Higher Nitrogen Uptakes Contribute to Growth Advantage of Invasive Solanum rostratum over Two Co-Occurring Natives Under Different Soil Nitrogen Forms and Concentrations

High nitrogen (N) uptake is one of the main reasons for invasive alien plant invasions. However, little effort has been made to compare the effects of different N forms on N uptake between invasive and native plants, especially those on N form acquisition strategies (preference and plasticity), which influence N uptake, and thus exotic plant invasions. Related studies are particularly few in barren habitats, where the effects of N deposition on invasiveness are considered to be much weaker than in fertile habitats. In this study, we grew Solanum rostratum, a noxious invader in barren habitats, and the native plants Leymus chinensis and Agropyron cristatum in both mono- and mixed cultures under nitrate and ammonium addition treatments, and analyzed the effects of the soil N availability and forms on the growth, N uptake, and N form acquisition strategies for these plants. The invader outperformed the natives in N uptake (in most cases) and growth (always) in both mono- and mixed cultures under all N treatments. N addition increased the N uptake and growth of the invader. The advantages of the invader over the natives were higher under ammonium relative to nitrate addition. The growth advantage of the invader was associated with its higher N uptake and higher N-use efficiency. Higher plasticity in N form uptake may contribute to the higher N uptake for the invader when grown in mixed cultures. Our findings indicate that N deposition, particularly in the form of ammonium, may accelerate exotic plant invasions in barren habitats.