This study assessed the effects of Reynoutria japonica , Rudbeckia laciniata , and Solidago gigantea invading sites within and outside river valleys on activity, biomass, and composition of soil microbial communities. Microbial properties such as soil respiration, urease and arylsulfatase activities, microbial biomass (based on substrate-induced respiration, or SIR, and phospholipid fatty acids, or PLFA), and community composition (based on PLFA) were determined. R. japonica encroached on sites characterized by the lowest values of microbiological properties and R. laciniata on sites with the highest microbiological quality. The effect of invasion on soil microbial properties depended on the invasive plant species. R. japonica significantly decreased microbial biomass, determined by both SIR and total PLFA, urease activity, fungal PLFA, fungal:bacterial PLFA ratio, gram-negative bacterial PLFA, and soil respiration in comparison to soil under adjacent native plant communities. Microbial community composition also differed between soils under R. japonica and those under native plants. In contrast, R. laciniata and S. gigantea did not influence most microbial properties, though S. gigantea significantly increased fungal PLFA and R. laciniata and S. gigantea increased fungal:bacterial PLFA ratio. The effects of plant invasion on microbial properties were basically similar in soils located within and outside river valleys, probably because initially (i.e., before invasion) soils from the two locations were largely similar in terms of basic properties such as texture, moisture, pH, C:N ratio, and most microbial properties.