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 speciesAmaranthus retroflexusand the native speciesPortulaca oleraceaandMedicago 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 ofA. retroflexusbut significantly increased the relative yield ofP. oleracea, thereby weakening the competitive advantage ofA. retroflexus. In contrast, nitrogen addition had no significant effect on the relative yield ofM. sativabut significantly increased the relative yield ofA. retroflexus, thereby enhancing the competitive advantage ofA. retroflexus. The effect of NO3−-N treatment varied markedly between the two mixed-culture systems: it strengthened the advantage ofA. retroflexuswhen grown withM. sativayet weakened the advantage when grown withP. oleracea. Further analysis revealed that the competitive advantage ofA. retroflexuswas 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.