Simulated nitrogen deposition increased herbivore resistance and competitive ability of an invasive plant species over a native co-occurring congeneric species

The deposition of atmospheric nitrogen has increased globally and interfered with plant growth and development. However, only a few studies have examined the influence of atmospheric nitrogen deposition on plant–herbivore interactions in the framework of plant invasion, and the underlying mechanisms remain unclear. In this study, we examined the changes in growth and herbivore resistance of the invasive weed Alternanthera philoxeroides and its native co-occurring congener Alternanthera sessilis in response to simulated nitrogen deposition. We further investigated the competitive ability of these two plant species grown in mono- and mix-culture conditions with or without herbivore pressures. The results indicated that simulated nitrogen deposition largely increased the growth of both plant species, while A. philoxeroides produced more biomass than A. sessilis. The specialist and generalist herbivores showed contrasting preferences for the two plant species in the controls, whereas A. philoxeroides showed greater resistance to both herbivore species under simulated nitrogen deposition. Further investigation of host plant traits related to leaf structural and chemical defences suggested that such difference in herbivore resistance between the two plant species could be attributed to the increased levels of structural defense in A. philoxeroides and reduced levels of defensive metabolites in A. sessilis. The inter-specific competition test revealed that simulated nitrogen deposition largely increased the competitive advantage of A. philoxeroides over A. sessilis, regardless of the presence or absence of herbivores. Our study suggests that the invasiveness of A. philoxeroides may be enhanced by increased atmospheric nitrogen deposition.