Background and aims <p>Alien plants can strongly alter soil properties in invaded ecosystems, but it remains unclear how much these effects stem from rhizosphere processes versus litter inputs. We aimed to disentangle the relative contributions of these two pathways in the case of Japanese knotweed (<i>Reynoutria japonica</i>).</p> Methods <p>We conducted a two-year pot experiment with four treatments (litter only, plant only, litter + plant, control) and three sampling times (autumn of year 1; spring and autumn of year 2). Soil samples were analyzed for 37 physicochemical and microbial variables, including pH, moisture, nutrient availability, phenolic compounds, and microbial biomass.</p> Results <p>Japanese knotweed significantly modified soil properties via both litter and rhizosphere pathways, each influencing distinct sets of variables. Litter addition mainly affected physicochemical parameters, especially soil moisture and exchangeable K and Mg. Plant growth predominantly altered phenolic content and microbial attributes, with a strong effect on bacterial biomass. When combined, the two pathways generated antagonistic (e.g., moisture), additive (most parameters), and synergistic (e.g., bacterial biomass) outcomes, suggesting a complex plant-soil feedback system. Effects intensified over time, and several variables exhibited seasonal fluctuations, highlighting the temporal dynamics of soil responses to Japanese knotweed.</p> Conclusion <p>Both litter and rhizosphere pathways of Japanese knotweed are crucial for its ecosystem-transforming capacity. They differ in nature and timing, underscoring the importance of multi-seasonal approaches to fully capture soil change mediated by invasive plants.</p>