Nitrogen (N) is one of the most important soil nutrients for plants, and the amount and frequency of N release in soils is heterogeneous. Many studies have focused on the effects of heterogeneous N addition on a single plant species, but little is known about the effects of N amount and short-term N pulse on interspecific competition between invasive and native plant species. A replacement series experiment was conducted under greenhouse conditions to examine the effects of N amount and frequency release on the growth and interspecific competition between an invasive wetland clonal plant, Alternanthera philoxeroides (alligator weed), which is native in South America but highly invasive in China, and its native congener Alternanthera sessilis (sessile joyweed). Plant materials of A. philoxeroides and A. sessilis were collected from five locations in Xixi Wetland Park in Hangzhou, Zhejiang Province, China. Stem fragments of similar length (20 cm), with a stem tip for each species, were grown in monoculture (12 ramets in one container, no interspecific competition) and in mixture (six invasive plants with six native plants, with interspecific competition) in five different N treatments: control (no N added), low amount and high frequency (a total of 15 g N m^-2 a^-1 added every 5 days), low amount and low frequency (a total of 15 g N m^-2 a^-1 added every 15 days), high amount and high frequency (a total of 30 g N m^-2 a^-1 added every 5 days), and high amount and low frequency (a total of 30 g N m^-2 a^-1 added every 15 days). The results showed that N addition significantly increased the growth of both A. philoxeroides and A. sessilis. No significant difference was observed between the control and treatments for the relative yield (yield of each species in mixture divided by yield in its monoculture) for both species, indicating that N addition did not significantly affect the interspecific competition between the two species. The probable reason is that both species are from the same functional group and may have comparable capacities to compete and utilize the soil nutrients. The N pulse did not significantly modify growth or interspecific competition for either species. The sufficient soil water content and availability of nutrients other than N may explain why there were no effects of a N pulse on competition between the two species. Alternatively, high tolerance of both species to low soil nutrients and water content may lead to weaker responsiveness to a N pulse. With interspecific competition, the invasive species, A. philoxeroides, invested more biomass to stems, i.e., the stem biomass increased by 60%, which significantly decreased the root to shoot ratio. The native species, A. sessilis, allocated more biomass to roots, i.e., the root biomass increased by 250%, which significantly increased the root to shoot ratio. The results indicate that increasing atmospheric N deposition in the context of climate change may change population structure and dynamics of both species, but may not affect the interspecific competition of these plants.