This dataset supports a study investigating the biogeochemical traits of the invasive plant Fallopia japonica compared to the native species Urtica dioica. Using stable isotope tracers (¹⁵N), we examined nitrogen accumulation patterns, biomass production, and C:N ratios under different nitrogen treatments (nitrate vs ammonium). The control data presented here reflect untreated soil and plant values, providing a baseline for interpreting tracer uptake and nutrient allocation. These data are integral to understanding species-specific nutrient use strategies in floodplain ecosystems.