Methane Flux Dynamics and Driving Mechanisms in Constructed Wetland

Abstract

Methane (CH4) is a greenhouse gas primarily produced through natural processes and human activities. Constructed wetlands (CWs), while effective for treating diffused agricultural runoff, can also emit CH4 due to the anaerobic decomposition of organic matter. This study investigates CH4 emissions and their driving mechanism in Vända CWs (designed to treat agricultural runoff) in Uhti village, southeast Estonia. The CWs have two sections (wetlands one and two). CH4 emissions were measured from October 2022 to December 2023. The result reveals that both wetlands emit CH4 emissions. However, the second wetland has higher emission with a median of 619.2 μg m⁻² hr⁻¹ (5306.1 μg m⁻² hr⁻¹ ± 936.5 μg m⁻² hr⁻¹ mean ± standard error SE) and a peak emission of about 35,000 μg m⁻² hr⁻¹ compare to the first wetland which shows a median of 218.9 μg m⁻² hr⁻¹ (2424.4 μg m⁻² hr⁻¹ ± 743.9 μg m⁻² hr⁻¹) and a peak of above 15,000 μg m⁻² hr⁻¹. Also, both wetlands exhibited seasonal variations, with summer months displaying the overall CH4 emission with a median of 2666.8 μg m⁻² hr⁻¹ (8393.3μg m⁻² hr⁻¹ ± 3351.6 μg m⁻² hr⁻¹) in the first wetland and a median of 11150.8 μg m⁻² hr⁻¹ (19430.7 μg m⁻² hr⁻¹ ± 3563.4 μg m⁻² hr⁻¹) in the second wetland. Winter months showed the lowest overall CH4 median emission of 4.9 μg m⁻² hr⁻¹ (103.5 μg m⁻² hr⁻¹ ±36.1 μg m⁻² hr⁻¹) in the first wetland and a median of 4.9 μg m⁻² hr⁻¹ (114.1 μg m⁻² hr⁻¹ ±47.4 μg m⁻² hr⁻¹) in the second wetland. Spearman's rank correlation analysis was used to assess the strength of the relationships between CH4 emission and various measured parameters. The analysis revealed that several measured parameters, including dissolved oxygen (O2), pH, oxidation-reduction potential (ORP), and conductivity, exhibited statistically significant correlations (<0.001) with CH4 emission when compared to other parameters. Based on these findings, we recommend regular harvesting of wetland biomass and installation of aeration techniques as mitigation strategies for reducing CH4 emissions from Vända-constructed wetlands.