Table 1. Overall nitrogen budget for the Potomac River basin and the four major tributary basins of Chesapeake Bay. The Potomac River basin has the largest net nitrogen (N) import in food and animal feed of the four major tributaries. Tributary basin of Chesapeake Bay Net atmospheric N deposition N fertilizer use N fixation in forestlands N fixation in agricultural lands Net N import in food and feed Total N inputs Potomac 769 1024 271 1173 1452 4689 Susquehanna 1138 615 179 1147 1095 4173 Rappahannock 893 1030 277 1439 607 4246 James 953 361 361 703 395 2773 Note: Units for inputs and exports of nitrogen are kg km−2 yr−1. All data are from Boyer et al. (2002).