Stable isotopes as a tool towards a better understanding of the biogeochemical traits of invasive <i>Fallopia japonica</i>

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 <i>Fallopia japonica</i>, 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 <i>F. japonica</i> upon ecosystem processes remain poorly understood. Prior studies have suggested that <i>F. japonica</i> may exhibit traits such as a unique preference for ammonium over nitrate, potentially altering nitrogen availability for native plants like <i>Urtica dioica</i>. 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, <i>F. japonica</i> showed an overall lower uptake of both ammonium and nitrate relative to the native competitor, <i>U. dioica</i>. Although we expected a preference for ammonium, <i>F. japonica</i> instead exhibited a slight preference for nitrate. In addition, <i>F. japonica</i> demonstrated higher nitrogen-use efficiency and allocated more freshly assimilated carbon and nitrogen to root growth than <i>U. dioica</i>. These findings suggest that traits such as efficient nitrogen use and strategic root allocation may contribute to <i>F. japonica</i>'s ability to establish itself in nitrogen-variable environments like riparian zones. By prioritising belowground biomass during early development, <i>F. japonica</i> 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.