In contrast to nirS, the DGGE analysis of nirK and
nosZ genes worked satisfactorily. The analyses were
reproducible. Also, there was no variation in the pattern
between duplicate DNA extracts run with separate PCR
amplification of nirK from samples of the same origin
(Fig. 3). The DGGE banding pattern of nirK and nosZ
genes was more composite in the soil samples than in the
activated sludge and peat samples, indicating a higher
number of denitrifying populations in soil (Fig. 2b and
c). At least 25 visible bands were detected for each soil
sample with equal intensity, although there also appeared
to be some dominant nirK and nosZ populations
within the denitrifying communities. The complexity of
the soil DGGE analysis resembled what has been shown
for 16S rDNA in soil [34]. The activated sludge samples
and the peat were composed of a few dominant populations
according to the DGGE banding pattern.