Dynamics of a bacterial population in winter wheat rhizosperes during a soil freezing and thawing period, and those in soil incubated under a freeze-thaw cycle imitating soil thawing in April were investigated to clarify the effects of soil freezing and thawing on microbes in soil. The bacterial population was divided into two subgroups using colony forming curve analysis: fast and slow-growing bacteria, respectively. Bacterial population decreased significantly during the soil freezing period and then increased in a process of soil thawing. Fast-growing bacteria responded to the soil thawing more rapidly than slow-growing bacteria. Incubation experiments indicated that a cycle treatment at 20°C and -10°C significantly stimulated the growth of fast-growing bacteria in rhizosperes and caused a relative increase in psychrophilic bacteria. These findings suggested that soil thawing stimulated fast-growing bacteria, especially psychrophilic bacteria, which resulted in a sudden increase in the bacterial population in wheat rhizosperes during a soil thawing period.

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