This data set contains atmospheric nitrogen deposition rates and herbaceous plant species richness values across sample sites in the USA, originally used for the publication by Simkin et al. (2016. Conditional vulnerability of plant diversity to atmospheric nitrogen deposition across the United States. Proceedings of the National Academy of Sciences of the United States of America 113: 4086-4091. A portion of this data set was used for example quantile regression analyses in the book chapter Cade (2017. Quantile regression applications in ecology and the environmental sciences. R. Koenker, et al., eds. Handbooks of Modern Statistical Methods: Handbook of Quantile Regression. Chapman & Hall/CRC. The data set contains the herbaceous species counts used as the response variable and the various environmental predictors, including nitrogen deposition rate, used in the statistical models.