High denitrification potential but low nitrous oxide emission in a constructed wetland treating nitrate-polluted agricultural run-off

Constructed wetlands (CW) can efficiently remove nitrogen from polluted agricultural run-off, however, a potential caveat is nitrous oxide (N2O), a harmful greenhouse gas and stratospheric ozone depleter. During five sampling campaigns, we measured N2O fluxes froma 0.53 ha off-stream CW treating nitrate-richwater fromthe intensively fertilized watershed in Rampillon, France, using automated chambers with a quantum cascade laser system, and manual chambers. Sediment samples were analysed for potential N-2 flux using the He-O-2 incubation method. Both inlet nitrate (NO3-) concentrations and N2O emission varied significantly between the seasons. In the Autumn and Winter inlet concentrations were about 11 mg NO3--N L-1, and < 6.5 mg NO3--N L-1 in the Spring and Summer. N2O emission was highest in the Autumn (mean +/- standard error: 9.7 +/- 0.2 mu g N m(-2) h(-1)) and lowest in the Summer (wet period: 0.2 +/- 0.3 mu g N m(-2) h(-1)). The CW was a very weak source of N2O emitting 0.32 kg N2O-N ha(-1) yr(-1) and removing around 938 kg NO3--N ha(-1) yr(-1), the ratio of N2O-N emitted to NO3--N removed was 0.033%. The automated and manual chambers gave similar results. From the potential N2O formation in the sediment, only 9% was emitted to the atmosphere, the average N-2 N O-2 ratio was high: 89:1 for N-2-N-potential: N2O-N-potential and 1353:1 for N-2-N-potential: N2O-N-emitted. These results indicate complete denitrification. The focused principal component analysis showed strong positive correlation between the gaseous N2O fluxes and the following environmental factors: NO3--N concentrations in inlet water, streamflow, and nitrate reduction rate. Water temperature, TOC and DOC in thewater and hydraulic residence time showed negative correlations with N2O emissions. Shallow off-stream CWs such as Rampillon may have good nitrate removal capacity with low N2O emissions. (C) 2021 Elsevier B.V. All rights reserved.