Effects Of Simulated Saltwater Intrusion And Fe³⁺ Increase On CH₄ And CO₂ Emissions From Tidal Freshwater Wetland In The Minjiang River Estuary

Global warming and saltwater intrusion caused by sea level rise are the focus of today’s people.Estuarine freshwater tidal wetland are considered important natural sources of CH4 and CO2,which play an important role in global warming.This_ecosystem is characterized by high temporal and spatial variations involved with topographic feature,environmental factors and astronomic tidal fluctuation,and is very sensitive to saltwater intrusion caused by sea level rise.To understand the effect of the increase of salinity,Fe3+,and the coupled salinity and Fe3+ on CH4 and CO2 fluxes from freshwater wetlands dominated by Sgittaria sgittifolia in the Min River estuary of southeast China,we measured CH4 and CO2 fluxes from field mesocosms with different treatment(simulated saltwater intrusion,ferric concentration increase,and saltwater-ferric coupled)from December 2015 to October 2016.On each sampling date,meteorological data(e.g.Air temperature,humidity),soil physicochemical propertie and porewater physicochemical properties were also measured in situ from each mesocosms.The main research conclusions were as follows:(1)The fluxes of CH4 and CO2 across various treatments from the Tajiaozhou tidal freshwater wetland exhibited strong monthly and seasonal variations.We found that high CH4 fluxes occured during August and low fluxes occured during December(control,saltwater,and saltwater-ferric coupled treatments)and January(ferric treatment).CO2 fluxes across various treatments were the lowest in December and the highest in June.On a seasonal scale,we found that high fluxes(CH4 and CO2)across various treatments occurred during summer and low fluxes occurred during winter.(2)CH4 emissions fluxes in simulated saltwater intrusion,ferric concentration increase,and saltwater-ferric coupled treatment was clearly lower than in the control treatment,which suggest that saltwater intrusion and ferric concentration increase could decrease CH4 emissions.CO2 fluxes in simulated saltwater intrusion treatment was clearly lower than in the control treatment,while there were no differences between CO2 fluxes from ferric concentration increase(or saltwater-ferric coupled)treatment and the control treatment.(3)CH4 fluxes in the Tajiaozhou tidal freshwater wetland were mainly influenced by air temperature,soil temperature,the soil moisture content and porewater NO3’-N content.(4)CO2 fluxes in the Tajiaozhou tidal freshwater wetland were mainly influenced by air temperature,humidity,soil temperature,soil conductivity,the soil moisture content,porewater pH and NO33-N content.(5)The addition of saltwater,ferric,and saltwater+ferric had a significantly impact on the physicochemical properties of soil and porewater in the Tajiaozhou tidal freshwater wetland.Compared to the control,the addition of saltwater and saltwaterfferric showed increases in soil conductivity,and porewater SO42-and Cl-concentration,while the addition of ferric and saltwater+ferric showed increases in soil Fe3+ content.