Temporal dynamics of CH ₄ emission pathways in the subsaline reed wetland of Lake Neusiedl

Abstract. Wetlands are a natural source of methane (CH4) emissions and represent a substantial uncertainty in the global CH4 budget. Furthermore, wetlands dominated by reeds (Phragmites australis) have various CH4 emission pathways, some of which are challenging to quantify (e.g., ebullition) or require additional research (e.g., plant-mediated transport). Plant-mediated transport is often not considered in greenhouse gas balance models for wetlands, nor is the correct mode of gas transport in reeds (pressurized flow). Therefore, further field studies on CH4 emissions in wetlands, especially reed wetlands, are needed to reduce uncertainties in the global CH4 budget and to improve the parametrization and implementation of emission pathways in greenhouse gas balance models of wetlands. This field study investigates all assessable CH4 emission pathways and interfaces (diffusion, ebullition, plant-mediated transport) with various chamber types over four seasons and over the entire diel cycle (24 h) in the subsaline and dynamic reed wetland of Lake Neusiedl in Austria. The pathways of CH4 formation (methanogenesis) were examined in each season by determining δ13C source signatures and over the course of 1 year by investigating specific microbial groups (methanogens, methanotrophs, and sulfate reducers) in the sediments. The highest CH4 emissions were observed in summer, regardless of the emission pathway, with the highest emissions in all seasons occurring via the plant-mediated transport. Significant differences in CH4 fluxes were observed between the plant-mediated transport and diffusion pathway in each season. However, a distinct diel cycle of CH4 flux was exclusively observed via plant-mediated transport during summer. The source signatures δ13C-CH4 exhibit seasonal variation, with the highest 13C depletion occurring in fall. Despite the different seasonal source signatures, the dominant methanogenic pathway remains acetoclastic throughout all seasons. Desiccation of the reed ecosystem resulted in a reduction in methanogenic microbial diversity in the sediments over the course of 1 year. Concurrently, the drought resulted in an increase and dominance of oxygen-tolerant Methanomicrobiales.