Exploring the driving factors of CH4 and CO2 emissions in coastal wetlands: a case study in the Ravenna Province, Italy

Coastal wetlands play a strategic role in the context of mitigating climate-change thanks to their ability of sequestering large amounts of organic carbon (C) and store it in the ground. However, methane (CH4) may form in the sediments of freshwater wetlands, so that these ecosystems may switch from a net sink to a net source of greenhouse gases (GHGs). Salinity is known to be one of the main inhibitors of CH4 production; however, its influence in brackish water systems is still poorly studied. Our study aims at understanding how the consequences of climate change (sea-level rise, salinization, and temperature increase) may affect the C storage in vegetated coastal wetlands.

Here we present the results of almost one year of measurements performed in four wetlands located along the northeast Adriatic coast near Ravenna, Italy. Despite a very limited distance among the four sites (1-4 km), they present a significant salinity gradient, going from fresh- to brackish waters. Air and soil temperatures and solar irradiance were continuously monitored through a network of sensors. Carbon dioxide (CO2) and CH4 fluxes from soils and waters, water head levels, surface, and ground water physical-chemical parameters (redox potential (Eh), temperature (T), pH, conductivity (EC), sulphate and sulfide concentrations) were measured monthly. Finally, soil samples were collected at each site in order to determine soil properties, i.e. organic matter content, bulk density, granulometry. 

We used multivariate statistics to investigate emergent relationships between GHGs fluxes from water and soil and environmental factors. The results of the principal component analysis (PCA) suggest that air T, water T  and irradiance play a significant role in both CH4 and CO2 emissions from water and soil. On the other hand, water head level and EC have been found to be limiting factors of the GHGs emissions. Soil properties seem to be secondary factors both in soil and water emissions.

The results obtained from these and other analyses will be presented to provide a critical insight on correlations between GHGs emissions and the environmental drivers in temperate coastal wetlands. A remote-sensing approach to upscale the results obtained on the four studied wetlands, to the adjacent coastal wetland system will also be presented. Remote sensing turns out to be a key method to extend the assessment on C fluxes to areas difficult to access and that could not be characterized otherwise.