The bacterial communities in floodwater, from a rice-planted and an unplanted field were characterized at
the beginning (flooding stage) and at the end (harvest stage) of the rice cropping cycle. Most probable
number estimations and plate counts of aerobic and anaerobic heterotrophic bacteria and of several
metabolic bacterial groups (methanogens, sulfate-reducers, anaerobic sulfur and nonsulfur phototrophs,
denitrifiers and ammonifiers) were similar in rice and unplanted floodwater at both sampling times. The
analysis of denitrifiers and methanogens by fluorescent in situ hybridization revealed a shift in the
phylogenetic affiliation only of the former group in the rice-planted floodwater. Terminal restriction
fragment length polymorphism of 16S rRNA gene amplicons indicated that the bacterial communities of
the rice-planted and unplanted soils were consistently diverse and strongly influenced by the season.
