Spatial and temporal dynamics of methane and carbon dioxide fluxes in a constructed wetland

Abstract

Constructed wetlands (CWs) are increasingly adopted as a nature-based solution for wastewater treatment. However, given their affinity with natural wetlands and high degradation of organic pollutants, their classification as a sustainable ‘green’ technology is questioned due to potentially high greenhouse gas (GHG) emissions. While methane (CH₄) and carbon dioxide (CO₂) emissions from natural wetlands and ponds have been widely studied, fluxes from CWs remain underexplored. To address this knowledge gap, CH₄ and CO₂ fluxes were measured from the Ingoldisthorpe integrated CW and an adjacent natural pond, located in the UK using the floating chamber method with gas chromatography, alongside climate and water quality monitoring. Mean seasonal CH₄ fluxes ranged from 0.13 ± 0.18 mg CH₄ m⁻² h⁻¹ (autumn) to 20.52 ± 45.01 mg CH₄ m⁻² h⁻¹ (spring), with ebullition contributing 25–93 % of total spring-summer CH₄ flux. When CH₄ was converted to CO₂ equivalents, the CH₄:CO₂ seasonal ratio revealed a greater contribution of CH₄ to the CW emission potential in spring-summer and CO₂ (mg CO₂ m⁻² h⁻¹) in autumn-winter, resulting in a mean annual global warming potential (GWP) of 3.76 kg CO₂-eq m⁻² year⁻¹. Temporal temperature variability significantly influenced CH₄ ebullitive fluxes, while spatial variability of water depth and velocity were key drivers of CH₄ and CO₂ diffusive fluxes. The absence of ebullitive flux in the natural pond adjacent to the CWs was attributed to its lower eutrophic conditions than the CWs. However, no significant differences in overall GHG emissions were observed between the CWs and the natural pond. Overall, our findings capture the complex temporal and spatial variability of GHG fluxes in CWs, highlighting the importance of comprehensive sampling to inform more accurate GHG budgeting and support evidence-based design and management strategies for sustainable CW systems.