chlorate were tested with pure cultures of nitrite and ammonium oxidizers, as well as in soil and sediment slurries. It was concluded that chlorate had relatively little inhibitory effect on ammonium oxidation. However, under some conditions chlorate was not completely effective in blocking nitrite oxidation, and the causes of this were investigated. The technique was designed to check for incomplete blockage.
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 103 to 3.0 x 103 cells g-' in sediments and between 4.4 x 103 to 19 x 103 cells g-1 in soils. These counts were only 0.1 to 5.0% of the
Multiple genera of ammonia-oxidizing chemoautotrophic nitrifiers in a soil were detected, isolated, and studied by means of modified most-probable-number (MPN) techniques. The soil examined was a Waukegon silt loam treated with ammonium nitrate or sewage effluent. The genera Nitrosomonas and Nitrosospira were found to occur more commonly than the genus Nitrosolobus. Three different MPN media gave approximately the same overall ammonia oxidizer counts within statistical error after prolonged incubation but differed markedly in ratios of Nitrosomonas to Nitrosospira. Selectivity and counting efficiency of MPN media were studied by observing the growth response of representative pure cultures isolated from the soil. Selectivity was evident in each medium with respect to all strains tested, and the media differed greatly in incubation times required to obtain maximum counts.
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