Unlocking the Impact of Temperature and Salinity on Greenhouse Gas Emissions From Estuarine Salt Marsh Soils

Salt marshes hold substantial potential for nature-based climate solutions; yet their carbon sequestration potential is threatened by greenhouse gas (GHG) emissions under global warming and sea-level rise (SLR). The combined effects of temperature and salinity on soil GHG emissions still remain unclear. This study integrated in situ monitoring and laboratory-controlled incubation to quantify the fluxes of CO₂, CH₄, and N₂O from the salt marsh (Phragmites australis) soils in the Yangtze River Estuary, in response to temperature and salinity variations. The results demonstrated that warming magnified the influence of salinity, elevating the salinity threshold for GHG emissions, while at low temperatures (<20°C), emissions remained unaffected by salinity. At low salinity (0–4 ppt), temperature significantly stimulated emissions, whereas higher salinity suppressed them. Nevertheless, at elevated salinity, soil respiration becomes more temperature-sensitive, indicating heightened microbial activity under favorable thermal conditions. Soil temperature was identified by the random forest model as the dominant control on GHG emissions than salinity, with 25°C and 5 ppt identified as tipping points for temperature and salinity. Under SLR scenarios of 0.5, 1, and 2 m, GHG fluxes from the Yangtze River Estuary wetlands exhibited a slight decline in emissions with moderate salinity increases, but emissions intensify under more extreme SLR scenarios. This study underscores the combined impact of global warming, SLR, and saltwater intrusion on coastal GHG emissions, highlighting the potential reduction in the effectiveness of carbon sequestration by salt marshes. The findings offer valuable insights for evaluating future blue carbon dynamics under global change.