Fate of Water and Nitrate in a Field with Drainage Water Management: Denitrification?.

Drainage water management (DWM) is used to reduce discharge and therefore nitrate loss from agricultural fields. Studies made in North Carolina, the tile-drained Midwest, and Canada have shown a nitrate loss reduction between 30 to 70% from fields where DWM was applied compared with conventional subsurface drainage systems. Denitrification along the flow path to the stream is presumed to be the fate of the controlled drainage water, but this has not been confirmed. In this study we assessed nitrate export and changes in nitrate concentrations in shallow groundwater that resulted from the implementation of two drainage management systems: DWM and conventional subsurface drainage (CSD). Two adjacent patterned tile systems in the same field were located on a poorly drained soil (Mollisol) with a crop rotation of corn and soybean under no tillage. Both experimental fields were equipped with an Agri-Drain structure on the tile outlet to continuously measure discharge and to control the drainage of one tile system. Each field also had 13 shallow groundwater wells (3 with continuous pressure transducers to record water elevations) to compare the water table fluctuations in relation to the tile drain outlets. During the period of controlled drainage (late winter and spring) water table depths and samples for nitrate were measured or collected weekly for both tile systems. Wells located in the DWM had an average nitrate concentration of 8.2 mg N/L while wells in the CSD had and average concentration of 11.1 mg N/L. However, at the tile outlet of the CSD, the average concentration of nitrate was 10.2 mg N/L. The water table was on average 30 cm closer to the soil surface in the DWM than in the CSD. These results indicate a significant decrease in the nitrate load that is discharged to the stream through the tile outlet. Modeling was used with tile elevation data to estimate water flow paths. Our flow path studies suggest that water moved toward the ditch away from the outlet, and the saturated conditions may have led to denitrification of the nitrate. The adoption of DWM in conventional subsurface drainage systems could result in the best alternative solution to reducing nitrate export from the intensively tile drained areas of the Corn Belt.