Abstract: Soil samples were collected from five regions and 26 soil samples from the Xifeng Profile of the Loess Plateau.Total DNA was extracted from those samples with the aid of bacterial universal primer and amplified by means of PCR and its product analyzed with the DGGE technique to explore diversity of bacterial communities.Oriented towards the features of the microbes in the soil of the plateau,conditions of the DGGE experiment were optimized with an optimal denaturant gradient ranging between 40% and 70%,and an optimal electrophoresis duration being 17 hours(100 V).Analysis of DGGE profiles of bacteria in soils at various depths revealed that there are two different types of bacterial community structures in soils at various depths in the Loess Plateau, which is presumably attributed to changes of the warm-humid environment caused by monsoon climatic conditions,and ice age.In the age when monsoon got stronger in summers and became weak in winters,the climate was warm and humid,and weathering and soil formation were intensive,which showed multiple and dense bands in the DGGE profile,whereas conversely,weathering and soil formation are weak,which showed sparse bands in the profile.The determination of Shannon-Weiner indexes of soil bacteria in the five regions of the plateau indicated that the Shannon-Weiner indexes were influenced not only by the amount of bands,but also by brightness and moving rates of the bands

Key words: Loess Plateau, bacterial diversity, community structure, PCR-DGGE