Pathways of CH4 formation and emission in the subsaline reed wetland of Lake Neusiedl

Wetlands are a natural source of methane (CH4) emissions and represent a substantial uncertainty in the global CH4 budget. Furthermore, wetlands dominated by reed (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) to reduce uncertainties and improve the accuracy of greenhouse gas balance models for wetlands. This field study investigates all CH4 emission pathways (incl. diffusion) with various chamber types over four seasons and over the entire diel cycle (24 h) in the subsaline 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 a 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 one year. Concurrently, the drought resulted in an increase and dominance of oxygen-tolerant Methanomicrobiales.