REDUCING NITRATE LOSSES FROM AGRICULTURAL FIELDS WITH SUBSURFACE DRAINAGE
Title: Denitrification in Wood Chip Bioreactors at Different Water Flows
Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 5, 2009
Publication Date: June 25, 2009
Citation: Greenan, C., Moorman, T.B., Parkin, T.B., Kaspar, T.C., Jaynes, D.B. 2009. Denitrification in Wood Chip Bioreactors at Different Water Flows. Journal of Environmental Quality. 38:1664-1671.
Interpretive Summary: Subsurface drainage in agricultural watersheds exports a large quantity of nitrate-nitrogen (NO3-N) and concentrations frequently exceed 10 parts per million (ppm). Wood chip biofilters were shown to be effective in nitrate removal in the field, but their performance is governed by many interacting factors, particularly variations in water flow and nitrate concentration. A laboratory study was conducted to investigate the ability of a wood chip biofilter to promote denitrification at water flows which are representative of flows entering subsurface drainage tiles in the field. Denitrification (the microbial conversion of nitrate to dinitrogen gas) was shown to be the dominant nitrate removal mechanism. Complete nitrate removal was obtained at the lowest flow rate, but only 30% removal was obtained at the highest rate. Nitrous oxide is an important green house gas produced during denitrification, but the amounts of this gas produced during the nitrate removal process were not environmentally significant. The research provides some relationships between water flow and nitrate removal that are useful for design of wood chip biofilters. Research and extension scientists are working with producers and environmental groups to test these kinds of biofilters in production fields and this information will be relevant to their efforts.
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 biofilter 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 g of wood increased with increasing flow rates. Denitrification was considered 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 biofilters 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 a central Iowa subsoil.