Incubation experiments using soils from mixed oak ( Quercus sp.) stands showed accumulation of nitrate but standard most probable number (MPN) assays showed no NH 4 ‐oxidizing bacteria were present. This led to a preliminary conclusion that nitrification in these soils must be heterotrophic. However, further incubation studies indicated that NH 4 ‐N concentrations in standard MPN medium were too high for autotrophic NH 4 oxidizers that are adapted to soils with low NH 4 ‐N levels and exhibit low activities. Therefore, media with lower NH 4 ‐N concentrations were tested against the standard MPN medium. Each medium was tested using both 1 m M 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 NH 4 ‐oxidizing bacteria present in soils with low NH 4 ‐N concentrations.
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Donaldson et al. (1989) studied this question.