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Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Paper number  701P0304,  . (doi: 10.13031/2013.15713)
Authors:   Dan B. Jaynes, Tom C. Kaspar, Tom B. Moorman, and Tim B. Parkin
Keywords:   Nitrate, tile drainage, subsurface drainage, nitrogen fertilizer, water quality, denitrification, bioreactors

Nitrate in water leaving subsurface drain (tile) systems often exceeds the 10 mg-N L-1 maximum contaminant level (MCL) set by the U.S. EPA for drinking water and has been implicated in contributing to the hypoxia problem within the Gulf of Mexico. Much of the NO3 - from agricultural lands impacting surface waters within the Midwest cornbelt is from subsurface field drainage. Because previous research shows that N fertilizer management alone is not sufficient for reducing NO3 - concentrations in subsurface drainage below the MCL, additional approaches need to be devised. We are comparing the efficacy of several tile and cropping modifications for reducing NO3 - in tile drainage versus the nitrate concentration in drainage from a control treatment (CK) consisting of a free-flowing tile installed at 1.2 m below the surface. The modifications being tested include a) deep tile (DT) - a tile installed 0.6 m deeper than the control tile depth, but with the outlet maintained at 1.2 m; b) denitrification walls (DW) - trenches excavated parallel to the tile and filled with wood chips as an additional carbon source to increase denitrification; c) phyto-remediation (PR) - eastern gamagrass (Tripsacum dactyloides L.) grown in 3.81 m wide strips above the tile with the plant roots capable of developing below the water table and serving as a renewable carbon source for increasing denitrification; and d) winter cover crop (CC) - planting rye (Secale cereale L.) after soybean [Glycine max (L.) Merr.] and corn (Zea mays L.) harvest and chemically killing before planting the following spring. 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. For 2001 - 2003, the tile flow from the DW treatment had annual average NO3 - concentrations significantly lower than the control. Following a good cover crop stand in 2001, the flow-weighted NO3 - concentrations for the CC treatment were significantly lower than the control in 2002 and 2003. Poor initial establishment of the eastern gamagrass and lack of time for roots to proliferate below the water table probably have limited the effectiveness of the PR treatment. Average NO3 - concentration in tile drainage from the control was about 25 mg-N L-1 compared with less than 10 mg-N L-1 for the DW treatment. This represented an annual reduction in NO3 - mass loss of 50 kg-N ha-1 for the denitrification walls treatment.

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