An oxic-anoxic interface develops in the upper 2-mm layer of submerged paddy soil within six hours upon flooding. Changes in the bacterial community composition were assessed over time. Molecular fingerprinting techniques showed that both spatial distribution and temporal dynamics of bacterial populations were resolved more clearly at the RNA level than at the DNA level. At the RNA level, changes were most pronounced in both oxic and anoxic zones from 0 to 2 days after flooding, and less pronounced from 2 to 21 days after flooding. No spatio-temporal effects were observed during the later stages of incubation. We presume that the decrease of successional dynamics corresponded to a bacterial community shift from r- to K-strategy. To further examine our presumption, the culturability of bacteria from the early and late successional stages was investigated for the oxic zone using plate counting experiments. Most representatives of the early successional stage were identified as members of the Betaproteobacteria and the genus Bacillus. In contrast, isolates characteristic of late succession belonged, among others (e.g. Alphaproteobacteria, Actinobacteria), to bacterial lines of descent for which only a very few pure culture representatives have been reported so far, such as Verrucomicrobia and Acidobacteria.
