Ditches as key players in carbon emissions in managed Phragmites-dominated wetland

Abstract

Phragmites wetlands absorb carbon dioxide (CO₂) and store organic matter, acting as a greenhouse gas sink, but also emit methane (CH₄) from sediment, contributing to greenhouse gas emissions. Managed Phragmites wetlands for reserve purpose, with extensive drainage ditch networks, can be significant emission hotspots for greenhouse gases (GHGs), potentially countering terrestrial carbon sequestration efforts. However, there’s a lack of detailed data on GHG emissions and control from these ditches, and their role in ecosystem carbon budgets. To fill this gap, we conducted comprehensive monitoring of CH₄ and CO₂ emissions from ditches, rivers and ponds in Qilihai Wetland, a managed Phragmites-dominated Wetland Reserve in Tianjin, China. Our study identified ditches as significant emission hotspots for both CO₂ and CH₄, emitting CO₂ at 2.33 ± 0.71 μmol m⁻² h⁻¹, approximately 4.9 and 5.3 times higher than rivers and ponds, respectively. CH₄ emissions average 15.53 ± 9.77 μmol m⁻² h⁻¹, surpassing those from rivers and ponds by about 2.9 and 2.1 times, respectively. Methane ebullition was identified as a crucial contributor to emitted CH₄ from ditches, accounting for a fraction exceeding 70 %. Emissions of CO₂ and CH₄ in all waters were closely linked to DOC concentration and water temperature (T_w). Both gases increased with higher DOC levels, but while CH₄ rose with T_w, CO₂ declined. Additionally, CH₄ emission was negatively correlated with dissolved oxygen (DO) and the TN/TP (total phosphorus/total nitrogen) ratio. In ditches, it also negatively associated with chlorophyll-a (Chl-a) and positively linked with TP, different from other waters. Despite covering just 19 % of the water area, ditch emissions accounted for 53 % of the entire aquatic CO₂ equivalent emissions. The CO₂ and CH₄ emissions from ditches accounted for 55 % and 37 % of the entire aquatic CO₂ equivalent emissions for CO₂ and CH₄, respectively. Together, they offset 12 % of the net CO₂ fixation by vegetation, with ditch CH₄ emissions comprising 6 % of the total wetland CH₄ emission. This study highlights the substantial role of ditches in carbon emissions within managed Phragmites-dominated wetlands and their responses to environmental changes, providing valuable insights for future wetland management for conservation purposes.