Anthropogenic nitrogen (N) deposition exerts profound influence on the composition and function of native ecosystems, commonly resulting in reduced plant richness and increases in nonnative plant invasion. Identifying differences in plant N responses and functional traits could help to explain the ongoing invasion and biological impoverishment of ecosystems subject to N deposition.

We compared the growth, emergence phenology, and functional traits of thirty native and invasive annual plant species under high and low N availability, with the goal of evaluating if invasives exhibit functionally dissimilar responses to N compared to the native species they displace.

Invasives grew larger than natives, especially under increased N availability. Overall, invasives and natives did not differ in emergence phenology. However, these groups of species differed in relationships between growth and days to emergence, and N addition further strengthened these differences; invasives consistently grew larger than natives that emerged around the same time, while natives showed a strong negative effect of delayed emergence on plant growth.

Relative N responses for measures of plant growth and allocation did not differ between native and invasive species, but invasives exhibited greater plasticity in physiological traits. This included significantly lower leaf N content, higher CN ratios, and greater water-use efficiency (WUE) in plants grown under high N. Invasives also showed functionally different patterns of growth and water use. Across species, invasives showed a positive relationship between shoot growth and WUE. Native species showed a tradeoff between root allocation and WUE, while invasives displayed high values for WUE even despite reduced root:shoot ratios under high N.

Our results highlight the ability of invasives to rapidly accrue biomass, disproportionately benefit from early emergence and N addition, and use water efficiently despite rapid growth. Natives, on the other hand, pay a higher cost for delayed emergence and may experience tradeoffs between growth and water-use. The more pronounced functional differences between natives and invasives observed under high N likely play an important role in driving invasion and reduced species diversity under N enrichment. These differences could be used to inform successful management strategies for N-impacted and water-limited ecosystems.