Carbon Dosing Increases Nitrate Removal Rates in Denitrifying Bioreactors at Low‐Temperature High‐Flow Conditions

Nitrogen losses from croplands contribute to impairment of water bodies. This laboratory experiment evaluated various C sources for use in a denitrifying bioreactor, a conservation practice designed to reduce N losses. The nitrate removal efficiency of candidate treatments (corn cobs [CC], corn cobs with modified coconut coir [CC+MC], corn cobs with modified coconut coir and modified macadamia shell biochar [CC+MC+MBC], wood chips [WC], wood chips with hardwood biochar [WC+BC], and wood chips with continuous sodium acetate addition [WC+A]) were tested with up‐flow direction. Effluent was sampled after a repeated weekly flow regime with hydraulic residence times of 1.5, 8, 12, and 24 h. Column temperatures were 15°C for 14 wk (warm), 5°C for 13 wk (cold), and again 15°C for 7 wk (rewarm). Cumulative nitrate N load reduction was greatest for WC+A (80, 80, and 97% during the warm, cold, and rewarm runs, respectively). Corn cob treatments (CC, CC+MC, and CC+MC+MBC) had the second greatest cumulative load reductions for all three temperature experiments, and WC and WC+BC had the lowest performance under these conditions. The nitrate removal rate was optimum at the 1.5‐h hydraulic residence time for the WC+A treatment: 43, 30, and 121 g N m⁻³ d⁻¹ for the warm, cold, and rewarm runs, respectively. Furthermore, acetate addition greatly improved wood chip performance and could be used to enhance nitrate N removal under the cold and high‐flow‐rate conditions of springtime drainage for the north‐central United States. CORE IDEAS: Acetate dosing reduced nitrate load by one‐third at 5°C and <2‐h residence time. Acetate dosing increased nitrate removal rate by an order of magnitude. Biochars added to wood chips and corn cobs did not improve nitrate removal rates. Nitrate removal rate of corn cobs exceeded that of wood chips.