Nitrogen Forms Alter the Competitive Advantage of the Invasive Plant Amaranthus retroflexus over the Local Species

Nitrogen forms and native plant traits jointly regulate the competitive ability of invasive plants. This study investigated the invasive species Amaranthus retroflexus and the native species Portulaca oleracea and Medicago sativa. Using a pot experiment, we analyzed their competitive effects under NO3−-N, NH4+-N, CO(NH2)2-N and mixed nitrogen (Mix-N) treatments. The results showed that nitrogen addition had no significant effect on the relative yield of A. retroflexus but significantly increased the relative yield of P. oleracea, thereby weakening the competitive advantage of A. retroflexus. In contrast, nitrogen addition had no significant effect on the relative yield of M. sativa but significantly increased the relative yield of A. retroflexus, thereby enhancing the competitive advantage of A. retroflexus. The effect of NO3−-N treatment varied markedly between the two mixed-culture systems: it strengthened the advantage of A. retroflexus when grown with M. sativa yet weakened the advantage when grown with P. oleracea. Further analysis revealed that the competitive advantage of A. retroflexus was associated with the optimization of its photosynthetic traits and nitrogen absorption efficiency. Specifically, it included greater leaf number, leaf area, SPAD value, and leaf biomass. In summary, the competitive performance of invasive plants is not a fixed attribute but rather a dynamic outcome jointly regulated by the interplay between native plant traits and soil nitrogen forms. This provides new insight into the invasion mechanism of alien plants and aids in formulating targeted control strategies.