Low water level drives high nitrous oxide emissions from treatment wetland
2022
Kasak, Kuno | Kill, Keit | Uuemaa, Evelyn | Maddison, Martin | Aunap, Raivo | Riibak, Kersti | Okiti, Isaac | Teemusk, Alar | Mander, Ü. (Ülo)
Wetlands that are restored for carbon sequestration or created for water treatment are an important sources of greenhouse gases, especially methane. The emission of nitrous oxide (N₂O) from these systems is often considered negligible due to the inundation and anerobic conditions that support complete denitrification. We used closed chamber method to analyze N₂O fluxes over a long-term period across heterogeneous wetland ecosystem constructed for treating nitrate-rich agricultural runoff. Our results showed that the water depth and temperature were most important factors affecting high N₂O emissions. The shallow areas where water depth was less than 9 cm created N₂O hot spots that emitted 48.8% of the total wetlands annual emission while only covering 6% of the total area. The annual emission from shallow-water hot spots with dense helophytic vegetation was 4.85 ± 0.5 g N₂O–N m⁻² y⁻¹ while it was only 0.37 ± 0.01 g N₂O–N m⁻² y⁻¹ in deeper zones. While the water depth was the main factor for high N₂O emissions, the temperatures increased the magnitude of the flux and therefore summer droughts and water drawdown created even larger hot spots. These results also suggest that IPCC benchmarks could underestimate N₂O emission from shallow waterbodies. Thus, it is important that the shallow zones and water level drawdown in the created or restored wetlands is avoided to minimize the N₂O flux.
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