Wadden Sea salt marshes - sinks or sources of methane and nitrous oxide?

Salt marshes are vegetated coastal habitats recognised for their great potential to act as effective soil organic carbon sinks, driven by high rates of photosynthetic CO₂ uptake and effective long-term storage of organic matter under reducing soil conditions. However, it is poorly understood when and under which conditions salt marshes can act as sinks or sources of the powerful non-CO₂ greenhouse gases CH₄ and N₂O. A complex interplay of environmental factors characterises the biogeochemistry of these ecosystems. This interplay is in turn controlled by elevation in respect to mean high water level and thereby inundation frequency, forming three vegetation zones, which are on average flooded twice daily with every high tide (pioneer zone), twice per month with every spring tide (low marsh) and sporadically during storm surges (high marsh).

We measured land atmosphere fluxes of CH₄, N₂O and CO₂ at a salt-marsh site in Nordfriesland, Germany, combining a closed chamber approach with in situ measurements of portable infrared gas analysers. From June 2018 to September 2021 we conducted biweekly (Apr-Sept) and monthly (Oct-Mar) campaigns covering the elevational gradient throughout all vegetation zones from pioneer zone to high marsh.

All greenhouse gas fluxes indicated strong dependence on elevation. Ecosystem respiration CO₂ fluxes showed highest values in the high marsh. CH₄ emissions occurred mainly in the most frequently flooded pioneer zone (up to +0.60 µmol*h^-1*m^-2), whereas low and high marsh acted as net CH₄ sinks (down to -2.0 µmol*h^-1*m^-2). Contrastingly, N₂O mainly showed positive fluxes (up to +1.1 µmol*h^-1*m^-2) in the high marsh, and the more frequently flooded zones acted as net N₂O sinks (down to  0.21 µmol*h^-1*m^-2). Further analysis of environmental variables like soil temperature, flooding frequency, groundwater level fluctuations and plant community composition will follow to identify drivers of varying greenhouse gas fluxes.

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, emphasizing their important role in mitigating global warming.