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Subsurface Drain Modifications to Reduce Nitrate Losses in Drainage.

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

Citation:  Paper number  022015,  2002 ASAE Annual Meeting . (doi: 10.13031/2013.9703) @2002
Authors:   Dan B. Jaynes, Tom C. Kaspar, Tom B. Moorman, 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 - present in 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 modifications for reducing NO3 - in tile drainage versus the nitrate concentration in drainage from a control treatment consisting of a free-flowing tile installed at 1.2 m below the surface. The modifications being tested include a) a deep tile - a tile installed 0.6 m deeper than control tile depth, but with the outlet maintained at 1.2 m; b) denitrification walls - trenches excavated parallel to the tile and filled with wood chips as an additional carbon source to increase denitrification; and c) phyto remediation - eastern gama grass (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. 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. In 2001, only the tile flow from the denitrification wall treatment had NO3 - concentrations significantly lower than the control. Poor establishment of the eastern gama grass and lack of time for roots to proliferate below the water table probably limited the effectiveness of the phyto remediation treatment. Average NO3 - concentration in tile drainage from the control was about 25 mg-N L-1 but less than 10 mg-N L-1 for the treatment with the denitrification walls. This represented an annual reduction in NO3 - mass loss of from 70 kg-N ha-1 for the control to 20 kg-N ha-1 for the denitrification walls treatment.

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