Fate of water and nitrate using drainage water management on tile systems in East-Central Illinois

Nitrate from agricultural fields in the tile-drained upper Midwest is one of the primary causes of the hypoxic zone that forms each summer in the Gulf of Mexico. Drainage water management (DWM) is a potential edge-of-field technique that is being studied as a method to improve soil water management in agricultural fields, which would reduce nitrate losses to surface waters during the non-growing season. Agri Drain tile control structures and monitoring wells were installed on a 34 ha private farm located in the Upper Salt Fork River Watershed in central Illinois to evaluate DWM from 2011 to 2013. The overall objective was to determine the fate of water and nitrate that were held back in the field when DWM was applied. A paired watershed approach was used to compare the efficiency of DWM versus a conventional or free drainage (FD) tile system. The field was under a corn (Zea mays L.) and soybean (Glycine max L.) rotation with continuous no-till (27 years). In 2010 and 2012 corn was fertilized at 180 kg N ha-1 of urea-ammonium nitrate (UAN 28%). Soybean was planted in 2011 and 2013. During 2011 and 2012, DWM was able to greatly reduce tile flow compared to the FD tile system. However, based on runoff and nitrate yields from the entire field, there was no measureable reduction in nitrate loss and shallow ground-water wells showed little area of influence in the field. The held back water from the DWM tile flowed laterally to the nearby FD tile, increasing flow and nitrate loss from that tile. In 2013, when both tiles were under DWM, water was retained and the water table level was increased in a larger area of the field. However, at the end of the experiment when the control boards were lowered in the Agri Drain structure, the retained water was discharged through the tile lines with little apparent reduction in overall water and nitrate loss for the year. Measurements of tile and well nitrate concentrations suggested that nitrate was not denitrified in the shallow groundwater of the field at any time iii during the three-year study. Nitrate losses were directly proportional to tile flow each year of the study. Retrofitting DWM on an existing tile system was not found to have a water quality benefit