Diffusive CH4 emissions from wetlands: the Porta and Massaciuccoli (northern Tuscany, central Italy) case studies

Wetlands are among the main natural sources of CH4 emissions to the atmosphere. Nevertheless, large uncertainties still affect the estimates of their overall contribution at both local and global scales due to several knowledge gaps on how environmental drivers shape CH4 emissions from these delicate ecosystems. In this work, we present CH4 diffusive fluxes, along with chemical and isotopic data on dissolved ionic and gaseous species, from two wetlands located in northern Tuscany (Italy), i.e. (i) Porta, a small wetland invaded by Phragmites australis reeds and affected by reed die-back syndrome (RDBS) since 2016, and (ii) Massaciuccoli, a wide marsh area including both open water basins and channels affected by seawater intrusion and eutrophication. Both wetlands were found to be net sources of CH4 to the atmosphere. The magnitude of CH4 emission at the water-air interface was governed by CH4 production and consumption rates, which were depending on (i) water temperature and availability of labile carbon substrates, and (ii) water column depth, wind exposure, and dissolved O2 contents, respectively. Our results suggested that reed beds enhance diffusive CH4 fluxes, likely as a consequence of wind sheltering and availability of plant litter. The latter provides labile organic matter promoting acetoclastic methanogenesis as suggested by carbon isotopic signatures of CH4. These findings indicate that a relevant portion of CH4 emissions from waterbodies is likely underestimated in global carbon budgets, with relevant implications for Earth system and climate models. Moreover, while wetlands are among the most threatened ecosystems on Earth, the relationship between RDBS and high CH4 diffusive fluxes underlines the urgent need of wetland protection and restoration interventions.