The role of sediment transport in regulating ecosystem CO₂ fluxes during flood events in a restored floodplain grassland

The occurrence of flood events in wetlands has a significant impact on ecosystem dynamics, particularly regarding carbon cycling and storage. While there is a consensus that an understanding of the dynamics of restored wetlands is essential for the mitigation of climate change, the potential for carbon sequestration in floodplain grasslands remains understudied. Two floods in June and July 2024 in a restored floodplain grassland in Austria, exhibited distinct ecosystem responses, notably in gross primary productivity (GPP), net ecosystem exchange (NEE), and ecosystem respiration (Reco), as well as methane exchange, measured via the eddy covariance (EC) method with open path gas analysers (LI-7500 DS and LI-7700, LI-COR Biosciences, Lincoln, NE, USA).

Following the floods, carbon uptake significantly declined due to complete submersion of around 2m water levels above grounds continuing for six days for the first flood and 1m lasting three days for the second flood. However, ecosystem recovery varied, with slower recovery after the June flood. Sediment deposition during the first flood in June hindered photosynthesis in older plant parts, as evidenced by a brown sediment layer, while new growth remained green and photosynthetically active. This sediment layer contributed to reduced GPP during recovery. Conversely, the sediment-light July flood caused only a brief decline in NEE and GPP, with rapid recovery and no sediment deposition. The contrasting sediment loads stemmed from the floods’ origins: the first from sediment-heavy Danube backwater following upstream precipitation and the second from a sediment-light sluice opening on the Morava River. These differences explain the varying impacts on plant vitality, photosynthesis, and CO₂ exchange. One month after the first flood net CO₂ uptake remained below pre-flood levels, reflecting ecosystem stress and adaptation. In addition, climatic conditions also played a role in ecosystem responses. The favourable temperatures and abundant rainfall in May 2024 provided an environment conducive to plant growth. However, heat and the natural decline of the growing period in July led to an exacerbation of the flood impacts. These differing growth stages likely contributed to the varying plant sensitivities to the flood events, intensifying GPP reductions in response to sediment transport. The prolonged presence of water bodies during recovery, with slow withdrawal and evaporation, further influenced evapotranspiration dynamics during flooding.

These findings indicate that sediment transport dynamics during floods can significantly influence plant vitality and photosynthesis, with implications for ecosystem CO₂ exchange. The interplay between sediment deposition, plant recovery, and flood timing underscores the necessity for further research to elucidate these processes and their role in carbon cycling, as well as ecosystem resilience and vulnerability in floodplain grasslands. This study illuminates the intricate interactions between hydrology, plant processes, and carbon cycling in floodplain grasslands under climatic extremes by linking sediment dynamics with ecosystem recovery.

 

Funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or CINEA. Neither the European Union nor the granting authority can be held responsible for them.