Modeling Impacts of Tile Drain Spacing and Depth on Nitrate-Nitrogen Losses

61 S in the Gulf of Mexico have documented a large area with seasonally depleted O2 levels (<2 mg L−1). Most aquatic species cannot survive at such low O2 levels. A reduction in NO3–N loading by 30% was initially recommended to reduce hypoxia in the Gulf of Mexico (Mitsch et al., 1999). Following a scientifi c reassessment that recommended a reduction of 45% (USEPA, 2007), the USEPA developed an action plan (USEPA, 2008) that recommended that the longterm area of the hypoxic zone be reduced to 5000 km2. Nitrate-N loading from row cropland in the Upper Mississippi River Basin is the major source of nutrients leading to hypoxia in the Gulf of Mexico (Goolsby et al., 1997). About half of the N applied to lands draining to the Gulf comes from commercial fertilizer and about 15% comes from animal manure. Other sources of N to the Gulf include urban runoff , industrial point sources, and atmospheric deposition (USGS, 2000). While far away geographically, the hypoxia problem in the Gulf of Mexico is intimately linked to the Upper Midwest via the Mississippi River. Th e Upper Mississippi River Basin comprises only 15% of the total drainage area of the Mississippi River, but contributes a disproportionate 30% of the total NO3–N loading reaching the Gulf (Goolsby et al., 1997; Rabalais et al., 1999). Much of this NO3–N loading is linked to row crop areas in Minnesota, Iowa, Indiana, and Illinois, where a large percentage of cropland is tile drained (Zucker and Brown, 1998). Suitable agricultural management strategies to reduce NO3–N loadings from agricultural systems include a variety of practices, such as improved nutrient management, restoration of wetlands, and changes in tile drain spacing and depth. Subsurface tile drainage systems enhance crop yields on poorly drained but highly productive soils and help to reduce year-to-year variability in yields (Zucker and Brown, 1998). Lal and Fausey (1998) reported that corn crop yields increased with decreasing distance from the tile drain in central Ohio. Subsurface tile drainage improves aeration, increases the availability of nutrients, enhances crop productivity (Lal and Taylor, 1970; Cannell, 1979), and allows timely farm operations. It also reduces crop diseases, soil erosion, and surface runoff (Fausey et al., 1986). Consequently, it has become a routine practice across large areas of the U.S. Upper Midwest. Numerous studies have shown, however, that the presence of subsurface tile drainage systems increases NO3–N losses from fi elds through interception of NO3–N in the soil profi le (Baker and Johnson, 1981; Baker and Modeling Impacts of Tile Drain Spacing and Depth on Nitrate-Nitrogen Losses