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

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

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 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.

Original languageEnglish
Article number146614
Number of pages12
JournalScience of the Total Environment
Volume779
Early online date19 Mar 2021
DOIs
Publication statusE-pub ahead of print - 19 Mar 2021

Keywords

  • Automated chambers
  • Greenhouse gas
  • Manual chambers
  • N2
  • Nitrate removal
  • Quantum cascade laser absorption spectrometer

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