Predicting Subsurface Drainage, Corn Yield, and Nitrate Nitrogen Losses with DRAINMOD-N

Nitrate N from artificially drained soils of the upper Midwest USA is finding its way to the Mississippi River and then to the Gulf of Mexico. There is some concern that this nitrate N is causing hypoxia in the Gulf of Mexico. The DRAINMOD-N model was used to evaluate the long-term effect of N application rate and drain spacing on corn (Zea mays L.) yield and nitrate N losses. Prior to evaluation, the model was calibrated and then validated against long-term field data from southwestern Minnesota. A 24-yr simulation showed that climate plays a major role in determining drainage, yield, and nitrate N losses from a moderately well-drained Normania clay loam (fine-loamy, mixed, mesic Aquic Hapludoll) soil under continuous corn. April to August rainfall accounted for 82% of the variation in drainage and 66% of the variation in nitrate N losses during the growing season. Corn yield increased significantly when drain spacing was changed from 56 to 28 m but there was much less increase when changing from 28 to 14 m. During the growing season, drain spacing had little effect on nitrate N losses from this soil. Cost-benefit analysis showed that a 28 m drain spacing was a good design criterion for this soil. For a given drain spacing, an increase in N application rate significantly increased nitrate N losses through drainage. We conclude that efforts to reduce drainage-associated nitrate N losses from soils with perched water table conditions in the upper Midwest USA should concentrate on identifying N management strategies that increase N use efficiency.