A Dilute Medium to Determine Population Size of Ammonium Oxidizers in Forest Soils
Donaldson, Jennifer M. | Henderson, Gray S.
Incubation experiments using soils from mixed oak (Quercus sp.) stands showed accumulation of nitrate but standard most probable number (MPN) assays showed no NH₄-oxidizing bacteria were present. This led to a preliminary conclusion that nitrification in these soils must be heterotrophic. However, further incubation studies indicated that NH₄-N concentrations in standard MPN medium were too high for autotrophic NH₄ oxidizers that are adapted to soils with low NH₄-N levels and exhibit low activities. Therefore, media with lower NH₄-N concentrations were tested against the standard MPN medium. Each medium was tested using both 1 mM phosphate buffer and deionized water as diluent to examine responses to small changes in buffer capacity. A 0.1 × medium used with deionized water diluent was found to best estimate populations of low-activity NH₄-oxidizing bacteria present in soils with low NH₄-N concentrations. Contribution of the Missouri Agric. Exp. Stn. Journal no. 10760.
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