Abstract:Significant seasonal soil freezing and thawing could influence greatly on soil bacterial diversity in the alpine/subalpine forest. To understand the characteristics and dynamics of soil bacterial diversity in the alpine and subalpine forests of western Sichuan during one onset of soil freezing, a field experiment was conducted to investigate the soil bacterial diversity by PCR-DGGE in the primary fir (Abies faxoniana) forest, fir and birch (Betula albosinensis) mixed forest and secondary fir (A. faxoniana) forest, which were three representative forests in the subalpine and alpine forest region in western Sichuan. Soil samples were collected on Nov.5 and on Nov.25, 2008, which was defined as the onset of freezing based on the simultaneous monitoring of soil temperature. Relative rich soil bacterial groups were still observed in the three sampled forests after soil freezing. The ranked order of soil bacterial groups was MF> SF> PF before soil freezing, while the ranked order was PF> MF> SF after soil freezing. Soil freezing decreased soil bacterial group and diversity, but increased soil bacterial dominance. However, the impact degree of soil freezing declined with the increase of soil depth and elevation. The results suggested that seasonal freezing had obvious effects on soil bacterial diversity in these subalpine/ alpine forests. Meanwhile, the results also implied that the dynamics of soil bacterial community during the onset of freezing was of ecological significance to understand the wintertime ecological process in soils.