Deciphering CH4 emission pathways in a reed ecosystem employing chamber measurements and stable carbon isotope signatures

Wetlands dominated with common reed (Phragmites australis) can store carbon due to photosynthetic assimilation of carbon dioxide and sequestration of organic matter produced in the wetland soil or release it by emission of sediment-produced methane (CH₄). On an annual timescale about 15 % of the net carbon fixed by wetlands may be released to the atmosphere as CH₄. However, little is known about the effects of climate change on central European wetland ecosystems with reed and the quantification of the different pathways of CH₄ emissions in reed belts and their underlying processes.

With an area of approximately 181 km², the reed belt of Lake Neusiedl is the second largest coherent reed population in Europe after the Danube delta and forms a mosaic of water, reed and sediment patches, which varies between the seasons. Lake Neusiedl is the largest lake of Austria and the westernmost steppe lake of Europe with no natural outflow. It is a saline and very shallow lake with water levels of maximal 1.5 m but can differ strongly between the shorelines due to strong winds. Due to its shallowness, the lake is very sensitive to climate variations.

To investigate the different pathways of CH₄ emissions, 24-hour measurement campaigns were conducted in the reed belt near the Biological Station Illmitz on the east side of Lake Neusiedl every 3 months (seasonally) in 2021.

Various chamber measurement systems were used to capture the different pathways of CH₄ emissions in the reed belt: Ebullition traps for the ebullition of gas bubbles from supersaturated sediments, floating chambers for the molecular diffusion transport at the water-atmosphere interface, soil chambers for the molecular diffusion transport at the soil-atmosphere interface and vegetation chambers for the plant-mediated transport of P. australis.

Methane concentrations and stable carbon isotope values of methane (δ¹³C-CH₄) and carbon dioxide (δ¹³C-CO₂) were measured with an isotope measurement technique of Cavity Ring Down Spectroscopy (Picarro G2201-i). The δ¹³C-CH₄ can be used to differentiate biological and geological sources of CH₄ emissions and to examine the mechanisms of CH₄ production and oxidation. Additionally, sediment and water samples were taken every campaign and analysed for various parameters such as TOC, sulphate, nitrate, ammonium and phosphate.

Here, we quantify the different pathways of CH₄ emissions, evaluate the underlying factors being responsible for seasonal variations and examine the differences in diurnal pattern.

Preliminary results indicate (1) the highest CH₄ emissions in the summer season, (2) a significant difference in CH₄ fluxes between each emission pathway per season, (3) that only the δ¹³C-CH₄ values from the ebullition pathway differs clearly from all other pathways and (4) that the δ¹³C-CH₄ values from ebullitions are in the same range as δ¹³C-CH₄ values of the second sediment horizon.