Nitrate in water from tile drained corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] fields in the U.S. Midwest contributes to nitrate contamination of surface waters. Denitrification-based biofilters are a promising strategy for reducing nitrate concentrations, but these systems require an external carbon supply to sustain denitrification. The ability of four organic materials to serve as carbon substrates for denitrification biofilters was evaluated in this laboratory study. Wood chips, wood chips amended with soybean oil, cornstalks, and cardboard fibers were mixed with subsoil (oxidized till) and incubated anaerobically for 180 d. Periodically, 15NO3-N was added to maintain nitrate N concentrations between 10 and 100 mg L-1. All of the materials stimulated NO3-N removal and the degree of removal from highest to lowest was: cornstalks, cardboard fibers, wood chips with oil, and wood chips alone. Analysis of 15N showed that immobilization and dissimilatory nitrate reduction to ammonium accounted for <4% of NO3-N removal in all treatments, therefore denitrification was the dominant NO3-N removal process. Cardboard fibers, wood chips and oil, and wood chips alone did not support as much denitrification as cornstalks, but their rates of NO3-N removal were steady and would probably continue longer than cornstalks. The addition of soybean oil to wood chips significantly increased denitrification over wood chips alone.

Subsurface drainage in agricultural watersheds exports a large quantity of nitrate-nitrogen (NO(3)-N) and concentrations frequently exceed 10 mg L(-1). A laboratory column study was conducted to investigate the ability of a wood chip bioreactor to promote denitrification under mean water flow rates of 2.9, 6.6, 8.7 and 13.6 cm d(-1) which are representative of flows entering subsurface drainage tiles. Columns were packed with wood chips and inoculated with a small amount of oxidized till and incubated at 10 degrees C. Silicone sampling cells at the effluent ports were used for N(2)O sampling. (15)Nitrate was added to dosing water at 50 mg L(-1) and effluent was collected and analyzed for NO(3)-N, NH(4)-N, and dissolved organic carbon. Mean NO(3)-N concentrations in the effluent were 0.0, 18.5, 24.2, and 35.3 mg L(-1) for the flow rates 2.9, 6.6, 8.7, and 13.6 cm d(-1), respectively, which correspond to 100, 64, 52, and 30% efficiency of removal. The NO(3)-N removal rates per gram of wood increased with increasing flow rates. Denitrification was found to be the dominant NO(3)-N removal mechanism as immobilization of (15)NO(3)-N was negligible compared with the quantity of (15)NO(3)-N removed. Nitrous oxide production from the columns ranged from 0.003 to 0.028% of the N denitrified, indicating that complete denitrification generally occurred. Based on these observations, wood chip bioreactors may be successful at removing significant quantities of NO(3)-N, and reducing NO(3)-N concentration from water moving to subsurface drainage at flow rates observed in central Iowa subsoil.

Nitrate-nitrogen (N03-N) concentrations in water leaching from com-soybean rotated fields often exceed the 10 mg L-1 standard for drin~ng water set by the Environmental Protection Agency (EPA). A laboratory incubation evaluated the ability of four organic materials to serve as carbon substrates for denitrification biofilters. Wood chips, wood chips amended with soybean oil, com stalks, and cardboard fibers were mixed with oxidized till from the subsoil of a Central Iowa com-soybean field. Ground and unground substrates were compared to quantify the importance of particle size importance on N03-N removal. Nitratenitrogen was added to incubation jars to achieve a concentration of 100 mg L-1 and re-spiked when levels declined to < 10 mg L-1 • After 6 months of incubation, it was observed that all of the materials stimulated N03-N removal and the degree of removal from highest to lowest was: corn stalks, cardboard fibers, wood chips with oil, and wood chips alone. Cardboard fibers, wood chips and oil, and wood chips did not support as much denitrification as com stalks, but their rates of N03-N removal were steady and would probably outlast corn stalks. Denitrification was the dominant N03-N removal process, while N immobilization and dissimilatory nitrate reduction to ammonium accounted for< 4 % of N03-N removal in all treatments. Grinding of the materials enhanced denitrification, and addition of soybean oil significantly increased denitrification rates in early phases of the incubation. It seems that com stalks may be more appropriate in a denitrification system that can be replenished annually, while the other materials could be used in a more permanent denitrification system.