Increased nitrogen deposition may facilitate an invasive plant species through interfering plant–pathogen interactions

Abstract Anthropogenic activities have intensified atmospheric nitrogen deposition, influencing the growth and herbivore resistance of invasive plant species and potentially facilitating their invasion success. However, the extent to which elevated nitrogen deposition influences plant–pathogen interactions within the context of biological invasions, as well as the mechanisms involved, has yet to be investigated. This study initially investigated the differences in leaf lesion areas between the invasive weed Alternanthera philoxeroides and its native congener Alternanthera sessilis across terrestrial habitats with varying atmospheric nitrogen deposition levels. Additionally, greenhouse‐based single‐culture and competition experiments were conducted to assess the changes in pathogen resistance and competitive ability of these two species in response to simulated nitrogen deposition, following artificial infection with a leaf pathogen. Lastly, changes in the amounts of leaf defence chemical compounds and phyllosphere microbial diversity were also analysed to clarify underlying mechanisms. Our results revealed that A. philoxeroides demonstrated superior pathogen resistance compared to A. sessilis under both field and laboratory conditions. Leaf lesion areas of both species exhibited inverse correlations with atmospheric nitrogen deposition levels in the field and leaf nitrogen content in the laboratory. Further chemical analyses and in vitro toxicity assessments indicated that such enhanced pathogen resistance of A. philoxeroides could be attributed to increased levels of leaf total flavonoids and recruitment of antagonistic microbes within its endophytic phyllosphere. A competition experiment further proved that pathogen infection largely promoted the competitive advantage of A. philoxeroides over A. sessilis in the presence of increased nitrogen deposition. Synthesis . These findings imply that elevated nitrogen deposition may enhance the invasiveness of A. philoxeroides by influencing the interactions between the plant and its pathogens.