Woodchip denitrifying bioreactors (WDBR) reduce off-field tile drainage nitrogen (N) losses from agricultural fields. Limited evaluation exists regarding the influence of flow direction through WDBRs. Changing flow direction could reduce short circuiting. This study evaluated the dependency of nitrate-N removal and dissolved nitrous oxide (dN 2 O) production rates on vertical flow direction in triplicate column bioreactors at 12-h (without carbon dosing) and 2-h (with carbon dosing) hydraulic

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... sidence times. Results presented demonstrate that there was no significant difference in overall N removal rates from these column bioreactors as a function of flow direction. There was the suggestion of lower N 2 O production in the downflow direction, although this was not statistically significant due to the high variability of the N 2 O production observed in the upflow direction. Carbon addition led to bioclogging of downflow columns; future work needs to identify dosing rate, placement, and conditions that prevent biofilm formation.