Microbial controls on net production of nitrous oxide in a denitrifying woodchip bioreactor

Denitrifying woodchip bioreactors are potential low-cost technologies for the removal of nitrate (NO₃ ^- ) in water through denitrification. However, if environmental conditions do not support microbial communities performing complete denitrification, other N transformation processes will occur, resulting in the export of nitrite (NO₂ ^- ), nitrous oxide (N₂ O), or ammonium (NH₄ ⁺ ). To identify the factors controlling the relative accumulation of NO₂ ^- , N₂ O, and/or NH₄ ⁺ in denitrifying woodchip bioreactors, porewater samples were collected over two operational years from a denitrifying woodchip bioreactor designed for removing NO₃ ^- from mine water. Woodchip samples were collected at the end of the operational period. Changes in the abundances of functional genes involved in denitrification, N₂ O reduction, and dissimilatory NO₃ ^- reduction to NH₄ ⁺ were correlated with porewater chemistry and temperature. Temporal changes in the abundance of the denitrification gene nirS were significantly correlated with increases in porewater N₂ O concentrations and indicated the preferential selection of incomplete denitrifying pathways ending with N₂ O. Temperature and the total organic carbon/NO₃ ^- ratio were strongly correlated with NH₄ ⁺ concentrations and inversely correlated with the ratio between denitrification genes and the genes indicative of ammonification (Σnir/nrfA), suggesting an environmental control on NO₃ ^- transformations. Overall, our results for a denitrifying woodchip bioreactor operated at hydraulic residence times of 1.0-2.6 d demonstrate the temporal development in the microbial community and indicate an increased potential for N₂ O emissions with time from the denitrifying woodchip bioreactor.