Nitrous oxide and methane production and consumption at five full-size denitrifying bioreactors treating subsurface drainage water

Nitrate (NO₃ ⁻) removal in denitrifying bioreactors is influenced by flow, water chemistry, and design, but it is not known how these widely varying factors impact the production of nitrous oxide (N₂O) or methane (CH₄) across sites. Woodchip bioreactors link the hydrosphere and atmosphere in this respect, so five full-size bioreactors in Illinois, USA, were monitored for NO₃ ⁻, N₂O, and CH₄ to better document where this water treatment technology resides along the pollution swapping to climate smart spectrum. Both surface fluxes and dissolved forms of N₂O and CH₄ were measured (n = 7–11 sampling campaigns per site) at bioreactors ranging from <1 to nearly 5 years old and treating subsurface drainage areas from between 6.9 and 29 ha. Across all sites, N₂O surface and dissolved volumetric production rates averaged 1.0 ± 1.6 mg N₂O-N/m³-d and 24 ± 62 mg dN₂O-N/m³-d, respectively, and CH₄ production rates averaged 6.0 ± 26 mg CH₄-C/m³-d and 310 ± 520 mg dCH₄-C/m³-d for surface and dissolved, respectively. However, N₂O was consistently consumed at one bioreactor, and only three of the five sites produced notable CH₄. Surface fluxes of CH₄ were significantly reduced by the presence of a soil cover. Bioreactor denitrification was relatively efficient, with only 0.51 ± 3.5 % of removed nitrate emitted as N₂O (n = 48). Modeled indirect N₂O emissions factors were significantly lower when a bioreactor was present versus absent (EF₅: 0.0055 versus 0.0062 kg N₂O-N/kg NO₃-N; p = 0.0011). While further greenhouse gas research on bioreactors is recommended, this should not be used as an excuse to slow adoption efforts. Bioreactors provide a practical option for voluntary water quality improvement in the heavily tile-drained US Midwest and elsewhere.