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 from a 0.53 ha off-stream CW treating nitrate-rich water from the intensively fertilized watershed in Rampillon, France, using automated chambers with a quantum cascade laser system, and manual chambers. Sediment samples were analysed for potential N2 flux using the He[sbnd]O2 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 μg N m−2 h−1) and lowest in the Summer (wet period: 0.2 ± 0.3 μg 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 N2 N 2O ratio was high: 89:1 for N2-Npotential: N2O-Npotential and 1353:1 for N2-Npotential: N2O-Nemitted. 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 the water 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.
The study was conducted within the framework of several scientific projects: “Efficacité des Zones Tampons” by OFB (French Office for Biodiversity, and technical group “Zones Tampons “), and HydroGES (financed by the Agency for the Environment and Mastery of Energy, ADEME). The travel was supported by two French–Estonian Parrot RTD projects “Ecological engineering for nutrient control in rural catchments” and “Process-based approach and enhanced technologies of treatment wetlands” (2014–2016). The PIREN-Seine programme and the Fédération Ile-de-France de Recherche pour l'Environnement (FIRE) are also acknowledged for their support. The authors also thank AQUI'Brie association for their support and stakeholders' involvement. This study was also supported by the Estonian Research Council (grants IUT2 16, PRG352 and MOBERC20) and by the EU through the European Regional Development Fund (Centres of Excellence ENVIRON and EcolChange, and MOBTP101 returning researcher grant by the Mobilitas Pluss programme).
- Automated chambers
- Greenhouse gas
- Manual chambers
- Nitrate removal
- Quantum cascade laser absorption spectrometer