Invasive alien plants such as Reynoutria spp. can drastically affect the composition of plant communities. Yet, whether and how these species also affect soil physicochemical properties and microbial functioning is still an unresolved question in the literature. Using a space-for-time substitution approach comparing invaded to uninvaded adjacent plots, we estimated the impacts of Reynoutria on soil biochemistry across nine contrasted sites in France by measuring soil carbon content, nutrient availability and enzyme activities. Soil under Reynoutria displayed higher carbon, nitrogen and phosphorus contents but no differences were detected regarding enzyme activities between invaded and uninvaded sites. Moreover, the magnitude of Reynoutria's effects differed depending on local conditions, with greater effects when total carbon and phosphorus-related enzymes were relatively low. These data highlight that changes in soil nutrient availability might be primarily due to direct effects of Reynoutria on soil properties and microbial functioning. Higher impacts were observed in soils with low-nutrient status, suggesting a 'niche construction ability' of Reynoutria. Our results underscore the necessity of considering the context-dependency of Reynoutria on soil biochemistry and highlight that the impact of alien species belowground functioning depends on initial soil conditions.