Temporal and spatial variations of greenhouse gas fluxes from a tidal mangrove wetland in Southeast China

Tidal mangrove wetlands are a source of methane (CH4) and nitrous oxide (N2O); but considering the high productivity of mangroves, they represent a significant sink for carbon dioxide (CO2). An exotic plant Spartina alterniflora has invaded east China over the last few decades, threatening these coastal mangrove ecosystems. However, the atmospheric gas fluxes in mangroves are poorly characterized and the impact of biological invasion on greenhouse gas (GHG) fluxes in the wetland remains unclear. In this study, the temporal and spatial dynamics of key GHG fluxes (CO2, CH4, and N2O) at an unvegetated mudflat, cordgrass (S. alterniflora), and mangrove (Kandelia obovata) sites along an estuary of the Jiulong River in Southeast China were investigated over a 2-year period. The CO2 and CH4 fluxes demonstrated a seasonal and vegetation-dependent variation while N2O fluxes showed no such dependent pattern. Air temperature was the main factor influencing CO2 and CH4 fluxes. Cumulative global warming potential (GWP) ranked in the order of mangrove > cordgrass > mudflat and summer > spring > autumn > winter. Moreover, CH4 accounted for the largest proportion (68%) of GWP, indicating its dominant contribution to the warming potential in mangroves. Notwithstanding the lack of information on plant coverage, cordgrass invasion exhibited a minor influence on GHG emissions. These findings support the notion that mangrove forests are net accumulation sites for GHGs. As vegetation showed considerable effects on fluxes, more information about the significance of vegetation type with a special emphasis on the effects of invasive plants is crucial.