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Relating Net Nitrogen Input in the Mississippi River Basin to Nitrate Flux in the Lower Mississippi River

A Comparison of Approaches

^a University of Illinois, Department of Natural Resources and Environmental Sciences, W-503 Turner Hall, 1102 S. Goodwin Ave., Urbana, IL 61801
^b United States Geological Survey, Denver Federal Center, MS 406, Denver, CO 80225

View larger version (26K): [in a new window]   Fig. 1. Time series of Committee on Natural Resources and Environment (CENR) N budget residual (input - output), net anthropogenic N inputs (NANI), and estimated riverine nitrate flux in the lower Mississippi River based on nitrate concentration measurements at St. Francisville, LA and discharge measurements at Tarbert's Landing and the Old River outflow to the Atchafalaya River.

View larger version (38K): [in a new window]   Fig. 2. Estimated N inputs to and outputs from the 20-state region covering the Mississippi River basin used to (a) calculate net anthropogenic nitrogen input (NANI) and (b) estimate soil N mineralization resulting from conversion of hay and pasture to cropland, in addition to Committee on Natural Resources and Environment (CENR) estimates of net mineralization of soil organic N (mineralization - immobilization), crop senescence losses, and in-field denitrification.

View larger version (18K): [in a new window]   Fig. 3. Time series of annual water yield measured at Tarbert's Landing, and the Old River outflow.

View larger version (21K): [in a new window]   Fig. 4. Comparison of observed nitrate N flux in the Mississippi River at St. Francisville (1960–1998) to that predicted by the Caraco and Cole (1999) model (Eq. [1]) using watershed N inputs averaged over four years prior to the prediction year and lagged one year.

View larger version (14K): [in a new window]   Fig. 5. Difference between observed nitrate flux and that predicted by Eq. [1] (residuals) plotted as a function of net anthropogenic nitrogen input averaged for the previous two to five years (NANI2–5).