Abstract: In order to further understand the mechanisms of forest succession, the composition of soil microbes under forests at different succession stages in the Malan forest region of Loess Plateau and the effects of leaf litter from dominant tree species on the composition of soil microbial communities were investigated. The extracts of leaf litter were imposed to different types of soil samples. The number of soil microbes was determined by using solid medium added with the extracts of soil, and cluster and principal component analyses were applied to process the obtained data. The results showed that the total number of soil microbes and the number of bacteria increased in the order of Betula platyphylla forest→Pinus tabulaeformis forest→Quercus liaotungensis forest. From the pioneer community to the climax community, Shannon-Wiener index, a comprehensive indicator used to describe the microbial composition, showed a downward trend. Of three soil microbial groups, the group with a smaller number played a more important role in the changes of Shannon-Wiener index. According to the composition of culturable soil microbes, the test eight soil samples were divided into three groups. Compared with that from P. tabulaeformis, the leaf litter from Q. liaotungensis played a greater role on the composition of soil microbial communities. It was suggested that the composition of soil microbes in Malan forest region was closely related to the forest vegetation type and its succession stage. Through its specific dominance, the leaf litter from the dominant tree species at advanced succession stage could change the soil microbial composition gradually, and thereby, promote the occurrence of vegetation succession.

Key words: Forest-grassland boundary, Soil moisture content, Geostatistics, Spatial distribution