Elevated nitrogen deposition protects an invasive plant from herbivory through altering leaf volatile‐mediated tritrophic interactions

Atmospheric nitrogen deposition disrupts plant–herbivore interactions, but its impact on leaf volatile organic compounds (VOCs)‐mediated multi‐trophic herbivore resistance remains unclear within the context of plant invasion. In this study, we initially investigated the differences in leaf feeding damage and number of eggs laid by herbivores between the invasive weed Alternanthera philoxeroides and its native congener, Alternanthera sessilis , in different terrestrial habitats that experienced various levels of atmospheric nitrogen deposition. Furthermore, we compared the quantity and composition of leaf constitutive plant volatiles (CPVs) and herbivore‐induced plant volatiles (HIPVs) between the two plant species under simulated nitrogen deposition and tested olfactory and oviposition responses of herbivores and natural enemies at multi‐trophic levels. Field surveys showed less consumed leaf area and fewer herbivore eggs laid on A. philoxeroides than on A. sessilis in terrestrial habitats experiencing high levels of atmospheric nitrogen deposition. Laboratory bioassays revealed that simulated nitrogen deposition increased the emission and altered the composition of both CPVs and HIPVs in A. philoxeroides compared with A. sessilis . These changes reduced its attractiveness to specialist and generalist herbivores while enhancing its appeal to predatory beetles. Methyl salicylate was identified as the key compound driving these changes in insect behaviours, highlighting its critical role in mediating plant–insect interactions in A. philoxeroides under elevated nitrogen deposition. Synthesis. These findings suggest that leaf VOC‐mediated multi‐trophic interactions may facilitate A. philoxeroides invasion success under elevated atmospheric nitrogen deposition in this specific study system, providing a mechanistic basis for understanding how nitrogen enrichment could influence plant–herbivore–predator interactions. Read the free Plain Language Summary for this article on the Journal blog.