In Situ Bioreactors and Deep Drain-Pipe Installation to Reduce Nitrate Losses in Artificially Drained Fields

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.