Display omitted •Invasive Reynoutria japonica had greater biomass than resident species.•R. japonica had higher C, C/N, C/P ratios and lower N, P in aboveground biomass.•Element pools were generally greater in R. japonica than in resident species.•Differences in senescent biomass did not affect topsoil physicochemical properties. Biological invasions are an important element of ongoing global change. Reynoutria japonica is one of the most invasive plant species in Europe and North America, potentially affecting recipient ecosystems. However, little is known about the relationships between its senescing biomass and element concentrations/pools in soil. The aim of this study was to compare the quantity and quality of aboveground and belowground plant biomass, and the quality of topsoil between plots with invasive R. japonica and resident plant communities. The study was conducted at 25 paired R. japonica-native plots in southern Poland. Harvested biomass was dried and weighted, and characterized in terms of C, Ca, K, Mg, N and P concentrations and pools, as well as C/N and C/P ratios. These parameters were also measured in soil along with texture, bulk soil density, moisture and pH. R. japonica produced greater amounts of aboveground and belowground biomass relative to resident species. The concentrations and pools of most elements in plant biomass were affected by invasion (R. japonica vs. resident), biomass type (aboveground vs. belowground) and/or their interactions. Senescing aboveground biomass of R. japonica was characterized by poor quality – lower N, P, K, greater C, Ca concentrations and greater C/N, C/P ratios compared to resident vegetation. Belowground R. japonica biomass had either greater or similar element concentrations (with the exception of P) or ratios. Element pools were greater in R. japonica than in resident species, with the exception of N and P in aboveground biomass. Despite these differences, topsoil generally did not differ in physicochemical properties when analyzed across all study sites; only total N and H2O-extractable K pools were lower under R. japonica than resident vegetation. Our data suggest that ecosystem response to invasion may be site-specific and depend on initial ecosystem properties.