|CHRISTIANSON, LAURA - Freshwater Institute|
|Venterea, Rodney - Rod|
|COULTER, J - University Of Minnesota|
|TSCHIRNER, U - University Of Minnesota|
Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/25/2016
Publication Date: 4/25/2016
Citation: Feyereisen, G.W., Moorman, T.B., Christianson, L.E., Venterea, R.T., Coulter, J.A., Tschirner, U.W. 2016. Performance of agricultural residue media in laboratory denitrifying bioreactors at low temperatures. Journal of Environmental Quality. 45:779-787.
Interpretive Summary: Woodchip denitrifying bioreactors are an edge-of-field drainage conservation practice that can remove a substantial fraction of nitrate from agricultural tile drainage; however, questions about cold springtime performance persist. The objectives of this study were to improve the nitrate removal rate of denitrifying bioreactors at warm (summer) and cold (snowmelt/early spring) temperatures using available, agricultural materials in the bioreactor bed. Materials tested included corn cobs, corn stover, barley straw, wood chips, and a combination of corn cobs plus wood chips. Water with nitrate-nitrogen was pumped through PVC columns containing these materials for five months at 60 degF and for 5 months at 36 degF. At the warm temperature, nitrate removal rates were highest for corn cobs followed by the combination of corn cobs plus wood chips. The corn stover and barley straw were similar. All the agricultural materials had higher nitrate removal rates than wood chips. Nitrate removal was much lower for the colder temperature, with the agricultural materials again having higher removal rates than wood chips. The abundance of denitrifying bacteria was similar in the agricultural materials, which were all greater than in wood chips, which explains some of the performance difference. The corn cobs provided more bioavailable carbon to support the denitrification process. Production of nitrous oxide, a greenhouse gas, was highest for the barley straw and corn stover. The findings of this study inform researchers, policy makers, and conservation practitioners that the seasonal nitrate-reducing capacity of denitrifying bioreactors can be improved and contribute to water quality goals.
Technical Abstract: Woodchip denitrifying bioreactors can remove a substantial fraction of nitrate from agricultural tile drainage; however, questions about cold springtime performance persist. The objectives of this study were to improve the nitrate removal rate of denitrifying bioreactors at warm and cold temperatures using agriculturally derived media rather than wood chips. Wood chips (WC), corn cobs (CC), corn stover (CS), barley straw (BS), and WC+CC – were tested in laboratory flow-through columns for five months at a 12-h hydraulic residence time in separate experiments at 16 and 2 degC. Microbial samples were collected at the completion of each experiment. Dissolved gases in column effluent were measured. Warm and cold run nitrate-N removal rates (NNR) ranged from 1.4 – 35 and 1.6 – 7.4 g N m^-3 d^-1, respectively, with CC > CC+WC > BS = CS > WC and CC = CC+WC = CS = BS > WC, respectively. Nitrate-N load reductions for the warm and cold runs were significantly higher for the agricultural residues vs. wood chips: 46 – 73% vs. 5%, and 11 – 16% vs. 3.6%, respectively. The agricultural residues supported greater abundance of denitrifying genes (nosZ) within the microbial population than did WC for the warm run; for the cold run CS and BS were significantly greater than WC. Corn cobs exhibited potential to significantly increase nitrate-N removal rates relative to WC at 1.5 degC, but released a higher amount of dissolved carbon. Nitrous oxide produced relative to the amount of nitrate removed was lowest for CC.