Potential Methods for Reducing Nitrate Losses in Artificially Drained Fields

Authors: Jaynes, Dan; Kaspar, Thomas; Moorman, Thomas; Parkin, Timothy

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/10/2007
Publication Date: 3/10/2008
Citation: Jaynes, D.B., Kaspar, T.C., Moorman, T.B., Parkin, T.B. 2008. Potential Methods for Reducing Nitrate Losses in Artificially Drained Fields. Journal of Environmental Quality. 37:429-436.

Interpretive Summary: Much of the nitrate loss from agricultural lands that enters surface waters within the cornbelt and contributes to the hypoxic zone in the upper Gulf of Mexico comes from subsurface field drainage. Nitrate in water leaving subsurface drain ('tile') systems often exceeds the ten parts per million (ppm) maximum contaminant level (MCL) set by the U.S. Environmental Protectional Agency (EPA) for drinking water. Because previous research has shown that nitrogen (N) fertilizer management alone is not sufficient for reducing nitrate concentrations in subsurface drainage below the MCL, additional approaches need to be devised. We field tested the efficacy of two tile modifications for reducing nitrate losses; i) deep tile (DT) – tile installed 0.6 m deeper than typically installed, but with the outlet maintained at same elevation as traditionally installed tile and ii) denitrification walls (DW) – trenches excavated parallel to the tile and filled with wood chips as an additional carbon source to increase the natural conversion of nitrate to harmless di-nitrogen gas before it enters the tile. Our results over five years showed that the tile flow from the DW treatment had significantly lower nitrate concentrations resulting in a 55% reduction in annual nitrate mass loss. In contrast, the DT treatment did not consistently lower nitrate concentrations, nor reduce the annual nitrate mass lost in drainage. We also found no difference in crop yields among the treatments. Thus, denitrification walls may be a viable technology for reducing nitrate contributions to surface waters from tile drainage. These results are of use to other scientists and action agency personnel interested in reducing the environmental impact of farming.

Technical Abstract: Nitrate in water removed from fields by subsurface drain ('tile') systems is often at concentrations exceeding the ten mg N/L maximum contaminant level (MCL) set by the USEPA for drinking water and has been implicated in contributing to the hypoxia problem within the northern Gulf of Mexico. Because previous research shows that nitrogen (N) fertilizer management alone is not sufficient for reducing nitrate concentrations in subsurface drainage below the MCL, additional approaches are needed. In this field study, we compared the nitrate losses in tile drainage from a conventional drainage system (CN) consisting of a free-flowing pipe installed 1.2 m below the soil surface to losses in tile drainage from two alternative drainage designs. The alternative treatments were a deep tile (DT), where the tile drain was installed 0.6 m deeper than the conventional tile depth, but with the outlet maintained at 1.2 m, and a denitrification wall (DW), where trenches excavated parallel to the tile and filled with woodchips serve as additional carbon sources to increase denitrification. Four replicate 30.5 x 42.7-m field plots were installed for each treatment in 1999 and a corn/soybean rotation initiated in 2000. Over five years, 2001-2005, the tile flow from the DW treatment had annual average nitrate concentrations significantly lower than the CN treatment (8.8 vs. 22.1 mg N/L). This represented an annual reduction in nitrate mass loss of 29 kg-N/ha or a 55% reduction in nitrate mass lost in tile drainage for the DW treatment. The DT treatment did not consistently lower nitrate concentrations, nor reduce the annual nitrate mass loss in drainage. The DT treatment did exhibit lower nitrate concentrations in tile drainage than the CN treatment during late summer when tile flow rates were minimal. There was no difference in crop yields for any of the treatments. Thus, denitrification walls are able to substantially reduce nitrate losses in tile drainage for at least five years.