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TECHNICAL REPORTS

Surface Water Quality

Denitrification in Wood Chip Bioreactors at Different Water Flows

USDA-ARS, National Soil Tilth Lab., 2110 University Blvd., Ames, IA 50011

^* Corresponding author (tom.moorman{at}ars.usda.gov).

Received for publication September 16, 2008. Subsurface drainage in agricultural watersheds exports a large quantity of nitrate-nitrogen (NO₃–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°C. Silicone sampling cells at the effluent ports were used for N₂O sampling. ¹⁵Nitrate was added to dosing water at 50 mg L^–1 and effluent was collected and analyzed for NO₃–N, NH₄–N, and dissolved organic carbon. Mean NO₃–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 NO₃–N removal rates per gram of wood increased with increasing flow rates. Denitrification was found to be the dominant NO₃–N removal mechanism as immobilization of ¹⁵NO₃–N was negligible compared with the quantity of ¹⁵NO₃–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 NO₃–N, and reducing NO₃–N concentration from water moving to subsurface drainage at flow rates observed in central Iowa subsoil.

Abbreviations: DOC, dissolved organic carbon • DW, dry weight • HRT, hydraulic residence time