Preserving bare mudflats reduces methane emissions: Implications for coastal wetland management

Spartina alterniflora invasion and coastal ecological engineering may influence methane (CH₄) emissions by altering wetland ecosystem carbon cycling processes. However, there remains a paucity of systematic studies on the dynamics of CH₄ production and emissions resulting from these interconnected natural and anthropogenic processes. This study employed a combined method of static chambers and greenhouse gas online analyzers to conduct in situ monitoring of CH₄ exchange fluxes across four typical coastal wetland types (bare mudflat, S. alterniflora salt marsh, deep excavation-burial treatment zone for S. alterniflora eradication and artificial mangrove introduction areas). Concurrent measurements included sediment porewater CH₄ concentration, organic carbon content with stable isotopic composition, belowground biomass, Eh, and pH, aiming to elucidate CH₄ emission characteristics and their driving mechanisms in these distinct habitats. Research has demonstrated that the belowground biomass of S. alterniflora modulates CH₄ production pathways. In managed zones, the accumulation of CH₄ in sediments was attributed to plant debris decomposition-induced alterations in organic matter content, redox potential, pH, as well as sediment water content and bulk density. The study revealed that all habitat types function as atmospheric CH₄ sources, with emission intensities exhibiting the following gradient: bare mudflat < S. alterniflora salt marsh < artificial mangrove < S. alterniflora deep excavation-burial treatment zone. S. alterniflora invasion facilitates efficient CH₄ transport via aerenchyma, achieving a 40-fold enhancement in emission fluxes despite lower porewater CH₄ concentrations than mudflats. Notably, the engineered treatment zone demonstrated an average CH₄ flux of 7.421 mg m^-2·h^-1, representing a 285-fold enhancement compared to mudflat emissions. The substantial CH₄ emissions from the S. alterniflora eradication zone primarily arise from the synergistic effects of porewater CH₄ accumulation and subsequent pulsed ebullition. Regrettably, mangrove transplantation following S. alterniflora removal can't reduce CH₄ emissions to the level of bare mudflat in the short term. The bare mudflats exhibit significantly lower CH₄ emissions than restored areas, a feature that, from the perspective of CH₄ emissions alone, suggests a more climate-friendly alternative. Ecological restoration practices should fully consider the impact of anthropogenic disturbances on CH₄ production to avoid amplifying CH₄ emissions due to inappropriate engineering measures.