Flushing emissions of methane and carbon dioxide from mangrove soils during tidal cycles

Mangroves are transition areas connecting land, freshwater, and the ocean, where a great amount of organic carbon accumulates in the soil, forming a considerable carbon sink. However, the soil might also be a source of greenhouse gas (GHG) emissions. This study hypothesized that measuring GHG emissions solely during low tides can represent diurnal GHG emissions in mangroves. Methane (CH₄) and carbon dioxide (CO₂) emissions were quantified during tidal cycles using an ultraportable gas analyzer in Kandelia obovata (without pneumatophores) and Avicennia marina (with pneumatophores) mangroves in summer and fall. The results showed that the CH₄ fluxes varied greatly during tidal cycles, from -1.25 to 96.24 μmol CH₄ m^-2 h^-1 for K. obovata and from 2.86 to 2662.00 μmol CH₄ m^-2 h^-1 for A. marina. The CO₂ fluxes ranged from -4.23 to 20.65 mmol CO₂ m^-2 h^-1 for K. obovata and from 0.09 to 24.69 mmol CO₂ m^-2 h^-1 for A. marina. The diurnal variation in GHG levels in mangroves is predominantly driven by tidal cycles. The peak emissions of CH₄ and CO₂ were noted at the beginning of the flooding tide, rather than during daytime or nighttime. While the patterns of the CO₂ fluxes during tidal cycles were similar between K. obovata and A. marina mangroves, their CH₄ flux patterns during the tidal cycles differed. Possibly due to different transport mechanisms, CO₂ emissions are primarily influenced by surface soils, whereas CH₄ is predominantly emitted from deeper soils, thus being influenced by root structures. To reduce the uncertainty in measuring GHG emissions in mangrove soils during a tidal cycle, it is advisable to increase the number of GHG flux measurements during the period spanning 30 min before and after the beginning of the flooding and ebbing tides.