. Estimation of microbial populations
Aerobic heterotrophic bacteria were enumerated by plating soil dilutions to
agar media in petridishes. The agar medium used contained dilute (1:100 full
strength) trypticase soy broth (Difco), ten per cent soil extract (prepared as
described by Zuberer (1994)), and 1.5 per cent agar (difco). Plates were incubated
at 2872 1C for 72 h prior to enumeration. Ammonium oxidizers and dentrifiers
were enumerated using the MPN techniques of Schmidt and Belser (1994) and
Tiedje (1994), respectively.