Carbon Flux Dynamics in a Flood-Prone Grassland: Linking CO₂ Uptake and CH₄ Emission Pulses

Grasslands play a crucial role in the global carbon cycle, yet their greenhouse gas (GHG) dynamics are highly sensitive to environmental fluctuations, especially in flood-prone systems. This study provides a full year of continuous CO₂ and high-resolution CH₄ Eddy Covariance flux measurements from a seasonally flooded and grazed floodplain grassland in Marchegg, Austria – offering a rare insight into how repeated inundation events shape the net carbon balance.

In 2024, the grassland functioned as a modest net carbon sink (–17.6 g C-CO₂ eq m⁻² yr⁻¹). Annual CO₂ uptake (–27.3 g C m⁻² yr⁻¹) was dampened by reduced photosynthesis during floods, while CH₄ emissions (1.6 g C m⁻² yr⁻¹) increased sharply and predictably with each inundation. These flood-related CH₄ pulses, captured at high temporal resolution, accounted for the majority of annual CH₄ release and strongly influenced the overall carbon budget. Whereas CO₂ exchange was primarily driven by light availability and vegetation greenness, CH₄ fluxes were almost entirely controlled by soil moisture and standing water presence, showing minimal response to grazing. The timing of flood events within the growing season proved to be critical. Both early- and mid-season inundation substantially reduced CO₂ uptake, whereas late-season flooding had only a minimal impact. Inundation also triggered pronounced methane emission hot moments, underscoring the dominant role of hydrology in controlling annual greenhouse gas fluxes.

Overall, these findings demonstrate that flood events are the primary determinant of the annual GHG balance in this grassland ecosystem. They further highlight the necessity of year-round, multi-gas monitoring to accurately capture carbon dynamics in hydrologically variable systems. In addition, the results emphasize that adaptive management practices—such as water level regulation, grazing timing, and land-use planning—are crucial for mitigating GHG emissions and enhancing ecosystem resilience under increasingly variable hydrological conditions.