Crop Management and Corn Nitrogen Rate Effects on Nitrate Leaching

Excess N use in crop production is often identified as a major contributor to NO₃ enrichment of ground water. Little information is available to show the specific relationships between crop management systems and N fertilizer use on the amounts of NO₃ lost by leaching. This study determined the effect of several cropping systems and N rates, providing a range of N availability to corn (Zea mays L.), on soil water NO₃ concentrations and leaching below the root zone. Four cropping-manure management systems were established in 1993 and 1994 (8-site years) at Arlington, WI, on a Plano silt loam (fine-silty, mixed, superactive, mesic Typic Argiudoll). Ammonium nitrate (0 to 204 kg N ha⁻¹ in 34-kg increments) was broadcast at the time of corn planting. Economic optimum N rates (EONR) for corn ranged from 0 to 150 kg ha⁻¹ depending on site-year. Soil water NO₃ concentrations were determined for 18-mo using porous-cup samplers installed at a 120-cm depth in the 0 and 204 kg N ha⁻¹ treatments. Nitrate N concentrations in the samplers increased as the amount of N applied in excess of the observed EONR increased. Predicted soil water NO₃−N concentration at EONR was 18 mg L⁻¹. Average NO₃−N concentrations were <10 mg L⁻¹ where fertilizer N rates were >50 kg N ha⁻¹ below the EONR and >20 mg L⁻¹ where fertilizer N rates were >50 kg N ha⁻¹ above the EONR. Total NO₃−N leaching estimates based on water budget data and soil water NO₃ concentrations for the 18-mo study period ranged from 3 to 88 kg ha⁻¹ depending on crop and manure management system, N fertilizer rate, amount of water drainage, and time of drainage event relative to treatment establishment. An end-of-season soil NO₃ test appears to be capable of evaluating corn N management practices and indicating the amount of excess N fertilizer applied that may be leached from the root zone. These results illustrate the direct relationship between NO₃ loss by leaching and N application rates that exceed crop needs. Research supported by the USDA-CSRS Water Quality Research Program (Agreement 92-34214-8168); the Wis. Fertilizer Research Fund; the Univ. of Wisconsin Nonpoint Pollution and Demonstration Project; and the College of Agricultural and Life Sciences, Univ. of Wisconsin-Madison through project no. 3449.