Across the grasslands of N. America, there is a high correlation between aboveground net primary production and annual precipitation, suggesting that these systems are primarily structured by water availability. Why, then, might a biogeochemist (such as A. R. Mosier) choose to study soil nitrogen? If nitrogen dynamics in grasslands are important, what are the key questions that remain unanswered? In this paper, we synthesize long term monitoring and experimental data from grassland sites in the Central Grasslands of the U.S., evaluate regional scale data, and summarize simulation studies to assess the extent to which N influences key attributes of grassland ecosystem structure and function over a range of spatial and temporal scales. We examine the interactions of N with carbon storage and fluxes, plant community dynamics, soil food webs, and management practices, and the key changes in N availability that are occurring or are likely to occur in grasslands through human influence. Finally, we assess the state of our knowledge with respect to understanding and predicting the effects of such changes in N for grasslands.