Nitrogen enrichment differentially affects above‐ and belowground plant defense

• Premise of the Study: Human nitrogen (N) inputs to terrestrial ecosystems have greatly increased in recent years and may have important consequences for plant growth, reproduction, and defense. Although numerous studies have investigated the effects of nitrogen addition on plants, few have examined both above‐ and belowground responses within a range of predicted increase and apart from concomitant increases in other nutrients. • Methods: We conducted a greenhouse experiment to study the consequences of increased nitrogen inputs, such as those from atmospheric N deposition, on plant performance, chemical defenses, and allocation tradeoffs for an invasive species, Linaria dalmatica . This plant produces iridoid glycosides, which are a group of terpenoid compounds. • Key Results: Soil nitrogen enrichment increased growth, reproduction, and whole‐plant iridoid glycosides while decreasing some costs of defense. Interestingly, nitrogen addition had varying effects on defense allocation to above‐ and belowground tissues. Specifically, there was no change in iridoid glycoside concentrations of shoots, whereas concentrations decreased in flowers by ∼35% and increased in roots by >400%. • Conclusions: Observed increases in plant performance and chemical defenses may have implications for the invasion potential of L. dalmatica . Moreover, our results highlight the importance of evaluating both above‐ and belowground plant defenses. In particular, findings presented here indicate that research focused on leaf‐level defenses may not detect key allelochemical responses, including changes in plant resistance traits that could affect consumers (e.g., herbivores and pathogens) that specialize on different plant tissues as well as plant fitness and invasion success.