This dataset contains the first five years (2014-2018) of the effect of nitrogen addition on species richness, species evenness (Evar) and productivity for a long-term nitrogen addition gradient experiment in two North American grasslands: a semi-arid shortgrass steppe and a mesic tallgrass prairie. Fertilization with time-release urea has been on-going since 2014 in a gradient of eight levels: 0, 2.5, 5, 10, 15, 20, 30 g/m-2.