Biochar acting as an electron acceptor reduces nitrate removal in woodchip denitrifying bioreactors

Woodchip denitrifying bioreactors mitigate nitrate loading from agricultural tile effluents. Biochar is considered as an amendment to woodchips. Research findings are contradictory regarding the effect of biochar on nitrate removal. The current study is aimed at investigating the effect of biochar on the removal of nitrate in denitrifying bioreactors. For this purpose, laboratory and field experiments were carried out. In the laboratory, cylindrical up-flow bioreactors were used that were filled with woodchips and oxidized biochar at rates of 0%, 12.5%, 25%, 50% by volume. The removal of nitrate was investigated at two stages: unaged and aged for 16 weeks. The bioreactors were operated at four hydraulic retention times of 2, 4, 8, and 24 h. Additionally, we measured the concentrations of greenhouse gases in the laboratory bioreactors. In the field, nitrate removal over a 6-year period was investigated in two denitrifying bioreactors, one of which amended with 10% fresh biochar. The laboratory experiments showed that, on average, oxidized biochar (OB) amendment reduced nitrate removal by 9% in unaged and 13% in aged bioreactors while the respiration was equal or greater. This led us to conclude that oxidized biochar acted as an electron acceptor. In addition, oxidized biochar increased greenhouse gas concentrations. In the field bioreactors, fresh biochar initially increased nitrate removal. However, biochar became less effective over six years. In the final year, the biochar-amended bioreactor removed less nitrate than the woodchip bioreactor, which was ascribed to the electron-accepting ability of biochar due to aging. In sum, our study showed that biochar's electron acceptability significantly reduced denitrification in denitrifying bioreactors.