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

Ülo Mander; Julien Tournebize; Mikk Espenberg; Cedric Chaumont; Raili Torga; Josette Garnier; Mart Muhel; Martin Maddison; Jérémie D Lebrun; Emmanuelle Uher; Kalle Remm; Jaan Pärn; Kaido Soosaar

doi:10.1016/j.scitotenv.2021.146614

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

Sci Total Environ. 2021 Jul 20:779:146614. doi: 10.1016/j.scitotenv.2021.146614. Epub 2021 Mar 19.

Authors

Affiliations

¹ Department of Geography, Institute of Ecology and Earth Sciences, University of Tartu, Tartu, Estonia; UR 1462 HYCAR, University Paris Saclay, French National Institute for Agriculture, Food, and Environment (INRAE), Antony, France. Electronic address: ulo.mander@ut.ee.
² UR 1462 HYCAR, University Paris Saclay, French National Institute for Agriculture, Food, and Environment (INRAE), Antony, France.
³ Department of Geography, Institute of Ecology and Earth Sciences, University of Tartu, Tartu, Estonia.
⁴ UMR METIS, Sorbonne University CNRS EPHE, Paris, France.

PMID: 34030255
DOI: 10.1016/j.scitotenv.2021.146614

Abstract

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 HeO₂ 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-N_potential: N₂O-N_potential and 1353:1 for N₂-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 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.

Keywords: Automated chambers; Greenhouse gas; Manual chambers; N(2); Nitrate removal; Quantum cascade laser absorption spectrometer.