Factors influencing CO₂ and CH₄ emissions from coastal wetlands in the Liaohe Delta, Northeast China

Abstract. Many factors are known to influence greenhouse gas emissions from coastal wetlands, but it is still unclear which factors are most important under field conditions when they are all acting simultaneously. The objective of this study was to assess the effects of water table, salinity, soil temperature and vegetation on CH₄ emissions and ecosystem respiration (R_eco) from five coastal wetlands in the Liaohe Delta, Northeast China: two Phragmites australis (common reed) wetlands, two Suaeda salsa (sea blite) marshes and a rice (Oryza sativa) paddy. Throughout the growing season, the Suaeda wetlands were net CH₄ sinks whereas the Phragmites wetlands and the rice paddy were net CH₄ sources emitting 1.2–6.1 g CH₄ m⁻² yr⁻¹. The Phragmites wetlands emitted the most CH₄ per unit area and the most CH₄ relative to CO₂. The main controlling factors for the CH₄ emissions were water table, temperature, soil organic carbon and salinity. The CH₄ emission was accelerated at high and constant (or managed) water tables and decreased at water tables below the soil surface. High temperatures enhanced CH₄ emissions, and emission rates were consistently low (< 1 mg CH₄ m⁻² h⁻¹) at soil temperatures < 18 °C. At salinity levels > 18 ppt, the CH₄ emission rates were always low (< 1 mg CH₄ m⁻² h⁻¹) probably because methanogens were out-competed by sulphate-reducing bacteria. Saline Phragmites wetlands can, however, emit significant amounts of CH₄ as CH₄ produced in deep soil layers are transported through the air-space tissue of the plants to the atmosphere. The CH₄ emission from coastal wetlands can be reduced by creating fluctuating water tables, including water tables below the soil surface, as well as by occasional flooding by high-salinity water. The effects of water management schemes on the biological communities in the wetlands must, however, be carefully studied prior to the management in order to avoid undesirable effects on the wetland communities.