J Veg Sci. 2020;31:667–677. | 667 Journal of Vegetation Science wileyonlinelibrary.com/journal/jvs Received: 18 October 2019 | Revised: 6 March 2020 | Accepted: 9 March 2020 DOI: 10.1111/jvs.12879 RESEARCH ARTICLE Cascading effects from plant to soil elucidate how the invasive Amorpha fruticosa L. impacts dry grasslands Francesco Boscutti 1 | Elisa Pellegrini 1 | Valentino Casolo 1 | Maria de Nobili 1 | Massimo Buccheri 2 | Giorgio Alberti 1 © 2020 International Association for Vegetation Science 1 Department of Agricultural, Food, Environmental and Animal Sciences, University of Udine, Udine, Italy 2 Museo Friulano di Storia Naturale, Udine, Italy Correspondence Francesco Boscutti, Department of Agricultural, Food, Environmental and Animal Sciences, University of Udine, via delle Scienze 206, 33100 Udine, Italy Email: francesco.boscutti@uniud.it Co-ordinating Editor: Gillian Rapson Abstract Questions: Understanding the mechanisms underlying the impacts of exotic plant invasions is a central issue in plant ecology. Considering the invasion process, any alteration of the nutrient cycle is of fundamental importance. We hypothesized that the woody N-fixing invasive Amorpha fruticosa is indirectly depleting plant diversity by altering ecosystem functions of riverine grasslands, thus producing a conspicuous shift in species composition. Location: Temperate lowland riparian areas of northeast Italy. Methods: In 12 sites, we selected uninvaded, partially invaded, and invaded grass- lands (36 plots). In each plot, we performed a vegetation relevé, measured main A. fruticosa growth traits, light transmittance and soil features. We studied the effects of the invasion of A. fruticosa on grassland properties and plant diversity, examining the interactions between soil nutrient cycles, light availability and plant diversity. Results: Increased abundance of A. fruticosa affected main soil properties (i.e. in- crease in soil mineralization and nitrification) and light availability (i.e. decrease in light). The δ 15 N of soil organic matter was strongly related to A. fruticosa basal area, thus supporting the hypothesis of an increase in soil nitrogen (N) availability due to invasion by the N-fixer. Plant taxonomical and functional diversity decreased signifi- cantly as A. fruticosa cover increased. Plant diversity was more related with N than light availability or soil organic carbon. Similarly, the shift in species composition (i.e. beta diversity) of invaded grasslands was related to increased soil nitrification rates. Conclusions: Our findings show how a nitrogen-fixing invasive plant such as A. fruti- cosa may influence the main ecosystem properties, causing cascading effects on soil functions, decreasing plant diversity. Alteration of the N cycle represents the key process involved in this plant invasion, suggesting that further studies should focus on potential solutions to mitigate the soil N supply, curb the invasion and restore plant diversity of these riverine grasslands. KEYWORDS biodiversity, false indigo, functional trait, isotopic signature, N cycle, plant invasion, plant–soil interactions