Driving and limiting factors of CH ₄ and CO ₂ emissions from coastal brackish-water wetlands in temperate regions

Coastal wetlands play a fundamental role in mitigating climate change thanks to their ability to store large amounts of organic carbon in the soil. However, degraded freshwater wetlands are also known to be the first natural emitter of methane (inline-formulaCH₄). Salinity is known to inhibit inline-formulaCH₄ production, but its effect in brackish ecosystems is still poorly understood. This study provides a contribution to understanding how environmental variables may affect greenhouse gas (GHG) emissions in coastal temperate wetlands. We present the results of over 1 year of measurements performed in four wetlands located along a salinity gradient on the northeast Adriatic coast near Ravenna, Italy. Soil properties were determined by coring soil samples, while carbon dioxide (inline-formulaCO₂) and inline-formulaCH₄ fluxes from soils and standing waters were monitored monthly by a portable gas flux meter. Additionally, water levels and surface and groundwater physical–chemical parameters (temperature, pH, electrical conductivity, and sulfate concentrations of water) were monitored monthly by multiparametric probes. We observed a substantial reduction in inline-formulaCH₄ emissions when water depth exceeded the critical threshold of 50 inline-formulacm. Regardless of the water salinity value, the mean inline-formulaCH₄ flux was 5.04 inline-formula $M12inlinescrollmathmlunit\mathrm{normal\; g}{\mathrm{normal\; m}}^{-\mathrm{normal\; 2}}{\mathrm{normal\; d}}^{-\mathrm{normal\; 1}}$ 47pt15ptsvg-formulamathimgb790de44938efcdc0f4fd466e647e96a bg-21-73-2024-ie00001.svg47pt15ptbg-21-73-2024-ie00001.png in freshwater systems and 12.27 inline-formula $M13inlinescrollmathmlunit\mathrm{normal\; g}{\mathrm{normal\; m}}^{-\mathrm{normal\; 2}}{\mathrm{normal\; d}}^{-\mathrm{normal\; 1}}$ 47pt15ptsvg-formulamathimgcb43a188c95fe696e3461dcd2324554f bg-21-73-2024-ie00002.svg47pt15ptbg-21-73-2024-ie00002.png in brackish ones. In contrast, when water depth was shallower than 50 inline-formulacm, inline-formulaCH₄ fluxes reached an average of 196.98 inline-formula $M16inlinescrollmathmlunit\mathrm{normal\; g}{\mathrm{normal\; m}}^{-\mathrm{normal\; 2}}{\mathrm{normal\; d}}^{-\mathrm{normal\; 1}}$ 47pt15ptsvg-formulamathimgc5c90d706a9f2a32c73f90e483d87b36 bg-21-73-2024-ie00003.svg47pt15ptbg-21-73-2024-ie00003.png in freshwater systems, while non-significant results are available for brackish/saline waters. Results obtained for inline-formulaCO₂ fluxes showed the same behavior described for inline-formulaCH₄ fluxes, even though they were statistically non-significant. Temperature and irradiance strongly influenced inline-formulaCH₄ emissions from water and soil, resulting in higher rates during summer and spring.