Ten Ways to Reduce Nitrogen Loads from Drained Cropland in the Midwest

Agricultural drainage is an important contributing factor to high crop productivity in much of the Midwest. Modern crop production would not be possible in many parts of this region without artificial subsurface drainage. However, drainage is associated with an increase in nitrate loads to streams, rivers, and the Gulf of Mexico, where it contributes to the low oxygen or hypoxic zone (see Glossary). While the economic and environmental impacts of this "dead" zone make it one of the United States' largest national water quality concerns (USEPA, 2007), there is also reason to be concerned with nitrate in waters closer to home. For example, the city of Des Moines, Iowa operates one of the world's most expensive nitrate-removal facilities to treat local drinking water. Because of these water quality concerns at multiple scales, there is great interest in reducing nitrate loads from drained land. Nitrate loads from agricultural lands drained with subsurface drainage occur for a number of reasons. Corn production is inevitably "leaky," since the precise amount of fertilizer needed by the crop cannot be known in advance. During the seven months (October through April) when no crop is growing in conventional corn-based rotations, nitrate in the soil is not taken up, and this nitrate can leach into drainage water. One way to reduce nitrate loads would be to reduce the amount of drained land, but this is unlikely due to the important role of drainage in midwestern agriculture. Recent research is instead focusing on ways that cropping systems and drainage systems can be managed to reduce nitrate loads, while maintaining high agricultural productivity. This practice manual focuses on ten strategies for subsurface-drained cornsoybean systems that agricultural scientists and engineers have identified as being the most promising for reducing nitrate loads. The nitrate load (also called nitrate loss) in drainage is the total amount of nitrate lost through a drain and is the product of the drainage water volume and the concentration of nitrate in that water (flow volume × concentration). The alternative practices described here reduce nitrate loads through two primary means: (1) reducing the nitrate concentration in the drain flow, or (2) reducing the amount of drain flow. This publication first describes the complex processes that affect nitrate loads, and therefore need to be managed to decrease these loads and improve water quality. The ten promising practices are then described. The description for each practice includes an overview of the practice; how it reduces nitrate load; its effectiveness at doing so; where the practice is appropriate; the level of acceptance; and remaining questions and opportunities to make the practice more economical, more effective, or more likely to be adopted by agricultural producers.