We quantify the relationships between riverine nitrate–nitrogen (NN) concentration and agricultural land use in the continental United States – from the early 1900s through the end of the last century – on spatial scales ranging from the entire Mississippi River Basin to 1000-km2 watersheds. Cropland cover is linearly related to the NN concentration that exits a watershed at both the beginning and end of the 20th century. In addition, the slope of the relationship is higher at the end of the century, and the intercept of the regression analysis is not different from zero. These findings imply that agriculture was already affecting NN export by the early 1900s, that intensive management practices in modern agriculture have significantly increased the NN export per hectare of cropland, and that the baseline of exported NN has not shifted. We identify agricultural practices, principally associated with corn cultivation, that contribute substantially to NN concentrations and suggest that increasing cropland diversity and perennial plant cover can reduce NN loading.