Denitrification in wood chip bioreactors at different water flows
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.