Nitrogen addition increased resistance of resident plant community to Solidago canadensis invasion by altering allelopathic effect

Allelopathy plays an important role in the interaction between invasive and resident plants. Atmospheric nitrogen (N) deposition has become a global problem, but it is unclear whether N enrichment affects the interaction between invasive and resident plants by affecting their allelopathy. Thus, we performed a greenhouse experiment in which the resident plant community was grown under two levels of invasion by S. canadensis (invasion vs. no invasion) and fully crossed with two levels of allelopathy (with or without adding activated carbon) and two levels of N addition (with or without). The resident plant communities were constructed with eight herbaceous species that often co-occur with S. canadensis. The results showed that both allelopathy of S. canadensis and the resident plants had obvious positive effects on their own growth. Nitrogen addition had more obvious positive effects on the resident plants under invasion than those that were not invaded. Moreover, N addition also altered the allelopathy of resident plants. Specifically, N addition improved the allelopathy of resident plants when they were invaded but decreased the allelopathy of resident plants when they grew alone. Although nitrogen addition had no obvious effect on S. canadensis, it reduced the allelopathy of S. canadensis. These results suggest that N addition could improve the resistance of resident plants to invasion by improving the allelopathy of resident plants and reducing the allelopathy of S. canadensis. These findings provide a scientific basis to manage and control the S. canadensis invasion.