Greenhouse gas fluxes of Wadden Sea salt marshes strongly vary among different vegetation zones

Salt marshes are highly valuable Blue Carbon ecosystems in the transition zone between marine and terrestrial environments. They play an important role in mitigating climate change due to high carbon sequestration rates through photosynthetic CO₂ uptake. However, it is poorly understood when and under which conditions they act as sinks or sources for other greenhouse gases like CH₄ and N₂O.  A complex interplay of abiotic and biotic factors characterizes the biogeochemistry of these dynamic coastal wetland ecosystems. This interplay is in turn controlled by elevation in respect to mean high water level and the resulting inundation frequency.

We measured land‑atmosphere fluxes of CH₄, N₂O and CO₂ due to ecosystem respiration at Hamburger Hallig, North Frisia, Germany, combining a closed chamber approach with in situ‑measurements of a portable Fourier transform infrared absorption spectrometer (DX4015, Gasmet). Biweekly (Apr-Sept) and monthly (Oct-Mar) campaigns have started in December 2018 and cover the whole elevational gradient from the pioneer zone over the low marsh up to the high marsh.

While ecosystem respiration showed high variability over the seasonal course with fluxes up to +67 mmol*h^-1*m^-2, CH₄ and N₂O fluxes indicated a strong dependence on elevation and thus vegetation zone. Emissions of CH₄ occurred only in the most frequently flooded pioneer zone (+0.17 to +0.35 µmol*h^-1*m^-2), whereas the less frequently flooded zones of the low and high marsh acted as CH₄ sinks (down to -1.1 µmol*h^-1*m^-2). Contrastingly, N₂O solely showed positive fluxes (up to +1.0 µmol*h^-1*m^-2) in the high marsh and the more frequently flooded zones acted as sinks for N₂O (down to ‑0.21 µmol*h^-1*m^-2). Air temperature and tidal sea water level fluctuations could already be identified as additional environmental drivers of varying greenhouse gas fluxes. Further analysis of abiotic and biotic driver variables will elucidate their impact in detail.

Our findings show that salt marshes are not only effective in assimilating CO₂. They also show the ability to take up the strong greenhouse gases CH₄ and N₂O in certain vegetation zones, emphasizing their important role in mitigating global warming.