1. Exotic plant invasion has been changing the vegetation composition and function of terrestrial ecosystems. Nitrogen (N) and phosphorus (P) are often the limiting nutrients for terrestrial plants. However, under invasive pressure, in-situ plant N and P use mechanisms remain poorly understood but is pivotal for a better understanding of plant invasion and coexistence in invaded ecosystems. 2. Nitrogen and P concentrations, natural 15N abundance (δ15N values) were investigated in leaves and soils under different invasive pressures (here expressed as the biomass percentages of invasive plants in each plot) for two invasive species (Chromolaena odorata and Ageratina adenophora) in Xishuangbanna in tropical China. 3. Soil N and P concentrations revealed the relatively N-rich but P-poor status of our study site. Under invasion, soil inorganic N (dominated by ammonium) and available P increased. The leaf N and P of invasive plants also increased, while leaf N increased but P decreased for native species. Natural δ15N mass balance between leaves and soil inorganic N sources revealed that ammonium dominated N utilization in both natives and invaders. Invasive plants showed ammonium preference with increasing leaf N levels, while native plants under no invasion showed nitrate preference with increasing leaf N levels. 4. Synthesis. Increased soil ammonium availability contributed to preferential ammonium utilization by invasive plants and elevated ammonium utilization in natives, but the P competition of natives decreased in invaded ecosystems. These novel insights into nutrients dynamics in invaded ecosystems enhance our understanding of plant invasion and coexistence mechanisms.