Infusing Science into Nitrogen Management Policy Making.

Nitrate contamination threatens Minnesota’s surface water resources as well as marine waters in the Gulf of Mexico. Minnesota recently completed a comprehensive study of nitrogen sources and pathways for water pollution.  Nitrogen inputs to land surfaces are dominated by soil mineralization  and agricultural fertilizer.  A model was developed to estimate nitrate-N losses to surface waters under varying climatic regimes.  Total nonpoint source N loadings to Minnesota surface waters were estimated at 254 million lb during an average climatic year.  This is about 6% of the total inputs of N on all Minnesota cropland.  During wet years, N loadings were considerably larger.  A Watershed Nitrogen Reduction Tool based on the baseline N sources and pathways described above was subsequently developed for estimating N reductions to surface waters and associated costs when combinations of BMPs are adopted on suitable acres.  The Tool can be used to optimize selection of nine different agricultural “Best Management Practices” (BMPs) for reducing the N load from the highest contributing sources and pathways in a watershed.  The most cost-effective BMPs include optimizing rates of N fertilizer and manure on corn, shifting fall-applied N fertilizer applications to preplant spring applications or a split preplant-sidedress application, controlled drainage and wetland treatments.  Costs increase substantially with BMPs that take cropland out of production, such as riparian buffers and perennial crops.  A statewide analysis indicates that a combination of fertilizer and tile drain BMPs could reduce N loadings to surface waters in an average year by 20% at reasonable cost, but achieving higher reductions quickly becomes very expensive because of the need to change from row crops to perennial crops. This information was used by the Minnesota Pollution Control Agency to set interim and long term goals for reduction of nitrogen pollution in Minnesota surface waters.