Abstract:In order to understand the effect of continuous planting of Eucalyptus grandis on biomass and number of soil microbes, the relationships between soil microbial biomass, number of major microbial groups with soil physicochemical properties in Pinus massoniana forest and E. grandis plantations at different rotations were studied by using space-time interchange method. The results showed that the soil microbial biomass carbon, biomass nitrogen and number of bacteria and fungi in the first rotation plantations of E. grandis had not significant difference from those in P. massoniana forest, but the number of actinomycetes significantly increased. However, with the increasing rotation of Eucalyptus grandis continuous plantations, the soil microbial biomass carbon, biomass nitrogen and the number of bacteria, actinomycetes and fungi decreased. Stepwise regression analysis showed that soil total phosphorus could independently explain 71.7%, 86.1% and 63.0% variation of the microbial biomass nitrogen, the number of bacteria and fungi, respectively. Soil total phosphorus together with total porosity could explain 87.9% variation of microbial biomass carbon, and 89.6% variation of the number of actinomycetes together with total nitrogen. So, it could be concluded that soil total phosphorus well explained the variation of soil microbial characteristics in E. grandis plantations.