Stable isotopes as a tool towards a better understanding of the biogeochemical traits of invasive Fallopia japonica

Invasive plants pose a significant threat to ecosystems by disrupting the ecological balance, which includes the alteration of biogeochemical processes. Among the most aggressive invaders is Fallopia japonica, a species that thrives in riparian zones – critical interfaces between aquatic and terrestrial environments – where it significantly impacts biodiversity and ecosystem functions. Despite its success as an invader, the mechanisms that drive both the impact and success of F. japonica upon ecosystem processes remain poorly understood. Prior studies have suggested that F. japonica may exhibit traits such as a unique preference for ammonium over nitrate, potentially altering nitrogen availability for native plants like Urtica dioica. Additionally, it has been proposed that the species leverages phenolic compounds to influence soil biogeochemistry and nitrogen cycling. However, these processes lack comprehensive investigation. Using stable isotope labelling (15N and 13C), we found that, contrary to prior assumptions, F. japonica showed an overall lower uptake of both ammonium and nitrate relative to the native competitor, U. dioica. Although we expected a preference for ammonium, F. japonica instead exhibited a slight preference for nitrate. In addition, F. japonica demonstrated higher nitrogen-use efficiency and allocated more freshly assimilated carbon and nitrogen to root growth than U. dioica. These findings suggest that traits such as efficient nitrogen use and strategic root allocation may contribute to F. japonica's ability to establish itself in nitrogen-variable environments like riparian zones. By prioritising belowground biomass during early development, F. japonica may gain a competitive advantage that enables it to disrupt native plant communities and alter ecosystem dynamics. This study underscores the value of stable isotopes in understanding plant–soil interactions and informs strategies for managing invasive species in sensitive ecosystems.