•Invasive plant species affected soil microorganisms differently.•Contrary to expectations, Reynoutria japonica did not inhibit soil microbes.•Microbial (mainly fungal) biomass under Solidago gigantea was lower than expected.•Effect of invasion on most microbial parameters depended on soil type. Plant invasions can considerably modify soil microbial communities and processes. The direction and magnitude of these changes depend on invasive plant species. This problem has been studied in the field using a space-for-time substitution approach, but rarely in experimental conditions. Therefore, the aim of this study was to compare the effects on soil microbial properties in a mesocosm experiment of four invasive plant species in Europe, namely, Impatiens glandulifera, Reynoutria japonica, Rudbeckia laciniata and Solidago gigantea, and two expansive species native to Europe but invasive in North America, namely Artemisia vulgaris, Phalaris arundinacea, as well as a mixture of these two species grown in pots in two loamy sand soils differing in elemental concentrations and pH. After two years, microbial activity was measured using soil respiration, urease and arylsulfatase activity; total, bacterial and fungal biomass was measured using substrate-induced respiration (SIR) and/or phospholipid fatty acid analysis (PLFA); and community structure was measured using PLFA. Soil microbial parameters were significantly affected by plant species, soil type and their interaction. Contrary to expectations, R. japonica did not exhibit a strong negative effect on soil microorganisms. Microbial activity and/or biomass in soils under R. japonica, A. vulgaris, R. laciniata and/or species mixture were relatively high, though it depended to some extent on microbial parameters and soil type. In contrast, most microbial parameters had rather low values in bare (unplanted) soil and soils under P. arundinacea and S. gigantea. The largest differences between species were observed in the case of fungal biomass and fungi:bacteria ratio in both soils. Fungal biomass under A. vulgaris and R. japonica was a few times higher than in soils under S. gigantea, P. arundinacea and/or I. glandulifera. Our experiment proved that invasive plant species can modify soil microbial properties considerably during a period as short as two growing seasons. Alterations in the soil environment can potentially be considered as niche construction and support the growth and spread of invaders.