Technical Abstract: Differences in resource acquisition ability between native and exotic plants is one hypothesis to explain invasive plant success. Mechanisms include contrasting resource acquisition rates between native and invasive species, greater plasticity in resource acquisition by invasive species, and trade-offs between resource acquisition and long-term persistence. We assess the support for these mechanisms by comparing nitrate acquisition and seedling growth of invasive annual grasses and perennial grasses in western North America that differ in phenology, growth rates, and resource acquisition. Two invasive grasses (Bromus tectorum and Taeniatherum caput-medusae) and three perennial grasses (Pseudoroegneria spicata, Elymus elymoides, and Agropyron cristatum) were grown at various temperatures typical of spring growing seasons when resource are abundant and dominance is determined by rapid growth and resource acquisition. Bromus tectorum and perennial grasses had similar rates of nitrate acquisition at low temperature, but acquisition by B. tectorum significantly exceeded perennial grasses at higher temperature. Consequently, B. tectorum had the highest acquisition plasticity, showcasing its ability to take advantage of transient warm periods during spring. Nitrate acquisition by perennial grasses was limited either by root production or rate of acquisition per unit root mass, suggesting a trade-off between nutrient acquisition and allocation of growth to structural tissues. Our results indicate the importance plasticity in resource acquisition in the spring when temperatures change considerably within and among days. Highly flexible and opportunistic nitrate acquisition appears to be a mechanism whereby invasive annual grasses exploit soil nitrogen that perennials cannot use.